JP2015142423A - switching power supply - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a flyback switching power supply capable of turn-on at a bottom point in a PWM switching power supply.SOLUTION: The switching power supply includes: bottom point detection means 21 for comparing the voltage of a primary coil with a predetermined threshold, to detect the timing of a bottom point of free oscillation due to pseudo resonance generated at both voltages of a switching element 6; bottom signal generation means 22 for continuously outputting pulse signals synchronized with the free oscillation, after the bottom point detection means 21 detects the bottom point; and turn-on timing determination means 23 for supervising clock signals and the pulse signals, to turn on the switching element 6 through PWM control means 25 when a first pulse signal is detected after the start of a cycle of the clock signals.

Description

  The present invention relates to a flyback switching power supply device, and more particularly to a configuration for reducing switching loss by PWM control.

  Conventionally, the switching power supply device disclosed in Patent Document 1 is disclosed. As shown in FIG. 3, the flyback switching power supply employs a quasi-resonant circuit to reduce switching loss. Next, the configuration will be described.

  This switching power supply is output from the transformer 2, the switching element 3 for connecting / opening a DC power source connected to the primary winding 21 of the transformer 2, and the secondary winding 22 of the transformer 2. Based on the voltage output from the secondary side winding 23 of the transformer 2, the diode 4 and the electrolytic capacitor 5 for rectifying and smoothing the alternating current, the voltage detecting circuit 7 for detecting the smoothed direct current voltage, A timing circuit 83 that determines an on-timing, an on-time setting circuit 81 that sets an on-time of the switching element 3, an off-time setting circuit 82 that sets an off-time of the switching element 3, and a switching that controls the switching element 3 on and off And a drive circuit 84 for outputting a signal.

  The timing circuit 83 monitors the voltage output from the winding 23, and outputs a turn-on start signal c to the drive circuit 84 when this voltage becomes lower than the threshold voltage. The threshold voltage is a predetermined value, and is set to coincide with the timing of the bottom point of the voltage resonance waveform generated by the pseudo resonance generated in the voltage across the switching element 3. The pseudo resonance and the bottom point will be described later.

  In the timing circuit 83, a resistor 831 and a capacitor 830 connected in series are connected to both ends of the winding 23. A base terminal of the transistor 836 and an anode terminal of the diode 835 are connected to a connection point between the resistor 831 and the capacitor 830. Accordingly, when the voltage of the winding 23 is lowered, the electric charge accumulated in the capacitor 830 is discharged through the resistor 831. Therefore, the voltage across the capacitor 830 is gradually lowered, and when the voltage is lowered to the threshold voltage at which the transistor 836 is turned on, the turn-on is performed. The circuit outputs a start signal c.

Next, the pseudo resonance and the bottom point will be described with reference to an explanatory diagram shown in FIG.
In FIG. 4, Vtr is a voltage across the switching element 3, V830 is a voltage across the capacitor 830, c is a turn-on start signal output from the timing circuit 83, and d is a switching signal output from the drive circuit 84. Show.

  When the switching signal d is turned off at t1, the excitation energy stored in the transformer 2 via the primary winding 21 induces a voltage in the winding 22 and the winding 23. As indicated by V830 in FIG. 4, the voltage across the winding 23 rises from t1 and falls in the interval from t2 to t3. On the other hand, as indicated by Vtr, the voltage across the switching element 3 starts free oscillation in a period after t2 when the excitation energy of the transformer 2 is used up by the secondary winding and the secondary current stops flowing. .

  Free vibration is ringing in the form of a sine wave that always starts from a phase in which the voltage decreases after t2, and vibrates at a resonance frequency determined by the parasitic capacitance of the switching element 3 and the inductance of the winding 21. This resonance frequency is not affected by the load or voltage on the secondary side. This valley of ringing due to free vibration is called the bottom point. This ringing occurs during a period from when the switching element 3 is turned off until the next turn-on. Therefore, the frequency of ringing due to free vibration is higher than the switching frequency determined by turning on / off the switching element 3.

  When the switching element 3 is turned on at the timing of the bottom point, the voltage across the switching element 3 is lower than when the switching element 3 is turned on at other timings, so that switching loss can be reduced. Therefore, as described above, the timing circuit 83 knows the timing of the bottom point by matching the timing of the bottom point and the timing at which the capacitor 830 becomes the threshold voltage in advance with the time constant of the resistor 831 and the capacitor 830. Can do.

  The off time setting circuit 82 manages the off time (fixed time) of the switching element 3. On the other hand, the on-time setting circuit 81 determines the on-time of the switching element 3 in accordance with the output voltage detected by the voltage detection circuit 7. That is, if the output voltage is lower than the reference, the ON time of the switching element 3 is increased, and if the output voltage is higher than the reference, the ON time of the switching element 3 is decreased so that the output voltage becomes the reference voltage. Yes. For this reason, when the load is light and the change in the output voltage is small, the ON time of the switching element 3 is shortened, and as a result, the switching frequency is increased. On the other hand, when the load is heavy, the ON time of the switching element 3 becomes long, and as a result, the switching frequency becomes low.

  In addition to the quasi-resonant method in which the switching frequency changes depending on the load in this way, the PWM method in which the switching frequency is made constant, the pulse whose duty changes according to the size of the load is generated as a switching signal, and switching control is performed. There is a switching power supply.

  The quasi-resonant method and the PWM method have respective merits and demerits. Since the quasi-resonant system is turned on only at the bottom point, the switching loss can be reduced, but the switching frequency changes depending on the state of the load. Since the switching frequency decreases when the load is heavy, the number of times of switching per unit time decreases and the switching loss decreases. On the other hand, when the load is light, the switching frequency increases and the switching loss increases.

  In contrast, the PWM method has a fixed switching frequency, so the switching frequency does not increase even when the load is light. However, the duty of the pulse corresponding to the load is determined, and the voltage across the switching element is independent of the bottom point. However, there is a problem that the switching loss is large because the device is forcibly turned on even when the timing is high.

  For this reason, in the PWM switching power supply, a flyback switching power supply that can be turned on at the bottom point has been desired.

JP-A-8-289543 (page 4-5, FIG. 1)

  An object of the present invention is to solve the above-described problems and to provide a flyback switching power supply that can be turned on at a bottom point in a PWM switching power supply.

In order to solve the above-described problems, the present invention according to claim 1 of the present invention includes a rectifier that rectifies an input AC voltage and outputs a DC voltage, a primary winding, and a secondary winding. A transformer for transforming the DC voltage applied to the primary winding and outputting it from the secondary winding, a switching element arranged in series between the rectifier and the primary winding, and the switching element A flyback switching power supply device including switching control means for switching control by PWM control,
The switching control means includes
Clock signal generating means for generating a clock signal of the switching frequency;
PWM control means for outputting a switching signal generated based on the clock signal to the switching element;
Bottom point detection means for detecting the timing of the bottom point of free vibration due to pseudo-resonance generated in the voltage across the switching element when the voltage of the secondary winding is equal to or lower than a predetermined threshold value;
A bottom signal generating means for continuously outputting a pulse signal synchronized with the free vibration after the bottom point detecting means detects the bottom point;
A turn-on timing determining means for monitoring the clock signal and the pulse signal and turning on the switching element via the PWM control means when the first pulse signal is detected after the start of the cycle of the clock signal; It is characterized by that.

By using the above means, according to the switching power supply device of the present invention,
Among the plurality of bottom point pulse signals detected by the bottom point detection means, the first bottom point pulse signal after the turn-on timing determination means 23 detects the clock signal cycle, that is, after the rising edge of the clock signal is detected. Is detected, the switching element is turned on via the PWM control means. For this reason, even if it is a switching power supply device of a PWM control system, a switching element turns on at the bottom point where the both-ends voltage of a switching element is low. For this reason, a switching loss can be reduced rather than the switching power supply device using the conventional PWM control system.

It is a block diagram which shows the Example of the switching power supply device by this invention. It is explanatory drawing explaining operation | movement of the switching power supply device by this invention. It is a block diagram which shows the conventional switching power supply device. It is explanatory drawing explaining operation | movement of the conventional switching power supply apparatus.

  DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the present invention will be described in detail as examples based on the attached drawings.

FIG. 1 is a block diagram showing an embodiment of a flyback switching power supply 50 according to the present invention.
The switching power supply 50 includes an input end 1 and an input end 2 to which an AC power supply is connected, a rectifier 3 in which the input end 1 and the input end 2 are connected to respective input terminals, and an output end of the rectifier 3 + A smoothing capacitor 4 connected between a terminal and a negative terminal, a transformer 7 having a primary side winding 7a and a secondary side winding 7b, and an FET for switching a current flowing in the primary side winding 7a (Field A switching element 6 which is an effect transistor, and a capacitor 5 connected in parallel with the switching element 6.

  Further, the switching power supply device 50 includes a positive terminal 10 and a negative terminal 11 that output a DC voltage from the switching power supply device 50, one end of the secondary winding 7b connected to the anode terminal, and a cathode terminal connected to the positive terminal 10. The switching diode 6 is controlled, the smoothing capacitor 9 having one end connected to the cathode terminal of the diode 8 and the other end connected to the negative terminal 11 and the other end of the secondary winding 7b. Switching control means 20.

  One end of the primary winding 7a is connected to the + terminal of the rectifier 3, the other end of the primary winding 7a is connected to the drain terminal of the switching element 6, and the source terminal of the switching element 6 is connected to the-terminal of the rectifier 3. Each is connected. One end of the secondary winding 7 b and the gate terminal of the switching element 6 are connected to the switching control means 20.

  The switching control means 20 includes a bottom point detection means 21 that detects the timing of the bottom point when one end of the secondary side winding 7b is connected and a secondary side winding voltage is input, and a bottom point detection means 21. The bottom signal generation means 22 to which a signal for instructing the bottom signal output to be output is input, the clock signal generation means 26 to generate a clock signal at a predetermined frequency (switching frequency), and the clock signal is input to start the switching power supply device 50 A switching start means 27 for outputting an initial trigger signal for first turning on the switching element 6 and a turn-on timing determination means 23 for receiving a bottom signal and a clock signal output from the bottom signal generating means 22. Yes.

The switching control means 20 outputs a high level signal when either the turn-on trigger signal output from the turn-on timing determination means 23 or the initial trigger signal output from the switching start means 27 is at a high level. OR circuit 24 that performs DC voltage detection means 28 that detects the DC voltage of the smoothing capacitor 9, the detected DC voltage, the clock signal, and the trigger signal output from the OR circuit 24 are input, and the detected DC voltage is PWM control means 25 for outputting a switching signal whose pulse duty is controlled to a predetermined voltage to the switching element 6 is provided.
The turn-on trigger signal output by the turn-on timing determination unit 23 is also output to the bottom point detection unit 21.

Next, each means will be described in detail.
The switching start unit 27 includes a start start detection unit 27a and an AND circuit 27b. The activation start detection means 27a outputs an activation start state signal that is a high-level pulse having a length of not less than one cycle and not less than one cycle when the switching power supply device 50 is ready to start. To do. The startable state indicates a state in which the voltage of the smoothing capacitor 4 becomes equal to or higher than a certain level and the switching power supply device 50 can output a rated output voltage.

  The AND circuit 27b receives the clock signal and the start start state signal, and outputs a turn-on trigger signal to the OR circuit 24 when both signals become high level. The start-up detection means 27a includes a differentiating circuit (not shown) that detects a transient voltage change that rises when the voltage of the smoothing capacitor 4 is charged. The voltage signal detected by the differentiating circuit is used as the start-up state signal. Output as.

  The bottom point detection means 21 monitors the secondary winding voltage of the secondary winding 7b, and changes from a state in which this voltage is larger than the bottom voltage threshold, which is a predetermined threshold, to a state below the bottom voltage threshold. A bottom signal output instruction signal that changes from a low level to a high level when changed is output to the bottom signal generating means 22. Further, the bottom point detection means 21 resets the bottom signal output instruction signal from the high level to the low level in response to the fall of the turn-on trigger signal output from the turn-on timing determination means 23.

  In the secondary side winding voltage, the excitation energy generated by switching of the switching element 6 is generated as a flyback voltage, and this excitation energy is consumed by a load (not shown) on the secondary side, which has been described in the background art. Thus, when the current of the secondary winding 7b becomes zero, the voltage across the switching element 6 starts free vibration. In addition, since the frequency and phase of free vibration are not affected by the load on the secondary side, the secondary winding voltage at the timing of the first bottom point at which free vibration starts is defined in advance as the bottom voltage threshold. Thus, even when the load on the secondary side fluctuates, it is possible to detect the timing of the first bottom point at which free vibration has started.

  When the bottom signal generation means 22 recognizes the timing of the bottom point based on the bottom signal output instruction signal, the bottom signal generation means 22 generates a bottom signal that is a pulse signal having the same cycle synchronized with free vibration while the bottom signal output instruction signal is at a high level. Output continuously. Therefore, the turn-on timing determination means 23 to which the bottom signal is input can determine the timing of the bottom points generated by the bottom signal during the period when the bottom signal output instruction signal is at the high level.

  The clock signal generated by the clock signal generating means 26 is a signal that is the basis of the switching signal, and is a pulse signal having the same cycle as the switching cycle. The time when the clock signal changes from the low level to the high level within one cycle of the clock signal is defined as the start timing of the switching cycle. Therefore, the turn-on timing determination means 23 detects that the switching cycle has started at the rising timing of the clock signal, and then the turn-on trigger which is a pulse signal when the bottom signal first changes from low level to high level. Output a signal. The PWM control means 25 turns on the switching element 6 in response to the turn-on trigger signal, and turns off the switching element when the on-duty time corresponding to the DC voltage detected by the DC voltage detection means 28 elapses. For this reason, the PWM control means 25 executes switching (on / off) once within one cycle of the clock signal.

Next, the operation of the configuration of FIG. 1 will be described with reference to FIG. 2 which is an explanatory diagram for explaining the operation of the switching power supply device 50 according to the present invention.
2 (1) shows the clock signal that is the basis of the switching signal, FIG. 2 (2) shows the switching signal, FIG. 2 (3) shows the voltage across the switching element 6, and FIG. 2 (4) shows the secondary winding. The secondary winding voltage of the line 7b, FIG. 2 (5) is a bottom signal output instruction signal, FIG. 2 (6) is a bottom signal, FIG. 2 (7) is a trigger signal, and FIG. Respectively indicate start-up start state signals. The horizontal axis represents time, and t1 to t12 represent time.

  First, when AC power is applied to the switching power supply 50, the voltage across the smoothing capacitor 4 gradually increases due to the voltage rectified by the rectifier 3. The start-up detection means 27a that has detected that the voltage across the both ends exceeds a certain value outputs a start-up start state signal that is a high level signal for a predetermined period, as shown in FIG. 2 (8). The switching start means 27 outputs a trigger signal (initial trigger signal) as shown in FIG. 2 (7) at the timing t1 when the activation start state signal is at a high level and the clock signal rises.

  The PWM control means 25 to which this trigger signal is input via the OR circuit 24 outputs a switching signal for turning on the switching element 6. The PWM control means 25 turns off the switching signal when the on-duty time corresponding to the DC voltage detected by the DC voltage detection means 28 has elapsed after turning on the switching element 6, that is, at t 2. As a result, as shown in FIG. 2 (3), the voltage across the switching element 6 rises to approximately the voltage obtained by adding the flyback voltage to the voltage Vc 4 across the smoothing capacitor 4.

  Then, the excitation energy accumulated in the transformer 7 via the primary side winding 7a is output as the secondary side winding voltage from the secondary side winding 7b as shown in FIG. 2 (4). This voltage is rectified by the diode 8 and supplied to a load (not shown) connected to the + terminal 10 and the − terminal 11. When the excitation energy is eventually released and the current of the secondary winding 7b does not flow, the secondary winding voltage gradually decreases from t4.

  This voltage drop is due to free vibration, and when the current of the secondary winding 7b does not flow after t4, the voltage across the switching element 6 starts free vibration. As shown in FIG. 2 (4), when the secondary winding voltage becomes equal to or lower than the bottom voltage threshold at t5, the bottom point detection means 21 becomes high level after t5 as shown in FIG. 2 (5). Outputs a bottom signal output instruction signal.

  The bottom signal generation means 22 to which the bottom signal output instruction signal is input outputs a bottom signal that is a pulse signal having the same period synchronized with free vibration as shown in FIG. The bottom signal generating means 22 continuously outputs the bottom signal while the bottom signal output instruction signal is at a high level.

  As described above, the period of the free vibration and the period of the bottom signal are the same period, and each signal is designed to be synchronized, and the timing at which the secondary winding voltage falls below the bottom voltage threshold value. Since the timing at which the voltage resonance waveform due to free vibration (the waveform of the voltage across the switching element) becomes the bottom point is synchronized in advance, after t5, the rising timing of the bottom signal pulse and the bottom point timing of the voltage resonance waveform Is almost the same timing. In terms of circuit design, the bottom voltage threshold value includes a margin and is set slightly higher than the bottom point of the secondary winding voltage. For this reason, the bottom signal generating means 22 outputs the bottom signal at t5 that is slightly earlier in time than the actual bottom point t6.

  In the PWM control, the switching element 6 is turned on and off within one cycle of the clock signal. Therefore, the turn-on is performed at the start of the cycle of the clock signal. Since the turn-off timing depends on the DC voltage on the secondary side, the start time of the next clock signal period (turn-on time) does not necessarily match the timing of the bottom point of the voltage resonance waveform. Furthermore, the number of cycles of free vibration from the bottom point of t6 to the start time of the next clock signal cycle also changes. Therefore, it is necessary to turn on the first bottom signal after the start point of the cycle of the clock signal (rise of the clock signal) comes.

  For this reason, the turn-on timing determination means 23 monitors the timing of t9 when the clock signal rises and the timing when the bottom signal rises. Since free vibration vibrates at a constant cycle, the timing of the bottom point is continuous with t6, t7, t8, t10,. When the turn-on timing determination means 23 detects the bottom signal at the timing t10 when the clock signal rises and rises at the first bottom signal, the turn-on timing determination means 23 determines that it is the turn-on timing and sends the turn-on trigger signal to the OR circuit. The OR circuit 24 outputs the input turn-on trigger signal to the PWM control means 25 as a trigger signal. The PWM control means 25 to which this trigger signal is inputted turns on the switching element 6 by the switching signal.

  On the other hand, the turn-on trigger signal output from the turn-on timing determination means 23 is also output to the bottom point detection means 21. When the bottom point detection means 21 detects the fall of this turn-on trigger signal, the bottom signal is output at t11. Change the instruction signal from high to low.

  After turning on the switching element 6, the PWM control means 25 turns off the switching signal when the on-duty time calculated corresponding to the secondary side DC voltage has elapsed as described above. Thereafter, the switching power supply device 50 repeats the above-described operation, and operates so that the secondary side DC voltage becomes a predetermined voltage defined as the output voltage of the switching power supply device 50.

  The turn-on timing determination means 23 outputs a turn-on trigger signal when detecting the bottom signal at the timing t10 when the first bottom signal rises after the rising edge of the clock signal comes at t9. Since the generation period of the free vibration from t4 to t10 varies depending on the load of the switching power supply device 50, the number of bottom signals output after t6 is indefinite. For this reason, the turn-on timing determining means 23 turns on the switching element 6 with a delay of at most one cycle of the bottom signal from the rising edge of the clock signal. That is, the period from t9 to t10 is a delay time due to waiting for the bottom point. Since this time becomes an error in PWM control, it is necessary to make this delay time as small as possible.

  For example, when the switching period is 100 microseconds (10 kilohertz) and the delay time is less than 1%, which is an error that can be ignored in PWM control, the free vibration period is 1 / 100th of 100 microseconds. That is, less than 1 microsecond. As described above, since the period of the free vibration is determined by the inductance of the primary winding 7a, the parasitic capacitance of the switching element 6, and the time constant of the capacitor 5, the switching frequency is taken into consideration when designing the switching power supply 50. This time constant is determined in advance. Alternatively, when calculating the duty of the switching signal, the delay time actually generated may be subtracted. This delay time can be obtained by measuring the time from the rising timing of the clock signal to the rising timing of the trigger signal with a timer.

  As described above, the turn-on timing determination unit 23 detects the rising edge of the clock signal after the turn-on timing determination unit 23 starts from the plurality of bottom point pulse signals (bottom signals) detected by the bottom point detection unit 21. After the detection, when the first pulse signal (bottom signal) is detected, the switching element 6 is turned on via the PWM control means 25. For this reason, even if it is the switching power supply device 50 of a PWM control system, a switching element turns on at the bottom point where the both-ends voltage of a switching element is low. For this reason, a switching loss can be reduced rather than the switching power supply device using the conventional PWM control system.

  In this embodiment, the frequency of the free vibration is determined using the capacitor 5, but is not limited to this, and may be determined only by the inductance of the primary winding 7 a and the parasitic capacitance of the switching element 6. Moreover, although the case where only one set of secondary windings has been described, the present invention is not limited to this, and a plurality of secondary windings may be provided. Further, the secondary winding voltage input to the bottom point detection means 21 may be detected by a dedicated winding.

DESCRIPTION OF SYMBOLS 1 Input end 2 Input end 3 Rectifier 4 Smoothing capacitor 5 Capacitor 6 Switching element 7 Transformer 7a Primary side winding 7b Secondary side winding 8 Diode 9 Smoothing capacitor 10 + terminal 11-terminal 20 Switching control means 21 Bottom point detection means 22 Bottom signal generation means 23 Turn-on timing determination means 24 OR circuit 25 PWM control means 26 Clock signal generation means 27 Switching start means 27a Start-up start detection means 27b AND circuit 28 DC voltage detection means 50 Switching power supply device

Claims (1)

A rectifier that rectifies an input AC voltage and outputs a DC voltage, a primary winding and a secondary winding, and transforms and outputs the DC voltage applied to the primary winding from the secondary winding. A flyback type switching power supply device comprising a transformer, a switching element arranged in series between the rectifier and the primary winding, and switching control means for controlling the switching of the switching element by PWM control. ,
The switching control means includes
Clock signal generating means for generating a clock signal of the switching frequency;
PWM control means for outputting a switching signal generated based on the clock signal to the switching element;
Bottom point detection means for detecting the timing of the bottom point of free vibration due to pseudo-resonance generated in the voltage across the switching element when the voltage of the secondary winding is equal to or lower than a predetermined threshold value;
A bottom signal generating means for continuously outputting a pulse signal synchronized with the free vibration after the bottom point detecting means detects the bottom point;
A turn-on timing determining means for monitoring the clock signal and the pulse signal and turning on the switching element via the PWM control means when the first pulse signal is detected after the start of the cycle of the clock signal; The switching power supply device characterized by the above-mentioned.

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