JPS6222348B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a switching power supply device, and the protection circuit thereof has a circuit configuration that does not use a delay time constant circuit all at once to improve feedback response, thereby improving the original protection function. We aim to provide a more complete and simplified switching power supply with the aim of making it perfect and eliminating the need for a time constant circuit capacitor, thereby reducing external parts and pins when integrated into an IC. be.

First, a conventional device of this kind will be explained with reference to the drawing in FIG. 1. FIG. 1 shows an example of a device with a protection circuit against overcurrent. In the figure, 1 is an input terminal, 2 is a transformer, 3 is an output terminal, and 4 is a switching transistor, 5 is a pulse width modulator, 6 is an oscillator, 7 is a rectifier circuit, 8 is an error amplifier for output voltage control, 9 is a reference voltage source, and 10 to 13 are output voltage floating detection circuits. resistance to,
Reference numerals 14 and 15 denote capacitors and resistors constituting a phase compensation circuit, which are circuits that perform the basic functions of an ordinary switching power supply device. 16 is a resistor for detecting the output current, and 17 is an error amplifier provided to control the output current to a constant value. This is a general overcurrent protection circuit that protects against overcurrent. Note that 18 and 19 are capacitors and resistors that constitute a phase compensation circuit, and 20 is also a resistor. The whole is connected as shown in the figure.

Its output voltage-output current characteristics are as shown in FIG. 2, where a indicates the voltage control region and b indicates the constant current control region.

In the case of this overcurrent protection circuit, if the output is short-circuited, the current is detected by the resistor 16 and amplified by the error amplifier 17.
Since there is a phase compensation circuit consisting of a capacitor 18 and a resistor 19 and a resistor 20, the transient response of the circuit 7 is delayed by the time constant thereof. In this case, the switching transistor 4 is activated with a large pulse width during the delay time.
is ON and the output is short-circuited, so a very large stress is applied to the switching transistor 4. Therefore, it is necessary to use a large and expensive transistor with a large safe operating area (ASO) as the switching transistor 4. In this case, assuming a delay time of 5 msec, if the repetition frequency is 20kHz, 5 x 10 ^-3 x 20 x 10 ³ = 100
Therefore, a large peak power is applied to the transistor for 100 times.

The present invention has been made in response to such conventional problems, and the same parts as those in FIG. 1 are given the same reference numerals and only the different points will be explained below together with the drawing in FIG. 3. In the figure, 21 is the error amplifier 17 in FIG. 1, but in the present invention it is a voltage comparator. 22 is a discharge switch element, which is turned on when the output of the voltage comparator 21 is at a high level and turned off when the output is at a low level. 23 is a capacitor, and 24 is a charging resistor. and capacitor 2
The voltage across the capacitor 3 is connected to the input side of the pulse width modulator 5, and in this case, the polarity is such that the pulse width increases as the voltage across the capacitor 23 increases.

If the output is short-circuited, the output current flows through the current detection resistor 16, and if the detected voltage is higher than the reference voltage (of course, it should be higher in the case of a short-circuit), the output of the voltage comparator 21 will be at a high level,
Therefore, the switch element 22 is turned on, the charge on the capacitor 23 is rapidly discharged, the voltage drops rapidly, and the pulse width of the pulse width modulator 5 becomes zero. The output current then decreases to zero. Then,
The voltage generated across the resistor 16 becomes 0, the output of the voltage comparator 21 becomes low level, and the switch element 22
is turned OFF, and as a charging current flows through the resistor 24, the voltage of the capacitor 23 gradually increases, and accordingly, the pulse width applied to the switching transistor 4 also gradually widens from 0. go. Then, when the output current reaches the preset current again, the same operation will be repeated again. By setting this current slightly larger than the maximum load current, an overcurrent protection circuit can be constructed. The current waveform becomes a ramp waveform as shown in FIG. Therefore, when the output voltage-output current characteristic is measured using an effective value, a fold-back characteristic can be obtained as shown in c in FIG. 5. Note that a is the constant voltage control region, and b is the first
This is the overcurrent protection characteristic for the case shown in the figure.

In the case of the present invention, since 21 is simply a voltage comparator, the delay time is very small and is negligibly smaller than the repetition period. Also, capacitor 23
The discharge time of the switch element 22 can also be made sufficiently shorter than the repetition period. Therefore, when the output is short-circuited, the number of times that the maximum pulse width is applied to the switching transistor 4 is only once at the moment of short-circuiting, which is very small compared to 100 times in the conventional device shown in Fig. 1. Therefore, the ASO is small. This means that inexpensive small transistors can be used.

6A, 6B, and 6C are other embodiments of the circuit A surrounded by the broken line in FIG. 3.

FIG. 6A shows that when the output of the voltage comparator 21 becomes high level, the switch element 22 is turned on and the capacitor 23 is rapidly charged by the reference voltage source 9. Then, when the voltage comparator 21 becomes low level, the switch element 22 is turned on. The element 22 is turned OFF, discharge gradually occurs through the resistor 25, and the voltage waveform of the capacitor 23 becomes a in FIG.
become that way. In this case, the connection polarity with the pulse width modulator 5 is such that as the voltage of the capacitor 23 increases, the pulse width becomes narrower.

FIG. 6B shows that when the output of the voltage comparator 21 becomes a high level, the switch element 22 is turned on and discharges rapidly, and then when it becomes a low level, the constant current source 26 gradually charges the capacitor 23 with a constant current. The voltage waveform becomes a linear waveform as shown in FIG. 7c. In this case, the connection polarity with the pulse width modulator 5 is the same as in FIG.

Fig. 6 (c) shows that when the output of the voltage comparator 21 becomes high level, the switch element 22 is turned on to perform rapid charging, and when the output becomes low level, the constant current source 22
7 and the current mirror circuit (transistor 28,
29 and resistors 30 and 31), the charge in the capacitor 23 is gradually discharged at a constant current, and the voltage waveform is a linear one as shown in d in Figure 7. It becomes a waveform. In this case, the connection polarity with the pulse width modulator 5 is the same as in FIG. 6A.

Fig. 8 shows an example in which the output voltage is polar, and 32 and 33 are resistors for detecting the output voltage. When the output is polar, the voltage drop due to the output current generated in the resistor 16 is with respect to ground. Since the polarity is determined, floating detection is performed by resistors 34 to 37. A is the part A in FIG. 3 or the circuits A, B, and C in FIG. 6.

Figure 9 shows an example of a protection circuit against abnormally high voltage with no load in the case of a constant current power supply. In this case, since the output is polar, the resistors 10 to 13 are This constitutes a floating detection circuit and performs feedback for constant current. 38, 39, and 40 are diodes, capacitors, and resistors that constitute a rectifier circuit. N
_T is a tertiary winding for detecting the peak value of the output voltage. If the peak value of this output voltage exceeds a certain limit, the output of the voltage comparator 21 goes high, and the same operation as described above occurs. By setting the limit of this voltage to be slightly larger than the maximum load voltage, it is possible to suppress the occurrence of abnormally high voltage during no-load conditions. The voltage waveform in this case is as shown in FIG. 4, which is similar to the overcurrent protection. Therefore, the output voltage-output current characteristic is c in Fig. 10 at the effective value.
The peak value will be as shown in b. a
is the constant current control region.

Figure 11 shows an example in which the stress applied to the switching transistor 4 is reduced compared to when protection is provided against output short circuits in the case of a constant current output power supply. Although it is not necessary, there are cases where you want to minimize the stress applied to the switching transistor 4, or you want to minimize damage to the photosensitive drum due to load short circuits or arcs in high-voltage power supplies for copying machines, etc., or you want to minimize the risk of electric shock and In terms of flame prevention, it is very effective when it is desired to lower the effective value of current during short circuits and arcs. In the figure, 41 and 42 are resistors and capacitors that constitute a filter circuit, 15, 14, and 43 are resistors, capacitors, and resistors that constitute a phase compensation circuit,
44 is a resistor for detecting the peak value of output power; 45
is an AND circuit, and 46 is an RS flip-flop. As the load resistance is lowered lower and lower under constant current control, the pulse width becomes smaller and smaller, and near the short circuit, the pulse width cannot be reduced any further and an intermittent oscillation state occurs. The output current ripple then increases significantly, the peak voltage across resistor 44 becomes greater than the reference voltage, and the aforementioned protective operation occurs. The output voltage-output current characteristics in this case are as shown in a to c of FIG. 12. Without this protection, the results would be as shown in a to b. In the protective operation explained in FIG.
The time required for discharging is not zero, and stress is applied to the switching transistor 4 even during discharging. Circuits 45 and 46 in FIG. 11 are designed to reduce this stress by inputting a high level signal to the set input of flip-flop 46 at the same time the output of voltage comparator 21 goes high, and then outputting becomes a low level, and the pulse is cut off by the AND circuit 45. At that time, the charge in the capacitor 23 should still be in a state where it has begun to discharge.

FIG. 13 shows an example when it is desired to further reduce the stress applied to the switching transistor 4, or when it is desired to insulate between the input and output, such as when the input is directly connected to a line.
The output voltage is detected by a capacitor 48, a resistor 49, and a tertiary winding N _T and constant voltage feedback is applied. 50 is a resistor that detects the emitter current of the switching transistor 4, and 51 and 52 are resistors that divide the voltage so that since the voltage detected by the resistor 50 is small, the reference voltage is also small. Note that the collector current of the switching transistor 4 may be detected.

The switching power supply device of the present invention having the above configuration has the following advantages.

(1) A small, inexpensive switching transistor with a small ASO can be used.

(2) If the same switching transistors are used, the power output can be increased.

(3) When converting into an IC, there is no phase compensation capacitor, so external components can be reduced and the number of IC pins can be reduced. Switch element 22, capacitor 2
3. Since circuit A consisting of resistor 24 is also used as a soft start circuit, the number of capacitors 23 is not increased.

(4) Since the effective value of the current during short circuits and arcs can be reduced, it is very advantageous in terms of human electric shock from high voltage power supplies, flames caused by arcs, and safety standards. It is extremely effective in preventing short circuits between photosensitive drums and damage to the drums due to arcing.

(5) Since the effective value is small even in the case of abnormally high voltage under no load, it is advantageous in terms of safety standards.

(6) Once arc discharge occurs with a high voltage power supply, the arc does not stop even with a constant current power supply because the arc discharge voltage is low (60 to 100V). In the case of the present invention, if even one arc discharge occurs, the output voltage temporarily becomes 0, and then the voltage gradually rises again, so that the arc is no longer continuous, which is very effective in the case of a high voltage power supply.

[Brief explanation of the drawing]

FIG. 1 is an electric circuit diagram of a conventional switching power supply device, FIG. 2 is an output voltage-output current characteristic diagram of the same device, FIG. 3 is an electric circuit diagram of an embodiment of the switching power supply device according to the present invention, and FIG. The figure is a voltage or current characteristic diagram of the same device, Figure 5 is an output voltage-output current characteristic diagram of the same, Figure 6 A, B, and C are circuit diagrams of each embodiment of the main part of the same device, and Figure 7 8 and 9 are electrical circuit diagrams of other embodiments of the switching power supply device according to the present invention, and FIG. 10 is a voltage characteristic diagram of the device.
The figure is an output voltage-output current characteristic diagram of the device in Figure 9,
FIG. 11 is an electric circuit diagram of still another embodiment, the first
Figure 2 is the output voltage-output current characteristic diagram of the same device, the first
FIG. 3 is an electrical circuit diagram of still another embodiment. 1...Input terminal, 2...Transformer, 3...Output terminal, 4...Switching transistor, 5...
Pulse width modulation circuit, 6... Oscillator, 7... Rectifier circuit, 8... Error amplifier, 9... Reference voltage source, 16
... Resistor, 21 ... Voltage comparator, 22 ... Switch element, 23 ... Capacitor, 32, 33, 44
...Resistor, 47...Diode, 48...Capacitor, 49, 50, 51, 52...Resistance.

Claims (1)

[Claims] 1 In a switching power supply device that controls output by detecting an output voltage or output current, comparing it with a reference voltage, amplifying it, and inputting the signal to a pulse width modulation circuit, the output current, output voltage, or switching transistor Detects the collector current or emitter current, compares it with the reference voltage using a voltage comparator, and uses the output to control the switch element.
ON, rapidly discharging or charging the capacitor connected to the input side of the pulse width modulation circuit, rapidly lowering or increasing the voltage across the capacitor, temporarily reducing the pulse width, or making it zero. A switching power supply device characterized in that the pulse width is gradually widened by gradually charging or discharging the capacitor in the opposite manner to the above.