JP7790169B2 - Power supply device and image forming apparatus - Google Patents

Description

The present invention relates to a power supply device, and in particular to a power supply device with an overcurrent detection function, and an image forming apparatus equipped with such a power supply device.

Conventionally, there have been power supply devices that ensure supply voltage during short-term power outages by maintaining a low brownout threshold during a power outage so that the primary rectified/smoothed voltage does not fall below the brownout threshold even if it drops (for example, Patent Document 1).

JP 2016-24445 A (pages 9 and 10, FIG. 9)

However, power supply devices equipped with an overcurrent threshold to prevent overcurrent in the primary current were unable to prevent unnecessary overcurrent detection from occurring when the primary current fluctuated due to, for example, a momentary interruption in the AC power supply.

The power supply device according to the present invention comprises:
a PFC circuit having a PFC control unit that receives a full-wave rectified voltage of an AC input voltage, switches a switching element to boost a voltage with a coil, and determines that an overcurrent has occurred when a voltage across a current detection resistor through which a primary current flows becomes equal to or exceeds a predetermined value with respect to a primary current flowing through the switching element; a capacitor that smoothes the primary voltage boosted by the coil; and a voltage feedback unit that feeds back the primary voltage; a first overcurrent threshold switching circuit that detects when the AC input voltage is turned off, and configured such that a second current detection resistor is connected in parallel to a first current detection resistor as the current detection resistor; and a second overcurrent threshold switching circuit that detects when the primary voltage drops to a first reference value, and configured such that a third current detection resistor is connected in parallel to the first current detection resistor as the current detection resistor;
When the AC input voltage is turned off and the primary voltage drops to the first reference value, the current detection resistor is configured such that the first current detection resistor, the second current detection resistor, and the third current detection resistor are connected in parallel.

Another power supply device according to the present invention comprises:
The PFC control unit receives a full-wave rectified voltage of an AC input voltage, switches a switching element to boost the voltage with a coil, and determines that an overcurrent has occurred when a primary current flowing through the switching element exceeds a threshold value; a capacitor smoothes the primary voltage boosted by the coil; and an overcurrent threshold value switching unit switches the threshold value.
the overcurrent threshold switching unit includes a first overcurrent threshold switching circuit that detects the AC input voltage being turned off and switches the threshold from a first threshold to a second threshold that is higher than the first threshold by a first increment, and a second overcurrent threshold switching circuit that detects the primary voltage decreasing to a first reference value and switches the threshold from the first threshold to a third threshold that is higher than the first threshold by a second increment,
When the AC input voltage is turned off and the primary voltage drops to the first threshold, the threshold is switched to a fourth threshold obtained by adding the first increment and the second increment.

According to the present invention, the overcurrent threshold for the primary current can be appropriately switched when the load increases, when the AC power supply is momentarily interrupted, etc., thereby preventing overcurrent detection from being performed more than necessary.

1 is a diagram showing a configuration of a main part of an image forming apparatus according to a first embodiment of the present invention; 2 is a block diagram showing the configuration of a main part of a power supply unit and a control unit of a control system of the image forming apparatus; FIG. FIG. 3 is a block diagram showing a detailed configuration of the power supply unit described in FIG. 2 . 4 is a circuit diagram showing detailed configurations of a first overcurrent threshold switching circuit, a second overcurrent threshold switching circuit, and a third overcurrent threshold switching circuit described in FIG. 3. FIG. 5 is a time chart showing changes in voltage and current detected at each part shown in the block diagrams of FIGS. 2 and 3 and the circuit diagram of FIG. 4 when the AC input voltage (A) is 100 V during the operation of the power supply device. 5 is a time chart showing changes in voltage and current detected at each part shown in the block diagrams of FIGS. 2 and 3 and the circuit diagram of FIG. 4 when the AC input voltage (A) is 230 V during the operation of the power supply device. 3 is a control system block diagram of an image forming apparatus shown as a comparative example to the control system of the image forming apparatus of the present embodiment shown in FIG. 2. FIG. FIG. 8 is a block diagram showing a detailed configuration of the power supply unit described in FIG. 7. 9 is a time chart showing changes in voltage and current detected at each part shown in the block diagrams of FIGS. 7 and 8 during the operation of the power supply device of the comparative example. FIG. 11 is a block diagram showing the configuration of a main part of a power supply unit as a power supply device and a control unit in a control system of an image forming apparatus according to a second embodiment of the present invention. FIG. 11 is a block diagram showing a detailed configuration of the power supply unit described in FIG. 10 . 12 is a circuit diagram showing detailed configurations of the first overcurrent threshold switching circuit, the second overcurrent threshold switching circuit 131, the third overcurrent threshold switching circuit, the fourth overcurrent threshold switching circuit that detects a drop in AC input voltage, and the fifth overcurrent threshold switching circuit (A) that detects a drop in AC input voltage over a long period of time, all of which are described in FIG. 13 is a time chart showing changes in voltage, current, and temperature detected at each part shown in the block diagrams of FIGS. 10 and 11 and the circuit diagram of FIG. 12 during the operation of the power supply device. FIG. 9 is a time chart showing changes in voltage, current, and temperature detected at each part shown in the block diagrams of FIGS. 7 and 8 when the AC input voltage (H) similarly drops during the operation of the power supply device of the comparative example shown in FIGS. 7 and 8 . FIG. 10 is a circuit diagram showing detailed configurations of a first overcurrent threshold switching circuit, a second overcurrent threshold switching circuit, a third overcurrent threshold switching circuit, a fourth overcurrent threshold switching circuit, and a fifth overcurrent threshold switching circuit (B) that detects the temperature of a PFC coil in the third embodiment. 16 is a time chart showing changes in voltage, current, and temperature detected at each part shown in the circuit diagram of FIG. 15 during the operation of the power supply device.

Embodiment 1.
FIG. 1 is a diagram showing the configuration of a main part of an image forming apparatus according to a first embodiment of the present invention.

As shown in the figure, the image forming apparatus 100 is broadly divided into a paper feed unit 1, an image forming unit 2, a fixing unit 3, and a paper discharge unit 4. The paper feed unit 1 includes a paper cassette 5 for setting recording paper, pickup rollers 6, 7, and 8 that separate the recording paper into individual sheets, remove them from the paper cassette 5, and send them to the paper transport path (a roughly S-shaped path indicated by the bold line in Figure 1), and registration rollers 9 and 10 that transport the recording paper to the image forming unit 2.

The image forming unit 2 includes four toner image forming units 19K, 19Y, 19M, and 19C (which may simply be referred to as toner image forming units 19 when no particular distinction is required) arranged in series from upstream in the recording paper transport direction, LED heads 15K, 15Y, 15M, and 15C (which may simply be referred to as LED heads 15 when no particular distinction is required) arranged corresponding to each toner image forming unit 19, and a transfer unit 21 that transfers the toner images formed by the toner image forming units 19 onto the top surface of the paper using Coulomb force.

Toner image forming unit 19K forms a black (K) toner image, toner image forming unit 19Y forms a yellow (Y) toner image, toner image forming unit 19M forms a magenta (M) toner image, and toner image forming unit 19C forms a cyan (C) toner image.

Each toner image forming unit 19 includes a photosensitive drum 11, which is an electrostatic latent image carrier; a charging roller 12 that contacts the photosensitive drum 11 and uniformly charges the surface of the photosensitive drum 11 to a high voltage; a developing roller 13 that uses toner to develop the electrostatic latent image formed on the surface of the photosensitive drum 11 using an LED head 15 located above the photosensitive drum 11; a toner supply roller 14 that contacts the developing roller 13 and supplies toner to the developing roller 13; and a detachable toner cartridge 16 that contains and supplies toner. The LED head 15 selectively exposes the charged surface of the photosensitive drum 11 to light to form an electrostatic latent image.

The transfer unit 21 includes a transfer belt 17 that transports the recording paper transported from the paper feed unit 1 in the direction of the arrow, and four transfer rollers 18 arranged opposite the photosensitive drums 11 of each toner image forming unit 19 via the transfer belt 17. The toner images of each color formed on the photosensitive drums 11 of each toner image forming unit 19 are transferred onto the recording paper in succession, superimposed on top of each other, by Coulomb force.

The fixing unit 3 comprises a fixing roller 31, a halogen lamp 32 as a heating element located inside the fixing roller 31, a temperature detection sensor 33 such as a thermistor for detecting the surface temperature of the fixing roller 31, and a pressure roller 34 placed in pressure contact with the fixing roller 31. The toner image transferred to the recording paper is fixed onto the recording paper by heat and pressure. The paper discharge unit 4 comprises discharge rollers 41 and 42 for discharging the recording paper after fixing.

Figure 2 is a block diagram showing the main components of the control system of the image forming apparatus 100, including the power supply unit 120 as a power supply device and the control unit 160.

In the figure, the power supply unit 120 is broadly divided into a switch 121, a bridge diode 122, a PFC circuit 123, a DC-DC conversion unit 124, a first overcurrent threshold switching circuit 130 that responds to momentary interruptions in the AC input voltage, a third overcurrent threshold switching circuit 132 that responds to the AC input voltage, a second overcurrent threshold switching circuit 131 that responds to current loads such as motor startup, an AC zero-cross detection circuit 126, and a heater on/off circuit 127, and operates on the AC voltage output from the commercial power supply 205. The switch 121 is located at the input of the power supply unit 120 and is a manual switch that turns the input from the commercial power supply 205 on and off, but it may also be a switch that is turned on and off by the device control unit.

The heater on/off circuit 127 is a circuit that turns on and off the halogen lamp 32 (Figure 1) that serves as a heater inside the fixing unit 3 in response to a heater on/off signal output from the main control unit 161. The PFC circuit 123 constitutes an AC-DC converter that boosts AC voltage to DC voltage, and is a so-called PFC (Power Factor Correction) circuit designed to improve the power factor. It inputs the full-wave rectified voltage that is the output of the bridge diode 122, and outputs the boosted DC voltage to the DC-DC conversion unit 124.

The DC-DC conversion unit 124 supplies DC voltage to the control unit 160. Here, 24V DC is supplied to the actuator system and 5V DC is supplied to the logic system, but the type of DC voltage output from the power supply unit 120 is generally determined by the configuration of the control unit 160, and a DC 3.3V output is also common. The AC zero-cross detection circuit 126 is a circuit that outputs an AC zero-cross signal to the main control unit 161, which will be described later.

The input section of the first overcurrent threshold switching circuit (momentary interruption) 130, which responds to momentary interruptions in the AC input voltage, is connected to the AC zero-cross detection circuit 126 and the PFC circuit 123, and its output section is connected to the PFC circuit 123. The input section of the third overcurrent threshold switching circuit (AC input voltage) 132, which responds to the AC input voltage, is connected to the AC zero-cross detection circuit 126 and the PFC circuit 123, and its output section is connected to the PFC circuit 123. The input and output sections of the second overcurrent threshold switching circuit (motor start) 131, which responds to motor start, are both connected to the PFC circuit 123. The first overcurrent threshold switching circuit 130, second overcurrent threshold switching circuit 131, and third overcurrent threshold switching circuit 132, as described below, are circuits that switch the overcurrent threshold for the PFC circuit 123.

The control unit 160 is composed of a main control unit 161, ROM 162, RAM 163, temperature detection unit 164, sensor on/off circuit 165, high-voltage power supply 166, head control unit 167, and actuator drive unit 168.

The main control unit 161 is a device that operates according to programs written in ROM 162, a non-volatile storage component that stores programs and setting data. RAM 163 is memory that stores and reads data.

The temperature detection unit 164 resistively divides the output of the temperature detection sensor 33 inside the fixing unit 3 and outputs a temperature detection signal to the main control unit 161. The sensor on/off circuit 165 is composed of transistors, and basically outputs a sensor off signal from the main control unit 161 to turn off the power supplied to the various sensors 201 described below, except when the device is warming up after the power is turned on or when printing is performed in response to instructions from the host 206, etc. The high-voltage power supply 166 is a power supply that applies a high voltage to the photosensitive drum 11 and various rollers of the image forming unit 2, as described in Figure 1.

The head control unit 167 is a control unit that controls the on/off of the LED head 15, as described in Figure 1. The actuator drive unit 168 is a dedicated driver that outputs a drive signal to the actuator 202 (described below) based on a logic signal output from the main control unit 161. The paper feed unit 1, image forming unit 2, and paper discharge unit 4 are as described above in Figure 1.

The various sensors 201 include paper path sensors (not shown) for detecting paper position, and sensors for correcting image density and color misalignment, which are arranged in the paper feed unit 1, image forming unit 2, fixing unit 3, and paper discharge unit 4. The actuators 202 are driven by the actuator drive unit 168 and include motors, clutches, solenoids, cooling fans, and other devices (not shown) that are arranged in the paper feed unit 1, image forming unit 2, fixing unit 3, and paper discharge unit 4.

Figure 3 is a block diagram showing the detailed configuration of the power supply unit 120 described in Figure 2, and is broadly divided into a switch 121, a protection element 500, a filter 501, an inrush current prevention circuit 502, a bridge diode 122, a PFC circuit 123, a DC-DC conversion unit 124, a first overcurrent threshold switching circuit (momentary interruption) 130, a second overcurrent threshold switching circuit (motor start) 131, a third overcurrent threshold switching circuit (AC input voltage) 132, an AC zero-cross detection circuit 126, and a heater on/off circuit 127.

The protective element 500 consists of a fuse for overcurrent protection and a varistor for lightning surge protection. The filter 501 is generally composed of a common or normal choke coil and a capacitor. The capacitor consists of an X capacitor placed between the LINE and NEUTRAL, and a Y capacitor placed between the LINE or NEUTRAL and FG (frame ground). The inrush current prevention circuit 502 is a circuit that suppresses the inrush current of the PFC output electrolytic capacitor 604. A thermistor is an inexpensive component, but since it cannot suppress the inrush current when the thermistor is hot, a circuit combining a resistor and a switching element such as a triac or relay may also be used. The bridge diode 122 consists of four diodes, and a four-element bridge diode is often used.

The PFC circuit 123 is composed of a PFC power device 603 (FET) as a switching element, a PFC coil 601, a current detection resistor 605 as a first current detection resistor, a PFC diode 602, a PFC output electrolytic capacitor 604 as a smoothing capacitor, a voltage feedback unit 607, and a PFC control circuit 606 as a PFC control unit. The PFC control circuit 606 is the control unit of the PFC circuit 123 that receives AC voltage full-wave rectified by the bridge diode 122, converts it to DC voltage, and boosts it. A dedicated IC or microcomputer is typically used. Taking into account the maximum AC input, the boosted voltage is typically set to approximately 264V AC x √2 + 10V = 390V for worldwide input.

The PFC control circuit 606 receives the detection results of the voltage feedback unit 607 and the current detection resistor 605, determines the gate voltage of the PFC power device 603, and outputs it. The PFC coil 601 is a boost coil. The PFC power device 603, which is an FET, is a switching power device, and its gate input terminal receives input from the PFC control circuit 606. The current detection resistor 605 is a resistor that detects the drain current of the PFC power device 603, and the detection result is output to the PFC control circuit 606.

The PFC diode 602 is a rectifier diode that outputs to the PFC output electrolytic capacitor 604. The PFC output electrolytic capacitor 604 smoothes the PFC output voltage and delays the drop in output voltage during a momentary power outage. The voltage feedback unit 607 divides the PFC output voltage and outputs the detection result to the PFC control circuit 606, the first overcurrent threshold switching circuit 130, the second overcurrent threshold switching circuit 131, and the third overcurrent threshold switching circuit 132. Note that, in this example, the three output voltage values of the voltage feedback unit 607 are assumed to be the same.

The DC-DC conversion unit 124 is composed of a transformer 701, a main FET 704, a snubber circuit 703, a power supply control unit 702, a current detection resistor 705, a secondary rectifying and smoothing circuit 503, a voltage feedback unit 504, a protection circuit 505, a filter 506, and a DC-DC converter 507.

The transformer 701 insulates the primary and secondary sides and also transforms the PFC output voltage output by the PFC circuit 123. The main FET 704 turns on and off the power supplied to the primary winding of the transformer 701. The snubber circuit 703 is a circuit that suppresses surge voltage when the main FET 704 is turned off. It is often composed of diodes, resistors, and capacitors. The power supply control unit 702 determines the gate voltage on-duty of the main FET 704 mainly based on the feedback results of the DC output voltage on the secondary side.

The secondary rectifying and smoothing circuit 503 rectifies and smooths the output voltage from the secondary winding of the transformer 701. Here, a single winding outputs 24V DC, and a rectifying diode and electrolytic capacitor are installed. The voltage feedback unit 504 divides the output voltage and outputs the detection result to the power supply control unit 702. The protection circuit 505 is equipped with an overvoltage detection circuit and an overcurrent detection circuit. The overvoltage protection circuit is composed of a Zener diode and a photocoupler, and when an overvoltage is detected, the primary-side power supply control unit 702 latches or intermittently stops switching. The overcurrent detection circuit has various circuit configurations, such as current detection, DC output voltage droop detection, and fuses. It is also possible for the power supply control unit 702 to detect it as a primary current.

The secondary filter 506 is an LC filter. Although it is not necessary to install it, it is used to suppress ripple voltage and ripple noise voltage. The DC-DC converter 507 has the function of converting the output 24V to 5V.

The AC zero-cross detection circuit 126 is located before the PFC circuit 123 and is composed of a rectifier diode 801 and a photocoupler 802. It outputs a full-wave rectified voltage signal to the first overcurrent threshold switching circuit 130 and the third overcurrent threshold switching circuit 132, and also outputs a pulsed AC zero-cross signal that becomes high level [H] at the zero-cross point to the main control unit 161 (Figure 2).

The first overcurrent threshold switching circuit (momentary interruption) 130, which responds to momentary interruptions in the AC input voltage, is a circuit that changes the overcurrent threshold by switching on and off the parallel connection of current detection resistor 1009 (see Figure 4) as a second current detection resistor to current detection resistor 605 of PFC circuit 123 based on the output of AC zero-cross detection circuit 126 and voltage feedback unit 607. The second overcurrent threshold switching circuit (motor start) 131, which responds to motor start, is a circuit that changes the overcurrent threshold by switching on and off the parallel connection of current detection resistor 2008 (see Figure 4) as a third current detection resistor to current detection resistor 605 of PFC circuit 123 based on the output of voltage feedback unit 607. The third overcurrent threshold switching circuit (AC input voltage) 132 corresponding to the AC input voltage is a circuit that switches on and off the series connection of the current detection resistor 3008 (see Figure 4) as the fourth current detection resistor to the current detection resistor 605 of the PFC circuit 123 based on the outputs of the AC zero-cross detection circuit 126 and the voltage feedback unit 607, thereby changing the overcurrent threshold. Note that the AC zero-cross detection circuit configuration shown here is an example, and the configuration is not particularly limited.

Figure 4 is a circuit diagram showing the detailed configuration of the first overcurrent threshold switching circuit (momentary interruption) 130, the second overcurrent threshold switching circuit (motor start) 131, and the third overcurrent threshold switching circuit (AC input voltage) 132 described in Figure 3.

In the first overcurrent threshold switching circuit (momentary interruption) 130, one end of resistor 1004 is connected to the cathode output of rectifier diode 801 of AC zero-cross detection circuit 126 via resistor 1002 and to the anode output of rectifier diode 801 via resistor 1003, and the other end is connected to the base of transistor 1001. The emitter of transistor 1001 is connected to one end of resistor 1004 via capacitor 1005 and to the anode output of rectifier diode 801, and the collector is connected to output resistor 611 of voltage feedback unit 607 via resistor 1010. The base of transistor 1007 is connected to output resistor 611 via resistor 1006, the emitter is connected to output resistor 611 via capacitor 1008 and to the other end of current detection resistor 605 via current detection resistor 1009, and the collector is connected to one end of current detection resistor 605, i.e., the source of PFC power device 603.

In the above configuration, resistor 1004 determines the base current of transistor 1001, and resistor 1006 determines the base current of transistor 1007. As will be described later, capacitor 1005 keeps transistor 1001 on until the AC power supply is momentarily interrupted, and capacitor 1008, together with resistor 1006, works to delay the turn-off timing of transistor 1007. Furthermore, when transistor 1001 turns off, transistor 1007 turns on, current detection resistor 1009 is connected in parallel with current detection resistor 605, and the overcurrent threshold is changed.

In the second overcurrent threshold switching circuit (motor start) 131, the base of transistor 2004 is connected to output resistor 612 of voltage feedback section 607 via Zener diode 2001 and resistor 2002 connected in series, and is also connected to the emitter via resistor 2003. The emitter is directly connected to the anode output section of rectifier diode 801, and the collector is connected to output resistor 612 via resistor 2009. Transistor 2007 has its base connected to output resistor 612 via resistor 2005, its emitter connected to output resistor 612 via capacitor 2006 and the other end of current detection resistor 605 via current detection resistor 2008, and its collector connected to one end of current detection resistor 605, i.e., the source of PFC power device 603.

In the above configuration, resistor 2002 is a resistor that limits the base current of transistor 2004, and resistor 2005 is a resistor that determines the base current of transistor 2007. As will be described later, capacitor 2006, together with resistor 2005, works to delay the turn-off timing of transistor 2007. Furthermore, when transistor 2004 turns off, transistor 2007 turns on, current detection resistor 2008 is connected in parallel with current detection resistor 605, and the overcurrent threshold is changed.

In the third overcurrent threshold switching circuit (AC input voltage) 132, one end of resistor 3003 is connected to the cathode output of rectifier diode 801 of AC zero-cross detection circuit 126 via Zener diode 3001 and to the anode output of rectifier diode 801 via resistor 3002, and the other end is connected to the base of transistor 3004. Transistor 3004 has its emitter connected to one end of resistor 3003 via capacitor 3005 and to the cathode output of rectifier diode 801, and its collector connected to output resistor 611 of voltage feedback unit 607 via resistor 3010. The base of transistor 3007 is connected to output resistor 611 via resistor 3006, its emitter is connected to output resistor 611 via capacitor 3009 and to the primary side ground, its collector is connected to the other end of current detection resistor 605, and current detection resistor 3008 is connected between the collector and emitter.

In the above configuration, resistor 3003 is a resistor that limits the base current of transistor 3004, and resistor 3006 is a resistor that determines the base current of transistor 3007. As will be described later, capacitor 3005 works together with resistor 3003 to delay the turn-off timing of transistor 3004, and capacitor 3009 works together with resistor 3006 to delay the turn-off timing of transistor 3007. Furthermore, when transistor 3004 turns on, transistor 3007 turns off, current detection resistor 3008 is connected in series with current detection resistor 605, and the overcurrent threshold is changed.

Figure 5 is a time chart showing the changes in voltage and current detected at each part shown in the block diagrams of Figures 2 and 3 and the circuit diagram of Figure 4 when the AC input voltage (A) is 100V during the operation of the power supply device of this embodiment. The operation of the power supply device will be explained with reference to this time chart, Figures 3 and 4. Note that the symbols (A) through (G) attached to the voltage and current signal waveforms in this time chart are attached to the corresponding detection points in Figures 2, 3, and 4.

The waveforms shown in the time chart of FIG. 5 are as follows:
AC input voltage (A): This is the AC voltage output from the commercial power supply 205, that is, the AC voltage input to the power supply unit 120. In this example, the AC voltage is 100 V AC.
AC zero-cross signal (B): This is an output signal from the AC zero-cross detection circuit 126 that detects the zero-cross points of the AC voltage output from the commercial power supply 205.
PFC output voltage (C): This is a DC voltage output from the PFC circuit 123. Taking into account the maximum AC input, this DC voltage is generally boosted to approximately AC 264V×√2+10V=390V in the case of worldwide input.
PFC primary current (D): This is the primary current that flows through the PFC power device 603, which is an FET in the PFC circuit 123, and overcurrent detection is performed based on this primary current.
Gate voltage (E): This is the gate voltage of the main FET 704 output from the power supply control unit 702 of the DC-DC conversion unit 124.
DC output 5V (F): DC 5V output voltage output from the DC-DC converter 507 of the DC-DC conversion unit 124.
DC output 24V (G): This is the DC 24V output voltage output from the DC-DC conversion unit 124.

The horizontal axis of the time chart in Figure 5 is the time axis common to all of these signals, and explains how the power supply device's operation changes over time (from time t11 to time t17).

In this case, since the AC voltage is 100 V AC, the Zener diode 3001 of the third overcurrent threshold switching circuit (AC input voltage) 132 is set to remain off. As a result, during this time, transistor 3004 remains off and transistor 3007 remains on, so no current flows through current detection resistor 3008 and the effect of current detection resistor 3008 can be ignored.

At time t11, one (or more) of the motors of actuator 202 starts up, and as the PFC primary current (D) begins to rise, the PFC output voltage (C) starts to decrease from voltage value V1. Thereafter, at time t12, when the PFC output voltage (C) decreases to voltage value V2, Zener diode 2001 of second overcurrent threshold switching circuit (motor start) 131 turns off, transistor 2004 turns off, and transistor 2007 turns on. As a result, current detection resistor 2008 is connected in parallel with current detection resistor 605, and a portion of the PFC primary current (D), which is the switching current of PFC power device (FET) 603, also flows through current detection resistor 2008, causing the overcurrent threshold of PFC control circuit 606 to change from I6 to I4.

The PFC primary current (D) then continues to rise and reaches current value I5, but does not exceed overcurrent threshold I4. Therefore, overcurrent threshold I4 at this time is set to be higher than current value I5. Meanwhile, the PFC output voltage (C) drops to voltage value V3, but this voltage value V3 does not fall below the brownout threshold. The brownout threshold is the voltage value at which the power supply control unit 702 instructs the DC-DC conversion unit 124 to stop outputting when the PFC output voltage (C) drops below this value.

At time t13, when the PFC output voltage (C) rises again to voltage value V2, the first reference value, Zener diode 2001 turns on, turning on transistor 2004 and preventing current from being supplied to transistor 2007. However, because capacitor 2006 is charged, the time constant of resistor 2005 and capacitor 2006 causes transistor 2007 to turn off later than time t13. The time constant is set to approximately 100 ms, and transistor 2007 turns off at time t14, a predetermined time after time t13. At this time, current stops flowing through current detection resistor 2008, and the overcurrent threshold of the PFC control circuit 606 returns from I4 to I6.

Next, when the AC input voltage (A) turns off at time t15, the PFC output voltage (C) begins to discharge, and the AC zero-cross signal (B) stops outputting pulses. At this time, transistor 1001 of the first overcurrent threshold switching circuit (momentary interruption) 130 turns off, and transistor 1007 turns on. As a result, current detection resistor 1009 is connected in parallel with current detection resistor 605, and part of the PFC primary current (D), which is the switching current of PFC power device (FET) 603, also flows through current detection resistor 1009, causing the overcurrent threshold of PFC control circuit 606 to change from I6 to I3.

Note that capacitor 3005 is a small-capacity capacitor that prevents transistor 1001 from turning off at zero crossings, and therefore does not significantly affect the timing at which transistor 1001 turns off when the AC input voltage (A) is off.

When the PFC output voltage (C) drops to voltage value V2 at time t16, Zener diode 2001 of second overcurrent threshold switching circuit (motor start) 131 turns off, transistor 2004 turns off, and transistor 2007 turns on. As a result, current detection resistor 2008 is connected in parallel with current detection resistor 605, and part of the PFC primary current (D), which is the switching current of PFC power device (FET) 603, also flows through current detection resistor 2008, causing the overcurrent threshold of PFC control circuit 606 to change from I3 to I1.

Finally, current detection resistor 605, current detection resistor 1009, and current detection resistor 2008 are connected in parallel, and the value of this combined resistance determines the overcurrent threshold I1.

After that, at time t17, when the PFC output voltage (C) drops to voltage value V4, it falls below the brownout threshold (not shown). As a result, the power supply control unit 702 turns off the gate voltage of the main FET 704, the output of the DC-DC conversion unit 124 stops, and the DC output 24 V (G) begins to drop. Because the AC input voltage (A) has stopped, the PFC circuit 123 also stops, and the PFC primary current (D) also begins to drop. Then, the DC-DC converter 507 stops, the DC output 5 V (F) drops, and the power is turned off.

On the other hand, if the AC input voltage (A) is turned back on at time t16' before the PFC output voltage (C) falls below the brownout threshold (not shown), the PFC primary current (D) does not exceed the overcurrent threshold, so normal operation of the PFC control circuit 606 is maintained and the PFC output voltage (C) can be prevented from falling below the brownout threshold.

The PFC control circuit 606 monitors the drain voltage of the PFC power device 603, which is an FET, and stops the operation of the PFC power device 603 when this drain voltage exceeds a predetermined threshold voltage. Therefore, for example, the resistance value of current detection resistor 605 is set so that the threshold voltage is generated when a current of overcurrent threshold I6 flows, the resistance value of current detection resistor 2008 is set so that the threshold voltage is generated when a current of overcurrent threshold I4 flows through its parallel connection with current detection resistor 605, and the resistance value of current detection resistor 1009 is set so that the threshold voltage is generated when a current of overcurrent threshold I3 flows through its parallel connection with current detection resistor 605.

Figure 6 is a time chart showing the changes in voltage and current detected at each part shown in the block diagrams of Figures 2 and 3 and the circuit diagram of Figure 4 when the AC input voltage (A) is 230V during the operation of the power supply unit. The operation of the power supply unit will be explained with reference to this time chart, Figures 3 and 4. Note that the symbols (A) through (G) attached to the voltage and current signal waveforms in this time chart are attached to the corresponding detection points in Figures 2, 3, and 4.

The explanation of each waveform shown in the time chart in Figure 6 is the same as the explanation of each waveform described in the time chart in Figure 5, so it will not be repeated here. However, the AC input voltage (A) here is 230V.

The horizontal axis of the time chart in Figure 5 is the time axis common to all signals, and explains the operation of the power supply device as it changes over time (time t21 to time t26).

Here, because the AC voltage is 230V AC, Zener diode 3001 of third overcurrent threshold switching circuit (AC input voltage) 132 repeatedly turns on and off after power is applied. Therefore, due to the action of capacitor 3005, transistor 3004 remains on and transistor 3007 remains off during this time, allowing current to flow through current detection resistor 3008. Therefore, at time t21, current detection resistor 3008 is connected in series with current detection resistor 605, and the combined resistance value of these resistors changes the overcurrent threshold from I6 to I16. The overcurrent threshold waveform indicated by the dotted line is the waveform at 100V AC, as mentioned above. It should be noted that Zener diode 3001 is set to repeatedly turn on and off when the AC voltage exceeds a predetermined voltage value serving as a second reference value between 100V AC and 230V AC.

At time t21, one (or more) of the motors of actuator 202 starts up, and as the PFC primary current (D) begins to rise, the PFC output voltage (C) starts to decrease from voltage value V1. Thereafter, at time t22, when the PFC output voltage (C) decreases to voltage value V2, Zener diode 2001 of second overcurrent threshold switching circuit (motor start) 131 turns off, transistor 2004 turns off, and transistor 2007 turns on. As a result, current detection resistor 2008 is connected in parallel with current detection resistor 605, and a portion of the PFC primary current (D), which is the switching current of PFC power device (FET) 603, also flows through current detection resistor 2008, causing the overcurrent threshold of PFC control circuit 606 to change from I16 to I14.

The PFC primary current (D) then continues to rise and reaches a current value I15, but does not exceed the overcurrent threshold I14. Meanwhile, the PFC output voltage (C) drops to a voltage value V3, but this voltage value V3 does not fall below the brownout threshold.

At time t23, when the PFC output voltage (C) rises again to voltage value V2, Zener diode 2001 turns on, turning on transistor 2004 and preventing current from being supplied to transistor 2007. However, because capacitor 2006 is charged, the time constant of resistor 2005 and capacitor 2006 causes transistor 2007 to turn off later than time t23. The time constant is set to approximately 100 ms, and transistor 2007 turns off at time t24, a predetermined time after time t23. At this time, current stops flowing through current detection resistor 2008, and the overcurrent threshold of the PFC control circuit 606 returns from I14 to I16.

Next, when the AC input voltage (A) turns off at time t25, the PFC output voltage (C) begins to discharge, and the AC zero-cross signal stops outputting pulses. At this time, transistor 1001 of the first overcurrent threshold switching circuit (momentary interruption) 130 turns off, and transistor 1007 turns on. As a result, current detection resistor 1009 is connected in parallel with current detection resistor 605, and part of the PFC primary current (D), which is the switching current of PFC power device (FET) 603, also flows through current detection resistor 1009, causing the overcurrent threshold of PFC control circuit 606 to change from I16 to I13.

When the PFC output voltage (C) drops to voltage value V2 at time t26, Zener diode 2001 of second overcurrent threshold switching circuit (motor start) 131 turns off, transistor 2004 turns off, and transistor 2007 turns on. As a result, current detection resistor 2008 is connected in parallel with current detection resistor 605, and part of the PFC primary current (D), which is the switching current of PFC power device (FET) 603, also flows through current detection resistor 2008, causing the overcurrent threshold of PFC control circuit 606 to change from I13 to I11.

Ultimately, the combined resistance of current detection resistors 605, 1009, and 2008 connected in parallel and current detection resistor 3008 connected in series with these determines the overcurrent threshold I11.

After that, at time t27, when the PFC output voltage (C) drops to voltage value V4, it falls below the brownout threshold (not shown). As a result, the power supply control unit 702 turns off the gate voltage of the main FET 704, the output of the DC-DC conversion unit 124 stops, and the DC output 24V (G) begins to drop. Because the AC input voltage (A) has stopped, the PFC circuit 123 also stops, and the PFC primary current (D) also begins to drop. Then, the DC-DC converter 507 stops, the DC output 5V (F) drops, and the power is turned off.

As described above, when the AC input voltage (A) becomes 230V, the PFC primary current (D) inevitably decreases compared to when it is 100V. However, the third overcurrent threshold switching circuit (AC input voltage) 132 connects current detection resistor 3008 in series with current detection resistor 605 to lower each overcurrent threshold, enabling threshold switching operation similar to that at 100V.

As described above, the power supply unit 120 of this embodiment switches the overcurrent threshold for the PFC primary current, for example, for several hundred ms or more, in response to the PFC output voltage falling below a predetermined voltage value due to motor startup, etc., and monitors the presence or absence of a voltage waveform in the AC input voltage to switch the overcurrent threshold for the PFC primary current when the commercial power supply is cut off. Therefore, during a momentary interruption in the commercial power supply (e.g., a power outage of 20 ms or less), the overcurrent threshold is switched in two stages. Therefore, even if the PFC primary current fluctuates in synchronization with these timings, it is possible to prevent the detection of an overcurrent more than necessary. Furthermore, even when the AC input voltage is higher than a predetermined value and the PFC primary current decreases relatively, the power supply unit 120 operates to switch the overcurrent threshold to an appropriate value during motor startup and power supply interruption.

Figure 7 is a block diagram of the control system of an image forming apparatus shown as a comparative example to the control system of the image forming apparatus 100 of this embodiment shown in Figure 2.

The control system of this comparative example differs mainly from the control system of the present embodiment shown in Figure 2 in that it includes an overcurrent threshold switching circuit 150 that operates upon receiving a motor start signal from the control unit 160, and instead does not include circuits equivalent to the first overcurrent threshold switching circuit 130, second overcurrent threshold switching circuit 131, and third overcurrent threshold switching circuit 132 shown in Figure 2.

Figure 8 is a block diagram showing the detailed configuration of the power supply unit 320 described in Figure 7. This block diagram differs from the power supply unit 120 of the present embodiment shown in Figure 3 mainly in that it includes an overcurrent threshold switching circuit 150 that operates in response to a motor start signal received from the control unit 160, and instead does not include circuits equivalent to the first overcurrent threshold switching circuit 130, second overcurrent threshold switching circuit 131, and third overcurrent threshold switching circuit 132 shown in Figure 3.

Figure 9 is a time chart showing the changes in voltage and current detected at each part shown in the block diagrams of Figures 7 and 8 during the operation of the power supply device of the comparative example. The operation of the power supply device of the comparative example will be explained with reference to the time chart and Figures 7 and 8. Note that the symbols (H) through (N) attached to the voltage and current signal waveforms in the time chart are attached to the corresponding detection points in Figures 7 and 8.

The waveforms shown in the time chart of FIG. 9 are as follows:
AC input voltage (H): This is the AC voltage output from the commercial power supply 205 , that is, the AC voltage input to the power supply unit 320 .
Motor start signal (I): This signal is output from the control unit 160 to the power supply unit 120, and high [H] indicates that the motor is running, and low [L] indicates that the motor is in steady operation or stopped.
PFC output voltage (J): This is a DC voltage output from the PFC circuit 123. Taking into account the maximum AC input, this DC voltage is generally boosted to approximately AC 264V×√2+10V=390V in the case of worldwide input.
DC-DC converter primary current (K): This indicates the primary current flowing through the main FET 704 of the DC-DC converter 124, and overcurrent detection is performed based on this primary current.
Gate voltage (L): This is the gate voltage of the main FET 704 output from the power supply control unit 702 of the DC-DC conversion unit 124.
DC output 5V (M): DC 5V output voltage output from the DC-DC converter 507 of the DC-DC conversion unit 124.
DC output 24V (N): This is the DC 24V output voltage output from the DC-DC converter 124.

The horizontal axis of the time chart in Figure 9 is the time axis common to all of these signals, and explains the operation of the power supply device as it changes over time (from time t31 to time t32).

At time t31, the motor starts, and the DC-DC converter primary current (K) begins to rise. Accordingly, the PFC output voltage (J) begins to decrease from voltage value V1. At the same time, the motor start signal (I) changes from low [L] to high [H], turning on photocoupler 3501 and transistor 3502 of the overcurrent threshold switching circuit 150. This connects current detection resistor 3503 in parallel with current detection resistor 705, and the DC-DC converter primary current (K) also flows through current detection resistor 3503, changing the overcurrent threshold of the power supply control unit 702 from I26 to I24.

When the motor starts, the DC-DC converter primary current (K) rises from the steady-state current value I27 to current value I25, but since it does not exceed the overcurrent threshold I24 at this time, no overcurrent is detected. Therefore, the overcurrent threshold I24 at this time is set to be higher than the current value I25.

When the motor is stopped at time t32 after a predetermined time has elapsed and the motor start signal (I) goes low [L], the photocoupler 3501 and transistor 3502 of the overcurrent threshold switching circuit 150 turn off, current no longer flows through the current detection resistor 3503, and the overcurrent threshold of the power supply control unit 702 returns from I24 at motor start to I26 during steady state.

Here, the power supply control unit 702 monitors the drain voltage of the main FET 704, and when this drain voltage exceeds a predetermined threshold voltage, it stops the operation of the main FET 704 to suppress overcurrent. Therefore, the method for setting the current detection resistors 705 and 3503 is the same as the method for setting the current detection resistors 605, 2008, and 1009 described when the PFC control circuit 606 performs this, so a description of this will be omitted here.

As described above, the power supply unit 320 of the comparative example receives a motor start signal from the control unit 160 when the motor is started, and switches the overcurrent threshold for the DC-DC conversion unit primary current as appropriate, preventing unnecessary overcurrent detection even if the DC-DC conversion unit primary current fluctuates in synchronization with the timing of the motor start.

Embodiment 2.
FIG. 10 is a block diagram showing the configuration of the main parts of a control system of an image forming apparatus according to a second embodiment of the present invention, including a power supply unit 420 as a power supply device and a control unit 160. In FIG.

The main difference between the image forming apparatus employing this control system and the image forming apparatus 100 employing the control system of embodiment 1 shown in Figure 2 is that a fourth overcurrent threshold switching circuit 421 and a fifth overcurrent threshold switching circuit (A) 422 have been added to the power supply unit 420. Therefore, parts of the image forming apparatus employing this control system that are common to the image forming apparatus 100 of embodiment 1 described above will be given the same reference numerals or will not be shown in the drawings, and the following description will focus on the differences. Note that the essential configuration of the image forming apparatus of this embodiment is common to the essential configuration of the image forming apparatus 100 of embodiment 1 shown in Figure 1, except for the power supply unit of the control system, so Figure 1 will be referenced as necessary.

Figure 10 is a block diagram showing the main components of the power supply unit 420, which serves as a power supply device, and the control unit 160 of the control system of the image control device of this embodiment. As described above, the power supply unit 420 here has a configuration in which a fourth overcurrent threshold switching circuit 421 and a fifth overcurrent threshold switching circuit (A) 422 are added to the power supply unit 120 of the control system of embodiment 1 shown in Figure 2.

Figure 11 is a block diagram showing the detailed configuration of the power supply unit 420 described in Figure 10. As shown in the figure, apart from the added fourth overcurrent threshold switching circuit 421 and fifth overcurrent threshold switching circuit (A) 422, the power supply unit 420 is identical to the power supply unit 120 described in embodiment 1, with some exceptions described below. Therefore, a description of these identical parts will be omitted and the differences will be mainly described.

First, resistors 615 and 616 in PFC circuit 123 are brownout voltage resistors for PFC control circuit 606. They are connected to the brownout voltage terminal of PFC control circuit 606 and divide the voltage of bridge diode 122 using the PFC brownout voltage resistance ratio determined by the ratio of the resistance values of resistors 615 and 616, generating the brownout voltage for PFC control circuit 606. PFC control circuit 606 stops operating when this brownout voltage falls below a predetermined voltage value.

Figure 12 is a circuit diagram showing the detailed configuration of the first overcurrent threshold switching circuit (momentary interruption) 130, second overcurrent threshold switching circuit (motor start) 131, third overcurrent threshold switching circuit (AC input voltage) 132, fourth overcurrent threshold switching circuit 421 that detects a drop in AC input voltage, and fifth overcurrent threshold switching circuit (A) 422 that detects a long-term drop in AC input voltage, all of which are described in Figure 11. Of these, the added fourth overcurrent threshold switching circuit 421 and fifth overcurrent threshold switching circuit (A) 422 will be described below.

In the fourth overcurrent threshold switching circuit (input voltage drop) 421, the base of transistor 4004 is connected to the cathode output of rectifier diode 801 of AC zero-cross detection circuit 126 via resistor 4002 and Zener diode 4001, and to the anode output of rectifier diode 801 via resistor 4003. The emitter of transistor 4004 is connected to the cathode output of rectifier diode 801, and the collector is connected to output resistor 612 of voltage feedback unit 607 via resistor 4009. The base of transistor 4007 is connected to output resistor 612 via resistor 4005, the emitter is connected to output resistor 612 via capacitor 4006 and to the other end of current detection resistor 605, and the collector is connected to one end of current detection resistor 605, i.e., the source of PFC power device 603, via current detection resistor 4008, which serves as a fifth current detection resistor.

In the above configuration, resistor 4002 is a resistor that determines the base current of transistor 4004, and resistor 4005 is a base resistor that determines the base current of transistor 4007. Capacitor 4006, together with resistor 4005, works to delay the turn-off timing of transistor 4007. Furthermore, when transistor 4004 turns off, transistor 4007 turns on, current detection resistor 4008 is connected in parallel with current detection resistor 605, and the overcurrent threshold is changed.

In the fifth overcurrent threshold switching circuit (A) (long-term AC input voltage drop) 422, one end of resistor 5003 is connected to the cathode output of rectifier diode 801 of AC zero-crossing detection circuit 126 via Zener diode 5001 and to the anode output of rectifier diode 801 via resistor 5002, and the other end is connected to the base of transistor 5004. Transistor 5004 has its collector connected to the anode of Zener diode 5001, its emitter connected to the anode output of rectifier diode 801 via capacitor 5005 and resistor 5006, respectively, and connected to the base of transistor 5008 via resistor 5007. Transistor 5008 has its emitter connected to the other end of current detection resistor 605, and its collector connected to one end of current detection resistor 605, i.e., the source of PFC power device 603, via current detection resistor 5009, which serves as a sixth current detection resistor.

In the above configuration, resistor 5003 is a base resistor that determines the base current of transistor 5004. When transistor 5004 is turned off, the charge in capacitor 5005 is discharged through resistor 5006, reducing the base current of transistor 5008. When transistor 5008 is turned off, no current flows through current detection resistor 5009. In other words, the parallel state of current detection resistor 5009 with current detection resistor 605 is released, and the overcurrent threshold is changed.

Figure 13 is a time chart showing the changes in voltage, current, and temperature detected at each part shown in the block diagrams of Figures 10 and 11 and the circuit diagram of Figure 12 during the operation of the power supply device of this embodiment. The operation of the power supply device will be explained with reference to this time chart, Figures 11 and 12. Note that the symbols (A), (P), (D), (Q), and (R) attached to the voltage, current, and temperature waveforms in this time chart are attached to the corresponding detection points in Figures 10, 11, and 12.

The waveforms shown in the time chart of FIG. 13 are as follows:
AC input voltage (A): the AC voltage output from the commercial power supply 205 , that is, the AC voltage input to the power supply unit 120 .
AC full-wave rectified voltage (P): This voltage is obtained by full-wave rectifying the AC voltage output from the commercial power supply 205 by the AC zero-cross detection circuit 126 .
PFC primary current (D): This is the primary current that flows through the PFC power device 603, which is an FET of the PFC circuit 123, and overcurrent detection is performed based on this primary current.
Base current (Q) of transistor 5008: Indicates the base current value of the transistor 5008 of the fifth overcurrent threshold switching circuit (A) 422. When the base current becomes smaller than Ith, the transistor 5008 turns off.
Temperature of PFC coil 601 (R): Indicates the temperature of the PFC coil 601 as a representative of the component temperatures of the PFC circuit 123. The larger the current flowing through the PFC coil 601, the higher the temperature becomes.

The horizontal axis of the time chart in Figure 13 is the time axis common to all of these signals, and explains the operation of the power supply device as it changes over time (times t41, t42, etc.).

Note that, since the AC voltage is assumed to be 100V here, the Zener diode 3001 of the third overcurrent threshold switching circuit (AC input voltage) 132 is set to remain off. As a result, during this time, transistor 3004 remains off and transistor 3007 remains on, so no current flows through current detection resistor 3008 and the effect of current detection resistor 3008 can be ignored.

When the AC input voltage (A) drops at time t41, the AC full-wave rectified voltage (P) drops accordingly. The drop in AC input voltage (A) causes the PFC primary current (D) to increase from a current value of I34 to I32, but the drop in AC full-wave rectified voltage (P) activates the fourth overcurrent threshold switching circuit 421, and the overcurrent threshold also increases from I33 as the first threshold to I31 as the sixth threshold.

In other words, when the AC input voltage (A) drops below a predetermined set value serving as the third reference value, the drop in AC full-wave rectified voltage (P) causes Zener diode 4001 to turn off, transistor 4004 to turn off, and transistor 4007 to turn on. As a result, current detection resistor 4008 is connected in parallel with current detection resistor 605, and part of the PFC primary current (D), which is the switching current of PFC power device (FET) 603, also flows through current detection resistor 4008, causing the overcurrent threshold of PFC control circuit 606 to change from I33 to the higher I31.

As a result, the PFC circuit 123 continues to operate, causing the current value of the PFC primary current (D) to increase from I34 to I32, and the temperature (R) of the PFC coil 601 to rise above the saturation temperature Tsut when the AC input voltage is within the rated value.

Furthermore, when the AC full-wave rectified voltage (P) decreases, Zener diode 5001 of fifth overcurrent threshold switching circuit (A) 422 turns off, transistor 5004 turns off, and charge is no longer supplied to capacitor 5005. As a result, capacitor 5005 and resistor 5006 discharge, causing the base current (Q) of transistor 5008 to decrease. When transistor 5008 turns off at time t42, current no longer flows through current detection resistor 5009. In other words, the parallel state of current detection resistor 5009 and current detection resistor 605 is released, and the overcurrent threshold changes from I31 to I35, the seventh threshold lower than I33.

As a result, the PFC primary current (D) with a current value of I32 is detected as an overcurrent, the operation of the PFC circuit 123 stops, and the PFC primary current (D) drops to a current value of I36. Furthermore, the stopping of the operation of the PFC circuit 123 stops the current flowing through the PFC coil 601, preventing the temperature (R) of the PFC coil 601 from exceeding the component absolute maximum rated temperature Tmax.

Figure 14 is a time chart showing the changes in voltage, current, and temperature detected at each component shown in the block diagrams of Figures 7 and 8 when the AC input voltage (H) similarly drops during the operation of the comparative power supply device shown in Figures 7 and 8. The operation of the comparative power supply device at this time will be explained with reference to the time chart and Figures 7 and 8. The symbols (H), (S), (T), and (U) attached to the voltage, current, and temperature signal waveforms in the time chart are also attached to the corresponding detection points in Figures 7 and 8.

The waveforms shown in the time chart of FIG. 14 are as follows:
AC input voltage (H): represents the AC voltage output from the commercial power supply 205 , that is, the AC voltage input to the power supply unit 320 .
AC full-wave rectified voltage (S): This voltage is obtained by full-wave rectifying the AC voltage output from the commercial power supply 205 by the AC zero-cross detection circuit 126 .
PFC primary current (T): This is the primary current that flows through the PFC power device 603, which is an FET of the PFC circuit 123, and overcurrent detection is performed based on this primary current.
Temperature of PFC coil 601 (R): Indicates the temperature of the PFC coil 601 as a representative of the component temperatures of the PFC circuit 123. The larger the current flowing through the PFC coil 601, the higher the temperature becomes.

The horizontal axis of the time chart in Figure 14 is the time axis common to all of these signals, and explains the operation of the power supply device as it changes over time (times t51, t52, etc.).

When the AC input voltage (H) drops at time t51, the AC full-wave rectified voltage (S) drops accordingly. The drop in AC input voltage (H) causes the PFC primary current (D) to increase from a current value of I42 to I41, causing the temperature (R) of the PFC coil 601 to rise above the saturation temperature Tsut within the rated AC input voltage. If operation continues with a further drop in AC input voltage (H), the temperature (R) of the PFC coil 601 will exceed the absolute maximum rated component temperature Tmax at time t52.

For simplicity, the explanation has been given here assuming that the PFC primary current (T) does not exceed the overcurrent threshold of the PFC control circuit 606 at time t51, but the same applies if, for example, the fourth overcurrent threshold switching circuit 421 of this embodiment is provided and the overcurrent threshold of the PFC circuit 123 is configured to be raised as described above.

As described above, the power supply unit of this embodiment can continue operating the device even when an input voltage drop is detected by changing the overcurrent threshold using the fourth overcurrent threshold switching circuit 421. Furthermore, if the input voltage drop has continued for a certain period of time, the fifth overcurrent threshold switching circuit (A) 422 changes the overcurrent threshold again to stop the device, preventing an increase in component temperature due to the device continuing to operate with a large current flowing, allowing the use of small components with low current resistance.

Embodiment 3.
FIG. 15 is a circuit diagram showing detailed configurations of the first overcurrent threshold switching circuit (momentary interruption) 130, the second overcurrent threshold switching circuit (motor start) 131, the third overcurrent threshold switching circuit (AC input voltage) 132, the fourth overcurrent threshold switching circuit 421, and the fifth overcurrent threshold switching circuit (B) 423 that detects the temperature of the PFC coil 601 in the power supply unit of the present embodiment.

The power supply unit of this embodiment, which has these overcurrent threshold switching circuits, differs mainly from the power supply unit 420 of embodiment 2 shown in FIG. 11 above in that it has a fifth overcurrent threshold switching circuit (B) (coil temperature detection) 423 instead of the fifth overcurrent threshold switching circuit (A) (long-term AC input voltage drop) 422. Therefore, parts of the image forming apparatus employing this power supply unit that are common to the image forming apparatus 100 of embodiment 1 or embodiment 2 described above will be given the same reference numerals or will be omitted from the drawings, and the explanation will focus on the differences. Note that, since the essential configuration of the image forming apparatus of this embodiment is common to the essential configuration of the image forming apparatus 100 of embodiment 1 shown in FIG. 1, except for the power supply unit of the control system, reference will be made to FIG. 1 as necessary.

In the fifth overcurrent threshold switching circuit (B) (coil temperature detection) 423, the base of the transistor 6003 is connected to the output resistor 612 of the voltage feedback unit 607 via thermistor 6001, which serves as temperature detection means, and is also connected to the emitter via resistor 6002. The emitter is directly connected to the anode output of the rectifier diode 801, and the collector is connected to the output resistor 612 via resistor 6004. The thermistor 6001 is installed next to the PFC coil 601 and detects the temperature of the PFC coil 601.

Transistor 6007 has its base connected to output resistor 612 via resistor 6005, its emitter connected to output resistor 612 via capacitor 6006 and to the other end of current detection resistor 605, and its collector connected to one end of current detection resistor 605, i.e., the source of PFC power device 603, via current detection resistor 6008, which serves as a sixth current detection resistor.

In the above configuration, when the temperature of PFC coil 601 rises, and the temperature of thermistor 6001 rises accordingly and its resistance value decreases, transistor 6003 turns on, reducing the base current of transistor 6007 and turning transistor 6007 off. This prevents current from flowing through current detection resistor 6008. In other words, the parallel state of current detection resistor 6008 and current detection resistor 605 is released, and the overcurrent threshold is changed.

Note that the component whose temperature is detected by the thermistor 6001 is not limited to the PFC power device 603, but may be another component, such as a FET PFC power device 603, for which there is concern that the temperature may rise due to a drop in the input AC voltage.

Figure 16 is a time chart showing the changes in voltage, current, and temperature detected at each part shown in the circuit diagram of Figure 15 during the operation of the power supply device of this embodiment. The operation of the power supply device will be explained with reference to this time chart and Figure 15. Note that the symbols (A), (P), (D), (X), and (R) attached to the voltage, current, and temperature waveforms in this time chart are attached to the corresponding detection points in Figure 15.

The waveforms shown in the time chart of FIG. 16 are as follows:
AC input voltage (A): the AC voltage output from the commercial power supply 205 , that is, the AC voltage input to the power supply unit 120 .
AC full-wave rectified voltage (P): This voltage is obtained by full-wave rectifying the AC voltage output from the commercial power supply 205 by the AC zero-cross detection circuit 126 .
PFC primary current (D): This is the primary current that flows through the PFC power device 603, which is an FET of the PFC circuit 123, and overcurrent detection is performed based on this primary current.
Temperature (X) of the thermistor 6001: Indicates the temperature of the thermistor 6001 of the fifth overcurrent threshold switching circuit (B) 423. When the temperature of the thermistor 6001 rises and exceeds Tthmax as a predetermined temperature, the transistor 6007 turns off and the overcurrent threshold is changed.
Temperature of PFC coil 601 (R): Indicates the temperature of the PFC coil 601 as a representative of the component temperatures of the PFC circuit 123 (FIG. 11). The larger the current flowing through the PFC coil 601, the higher the temperature becomes.

The horizontal axis of the time chart in Figure 16 is the time axis common to all of these signals, and explains the operation of the power supply device as it changes over time (times t61, t62, etc.).

Note that, since the AC voltage is assumed to be 100V here, the Zener diode 3001 of the third overcurrent threshold switching circuit (AC input voltage) 132 is set to remain off. As a result, during this time, transistor 3004 remains off and transistor 3007 remains on, so no current flows through current detection resistor 3008 and the effect of current detection resistor 3008 can be ignored.

When the AC input voltage (A) drops at time t61, the AC full-wave rectified voltage (P) drops accordingly. The drop in AC input voltage (A) causes the PFC primary current (D) to increase from a current value of I34 to I32, but the drop in AC full-wave rectified voltage (P) activates the fourth overcurrent threshold switching circuit 421, and the overcurrent threshold also increases from I33 as the first threshold to I31 as the sixth threshold.

In other words, when the AC input voltage (A) drops below a predetermined set value serving as the third reference value, the drop in AC full-wave rectified voltage (P) causes Zener diode 4001 to turn off, transistor 4004 to turn off, and transistor 4007 to turn on. As a result, current detection resistor 4008 is connected in parallel with current detection resistor 605, and part of the PFC primary current (D), which is the switching current of PFC power device (FET) 603, also flows through current detection resistor 4008, causing the overcurrent threshold of PFC control circuit 606 to change from I33 to the higher I31.

As a result, the PFC circuit 123 continues to operate, causing the current value of the PFC primary current (D) to increase from I34 to I32, and the temperature (R) of the PFC coil 601 to rise above the saturation temperature Tsut within the rated AC input voltage. As the temperature (R) of the PFC coil 601 rises, the temperature (X) of the thermistor 6001 also rises above the saturation temperature Tthsut within the rated voltage.

At time t62, when the temperature (X) of thermistor 6001 reaches Tthmax, the fifth overcurrent threshold switching circuit (B) 423 operates and the overcurrent threshold switches from I31 to I35, the seventh threshold lower than I33. That is, as the temperature of thermistor 6001 rises, the resistance value decreases, transistor 6003 turns on, and transistor 6007 turns off, thereby releasing current detection resistor 6008 from its parallel connection with current detection resistor 605 and switching the overcurrent threshold from I31 to I35.

As a result, the PFC primary current (D) is detected as an overcurrent, the operation of the PFC circuit 123 stops, and the PFC primary current (D) drops to a current value of I36. Furthermore, the stopping of the operation of the PFC circuit 123 stops the current flowing through the PFC coil 601, preventing the temperature (R) of the PFC coil 601 from exceeding the component absolute maximum rated temperature Tmax.

As described above, the power supply unit of this embodiment can continue operating the device even when an input voltage drop is detected by changing the overcurrent threshold using the fourth overcurrent threshold switching circuit 421. Furthermore, if the temperature (R) of the PFC coil 601 rises above a predetermined temperature due to the increased primary current caused by the input voltage drop, this is detected by the fifth overcurrent threshold switching circuit (B) 423 and the device is shut down before the component absolute maximum rated temperature Tmax is reached. This prevents component temperatures from rising due to continued device operation with a large current flowing, allowing the use of small components with low current resistance.

As described above, with the power supply unit of this embodiment, even if the PFC primary current fluctuates in situations where an abnormal shutdown of the device is undesirable, such as when there is a momentary interruption in the commercial power supply, when the motor starts, or when the AC input voltage changes (for example, between 100V and 230V), the overcurrent threshold is switched to an appropriate value depending on the situation, preventing the device from shutting down unnecessarily due to the detection of an overcurrent.

In addition, because the overcurrent threshold switches to an appropriate value depending on the situation, there is no need to set the overcurrent threshold higher than necessary to prevent unnecessary overcurrent detection. If the current threshold were set higher than necessary, overcurrents would be tolerated more frequently even when they should be detected, and the transformer would need to be increased in size to prevent overheating. However, with the power supply unit of this embodiment, the overcurrent threshold can always be set to an appropriate value, and when an overcurrent is properly detected, the current supply is limited (including being stopped), making it less likely for the transformer to overheat, and there is no need to increase the transformer size more than necessary.

In addition, if the AC input voltage temporarily drops and the PFC primary current increases, the current threshold is changed to prevent unnecessary overcurrent detection. However, if the voltage drop lasts for a long time, the device can be shut down at a stage where no load is placed on the power supply unit, depending on the time that has passed since the voltage drop.

In addition, since there is no need for a motor start signal like in the comparative example circuit, there is no need for cables, patterns, photocouplers, etc. for signal distribution, simplifying the configuration.

In the above-described embodiment, the present invention is applied to a printer, particularly a tandem printer, but it is not limited to this and can also be used in other devices such as MFPs (multi-function printers), facsimiles, and copiers. It can also be used as a power source for devices that control the on/off of DC motors, etc.

1 Paper feed section, 2 Image forming section, 3 Fixing section, 4 Paper discharge section, 5 Paper cassette, 6 Pickup roller, 7 Pickup roller, 8 Pickup roller, 9 Registration roller, 10 Registration roller, 11 Photosensitive drum, 12 Charging roller, 13 Developing roller, 14 Toner supply roller, 15 LED head, 16 Toner cartridge, 17 Transfer belt, 18 Transfer roller, 19 Toner image forming section, 21 Transfer section, 31 Fixing roller, 32 Halogen lamp, 33 Temperature detection sensor, 34 Pressure roller, 41 Discharge roller, 42 Discharge roller, 100 Image forming device, 120 Power supply section, 121 Switch, 122 Bridge diode, 123 PFC circuit, 124 DC-DC conversion section, 126 AC zero-cross detection circuit, 127 Heater on/off circuit, 130 First overcurrent threshold switching circuit, 131 Second overcurrent threshold switching circuit, 132 Third overcurrent threshold switching circuit, 150 Overcurrent threshold switching circuit, 160 Control unit, 161 Main control unit, 162 ROM, 163 RAM, 164 Temperature detection unit, 165 Sensor on/off circuit, 166 High voltage power supply, 167 Head control unit, 168 Actuator drive unit, 201 Various sensors, 202 Actuator, 205 Commercial power supply, 206 Host, 320 Power supply unit, 420 Power supply unit, 421 Fourth overcurrent threshold switching circuit, 422 Fifth overcurrent threshold switching circuit (A), 423 Fifth overcurrent threshold switching circuit (B), 500 Protection element, 501 Filter, 502 Inrush current prevention circuit, 503 secondary rectifying and smoothing circuit, 504 voltage feedback section, 505 protection circuit, 506 filter, 507 DC-DC converter, 520 power supply section, 601 PFC coil, 602 PFC diode, 603 PFC power device, 604 PFC output electrolytic capacitor, 605 current detection resistor, 606 PFC control circuit, 607 voltage feedback section, 611 output resistor, 612 output resistor, 615 resistor, 616 resistor, 701 transformer, 702 power supply control section, 703 snubber circuit, 704 main FET, 705 current detection resistor, 801 rectifying diode, 802 photocoupler, 1001 transistor, 1002 resistor, 1003 resistor, 1004 Resistor, 1005 Capacitor, 1006 Resistor, 1007 Transistor, 1008 Capacitor, 1009 Current detection resistor, 1010 Resistor, 2001 Zener diode, 2002 Resistor, 2003 Resistor, 2004 Transistor, 2005 Resistor, 2006 Capacitor, 2007 Transistor, 2008 Current detection resistor, 2009 Resistor, 3001 Zener diode, 3002 Resistor, 3003 Resistor, 3004 Transistor, 3005 Capacitor, 3006 Resistor, 3007 Transistor, 3008 Current detection resistor, 3009 Capacitor, 3010 Resistor, 4001 Zener diode, 4002 Resistor, 4003 Resistor, 4004 Transistor, 4005 Resistor, 4006 Capacitor, 4007 Transistor, 4008 Current detection resistor, 4009 Resistor, 5001 Zener diode, 5002 Resistor, 5003 Resistor, 5004 Transistor, 5005 Capacitor, 5006 Resistor, 5007 Resistor, 5008 Transistor, 5009 Current detection resistor, 6001 Thermistor, 6002 Resistor, 6003 Transistor, 6004 Resistor, 6005 Resistor, 6006 Capacitor, 6007 Transistor, 6008 Current detection resistor.

Claims (11)

a PFC circuit having a PFC control unit that receives a full-wave rectified voltage of an AC input voltage, switches a switching element to boost the voltage with a coil, and determines that an overcurrent occurs when a voltage across a current detection resistor through which a primary current flows becomes equal to or exceeds a predetermined value with respect to a primary current flowing through the switching element; a capacitor that smooths the primary voltage boosted by the coil; and a voltage feedback unit that feeds back the primary voltage;
a first overcurrent threshold switching circuit configured such that, when the AC input voltage is turned off, a second current detection resistor is connected in parallel to a first current detection resistor as the current detection resistor;
a second overcurrent threshold switching circuit that detects the primary voltage decreasing to a first reference value, and that has a configuration in which a third current detection resistor is connected in parallel to the first current detection resistor,
a first current detection resistor connected in parallel to the first, second, and third current detection resistors when the AC input voltage is turned off and the primary voltage drops to the first reference value; The power supply unit of claim 1, further comprising a third overcurrent threshold switching circuit that detects when the AC input voltage exceeds a second reference value and has a fourth current detection resistor value connected in series with the first current detection resistor. The power supply device of claim 1 or 2, characterized in that the second overcurrent threshold switching circuit delays the timing of releasing the parallel connection of the third current detection resistor with the first current detection resistor when the primary voltage exceeds the first reference value. The power supply device of claim 2, wherein the second reference value is higher than AC 100V and lower than AC 230V. a fourth overcurrent threshold switching circuit configured to connect a fifth current detection resistor in parallel with the first current detection resistor when the AC input voltage becomes equal to or lower than a third reference value;
and a fifth overcurrent threshold switching circuit configured to prevent current from flowing through a sixth current detection resistor connected in parallel to the first current detection resistor at a predetermined timing after the first current detection resistor and the fifth current detection resistor are connected in parallel. The power supply device of claim 5, wherein the fifth overcurrent threshold switching circuit is configured so that no current flows through the sixth current detection resistor after a predetermined time has elapsed since the first current detection resistor and the fifth current detection resistor are connected in parallel. 6. The power supply device according to claim 5, wherein the fifth overcurrent threshold switching circuit is configured so that when the temperature of the temperature detection means reaches or exceeds a predetermined temperature, no current flows through the sixth current detection resistor. A power supply device as described in any one of claims 1 to 7, characterized in that it is equipped with a DC-DC conversion unit that converts the primary voltage to DC. 8. The power supply device according to claim 7 , wherein the temperature detection means detects the temperature of the coil or a component that may exceed its maximum rated temperature due to a drop in the AC input voltage. an AC zero-crossing detection circuit that outputs pulses corresponding to AC zero-crossing points of the AC input voltage;
10. The power supply device according to claim 1, wherein the power supply device detects that the AC zero-cross detection circuit has been turned off when the pulses generated by the AC zero-cross detection circuit stop. A power supply device according to any one of claims 1 to 10 ;
A paper feed unit;
an image forming unit;
A fixing portion;
and a paper discharge unit.