JP7687866B2 - Switching Power Supply Unit - Google Patents

Description

The present invention relates to a switching power supply device that converts AC input power into desired DC output power and supplies it to an LED load.

The applicant of the present invention has provided a switching power supply device that reduces the current ripple of the current flowing through an LED load with a simple power supply configuration in the following Patent Document 1.
A feedback type constant current control circuit equipped with a variable impedance element composed of a MOSFET etc. is used as a ripple current reduction circuit connected in series with the load. With this configuration, the voltage across the LED load is kept constant, and the current ripple of the current flowing through the LED load is controlled to be reduced.

JP 2012-164633 A

In the conventional technology, the DC output power is controlled to a predetermined value by controlling the on/off of a switching element so that the average voltage of the feedback voltage _VFB at the connection point between the LED load and the ripple current reduction circuit becomes a preset target average voltage _Vref . As shown in FIG. 10, this feedback voltage _VFB contains a voltage ripple (voltage fluctuation) similar to that of the output voltage _VOUT . The voltage ripple of the feedback voltage _VFB is as shown in FIG. 10(a) and (b).
As shown in Fig. 10(c) and (d), the larger the load, the larger the
It increases as the output capacitor deteriorates.

10(d), when the feedback voltage _VFB falls below a voltage at which the variable impedance element cannot operate at a constant current due to voltage ripple, the voltage across the LED load cannot be maintained at the required voltage, and as a result, the value of the current flowing through the LED load periodically drops, becoming visible as flicker. Therefore, an operating margin voltage VM is set in consideration of the minimum voltage at which the variable impedance element can perform constant current control, various variations (target average voltage _Vref , etc.), and the undershoot of the feedback voltage _VFB during _transient operation, and the target average voltage _Vref is set so that the feedback voltage _VFB does not fall below this operating margin voltage _VM .

However, the target average voltage _Vref thus set is set in anticipation of a certain degree of degradation of the output capacitor and a state in which the voltage ripple is large at maximum load. Therefore, when the output capacitor is not degraded or when the voltage ripple is small at light or medium load, as shown in Figures 10(a) and 10(b), the feedback voltage _VFB will remain at a high voltage with a considerable margin above the operating margin, and this margin will result in a power loss.
In addition to the power loss, there is another problem that occurs when the LED load flickers for a moment when the variable impedance element starts to operate during the rise in output voltage at power-on startup. This occurs when the LED load is turned on from a state where the voltage applied to it is insufficient, and the I-V
The current increases suddenly according to the characteristics, causing false lighting. This problem is likely to occur when the feedback voltage _VFB is close to the operating margin, and also when deep dimming conditions such as 1% dimming are also present. One possible solution to this problem is to slow down the response of the variable impedance element at startup, but this would result in a longer startup time, which would reduce the product value.

The present invention has been made in consideration of the above problems, and has an object to provide a switching power supply device that can solve the problems, further improve power supply efficiency, and shorten start-up time.

The switching power supply of the present invention is a switching power supply that converts AC input power into desired DC output power and supplies it to an LED load, and includes: a switching element that is controlled to be on and off; a ripple current reduction circuit that is connected in series to the LED load and reduces the current ripple flowing through the LED load by variably controlling impedance; and a control circuit that controls the on and off of the switching element based on a feedback voltage at a connection point between the LED load and the ripple current reduction circuit, wherein the control circuit raises the voltage to a preset target voltage at start- up , and operates the ripple current reduction circuit after the target voltage is reached, to average value control the feedback voltage at a target average voltage set in accordance with the magnitude of the voltage ripple of the feedback voltage, and sets the target average voltage lower as the LED current flowing through the LED load due to a dimming signal becomes smaller .

According to the present invention, in an operating environment where the voltage ripple of the feedback voltage _VFB is small, the target average voltage _Vref can be set to a low value, and the power supply efficiency can be improved.

1 is a circuit diagram showing a configuration of a first embodiment of a switching power supply device according to the present invention; 2 is a block diagram showing a configuration of a control circuit shown in FIG. 1 . 2 is a waveform diagram of each part at the time of start-up of the switching power supply device shown in FIG. 1. FIG. 11 is a circuit diagram showing a configuration of a switching power supply device according to a second embodiment of the present invention. FIG. 5 is a block diagram showing a configuration of a computing unit shown in FIG. 4 . 5 is a waveform diagram of each part in the switching power supply device shown in FIG. 4. FIG. 11 is a circuit diagram showing an example in which a second embodiment of a switching power supply device according to the present invention is configured as a buck-boost converter. FIG. 8 is a block diagram showing a configuration of a control IC shown in FIG. 7 . 2 is a waveform diagram of an output voltage and a feedback voltage in the switching power supply device shown in FIG. 1 . 1A and 1B are waveform diagrams of an output voltage and a feedback voltage in a conventional switching power supply device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the following embodiments, components having similar functions are denoted by the same reference numerals and descriptions thereof will be omitted as appropriate.

The switching power supply device of this embodiment converts AC input power into DC output power and outputs
A switching power supply device for supplying a D load uses a feedback type constant current control circuit equipped with a variable impedance element constituted by a MOSFET or the like as a ripple current reduction circuit connected in series with an LED load, and increases the output voltage Vout of the LED load and the ripple current reduction circuit connected in series to a preset target voltage at start-up, and after reaching the target voltage, changes the average voltage of the feedback voltage _VFB at the connection point between the LED load and the ripple current reduction circuit in accordance with the magnitude of the voltage ripple of the feedback voltage _VFB .

(First embodiment)
The switching power supply device 1 of the first embodiment includes n LED elements (L
The present invention is a flyback type converter that drives an LED load 2 consisting of an LED 21 to an LED 2n (ED 21 to LED 2n). Referring to FIG. 1, the converter includes a rectifier circuit DB, a transformer TR, a switching element Q1, a rectifying and smoothing circuit 3, a control circuit 4, a ripple current reduction circuit 5, and an auxiliary power supply 6.

The rectifier circuit DB is a known diode bridge circuit that is connected to an AC input power source AC, rectifies the AC input power into a unidirectional pulsating current, and outputs the rectified current to the transformer TR.

The transformer TR includes a primary winding W1, a secondary winding W2, and a tertiary winding W3. One end of the primary winding W1 is connected to a rectifier circuit DB, and the other end is connected to a drain terminal of a switching element Q1. A rectifier smoothing circuit 3 is connected between both ends of the secondary winding W2, and an auxiliary power supply 6 is connected between both ends of the tertiary winding W3.

The switching element Q1 is a FET (Field Effect Transistor) or an IGBT (Insulated Gate Bipolar Transistor) driven by a drive signal (PWM signal) generated by the control circuit 4.
In this embodiment, the switching element Q1 is a MOSFET.
The source terminal of the switching element Q1 is grounded, and the gate terminal is connected to the VG terminal of the control circuit 4.

The rectifying and smoothing circuit 3 is composed of a diode D1 and a capacitor C1.
A connection point A between the cathode of the LED 21 and one end of the capacitor C1 is connected to the anode of the LED 21 constituting the LED load 2, and the other end of the capacitor C1 is grounded. The capacitor C1 is an output capacitor, and in this embodiment, it is configured as an electrolytic capacitor, but other capacitors may be used. The life and capacity of an electrolytic capacitor are generally dominated by the phenomenon in which the electrolyte evaporates to the outside through the sealing part, which appears as a decrease in capacitance and an increase in the tangent of the loss angle.

The ripple current reducing circuit 5 functions as a feedback type constant current control circuit that variably controls impedance, and includes a variable impedance element Q2 and a detection resistor Rs. The variable impedance element Q2 is composed of an element such as a FET, an IGBT, or a BiTr. In this embodiment, the variable impedance element Q2 will be described as a MOSFET.

The drain terminal of the variable impedance element Q2 is connected to the cathode of the LED2n that constitutes the LED load 2, the source terminal is grounded via the detection resistor Rs, and the gate terminal is connected to the output terminal of the error amplifier AMP1 via the ZR terminal of the control circuit 4.

The connection point between the detection resistor Rs and the source terminal of the variable impedance element Q2 is connected via the cc terminal to the inverting input terminal of the error amplifier AMP1 of the control circuit 4. The detection resistor Rs converts the LED current I _LED flowing through the LED load 2 into a voltage signal and outputs it to the error amplifier AMP1.

The non-inverting input terminal of the error amplifier AMP 1 of the control circuit 4 is connected to a reference voltage (PWM signal) generated by a reference voltage 49 via a DAC (digital-to-analog converter) 48 .
In other words, the reference voltage generated by the PWM signal generated by the reference voltage 49 is
At 48, the signal is converted into an analog signal and input to the non-inverting input terminal of the error amplifier AMP1.

The output terminal of the error amplifier AMP1 is connected to the gate terminal of the variable impedance element Q2 via the ZR terminal of the control circuit 4.
D An error signal based on the current I _LED and a reference value (reference voltage) is input to the variable impedance element Q2
In detail, the error amplifier AMP1 increases the voltage level of the error signal as the LED current _ILED becomes smaller than the reference value, and
Reduce the resistance between the sources.
Moreover, the error amplifier AMP1 operates to reduce the voltage level of the error signal as the LED current ILED becomes larger than the reference value, thereby increasing the resistance value between the drain and source of the variable impedance element Q2.

That is, the variable impedance element Q2 of the ripple current reduction circuit 5 is
Depending on the output of 1, the _drain and drain
The control circuit 4 controls the current ripple of the LED current I LED to be reduced. The control circuit 4 controls the current ripple of the LED current I _LED to be reduced. The response speed of the ripple current reduction circuit 5 is set to be higher than the response speed of the control circuit 4, and is preferably set to be higher than the frequency of the AC input power supply AC.

The control circuit 4 includes a start section 41, an internal power supply section 42, ADCs (analog-digital converters) 431 and 432, a calculator 44, a PWM generating section 45, and a driver 46. The control circuit 4 may be entirely a digital control circuit (including a circuit operated by software), some of its components may be digital control circuits, or the entire control circuit may be an analog control circuit.

The starter 41 is connected to a connection point between the rectifier circuit DB and the primary winding W1 of the transformer TR via an ST terminal and a resistor R2, and is also connected to an auxiliary power supply 6 consisting of a diode D2 and a capacitor C2 via a Vcc terminal. The starter 41 charges the capacitor C2 of the auxiliary power supply 6 at startup, and after startup, supplies power from the auxiliary power supply 6 to an internal power supply circuit (REG) 42 that generates an internal power supply in the control circuit 4.

The ADC 431 is connected via a CV1 terminal to the connection point of resistors R3 and R4 that constitute a voltage detection circuit 7 that detects the voltage of the rectifying and smoothing circuit 3, and the voltage at this connection point is input as a feedback voltage _VFB1 .
The ADC 432 is connected to a connection point B between the LED load 2 and the ripple reduction circuit 5 (the drain terminal of the variable impedance element Q2) via a CV2 terminal, and the voltage at the connection point B is input as a feedback voltage _VFB2 .
The ADCs 431 and 432 then detect the feedback voltages _VFB1 and VFB2 at intervals (e.g., 20 μs) that are sufficiently shorter than the period of the voltage ripples contained in the feedback voltages VFB1 and _VFB2 .
The signals _VFB1 and _VFB2 are sampled, converted into digital voltages, and output to the calculator 44.

Referring to Figure 2, the calculator 44 functions as an initial voltage setting unit 440, an average voltage calculation unit 441, an average value control unit 442, an operation amount calculation unit 443, a bottom voltage detection unit 444, a bottom voltage comparison unit 445, and a target value correction unit 446.

The initial voltage setting unit 440 compares an initial voltage that is initially set at the time of startup with a feedback voltage _VFB1 input from the ADC 431, thereby determining whether the initial voltage is equal to or smaller than the feedback voltage _VFB1.
B1 , and outputs the calculated error to the operation amount calculation unit 443 as an error signal _Vst .
Here, the initial voltage is set to a minimum voltage or higher at which the LED load 2 and the ripple reduction circuit 5 can perform constant current control.

The average voltage calculation unit 441 calculates an average voltage V _Ave of the feedback voltage V _FB2 for each period of the AC input power source AC based on the feedback voltage V _FB2 input from the ADC 432, and outputs the calculated average voltage V _Ave to the average value control unit 442.

The average value control unit 442 compares the target average voltage _Vref with the average voltage _VAve calculated by the average voltage calculation unit 441 to calculate the error between the target average voltage _Vref and the average voltage _VAve , and outputs the calculated error to the operation amount calculation unit 443 as an error signal.

When the output voltage at point A rises to the initial voltage at startup, the operation amount calculation unit 443 switches from the error signal of the initial voltage setting unit 440 to the error signal of the average value control unit 442. In order to smooth the output voltage fluctuation during this switching transition and prevent malfunctions such as flickering of the LED load, the error signal from the average value control unit 442 is set to a preset upper limit value.
At the time of start-up, feedback control is performed by the initial voltage setting unit 440, and the output voltage _Vout at the connection point A of the rectifying and smoothing circuit 7 rises toward the initial voltage.
When _out reaches the initial voltage, the error signal from the average value control unit 442 has already reached a preset upper limit value. At this point, the operation amount calculation unit 443 switches from feedback control by the initial voltage setting unit 440 to feedback control by the average value control unit 442. This allows the average value control unit 442 to perform gentle feedback control from the upper limit value of the error signal toward the lower limit value.
Next, the error signal of the average value control unit 442 is gradually decreased from the upper limit value based on the error signal between the target average voltage _Vref and the average voltage _VAve , until it reaches a preset lower limit value. At the point where the lower limit value is reached, the target average voltage _Vref is corrected with a correction value from the target value correction unit 446.

The operation amount calculation unit 443 first calculates an operation amount Δ for increasing or decreasing the on time of a PWM (pulse width modulation) signal that controls the on/off of the switching element Q1 based on the error signal input from the initial voltage setting unit 440 at startup, and outputs the calculated operation amount Δ to the PWM generation unit 45.
When the calculated manipulated variable Δ falls within a predetermined range of approximately zero, the error signal input from the initial voltage setting unit 440 is separated, and based on the error signal input from the average value control unit 442,
A control amount Δ for increasing or decreasing the on-time of a PWM (pulse width modulation) signal that controls the on-off of the switching element Q1 is calculated, and the calculated control amount Δ is output to the PWM generating unit 45 .

The bottom voltage detection unit 444 detects a bottom voltage _VB of the feedback voltage _VFB2 input from the ADC 43. For example, when the voltage value of the feedback voltage _VFB2 input from the ADC 43 falls below both the voltage value one sampling before and the voltage value one sampling after, the bottom voltage detection unit 444 detects the voltage value as the bottom voltage _VB .

The bottom voltage comparator 445 compares a preset reference bottom voltage _VBref with the bottom voltage _VB detected by the bottom voltage detector 444, and outputs the comparison result to the target value corrector 446.
The reference bottom voltage _VBref is set to a value that exceeds the minimum voltage at which the variable impedance element Q2, which is a variable impedance element, can perform constant current control, even taking into consideration variations in various elements and undershoot of the feedback voltage _VFB2 during transient operation.

When the bottom voltage _VB is lower than the reference bottom voltage _VBref , the target value correction unit 446 corrects the target average voltage Vref, which the average value control unit 442 compares with the average voltage _{VAve ,} in a direction to increase the target average voltage Vref, and when the bottom voltage _VB is higher than the reference bottom voltage _VBref , the target value correction unit 446 corrects the target average voltage _Vref , which the average value control unit 442 compares with the average voltage _VAve , in a direction to decrease the target average voltage _Vref . The correction width of the target average voltage Vref may be a preset correction value, or may be a correction value calculated based on the difference between the bottom voltage _VB and the reference bottom voltage _VBref .

PWM generation unit 45 generates a PWM signal by increasing or decreasing the on-time based on the operation amount Δ from operation amount calculation unit 443, and controls on and off of switching element Q1 by the generated PWM signal via driver 46. Note that, when the operation amount Δ of the on-time is calculated based on the error between average voltage V _Ave calculated for each cycle of the alternating current input power supply AC and target average voltage V _ref as in the present embodiment, the on-time of the PWM signal becomes constant during one cycle of the alternating current input power supply AC.

3 shows a sequence diagram of the output voltage _VOUT from the start of startup (time t0) of the switching power supply device 1 shown in FIG. 1 to steady state operation (time t4). As shown in FIG.
From t1 to t2, the output voltage _VOUT is controlled to target an initial voltage, and after reaching the initial voltage at time t1, control is switched to average value control. The output voltage _VOUT is controlled to gradually decrease toward the lower limit of a preset target value for average value control. When a current starts to flow through the LED load 2 at time t2, the lower limit of the target value is reached at time t3. At time t3 when the lower limit of the target value is reached, the average value control unit 442 switches to correcting the target average voltage _Vref with a correction value from the target value correction unit 446. The output voltage _VOUT at the connection point A is detected by the bottom voltage detection unit 44
The average value is controlled so that the reference bottom voltage becomes the reference bottom voltage based on the reference bottom voltage _VBref signal from the inverter 4.

As described above, by switching the target value of the output voltage _VOUT from the initial voltage to average value control to average value control of the reference bottom voltage at startup, it is possible to shorten the startup time and prevent flickering of the LED load. In particular, by switching from average value control to average value control of the reference bottom voltage, the difference between the target voltage and the feedback voltage VFB does not become large. This is because average value control performs control so that the target average value is updated every switching period, and the target average value can be gradually lowered. That is, in average value control of the reference bottom voltage, the target average value is updated every 8 to 10 ms with the commercial frequency of the AC input power, making it difficult to gradually lower the target average value from the initial voltage, and a stable transition can be achieved by going through average value control once.
As a result, the Vcc voltage of the control circuit 4 shown in FIG. 1 also changes in the same manner as the output voltage _VOUT , which has the advantage that a stable power supply voltage can be ensured.

Next, referring to FIG. 2 and FIG. 3, the operation of the ripple current reduction circuit 5 at the time of start-up is as follows:
At time t1 when out reaches the initial voltage, a reference voltage signal (PWM signal) from a reference voltage 49 is output as an analog signal from a DAC (digital-analog converter) 48 to a ZR terminal as a gate signal _VZR of a variable impedance element Q2.

In this case, in order to raise the reference voltage signal (PWM signal) from the reference voltage 49 during the time period t1 to t2 to the vicinity of the gate voltage threshold value of the variable impedance element Q2, an on-pulse signal longer than the specified on-width is sent.
The gate signal V _ZR of No. 2 rises to near the gate voltage threshold value.

Next, in the period from time t2 to t4, the reference voltage signal (PWM signal) is changed from the reference voltage 49 to a specified reference voltage, and the on-width is gradually widened from the narrowed on-width state. As a result, the gate signal V _ZR of the variable impedance element Q2 gradually rises from near the gate voltage threshold value, and in conjunction with this, the LED load current I _LED can also smoothly increase from time t2.

As described above, by controlling the reference voltage signal (PWM signal) from time t1 to t2 and from time t2 to t4, it is possible to shorten the start-up time of the LED load current I _LED and to achieve stable start-up.

Second Embodiment
4 and 5, a switching power supply device 1 a of the second embodiment is configured by adding a dimming calculator 47 instead of the reference voltage 49 to a control circuit 4 a of the switching power supply device 1 .

The calculator 44a further includes a target value setting unit 447.
The target value setting unit 442 sets the target average voltage _Vref converted according to the reference voltage value generated by the dimming calculator ₄₇ to the average value control unit 442.
The LED current I is corrected by a correction value based on the difference between the bottom voltage _VB and the reference bottom voltage _VBref via a bottom voltage detection unit 444, a bottom voltage comparison unit 445, and a target value correction unit 446.
The lower _{the LED} , the lower the target average voltage _Vref is set.
In the switching power supply device 1b according to the embodiment, the average voltage V _Ave of the feedback voltage V _FB2 is controlled in accordance with the magnitude of the voltage ripple that changes due to the dimming signal so that it becomes higher as the voltage ripple increases and becomes lower as the voltage ripple decreases.

Next, referring to FIG. 6, the operation of the ripple current reduction circuit 5 at the time of start-up is as follows:
At time t1 when the voltage VZR reaches the initial voltage, the dimming calculator 47 outputs a reference voltage signal (PWM signal) as an analog signal via a DAC (digital-analog converter) 48 from the ZR terminal as the gate signal _VZR of the variable impedance element Q2.
Here, in order to raise the reference voltage signal (PWM signal) from dimming calculator 47 to near the gate voltage threshold of variable impedance element Q2 during the time period t1 to t2, an on-pulse signal longer than the specified on-width is sent out. As a result, at time t2, the gate signal _VZR of variable impedance element Q2 rises to near the gate voltage threshold.
During the period from time t2 to t4, the reference voltage signal (PWM signal) from the dimming calculator 47 is changed to the original dimming signal value, and the on-width is gradually widened from the narrowed on-width state. As a result, the gate signal V _ZR of the variable impedance element Q2 gradually rises from near the gate voltage threshold, and in conjunction with this, the LED load current I _LED can also increase smoothly from time t2.
As described above, the reference voltage signal (P
By controlling the current I LED of the LED load, it is possible to reduce the start-up time of _{the LED} and to start the LED stably.

Alternatively, the input voltage may be detected, and the target average voltage _Vref converted according to the input voltage may be set in the average value control unit 442. In this case, the smaller (lower) the input voltage, the larger the voltage ripple. Therefore, the larger (higher) the input voltage, the lower the target average voltage _Vref is converted and set. In this way, the average voltage _VAve of the feedback voltage _VFB2 is controlled in accordance with the magnitude of the voltage ripple that changes with the input voltage so that it is higher as the voltage ripple increases and lower as the voltage ripple decreases.

Alternatively, the total operating time may be recorded in the control circuit 4a, and the target average voltage _Vref converted according to the total operating time may be set in the average value control unit 442. In this case, the longer the total operating time, the more the capacitor C1 deteriorates and the larger the voltage ripple becomes. Therefore, the longer the total operating time, the higher the target average voltage _Vref is converted and set. In this way, the average voltage _VAve of the feedback voltage _VFB2 is controlled to be low during a period when the voltage ripple is small and to be high as the elapsed time increases and the voltage ripple increases, depending on the magnitude of the voltage ripple that changes depending on the total operating time.

Furthermore, by using a temperature sensor, the total operating time taking into account the environmental temperature is recorded in the control circuit 4a, and the target average voltage _Vre converted according to the total operating time taking into account the environmental temperature is calculated.
_f may be set in the average value control unit 442. In this case, the higher the ambient temperature is, the faster the deterioration of the capacitor C1 will be, and the larger the voltage ripple will be. Therefore, the longer the total operating time at a high ambient temperature is, the higher the target average voltage _Vref is converted and set. As a result, the average voltage _VAve of the feedback voltage _VFB2 is low during periods when the voltage ripple is small, depending on the magnitude of the voltage ripple that changes depending on the total operating time taking the ambient temperature into account,
It is controlled to be higher as the voltage ripple increases.

Although an example of a flyback converter has been described for the switching power supply 1a shown in Fig. 4, the present invention may also be applied to a buck converter, a boost converter, or a buck-boost converter, and a switching power supply 1a configured as a buck-boost converter is shown in Fig. 7 and Fig. 8. In the switching power supply 1a, components exhibiting the same functions as those in the switching power supply 1 are given the same reference numerals and descriptions thereof will be omitted as appropriate.

(Application example of the second embodiment)
The switching power supply device 1b includes a synchronous rectifier element Q3 that functions as a diode D1, and the control circuit 4b includes a PWM generating unit 45 that generates a PWM signal for driving the synchronous rectifier element Q3.
a and a driver 46a.

The control circuit 4 b also includes an error amplifier AMP 1 that constitutes the ripple current reducing circuit 5 , and is equipped with a dimming calculator 47 and a DAC (digital-to-analog converter) 48 .
The dimming calculator 47 generates a digitized reference voltage value according to the dimming signal input from the DIM terminal, and the DAC 48 converts the digitized reference voltage value generated by the dimming calculator 47 into an analog signal and inputs it to the non-inverting input terminal of the error amplifier AMP1. This makes it possible to control the LED current I _LED supplied to the LED load 2 according to the dimming signal.

FIG. 9 shows the output voltage Vout at the connection point A and the feedback voltage V
9, (a) shows the waveforms of the output voltage Vout and the feedback voltage _VFB2 at a medium load with little degradation of the capacitor C1, (b) shows the waveforms of the output voltage Vout and the feedback voltage VFB2 at a rated load with little degradation of the capacitor C1, (c) shows the waveforms of the output voltage Vout and the feedback voltage VFB2 at a rated load with much degradation of the capacitor C1, and (d) shows the waveforms of the output voltage Vout and the feedback voltage _VFB2 at a rated load with much more degradation of the capacitor C1.

In this embodiment, as shown in FIGS. 9(a) to 9(d), regardless of the magnitude of the voltage ripple,
The feedback voltage _VFB2 is set so that the bottom voltage _VB becomes the reference bottom voltage _VBref .
Therefore, the average voltage V _Ave of the feedback voltage V _FB2 is expressed as follows:
Depending on the magnitude of the voltage ripple, which changes depending on the operating environment, the output voltage is controlled to be higher as the voltage ripple increases and lower as the voltage ripple decreases.

As a result, when the deterioration of the capacitor C1 is small and the voltage ripple is small, the power supply efficiency can be improved in all ranges regardless of the magnitude of the load. For example, in FIG.
The average voltage _VAve of the feedback voltage _VFB2 in the conventional average value control shown in Fig. 9(b) is 1.5 V both at medium load and rated load, whereas the average voltage _VAve of the feedback voltage _VFB2 in the average value control of this embodiment is 1.0 V at medium load as shown in Fig. 9(a) and 1.2 V at rated load as shown in Fig. 9(b). In other words, at medium load, the power loss can be reduced by Δ0.5 V × LED current _ILED , and at rated load, the power loss can be reduced by Δ0.3 V × LED current _ILED .

Furthermore, in the conventional method of fixing the average voltage V _Ave of the feedback voltage V _{FB ,} it was necessary to design in anticipation of the occurrence of end-of-life problems and variations in the capacitance of the output capacitor as shown in FIGS. 9( c) and 9(d) so that an appropriate operating margin would ultimately be secured.
In contrast, in this embodiment, even when the capacitor C1 varies or is at the end of its life, the bottom voltage _VB of the feedback voltage _VFB2 is controlled to the reference bottom voltage _VBref as a lower limit as shown in Figures 9(c) and 9(d), so that the feedback voltage _VFB2 is stable and there is little variation due to mass production. In other words, in this embodiment, simplification of the power supply design is expected.

Furthermore, in the conventional method of fixing the average voltage V _Ave of the feedback voltage V _FB , the output capacitor deteriorates significantly, and as shown in Figures 9(c) and (d), when the feedback voltage V _FB falls below the operating margin voltage V _M , there is a risk that the rated current cannot be obtained or flickering occurs. In contrast, in this embodiment, even if the output capacitor deteriorates significantly, the bottom voltage V _B of the feedback voltage V _FB2 is controlled to the reference bottom voltage V _Bref as shown in Figures 9(c) and (d), so that the feedback voltage V
The bottom voltage _VB of _FB2 does not fall below the reference bottom voltage _VBref by a large amount.
The ED load 2 can be driven without any problems, and the life of the power supply can be extended in an emergency.

In addition, as shown in FIG. 9D, if the average voltage V _Ave of the feedback voltage V _FB2 becomes too high, it may become inoperable or have a negative effect on other power supply elements depending on the design of other power supply parts. Therefore, the calculator 44 compares the average voltage V _Ave of the feedback voltage V _FB2 with a preset operation stop threshold value and calculates the average voltage V _A
It is preferable to configure the LED load 2 to stop driving when the voltage _Vv exceeds a preset operation stop threshold value ve.

In addition, when the load is light, the voltage ripple of the feedback voltage _VFB2 becomes small, and it may be difficult to detect the voltage ripple.
In environmental conditions where the voltage ripple of _FB2 is small, stability may be improved by switching to other control, such as conventional average value control with a fixed target average voltage _Vref .

As described above, this embodiment provides a switching power supply device 1 that converts AC input power AC into desired DC output power and supplies it to an LED load 2, and includes a switching element Q1 that is an on/off controlled switching element, a ripple current reduction circuit 5 that is connected in series with the LED load 2 and reduces a current ripple flowing through the LED load 2 by variably controlling impedance, and a control circuit that controls the on/off of the switching element Q1 based on a feedback voltage _VFB2 at a connection point B between the LED load 2 and the ripple current reduction circuit 5,
The control circuit controls the average value of the feedback voltage _VFB2 with a target average voltage _Vref that is set according to the magnitude of the voltage ripple of the feedback voltage _VFB2 .
With this configuration, in an operating environment where the voltage ripple of the feedback voltage _VFB2 is small, the target average voltage _Vref can be set to a low value, thereby improving the power supply efficiency.

Furthermore, in this embodiment, the control circuit detects the bottom voltage _VB of the feedback voltage _VFB2 , and corrects the target average voltage so that the detected bottom voltage _VB becomes a preset reference bottom voltage _VBref .
With this configuration, when the deterioration of the capacitor C1 is small and the voltage ripple is small, the target average voltage _Vref can be corrected to a low value, so that the power supply efficiency can be improved in all ranges regardless of the load. In addition, even when the capacitor C1 has variations or is at the end of its life, the bottom voltage _VB of the feedback voltage _VFB2 is stable because it is controlled to the lower limit of the reference bottom voltage _VBref , and the power supply design can be simplified.
Furthermore, even if the degradation of the output capacitor becomes significant, the bottom voltage _VB of the feedback voltage _VFB does not significantly exceed or fall below the reference bottom voltage _VBref , so that the LED load 2 can be driven without any problems, and the life of the power supply can be extended in an emergency.

Furthermore, in this embodiment, the control circuits 4a and 4b control the LED load 2 by the dimming signal.
The smaller the LED current I _LED flowing through the MOSFET 1 is, the lower the target average voltage V _ref is set.
With this configuration, in a region where the voltage ripple is small and the load is small, the target average voltage _Vref can be set to a low value, thereby improving the power supply efficiency.

Furthermore, in this embodiment, the control circuit sets a lower target average voltage _Vref as the input voltage increases.
With this configuration, at large input voltages where the voltage ripple is small, the target average voltage V _ref
can be set to a lower value, improving power supply efficiency.

Furthermore, in this embodiment, the control circuit sets a higher target average voltage _Vref as the total operating time becomes longer.
With this configuration, in an operating environment where the total operating time is short and the voltage ripple is small, the target average voltage _Vref can be set to a low value, thereby improving the power supply efficiency.

Furthermore, in this embodiment, the control circuit sets a higher target average voltage _Vref as the total operating time in a high environmental temperature state becomes longer.
With this configuration, the reference bottom voltage V _Bref can be set taking into account the operating time at the ambient temperature.

The present invention has been described above based on the embodiment. This embodiment is merely an example, and it will be understood by those skilled in the art that various modifications are possible in terms of the combination of the respective components, and that such modifications are also within the scope of the present invention.

Reference Signs List 1, 1a, 1b Switching power supply device 2 LED load 3 Rectification smoothing circuit 4, 4a, 4b Control circuit 5 Ripple current reduction circuit 6 Auxiliary power supply 7 Voltage detection circuit 21 to 2n LED
41 Start section 42 Internal power supply section 431, 432 ADC
44, 44a Calculator 45, 45a PWM generator 46, 46a Driver 47 Light control calculator 48 DAC
49 Reference voltage 440 Initial voltage setting section 441 Average voltage calculation section 442 Average value control section 443 Operation amount calculation section 444 Bottom voltage detection section 445 Bottom voltage comparison section 446 Target value correction section 447 Target value setting section AC AC input power supply AMP1 Error amplifiers C1, C2 Capacitor D1 Diode DB Rectifier circuit Q1 Switching element Q2 Variable impedance element Q3 Synchronous rectifier elements R2 to R4 Resistor Rs Detection resistor TR Transformer W1 Primary winding W2 Secondary winding W3 Tertiary winding

Claims (2)

A switching power supply device that converts AC input power into desired DC output power and supplies the DC output power to an LED load,
A switching element that is on/off controlled;
a ripple current reduction circuit connected in series to the LED load and reducing a current ripple flowing through the LED load by variably controlling an impedance;
a control circuit that controls the switching element to be turned on and off based on a feedback voltage at a connection point between the LED load and the ripple current reduction circuit,
the control circuit raises the voltage to a preset target voltage at start-up, and operates the ripple current reduction circuit after the target voltage is reached to average -value-control the feedback voltage at a target average voltage set in accordance with the magnitude of the voltage ripple of the feedback voltage, and sets the target average voltage lower as the LED current flowing through the LED load by a dimming signal becomes smaller . The switching power supply device according to claim 1, characterized in that after the control circuit transitions the feedback voltage to average value control at the target average voltage, it detects the bottom voltage of the feedback voltage and corrects the target average voltage so that the detected bottom voltage becomes a preset reference bottom voltage.

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