JP6015947B2 - Switching power supply device and control method thereof - Google Patents

Description

  The present invention relates to a switching power supply device, and more particularly, to a synchronous rectification switching power supply device and a method for controlling a synchronous rectification switching power supply device.

  Some conventional synchronous rectification switching power supply devices include a soft start circuit (see Patent Document 1). In the switching power supply device described in Patent Document 1, on / off operation of a switch element that supplies power to the primary coil side of the transformer is started when the device is activated, and is induced on the secondary coil side of the transformer during the soft start period. The alternating current voltage is rectified by a diode, and after the soft start period has elapsed, the on / off operation of the switch element for synchronous rectification on the secondary coil side of the transformer is started. This prevents an excessive current from flowing through the synchronous rectification switching element.

JP 2004-215356 A

  By the way, like the above-described switching power supply device described in Patent Document 1, when the on / off operation of the synchronous rectification switch element on the secondary coil side of the transformer is started after the soft start period has elapsed, the output voltage May vary.

This variation in output voltage will be described below with a specific example. FIG. 6 is a diagram showing a configuration of a conventional switching power supply apparatus. Moreover, it is a figure for demonstrating the fluctuation | variation of the output voltage in the conventional switching power supply device.
A switching power supply 1 ′ shown in FIG. 6 has a widely known general configuration. The switching power supply 1 ′ includes a power supply circuit unit 2 and switch elements Q1 to Q6 in the power supply circuit unit 2. And a control circuit 10 'for controlling the on / off operation.

  The power supply circuit unit 2 is configured to insulate the primary / secondary by a transformer Ts, and on the primary coil side of the transformer Ts, a full bridge circuit 3, an LLC resonance circuit (a choke coil L11, a capacitor C12, A resonance circuit with a primary inductance of the transformer Ts). A synchronous rectification circuit 4 is provided on the secondary coil side of the transformer Ts. The synchronous rectification circuit 4 includes a synchronous rectification switch circuit 5, a rectification smoothing circuit (choke coils L21 and L22, and a capacitor C21). It consists of

  In this power supply circuit unit 2, the primary coil side of the transformer Ts is AC driven by the full bridge circuit 3 and the LLC resonance circuit, and the AC voltage induced on the secondary coil side is full-wave rectified by the synchronous rectifier circuit 4. By smoothing, a desired DC output voltage Vout is generated.

  The control circuit 10 ′ includes a soft start processing unit 11, a voltage detection circuit 12, a constant voltage control unit 13, a rectification method control unit 14, a PWM output conversion unit 15, and a driver circuit 16. Configured.

The soft start processing unit 11 generates a signal of a target value Vref that gradually increases from an initial value to a constant value in a predetermined soft start period from the start of the device, and the switching power supply device 1 ′ generates the signal of the target value Vref. The signal is output to the constant voltage control unit 13 as a reference set value signal of the output voltage Vout to be output.
An example of a signal waveform of the target value Vref output from the soft start processing unit 11 is shown in FIG.
As shown in FIG. 7A, the soft start processing unit 11 sets the target value Vref from the initial value Vo to a predetermined reference with a predetermined soft start period LT after activation of the switching power supply device 1 ′ at time t1. The voltage is gradually increased to the set value Vc and output to the constant voltage control unit 13 as a signal of the target value Vref.

The voltage detection circuit 12 detects the voltage level of the output voltage Vout, generates a voltage feedback signal Vfbk, and outputs the voltage feedback signal Vfbk to the constant voltage control unit 13.
The constant voltage control unit 13 receives the signal of the target value Vref and the voltage feedback signal Vfbk, compares the signal of the target value Vref with the voltage feedback signal Vfbk, and switches elements Q1 to Q6 according to the comparison result. The operation amount Uv for controlling the on / off operation of the is generated. The constant voltage control unit 13 outputs the operation amount Uv signal to the PWM output conversion unit 15.

  The PWM output conversion unit 15 is a PWM signal Upwm for controlling the on / off timing (duty ratio (Duty)) of the switch elements Q1 to Q6 based on the signal of the operation amount Uv input from the constant voltage control unit 13. Is output to the driver circuit 16. The driver circuit 16 generates and outputs gate signals G1 to G6 for driving the switch elements Q1 to Q6 on / off based on the PWM signal Upwm input from the PWM output conversion unit 15.

  The rectification method control unit 14 is a processing unit for controlling the rectification method in the synchronous rectification circuit 4. The rectification method control unit 14 outputs a control signal DSEL for performing diode rectification or synchronous rectification to the PWM output conversion unit 15.

When the PWM output conversion unit 15 is controlled to perform diode rectification from the rectification method control unit 14 during the soft start period LT, as shown in the PWM signals in FIGS. The switch elements Q5 and Q6 in the rectifier switch circuit 5 are turned off, and the switch elements Q1 to Q4 in the full bridge circuit 3 are turned on based on the operation amount Uv signal input from the constant voltage control unit 13. / Generates a PWM signal Upwm that controls the off-operation (duty ratio).
Thereby, during the soft start period, the power supply circuit unit 2 rectifies the AC voltage induced on the secondary coil side of the transformer by the body diodes D5 and D6 ("body diode" is also simply referred to as "diode"). By performing diode rectification and controlling the on / off timing (duty ratio) of the switch elements Q1 to Q4, the output voltage Vout can be controlled to a value corresponding to the target value Vref.

  When the PWM output conversion unit 15 is controlled to perform synchronous rectification by the rectification method control unit 14 after the soft start period LT has elapsed, as shown in FIG. 7D, the synchronous rectification switch circuit starts from time t2. The switch elements Q5 and Q6 in 5 are turned on / off. The PWM output conversion unit 15 also turns on / off timing (duty ratio) of the switch elements Q5 and Q6 in the synchronous rectifier switch circuit 5 and on / off of the switch elements Q1 to Q4 in the full bridge circuit 3. A PWM signal Upwm for controlling the timing (duty ratio) is generated based on the signal of the operation amount Uv input from the constant voltage control unit 13.

  Thus, after the soft start period has elapsed, the power supply circuit unit 2 performs synchronous rectification in which the AC voltage induced on the secondary coil side of the transformer is rectified by the switch elements Q5 and Q6, and the switch elements Q1 to Q6. By controlling the on / off timing (duty ratio), the output voltage Vout can be controlled to a value corresponding to the target value Vref.

  By the way, in the switching power supply 1 ', during the soft start period, the switch elements Q5 and Q6 in the synchronous rectification switch circuit 5 are in the off state, and the body diodes D5 and D6 are alternately turned on to perform diode rectification. On the other hand, when the soft start period LT elapses and the switch element Q5 or Q6 in the synchronous rectifier circuit 4 is turned on, the body diode D5 or D6 is short-circuited at that moment (more precisely, via the ON resistance of the switch element Q5 or Q6). Are short-circuited).

  For this reason, at the moment when the switching element Q5 or Q6 is turned on, the forward voltage drop due to the body diode D5 or D6 disappears, and the voltage rise (fluctuation) Vup occurs in the output voltage Vout. For example, as shown in FIG. 7B, an instantaneous voltage increase Vup occurs in the output voltage Vout at the timing when the switch element Q5 or Q6 is turned on at time t2.

  In the case of a normal control state, the operation amount Uv output from the constant voltage control unit 13 is a signal for adjusting increase / decrease in the output voltage Vout so that the output voltage Vout approaches the target value Vref. If the output voltage Vout is to be increased, the manipulated variable Uv is changed so that the output voltage Vout approaches the target value Vref, thereby suppressing an increase in the output voltage Vout.

However, when the body diode D5 or D6 is suddenly shorted by turning on the switch element Q5 or Q6, as shown in FIG. 7E, a delay occurs in the response of the operation amount Uv, and the output voltage Vout Voltage rise Vup occurs. Thereafter, the signal of the operation amount Uv gradually changes, so that the output voltage Vout approaches the target value Vref at time t3.
As described above, in the switching power supply device 1 ′, when the soft start period elapses and synchronous rectification is started, there is a problem that the voltage rise (fluctuation) Vup occurs in the output voltage Vout, and this problem is solved. Was desired.

  The present invention has been made in view of such circumstances, and an object of the present invention is that a voltage increase (fluctuation) occurs in the output voltage when synchronous rectification is started after the soft start period has elapsed. It is an object to provide a switching power supply apparatus and a control method that can avoid the above.

  The present invention has been made to solve the above-described problems, and a switching power supply apparatus according to the present invention is a first switch that switches a current flowing to a primary coil side of a transformer and supplies electric power to the primary coil side. A circuit, a second switch circuit for synchronously rectifying an AC voltage output from the secondary coil side of the transformer, a diode connected in parallel to a switch element in the second switch circuit, the first and first And a control circuit that controls the on / off operation of the switch elements in the switch circuit, wherein the control circuit has an output of the switching power supply device with a predetermined soft start period when activated. A soft start processing unit that gradually increases a target output serving as a reference from an initial output to a predetermined output, the target output and the switch For controlling the on / off operation of the switch elements in the first and second switch circuits so that the output of the switching power supply device becomes a value corresponding to the target output. An output control unit for generating a manipulated variable of the first switch circuit, and an on / off operation of the switch element in the first switch circuit with the switch element in the second switch circuit turned off during the soft start period Is controlled based on the manipulated variable, and diode rectification is performed by the diode to rectify an AC voltage generated on the secondary coil side of the transformer. After the soft start period has elapsed, the second switch circuit On / off operation of the switch elements in the first and second switch circuits is started, and the on / off operation of the switch elements in the first and second switch circuits is started based on the operation amount. A rectification method control unit that performs synchronous rectification under control, and when the soft start period has elapsed and the rectification method control unit starts the synchronous rectification, the operation amount is temporarily set to the operation amount. And an operation amount replacement unit that replaces the operation amount with the correction amount added thereto.

  In the switching power supply device of the present invention, the control circuit includes a voltage detection circuit that detects an output voltage of the switching power supply device, and the soft start processing unit performs switching with a predetermined soft start period when starting up. A target value serving as a reference for the output voltage of the power supply device is gradually increased from an initial value to a predetermined value, and the output control unit compares the target value with a detected value of the output voltage of the switching power supply device, An operation amount for controlling the on / off operation of the switch elements in the first and second switch circuits is generated so that the output voltage of the switching power supply device becomes a value corresponding to the target value. And

  In the switching power supply of the present invention, the predetermined correction amount is set according to a value of a voltage drop in the diode and a value of a voltage drop due to an on-resistance of a switch element in the second switch circuit. It is characterized by that.

  In the switching power supply device of the present invention, on the primary coil side of the transformer, a full bridge circuit as the first switch circuit, a choke coil and a capacitor connected in series to the primary coil of the transformer, The full bridge circuit is controlled to supply power to the primary coil by an LLC resonant circuit via a choke coil and a capacitor connected in series to the primary coil, and the secondary coil side of the transformer Includes two switch elements in the second switch circuit connected between the terminals of the secondary coil and the reference potential terminal of the DC output voltage, respectively, and one end of each terminal of the secondary coil. Are connected, and the other end is connected in common, one end is the output terminal on the positive side of the DC output voltage, and the two choke coils are connected. A smoothing capacitor connected to the other end of the coil coil and the other end connected to the reference potential terminal, and the two switch elements in the second switch circuit are connected to the secondary coil side. And a full-wave rectification of the AC voltage generated at the same time, and a current doubler type synchronous rectification in which the full-wave rectified voltage is smoothed by the choke coil and the smoothing capacitor. .

  The switching power supply device control method according to the present invention includes a first switch circuit that switches a current flowing to the primary coil side of the transformer and supplies power to the primary coil side, and is output from the secondary coil side of the transformer. ON / OFF of a second switch circuit for synchronously rectifying AC voltage to be generated, a diode connected in parallel to the switch element in the second switch circuit, and the switch elements in the first and second switch circuits A control method for a switching power supply device comprising a control circuit for controlling the operation, wherein the control circuit provides a target output as a reference for the output of the switching power supply device with a predetermined soft start period at the time of start-up by the control circuit. A soft start processing procedure for gradually increasing from an output to a predetermined output, and the target output and the output of the switching power supply device. In comparison, an output for generating an operation amount for controlling the on / off operation of the switch elements in the first and second switch circuits so that the output of the switching power supply device has a value corresponding to the target output. During the control procedure and the soft start period, the on / off operation of the switch element in the first switch circuit is performed based on the operation amount with the switch element in the second switch circuit turned off. And performing diode rectification by the diode to rectify an AC voltage generated on the secondary coil side of the transformer. After the soft start period has elapsed, the switch element in the second switch circuit is turned on / off. An off operation is started, and synchronous rectification is performed by controlling the on / off operation of the switch elements in the first and second switch circuits based on the manipulated variable. A rectification method control procedure and an operation amount obtained by temporarily adding a predetermined correction amount to the operation amount when the soft start period has elapsed and the synchronous rectification is started by the rectification method control procedure. And an operation amount replacement procedure for replacing the operation amount.

In the switching power supply device and the control method of the present invention, rectification of the AC voltage induced on the secondary coil side of the transformer is performed by diode rectification during the soft start period, and by synchronous rectification after the soft start period has elapsed. When the rectification method is switched from diode rectification to synchronous rectification after the soft start period has elapsed, the amount of operation for controlling the on / off operation of the switch elements in the first and second switching circuits is set to synchronous rectification. The operation amount immediately before the start is replaced with an operation amount obtained by temporarily adding a predetermined correction amount.
Thereby, in the switching power supply device and the control method of the present invention, it is possible to avoid a voltage increase (fluctuation) in the output voltage when synchronous rectification is started after the soft start period has elapsed.

It is a figure which shows the structure of the synchronous rectification type | mold switching power supply concerning Embodiment of this invention. It is a figure for demonstrating the rectification operation | movement in a synchronous rectification circuit. 4 is a waveform diagram for explaining the operation of each part of the switching power supply device 1; FIG. It is a figure for demonstrating the relationship between the operation amount Uv and the duty ratio of a PWM signal. It is a figure which shows the flow of control in the switching power supply device 1. It is a figure which shows the structure of the conventional switching power supply device. It is a figure for demonstrating the fluctuation | variation of the output voltage in the conventional switching power supply device.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

FIG. 1 is a diagram illustrating a configuration of a synchronous rectification switching power supply according to an embodiment of the present invention. FIG. 2 is a diagram for explaining the rectification operation in the synchronous rectification circuit.
A switching power supply device 1 shown in FIG. 1 includes a power supply circuit unit 2 and a control circuit 10 that controls the operation of the power supply circuit unit 2.
In the switching power supply 1 shown in FIG. 1, the power supply circuit unit 2 is an LLC resonance type power supply circuit and has a widely known general configuration as a power supply circuit using a current doubler type full-wave rectifier circuit. Circuit. The switching power supply device 1 of this embodiment is characterized by the configuration of the control circuit 10 and the control operation of the power supply circuit unit 2 by the control circuit 10.

  Note that the switching power supply device 1 of the present embodiment shown in FIG. 1 is newly equipped with a manipulated variable replacement unit 17 in the control circuit 10 shown in FIG. 1 compared to the conventional switching power supply device 1 ′ shown in FIG. The other configuration is the same as that of the switching power supply device 1 ′ shown in FIG. For this reason, the same code | symbol is attached | subjected to the same component.

(Configuration of power supply circuit unit 2)
In the switching power supply device 1, the power supply circuit unit 2 is a circuit having a general configuration that is widely known as described above, and therefore the configuration and operation thereof will be briefly described below.
The power supply circuit unit 2 is configured to insulate the primary / secondary by the transformer Ts. The primary coil side of the transformer Ts is connected to the DC power source Vin and the primary coil side of the transformer Ts is AC-driven. A full bridge circuit (first switch circuit) 3 and an LLC resonance circuit (a resonance circuit including an inductance of a choke coil L11, a capacitor C12, and a primary coil) are provided.
Further, in the power supply circuit unit 2, a synchronous rectification circuit 4 is provided on the secondary coil side of the transformer Ts. The synchronous rectification circuit 4 includes a synchronous rectification switch circuit 5, choke coils L21 and L22, and a smoothing circuit. And a current doubler type full-wave rectifier circuit.

In the power supply circuit unit 2, the full bridge circuit 3 is a bridge circuit that AC drives the primary side coil of the supply transformer Ts, and includes four switch elements Q1 and switch elements using MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). It is composed of Q2, a switch element Q3, and a switch element Q4. Each of the switch elements Q1 to Q4 includes body diodes D1 to D4 connected in parallel.
The drains of the switch elements Q1 and Q3 are connected to the power supply line Vin + on the positive voltage side of the DC power supply Vin, and the sources of the switch elements Q2 and Q4 are connected to the power supply line Vin− on the negative voltage side of the DC power supply Vin. ing.

One electrode of the capacitor C12 is connected to a connection point between the source of the switch element Q1 and the drain of the switch element Q2, and the other electrode of the capacitor C12 is connected to the primary coil side terminal a of the transformer Ts. ing.
Further, one end of the choke coil L11 is connected to a connection point between the source of the switch element Q3 and the drain of the switch element Q4, and the other end of the choke coil L11 is connected to the terminal b of the primary coil of the transformer Ts. .
Thereby, the LLC resonance circuit which carries out alternating current drive of the primary coil side of transformer Ts by full bridge circuit 3 is constituted.

  Each of the switch elements Q1 to Q4 in the full bridge circuit 3 is controlled by the control circuit 10 so that the switch elements Q1 and Q4 are in the on state and the switch elements Q2 and Q3 are in the off state, and the switch element Q1 , Q4 is in an off state and the switching elements Q2 and Q3 are in an on state, and the period is alternately provided. As a result, the resonance current generated by the resonance circuit including the choke coil L11 and the capacitor C12 flows from one end of the primary coil of the transformer Ts to the other end, or flows from the other end of the primary coil of the transformer Ts to one end. When a current flows on the primary coil side of the transformer Ts, an AC voltage V1 is induced on the secondary coil side of the transformer Ts.

  In this LLC resonant circuit, the resonance effect between the choke coil L11 and the capacitor C12 is utilized, and the switching elements Q1 to Q4 are timings when the charge accumulated in the parasitic capacitances of the switching elements Q1 to Q4 is minimized. By turning on, switching loss in the switch elements Q1 to Q4 can be suppressed.

The anode b of the diode D11 is connected to the terminal b of the primary coil of the transformer Ts, and the cathode terminal of the diode D11 is connected to the power supply line Vin + on the positive voltage side of the DC power supply Vin. The diode D11 is a clamping diode for clamping the potential of the terminal b to the positive potential of the DC power supply Vin.
Further, the cathode terminal of the diode D12 is connected to the terminal b of the primary coil, and the anode terminal of the diode D12 is connected to the power supply line Vin− on the negative voltage side of the DC power supply Vin. This diode D12 is a clamping diode for clamping the potential of the terminal b to the negative potential of the DC power supply Vin.
Further, a noise absorbing capacitor C11 is provided between the positive voltage side power supply line Vin + and the negative voltage side power supply line Vin−.

Next, the configuration of the synchronous rectifier circuit 4 on the secondary coil side of the transformer Ts will be described.
The synchronous rectifier circuit 4 includes a synchronous rectifier switch circuit (second switch circuit) 5 and a rectifying / smoothing circuit (choke coils L21 and L22 and a capacitor C21), and rectifies an AC voltage induced in the secondary coil. The full-wave rectifier circuit converts the voltage into a DC voltage.
In this synchronous rectifier circuit 4, the on / off timing of the switch elements Q5 and Q6 constituting the synchronous rectifier switch circuit 5 is controlled by the control circuit 10 to generate the electromotive force generated in the first secondary coil of the transformer Ts. Synchronous rectification is performed, and the DC voltage subjected to synchronous rectification is smoothed by the choke coils L21 and L22 and the capacitor C21, and power is supplied to the load RL.

  In the present embodiment, the switch elements Q5 and Q6 in the synchronous rectification switch circuit 5 are constituted by MOSFETs. A body diode D5 is connected in parallel to the switch element Q5, and a body diode D6 is connected in parallel to the switch element Q6 (hereinafter, “body diode” is also simply referred to as “diode”).

In the synchronous rectifier circuit 4, one end of the choke coil L21 is connected to the terminal c of the secondary coil of the transformer Ts. The other end of the choke coil L21 is connected to an electrode on the positive voltage side of the capacitor C21, and is connected to the output terminal Out1 on the positive voltage side of the DC voltage and one end of the load RL via the power line DC +. Yes. The other end of the load RL is connected to one end of the open / close switch SW1, and the other end of the switch SW1 is connected to the reference potential line GND.
Further, the drain of the switch element Q5 in the synchronous rectification switch circuit 5 is connected to the terminal c of the secondary coil of the transformer Ts, and the source of the switch element Q5 is connected to the reference potential of the DC voltage via the reference potential line GND. Side output terminal Out2 (reference potential terminal).

One end of the choke coil L22 is connected to the terminal d of the secondary coil of the transformer Ts. The other end of the choke coil L22 is connected to the positive voltage side electrode of the capacitor C21, and is connected to the output terminal Out1 on the positive voltage side of the DC voltage and one end of the load RL via the power line DC +. Yes. The electrode on the negative voltage side of the capacitor C21 is connected to the output terminal Out2 (reference potential terminal).
The drain of the switch element Q6 in the synchronous rectification switch circuit 5 is connected to the terminal d of the secondary coil of the transformer Ts, and the source of the switch element Q6 is connected to the output terminal Out2 via the reference potential line GND. Has been.

  In the above configuration, when the switching power supply device 1 is operated as a diode rectification type power supply device during the soft start period, the switch elements Q5 and Q6 in the synchronous rectification switch circuit 5 are turned off, and the secondary coil of the transformer Ts. The AC voltage induced on the side is rectified by the body diodes D5 and D6 of the switch elements Q5 and Q6.

For example, as shown in FIG. 2, when a voltage is induced between terminals cd of the secondary coil of the transformer Ts in the direction indicated by V <b> 1 in FIG. 2, a current is supplied to the load RL through a path A indicated by a one-dot chain line. Flows. That is, when a voltage is induced in the direction of V1, a current flows through a path of “secondary coil terminal c → choke coil L21 → load RL → switch SW1 → diode D6 → secondary coil terminal d”.
Further, when a voltage is induced between terminals cd of the secondary coil of the transformer Ts in the direction opposite to V1 in the figure, a current flows through the load RL along a path B indicated by a broken line. That is, when a voltage is induced in the direction opposite to V1, a current flows through a path of “secondary coil terminal d → choke coil L22 → load RL → switch SW1 → diode D5 → secondary coil terminal c”.

  In the case of this diode rectification, the ON / OFF timing (duty ratio) of the switch elements Q1 to Q4 on the primary coil side of the transformer Ts is controlled based on the operation amount Uv output from the constant voltage control unit 13. Thus, the output voltage Vout diode-rectified by the body diodes D5 and D6 can be controlled to be a value corresponding to the target value Vref.

  On the other hand, when the switching power supply 1 is operated as a synchronous rectification type power supply, the ON / OFF timing (duty ratio) of the switch elements Q5 and Q6 is synchronized with the timing at which the AC voltage V1 is induced on the secondary coil side. Ratio). For example, when a voltage is induced in the direction of V1 on the secondary coil side, a path “secondary coil terminal c → choke coil L21 → load RL → switch SW1 → switch element Q6 → secondary coil terminal d”. A current is passed at A. Further, when a voltage is induced in the direction opposite to V1, a current is caused to flow through a path of “secondary coil terminal d → choke coil L22 → load RL → switch SW1 → switch element Q5 → secondary coil terminal c”. .

  The on / off timing (duty ratio) of the switch elements Q1 to Q4 on the primary coil side of the transformer Ts is controlled based on the operation amount Uv output from the constant voltage control unit 13, and the secondary of the transformer Ts. By controlling the on / off timing (duty ratio) of the coil side switching elements Q5 and Q6, the output voltage Vout can be controlled to a value corresponding to the target value Vref.

  In the case of synchronous rectification, the switching elements Q5 and Q6 are used as rectifying elements instead of the diodes D5 and D6 in order to reduce power loss due to the forward voltage of the diodes D5 and D6. That is, the source-drain voltage drop due to the on-resistance of the switch elements Q5 and Q6 (MOSFET) can be made smaller than the forward voltage of the diode.

  The on / off timing of the switch elements Q5 and Q6 is, for example, when the switch element Q6 is turned on and a current flows through the load RL through the path A indicated by the alternate long and short dash line, the switch element Q5 is turned off and the transformer Ts is turned on. Control is performed so that the secondary coil side is not short-circuited. Further, when the switch element Q5 is turned on and a current flows through the load RL through the path B indicated by the broken line, the switch element Q6 is turned off and the secondary coil side of the transformer Ts is controlled not to be short-circuited.

(Configuration and operation of control circuit 10)
Next, returning to FIG. 1, the configuration and operation of the control circuit 10 will be described with reference to FIG. FIG. 3 is a waveform diagram for explaining the operation of each part of the switching power supply device 1.
As shown in FIG. 1, the control circuit 10 includes a soft start processing unit 11, a voltage detection circuit 12, a constant voltage control unit 13, a rectification method control unit 14, a PWM output conversion unit 15, and a driver circuit 16. The operation amount replacement unit 17 is configured.
The control circuit 10 supplies the gate signals G1 to G4 to the switch elements Q1 to Q4 in the full bridge circuit 3 on the primary coil side of the transformer Ts, respectively, and the on / off timing (duty ratio) of the switch elements Q1 to Q4. To control. Further, the control circuit 10 supplies gate signals G5 and G6 to the switch elements Q5 and Q6 in the synchronous rectification switch circuit 5 on the secondary coil side of the transformer Ts, and the on / off timing of the switch elements Q5 and Q6 ( (Duty ratio) is controlled.

  The soft start processing unit 11 is a circuit for gradually increasing the output voltage of the output voltage Vout when the switching power supply device 1 is started up. The soft start processing unit 11 detects that an excessively large current flows to the primary coil side of the transformer Ts. To avoid. The soft start processing unit 11 includes, for example, an integration circuit that charges a capacitor element at a constant current, thereby generating a target value Vref of the output voltage Vout, and sends a signal of the target value Vref to the constant voltage control unit 13. Output.

An example of the signal waveform of the target value Vref output from the soft start processing unit 11 is shown in FIG.
3 shows the passage of time in the horizontal direction, and in the vertical direction, the signal of the target value Vref of the soft start processing unit 11 (FIG. 3A) and the output voltage Vout (FIG. 3B). A PWM signal for controlling the on / off operation of the switch elements Q1 to Q4 (FIG. 3C), a PWM signal for controlling the on / off operation of the switch elements Q5 and Q6 (FIG. 3D), The signals of the manipulated variable Uv (FIG. 3E) and their respective schematic waveforms are shown side by side.

  As shown in FIG. 3A, the soft start processing unit 11 sets the target value Vref to the initial value Vo (for example, approximately 0) with a predetermined soft start period LT after the switching power supply device 1 is started at time t1. The output is gradually increased from an arbitrary value such as [V] to a predetermined reference set value Vc. Further, when the soft start period LT elapses, the soft start processing unit 11 outputs a soft start completion signal Tup to the rectification method control unit 14 and the operation amount replacement unit 17.

  The voltage detection circuit 12 detects the voltage level of the output voltage Vout and generates a voltage feedback signal Vfbk. For example, as illustrated in FIG. 3B, the output voltage Vout changes according to the signal of the target value Vref output from the soft start processing unit 11. The voltage detection circuit 12 detects the voltage level of the output voltage Vout, generates a voltage feedback signal Vfbk, and outputs it to the constant voltage control unit 13.

  The constant voltage control unit 13 compares the target value Vref output from the soft start processing unit 11 with the voltage feedback signal Vfbk output from the voltage detection circuit 12, and sets the output voltage Vout to a value corresponding to the target value Vref. As described above, an operation amount Uv signal for controlling the on / off timing of the switch elements Q1 to Q6 is generated, and the operation amount Uv signal is output to the PWM output converter 15.

  The rectification method control unit 14 is a processing unit for controlling whether the rectification operation of the AC voltage induced on the secondary coil side of the transformer Ts is performed by the diode rectification method or the synchronous rectification method. The rectification method control unit 14 outputs a rectification method control signal DSEL for selecting a rectification operation to the PWM output conversion unit 15, thereby controlling the generation operation of the PWM signal Upwm in the PWM output conversion unit 15.

  For example, the rectification method control unit 14 outputs a low signal (L signal) rectification method control signal DSEL to the PWM output conversion unit 15 so that the PWM output conversion unit 15 performs a diode rectification operation. (PWM signal for Q1 to Q4) is generated. Further, the rectification method control unit 14 outputs a high signal (H signal) rectification method control signal DSEL to the PWM output conversion unit 15 so that the PWM output conversion unit 15 performs a synchronous rectification operation. (PWM signal for Q1 to Q6) is generated.

  The rectification method control unit 14 controls the PWM output conversion unit 15 so as to generate a PWM signal by the diode rectification method until the soft start completion signal Tup is input from the soft start processing unit 11 after the switching power supply device 1 is started. To do. Then, after receiving the soft start completion signal Tup from the soft start processing unit 11, the rectification method control unit 14 controls the PWM output conversion unit 15 to generate a PWM signal by the synchronous rectification method.

The PWM output conversion unit 15 receives the operation amount Uv signal from the constant voltage control unit 13 and the rectification method control signal DSEL from the rectification method control unit 14. The PWM output conversion unit 15 turns on / off the switch elements Q1 to Q6 based on the signal of the operation amount Uv input from the constant voltage control unit 13 and the rectification method control signal DSEL input from the rectification method control unit 14. A PWM signal Upwm for controlling the timing (duty ratio) is generated. The PWM output conversion unit 15 outputs the PWM signal Upwm to the driver circuit 16.
The driver circuit 16 generates gate signals G1 to G6 (PWM signals) for turning on / off the switch elements Q1 to Q6 according to the PWM signal Upwm input from the PWM output conversion unit 15, and the gate signals G1 to Each of G6 is output to the gate terminals of the corresponding switch elements Q1 to Q6.

  For example, as shown in FIGS. 3C and 3D, after the switching power supply device 1 is activated at time t1, the soft start period LT elapses (time t1 to time t2). A PWM signal is output to the gate terminals of the elements Q1 to Q4, and no PWM signal is output to the switch elements Q5 and Q6. That is, in the soft start period LT, diode rectification by the diodes D5 and D6 is performed by turning off the switch elements Q5 and Q6.

  After the soft start period LT has elapsed (after time t2), a PWM signal is output to the gate terminals of the switch elements Q1 to Q4 to control the on / off timing, and the switch elements Q5 and Q6 are switched on. In contrast, the PWM signal is output to control the on / off timing. That is, after the soft start period LT has elapsed, synchronous rectification is performed by controlling the on / off timing of the switch elements Q5 and Q6.

The operation amount replacement unit 17 performs control so that the correction amount ΔUv is added to the operation amount Uv output from the constant voltage control unit 13 when the soft start period has elapsed and synchronous rectification is started. It is a processing part. For example, as shown in FIG. 3E, the manipulated variable replacement unit 17 sets the manipulated variable Uv immediately before the switch element Q5 or Q6 in the synchronous rectifier switch circuit 5 to be turned on (time t2). On the other hand, the operation amount Uv (t) is corrected by subtracting the correction amount ΔUv corresponding to the forward voltage drop of the body diodes D5 and D6.
More precisely, the correction amount ΔUv is the difference between the value of the forward voltage drop in the diodes D5 and D6 and the value of the voltage drop due to the on resistance of the switch elements Q5 and Q6 in the synchronous rectification switch circuit 5. It is set in advance accordingly.

  For example, FIG. 4 is a diagram for explaining the relationship between the operation amount Uv and the duty ratio of the PWM signal. In FIG. 4, FIGS. 4A to 4E are the same as FIGS. 3A to 3E, and switch elements Q1 and Q4 (or Q2 and Q3) of FIG. 4F are newly added. 4 and the PWM signal of the switch element Q5 (or Q6) in FIG. 4G are shown side by side. Further, the PWM signal of the switch elements Q1 and Q4 (or Q2 and Q3) in the case of the prior art of FIG. 4 (H) and the PWM signal of the switch element Q5 (or Q6) in the case of the prior art of FIG. Signals are shown side by side.

As shown in FIG. 4E, at the timing (time t2) when the switch element Q5 (or Q6) is turned on at time t2, the correction amount ΔUv corresponding to the body diode D5 (or D6) from the previous operation amount Uv. Is deducted.
As a result, as shown in FIG. 4F, the body diodes D5 and D6 are sequentially switched from the PWM signal P1 that drives the switch elements Q1 and Q4 (or Q2 and Q3) immediately before the soft start process is completed at time t2. A PWM signal P2 is generated by subtracting a duty ratio ΔDu1 corresponding to the voltage drop. Then, the PWM signal P2 with the reduced duty ratio is output from time t2, and the switch elements Q1 and Q4 (or Q2 and Q3) are turned on by the PWM signal P2.

Further, the PWM output converter 15 also changes the duty ratio of the PWM signal for driving the switch element Q5 (or Q6) in the synchronous rectification switch circuit 5 in response to the duty ratio of the PWM signal P2 changing so as to become small. Change.
For example, as shown in FIG. 4G, the PWM signal P12 for driving the switch element Q5 (or Q6) has a longer duty ratio ΔDu11 than the conventional PWM signal (the PWM signal shown in FIG. 4I). The PWM signal P12 that has been made (the OFF period is shortened) is output from time t3.
Thereby, in switching power supply 1, it can control that voltage rise Vup occurs when switch element Q5 (or Q6) turns on after the end of a soft start period.

(Control flow in the switching power supply 1)
FIG. 5 is a diagram illustrating a control flow in the switching power supply device 1 described above. Hereinafter, the flow of control in the switching power supply device 1 will be described with reference to FIG.
First, the reference setting value Vc for setting the target value Vref of the output voltage Vout is input to the soft start processing unit 11 (step ST1). Subsequently, an output start timing signal St (activation signal) for activating the switching power supply device 1 is input (step ST2).

  When the output start timing signal St (start signal) is input, the soft start processing unit 11 starts the soft start process (step ST3). In this soft start process, the target value Vref is gradually increased from the initial value Vo to the predetermined reference set value Vc in a predetermined soft start period LT based on the input reference set value Vc, and used as a signal of the target value Vref. Output to the constant voltage control unit 13. The constant voltage control unit 13 inputs a signal of the target value Vref from the soft start processing unit 11 (step ST4).

  When the constant voltage control unit 13 receives the signal of the target value Vref of the output voltage Vout from the soft start processing unit 11, the constant voltage control unit 13 compares the target value Vref and the voltage feedback signal Vfbk (for example, PI (proportional / integral) calculation). (Step ST5). Then, during the soft start period LT, the constant voltage control unit 13 performs an operation without performing a correction process (a process of subtracting the correction amount ΔUv from the operation amount Uv) after the comparison calculation in step ST5. A signal of the amount Uv is output to the PWM output converter 15.

  The PWM output conversion unit 15 receives the operation amount Uv signal from the constant voltage control unit 13 and the rectification method control signal DSEL for selecting the diode rectification method or the synchronous rectification method from the rectification method control unit 14. . The PWM output conversion unit 15 generates the PWM signal Upwm based on the signal of the operation amount Uv input from the constant voltage control unit 13 and the rectification method control signal DSEL input from the rectification method control unit 14 (step ST6). That is, the PWM output conversion unit 15 selects the diode rectification method during the soft start period LT, and operates the PWM signal Upwm for controlling the on / off timing (duty ratio) of the switch elements Q1 to Q4. Generated according to the amount Uv. The PWM output conversion unit 15 outputs the PWM signal Upwm to the driver circuit 16.

  On the other hand, when the soft start period LT elapses in the soft start processing unit 11 and the soft start processing is completed, the soft start processing unit 11 sends the soft start completion signal Tup to the rectification method control unit 14 and the operation amount replacement unit 17. On the other hand, the data is output (step ST7).

  When the soft start processing unit 11 receives the soft start completion signal Tup, the operation amount replacement unit 17 outputs a correction amount ΔUv corresponding to the forward voltage drop of the body diodes D5 and D6 to the constant voltage control unit 13, The constant voltage control unit 13 requests to subtract the correction amount ΔUv from the previous operation amount Uv (step ST8). The constant voltage control unit 13 subtracts the correction amount ΔUv from the previous operation amount Uv and outputs the corrected operation amount Uv to the PWM output conversion unit 15 (step ST9).

  Further, when the soft start completion signal Tup is input from the soft start processing unit 11, the rectification method control unit 14 outputs a signal DSEL for instructing start of synchronous rectification for on / off control of the switch elements Q5 and Q6 to the PWM output conversion unit. 15 (step ST10).

When the PWM output conversion unit 15 receives the signal DSEL for instructing the start of synchronous rectification from the rectification method control unit 14, the PWM output conversion unit 15 starts synchronous rectification for performing on / off operations of the switch elements Q 5 and Q 6 in the synchronous rectification switch circuit 5. . Then, when starting this synchronous rectification, the PWM output conversion unit 15 determines that the duty ratio is the previous PWM based on the corrected operation amount Uv (the operation amount Uv obtained by subtracting the correction amount ΔUv from the previous operation amount Uv). A PWM signal corrected to be shortened by ΔDU1 from the signal is generated and output (step ST11).
Thereby, in the switching power supply device 1 of this embodiment, it is possible to avoid the output voltage Vout from rising (fluctuating) at the timing when the switch element Q5 or Q6 of the synchronous rectifier circuit 4 is turned on after the soft start period has elapsed. .

  In the above-described embodiment, the operation amount replacement unit 17 has been described as performing the timing at which the operation amount Uv is corrected by the correction amount ΔUv immediately before the switch element Q5 or Q6 is turned on. The timing when 17 corrects the operation amount Uv may be the same as the timing when the switch element Q5 or Q6 is turned on, or may be immediately after the timing when the switch element Q5 or Q6 is turned on.

  The embodiment of the present invention has been described above. Here, the correspondence relationship between the present invention and the above-described embodiment will be supplementarily described. That is, the switching power supply device according to the present invention corresponds to the switching power supply device 1, the first switch circuit according to the present invention corresponds to the full bridge circuit 3, and the switch elements in the first switch circuit according to the present invention are: The switch elements Q1 to Q4 correspond, the second switch circuit in the present invention corresponds to the synchronous rectification switch circuit 5 in the synchronous rectification circuit 4, and the switch elements in the second switch circuit in the present invention correspond to the switch elements. Q5 and Q6 correspond to each other, and the body diodes D5 and D6 correspond to the diodes connected in parallel to the switch elements of the second switch circuit in the present invention.

  The control circuit in the present invention corresponds to the control circuit 10, the soft start processing unit in the present invention corresponds to the soft start processing unit 11, and the voltage detection circuit in the present invention corresponds to the voltage detection circuit 12. The output control unit in the present invention corresponds to the constant voltage control unit 13, and the rectification method control unit in the present invention is a rectification method control unit 14 and a PWM output conversion unit whose operation is controlled by the rectification method control unit 14. 15 corresponds to the following (represented by the rectification method control unit 14). In addition, the operation amount replacement unit in the present invention includes a function in the constant voltage control unit 13 that corrects the operation amount Uv by the operation amount replacement unit 17 and the correction amount ΔUv output from the operation amount replacement unit 17. (Hereinafter, represented by the operation amount replacement unit 17).

  (1) In the above-described embodiment, the switching power supply device 1 includes a first switch circuit (full bridge circuit 3) that switches the current flowing to the primary coil side of the transformer Ts and supplies power to the primary coil side; A second switch circuit (synchronous rectification switch circuit 5) for synchronously rectifying the AC voltage output from the secondary coil side of the transformer Ts, and a diode connected in parallel to the switch elements Q5 and Q6 in the second switch circuit (A body diode D5 and D6) and a control circuit 10 for controlling the on / off operation of the switch elements Q1 to Q6 in the first and second switch circuits. At the time of start-up, a target output (target value Vr) serving as a reference for the output of the switching power supply device 1 with a predetermined soft start period. f) is gradually increased from the initial output to the predetermined output, and the target output (target value Vref) is compared with the output (output voltage Vout) of the switching power supply 1 to determine the switching power supply 1 That generates an operation amount Uv for controlling the on / off operation of the switch elements Q1 to Q6 in the first and second switch circuits so that the output of the first and second switch circuits becomes a value corresponding to the target output (target value Vref). During the soft start period with the control unit (constant voltage control unit 13), the switch elements Q1 and Q4 in the first switch circuit are turned on while the switch elements Q5 and Q6 in the second switch circuit are turned off. / Off operation is controlled based on the manipulated variable Uv, and AC voltage generated on the secondary coil side of the transformer Ts is applied to the diodes (body diodes D5 and D6). After the soft start period elapses, the on / off operation of the switch elements Q5 and Q6 in the second switch circuit is started, and the switch elements in the first and second switch circuits are started. A rectification method control unit 14 that performs on / off operation of Q1 to Q6 based on the operation amount Uv to perform synchronous rectification, and when the soft start period has elapsed and the rectification method control unit 14 starts synchronous rectification, And an operation amount replacement unit 17 that replaces the operation amount Uv with an operation amount obtained by temporarily adding a predetermined correction amount to the operation amount.

In the switching power supply 1 having such a configuration, the AC voltage induced on the secondary coil side of the transformer Ts is rectified by diode rectification using diodes (body diodes D5 and D6) during the soft start period. After the start period has elapsed, synchronous rectification is performed using the switch elements Q5 and Q6 in the second switch circuit (synchronous rectification switch circuit 5). Then, when the soft start period has elapsed and the rectification method is switched from diode rectification to synchronous rectification, the on / off timing (duty ratio) of the switch elements Q1 to Q6 in the first and second switch circuits is controlled. The operation amount Uv to be replaced is replaced with an operation amount obtained by temporarily adding a predetermined correction amount ΔUv to the operation amount Uv immediately before starting the synchronous rectification.
Thereby, in the switching power supply device 1, when the synchronous rectification is started after the soft start period has elapsed, it is possible to avoid the voltage increase (fluctuation) Vup from occurring in the output voltage Vout.

(2) In the above-described embodiment, the control circuit 10 includes the voltage detection circuit 12 that detects the output voltage Vout of the switching power supply device 1, and the soft start processing unit 11 has a predetermined soft start period at startup. The target value Vref serving as a reference for the output voltage Vout of the switching power supply 1 is gradually increased from the initial value to a predetermined value (reference set value Vc), and the output control unit (constant voltage control unit 13) The value Vref and the detected value (voltage feedback signal Vfbk) of the output voltage Vout of the switching power supply device 1 are compared, and the first and second values are set so that the output voltage Vout of the switching power supply device 1 becomes a value corresponding to the target value Vref. The operation amount Uv for controlling the on / off operation of the switch elements Q1 to Q6 in the switch circuit is generated.
Thereby, in the switching power supply device 1 of the present invention, the output voltage Vout can be gradually increased during the soft start period, and output when the synchronous rectification starts after the soft start period elapses. It is possible to avoid a voltage increase (fluctuation) Vup from occurring in the voltage Vout.

(3) In the above embodiment, the predetermined correction amount ΔUv is determined by the value of the voltage drop in the diode (body diodes D5 and D6) and the switch element Q5 in the second switch circuit (synchronous rectification switch circuit 5). It is set according to the voltage drop value due to the on-resistance of Q6.
As described above, in the switching power supply device 1, when the soft start period elapses and synchronous rectification is started, a predetermined correction corresponding to the voltage drop of the diodes (body diodes D5 and D6) with respect to the immediately preceding operation amount Uv. Since the amount ΔUv can be added, an increase in the output voltage Vout that occurs when the diodes (body diodes D5 and D6) are short-circuited by the switching elements Q5 and Q6 can be suppressed.

Although the embodiments of the present invention have been described above, in the switching power supply device 1 described above, each processing unit in the control circuit 10 may be realized by dedicated hardware, and each processing unit The program for realizing the above function may be recorded on a computer-readable recording medium, and the program recorded on the recording medium may be read into a computer system and executed to realize the function. .
That is, a computer system such as a microcontroller or a microcomputer having a CPU, a ROM, a RAM, and the like is mounted in the control circuit 10 described above, and the CPU reads and executes a software program so that a part of the control circuit 10 is obtained. Alternatively, all processing functions may be realized.

  In addition, the switching power supply device of the present invention is not limited to the above-described illustrated examples, and it is needless to say that various changes can be made without departing from the gist of the present invention.

  For example, in the above-described embodiment, the example in which the switching circuit on the primary coil side is configured by an LLC resonance type switching circuit has been described. However, in the switching power supply device of the present invention, the circuit configuration on the primary coil side is described. The configuration is not specified, and the rectifier circuit on the secondary coil side only needs to be configured by a synchronous rectification circuit. For example, the primary side circuit may be of a non-resonant type PWM control system, or may be a switching power supply device configured by a phase shift type switch circuit.

  Further, in the above-described embodiment, the example in which the target value Vref is gradually increased by the soft start processing unit 11 and the output voltage Vout is raised when the switching power supply device is started up is described. The present invention can also be suitably applied to the case where the current limit mode in which the output current is gradually increased until the soft start period elapses.

  DESCRIPTION OF SYMBOLS 1 ... Switching power supply device, 2 ... Power supply circuit part, 3 ... Full bridge circuit (1st switch circuit), 4 ... Synchronous rectification circuit, 5 ... Synchronous rectification switch circuit (2nd switch circuit), 10 ... Control circuit, DESCRIPTION OF SYMBOLS 11 ... Soft start process part, 12 ... Voltage detection circuit, 13 ... Constant voltage control part, 14 ... Rectification system control part, 15 ... PWM output conversion part, 16 ... Driver circuit, 17 ... Manipulation amount replacement part, Q1-Q6 ... Switch elements, D1 to D6 ... Body diodes, C11, C12, C21 ... Capacitors, L11, L21, L22 ... Choke coils

Claims (5)

A first switch circuit that switches current flowing to the primary coil side of the transformer and supplies power to the primary coil side; and a second switch circuit that synchronously rectifies the AC voltage output from the secondary coil side of the transformer; A switching power supply comprising: a diode connected in parallel to a switch element in the second switch circuit; and a control circuit for controlling on / off operation of the switch element in the first and second switch circuits. There,
The control circuit includes:
A soft start processing unit that gradually increases a target output serving as a reference of the output of the switching power supply device from an initial output to a predetermined output with a predetermined soft start period at the time of startup;
The target output and the output of the switching power supply device are compared, and the switch elements in the first and second switch circuits are turned on / off so that the output of the switching power supply device has a value corresponding to the target output. An output control unit that generates an operation amount for controlling the operation;
During the soft start period, with the switch element in the second switch circuit turned off, the on / off operation of the switch element in the first switch circuit is controlled based on the operation amount, and Perform diode rectification to rectify the AC voltage generated on the secondary coil side of the transformer with the diode,
After the soft start period elapses, the switch element in the second switch circuit is turned on / off, and the switch element in the first and second switch circuits is turned on / off. A rectification method control unit that performs synchronous rectification by controlling based on a quantity;
When the soft start period elapses and the rectification method control unit starts the synchronous rectification, the operation amount is replaced with an operation amount obtained by adding a predetermined correction amount to the operation amount temporarily. And
A switching power supply device comprising: The control circuit includes:
It has a voltage detection circuit that detects the output voltage of the switching power supply,
The soft start processing unit
At the time of start-up, with a predetermined soft start period, the target value serving as a reference for the output voltage of the switching power supply device is gradually increased from an initial value to a predetermined value,
The output control unit
The switch in the first and second switch circuits compares the target value with the detected value of the output voltage of the switching power supply device so that the output voltage of the switching power supply device becomes a value corresponding to the target value. The switching power supply according to claim 1, wherein an operation amount for controlling the on / off operation of the element is generated. The predetermined correction amount is set according to a value of a voltage drop in the diode and a value of a voltage drop due to an on-resistance of a switch element in the second switch circuit. The switching power supply device according to claim 2. On the primary coil side of the transformer,
A full bridge circuit as the first switch circuit;
A choke coil and a capacitor connected in series to the primary coil of the transformer;
With
The full bridge circuit is
Controlled to supply power to the primary coil by an LLC resonant circuit via a choke coil and a capacitor connected in series to the primary coil,
On the secondary coil side of the transformer,
Two switch elements in the second switch circuit connected between each terminal of the secondary coil and a reference potential terminal of the DC output voltage;
Two choke coils each having one end connected to each terminal of the secondary coil and the other end connected in common;
A smoothing capacitor having one end connected to the output terminal on the positive voltage side of the DC output voltage and the other end of the two choke coils, and the other end connected to the reference potential terminal;
With
Two switch elements in the second switch circuit are:
The AC voltage generated on the secondary coil side is subjected to full-wave rectification, and controlled to perform current doubler type synchronous rectification in which the full-wave rectified voltage is smoothed by the choke coil and the smoothing capacitor. The switching power supply device according to claim 2 or claim 3, wherein A first switch circuit that switches current flowing to the primary coil side of the transformer and supplies power to the primary coil side; and a second switch circuit that synchronously rectifies the AC voltage output from the secondary coil side of the transformer; A switching power supply comprising: a diode connected in parallel to a switch element in the second switch circuit; and a control circuit for controlling on / off operation of the switch element in the first and second switch circuits. A control method,
By the control circuit,
A soft start processing procedure for gradually increasing a target output serving as a reference of the output of the switching power supply device from an initial output to a predetermined output with a predetermined soft start period when starting;
The target output and the output of the switching power supply device are compared, and the switch elements in the first and second switch circuits are turned on / off so that the output of the switching power supply device has a value corresponding to the target output. An output control procedure for generating an operation amount for controlling the operation;
During the soft start period, with the switch element in the second switch circuit turned off, the on / off operation of the switch element in the first switch circuit is controlled based on the operation amount, and Perform diode rectification to rectify the AC voltage generated on the secondary coil side of the transformer with the diode,
After the soft start period elapses, the switch element in the second switch circuit is turned on / off, and the switch element in the first and second switch circuits is turned on / off. A rectification method control procedure for performing synchronous rectification by controlling based on a quantity;
When the soft start period has elapsed and the synchronous rectification is started by the rectification method control procedure, the operation amount is replaced with an operation amount obtained by temporarily adding a predetermined correction amount to the operation amount. Procedure and
A control method for a switching power supply device, wherein:

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