Flyback converter and turn-on time control circuit thereof

Abstract

According to the turn-on time control circuit of the flyback converter and the flyback converter, after an auxiliary switch tube is turned on, the turn-off of the auxiliary switch tube is controlled through the voltage of an auxiliary capacitor and the volt-second integral value of the turn-on time of the auxiliary switch tube; the voltage-second integral value of the conduction time of the auxiliary capacitor and the auxiliary switch tube is controlled to be a preset value, so that the voltage at the drain end and the source end of the main switch tube is reduced to zero before the main switch tube is conducted, and the zero-voltage conduction of the main switch tube is accurately realized. According to the turn-on time control circuit of the flyback converter, the most appropriate turn-on time of the auxiliary switch tube under different output voltages is decoupled in a volt-second integration mode, zero-voltage turn-on of the main switch tube is accurately controlled, and the efficiency of a system is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of flyback converter control, and more specifically, to a flyback converter conduction time control circuit and a flyback converter. Background Art

[0002] If the primary main switch of the flyback converter is not turned on when the drain-source voltage is close to zero, it is called hard turn-on, and the turn-on loss is large. As the adapter has higher and higher requirements for small size and high power density, the switching frequency needs to be increased, but the turn-on loss will be further increased, so the traditional hard-switched flyback converter is limited in this application.

[0003] In view of the above problems, in order to achieve zero voltage (ZVS) switching, such as Figure 1 The auxiliary switch tube Sa in the flyback converter is connected to the zero-crossing winding N ZVS and capacitor C1 in series to form a loop, the zero-crossing winding N ZVS The auxiliary switch is coupled in phase with the secondary winding of the transformer of the flyback converter, and a certain negative excitation current is generated by turning on the auxiliary switch tube, and then the auxiliary switch tube is turned off. The negative excitation current discharges the voltage on the parasitic capacitor Coss of the main switch tube to zero, and then the main switch tube is turned on to achieve zero voltage turn-on of the main switch tube. However, in this scheme, the auxiliary switch tube is often set with a fixed turn-on time, and the fixed turn-on time may not be able to completely discharge the voltage on the parasitic capacitor Coss to zero, resulting in the main switch tube actually being turned on above zero voltage.

[0004] Therefore, it is necessary to provide an improved technical solution to overcome the above technical problems existing in the prior art. Summary of the invention

[0005] In view of this, an object of the present invention is to provide a flyback converter on-time control circuit and a flyback converter, so as to solve the technical problem in the prior art that the flyback converter cannot accurately achieve zero voltage conduction.

[0006] Provided is a conduction time control circuit for a flyback converter, the flyback converter comprising a main switch tube, a transformer and an auxiliary switch tube, the auxiliary switch tube being connected in series with a zero-crossing winding and an auxiliary capacitor to form a loop, the zero-crossing winding being coupled to the transformer winding of the flyback converter, the conduction time control circuit being used to control the conduction time of the auxiliary switch tube, comprising an on-signal circuit receiving a compensation signal and an inductor current sampling signal to generate an on-signal for controlling the auxiliary switch tube to be turned on, and an off-signal circuit for performing a volt-second integration on a voltage signal of the auxiliary capacitor after the auxiliary switch tube is turned on, and generating an off-signal for controlling the auxiliary switch tube to be turned off when the volt-second integration value reaches a preset value, wherein the compensation signal is obtained according to an output feedback signal of the flyback converter.

[0007] Preferably, the preset value is determined according to the energy consumed by the flyback converter to achieve zero voltage conduction.

[0008] Preferably, the preset value is adjustable according to an input voltage of the flyback converter.

[0009] Preferably, the turn-on signal circuit includes a frequency detection circuit and a current zero point detection circuit, the frequency detection circuit detects the operating frequency of the main switch tube, and generates a valid first detection signal when the operating frequency reaches a maximum frequency, wherein the maximum frequency is obtained according to the compensation signal, and the current zero point detection circuit detects whether the inductor current passes through zero, and generates a valid second detection signal when the inductor current passes through zero, and when both the first detection signal and the second detection signal are valid, the turn-on signal is generated to control the auxiliary switch tube to turn on.

[0010] Preferably, the shutdown signal circuit includes an integration circuit, a preset value generation circuit and a comparison circuit.

[0011] After the auxiliary switch tube is turned on, the integration circuit obtains an integration signal by charging the first capacitor with the voltage signal of the auxiliary capacitor. The preset value generating circuit obtains the preset value by generating a voltage drop on the first resistor through the first voltage source. The comparison circuit compares the integration signal with the preset value. When the integration signal reaches the preset value, the shutdown signal controls the auxiliary switch tube to shut down.

[0012] Preferably, the resistance value of the first resistor is determined according to the energy consumed by the flyback converter to achieve zero voltage conduction.

[0013] Preferably, the shutdown signal circuit also includes a volt-second reference conversion module, which is connected between the first resistor and the comparison circuit. The volt-second reference conversion module receives the input voltage of the flyback converter, adjusts the preset value according to the value of the input voltage, and outputs the adjusted preset value to the comparison circuit.

[0014] Preferably, the on-time control circuit controls the on-time of the auxiliary switch tube to satisfy:

[0015] L P *I neg =K*V ZVS *t on

[0016] Among them, Lp is the primary excitation inductance of the transformer, Ineg is the negative current of the excitation, VZVS is the voltage of the auxiliary capacitor, and K is the turns ratio of the primary / secondary winding and the zero-crossing winding of the transformer.

[0017] In a second aspect, a flyback converter is provided, comprising a main switch tube, an auxiliary switch tube and a control unit, and also comprising the above-mentioned on-time control circuit, wherein the control unit comprises the on-time control circuit, and the control unit controls the switching action of the main switch tube and the auxiliary switch tube.

[0018] Preferably, the control unit is integrated into a chip, which includes a resistance pin, a sampling pin and a compensation pin. The resistance pin is connected to an external resistor, and the preset value is obtained according to the resistance value of the external resistor. The sampling pin detects the voltage across the sampling winding to obtain a current sampling signal. The compensation pin receives a feedback signal transmitted by the secondary side to obtain the compensation signal.

[0019] The on-time control circuit of the flyback converter of the present invention controls the turn-off of the auxiliary switch tube by the volt-second integral value of the on-time of the auxiliary capacitor and the auxiliary switch tube after the auxiliary switch tube is turned on, so as to control the volt-second integral value of the on-time of the auxiliary capacitor and the auxiliary switch tube to a preset value, so that before the main switch tube is turned on, the voltage across its drain and source drops to zero, and the zero-voltage turn-on of the main switch tube is accurately achieved. The on-time control circuit of the flyback converter of the present application can decouple the most suitable on-time of the auxiliary switch tube under a predetermined output voltage through the volt-second integral method, accurately control the zero-voltage turn-on of the main switch tube, and can flexibly set the auxiliary switch turn-on pulse width in combination with the compensation of the input voltage, so as to maximize the zero-voltage turn-on under the full input voltage range, thereby improving the efficiency of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 A circuit block diagram of a flyback converter in the prior art;

[0021] Figure 2 A circuit diagram of an implementation of a conduction time control circuit of a flyback converter according to the present invention;

[0022] Figure 3 According to the present invention Figure 2 A circuit diagram for realizing an opening signal circuit in;

[0023] Figure 4 According to the present invention Figure 2 A first implementation circuit diagram of the shutdown signal circuit;

[0024] Figure 5 According to the present invention Figure 2 The second implementation circuit diagram of the shutdown signal circuit. DETAILED DESCRIPTION

[0025] The preferred embodiments of the present invention are described in detail below in conjunction with the accompanying drawings, but the present invention is not limited to these embodiments. The present invention covers any substitution, modification, equivalent method and scheme made within the spirit and scope of the present invention.

[0026] In order to make the public have a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the present invention, but those skilled in the art can fully understand the present invention without these detailed descriptions.

[0027] The present invention is described in more detail in the following paragraphs by way of example with reference to the accompanying drawings. It should be noted that the accompanying drawings are all simplified and not in exact proportion, and are only used for the purpose of conveniently and clearly assisting in explaining the embodiments of the present invention.

[0028] Figure 2 A circuit diagram of an implementation of a conduction time control circuit of a flyback converter according to the present invention; Figure 3 According to the present invention Figure 2 A circuit diagram for realizing an opening signal circuit in; Figure 4 According to the present invention Figure 2 The first implementation circuit diagram of the shutdown signal circuit. Figure 2 The topology of the flyback converter in Figure 1 Similar to the flyback converter, the flyback converter includes a main switch tube Qm, a transformer and an auxiliary switch tube Sa, wherein the auxiliary switch tube is connected to the zero-crossing winding N ZVS , the auxiliary capacitor C1 is connected in series to form a loop, the zero-crossing winding is coupled with the transformer winding of the flyback converter, and the transformer includes a primary winding Np and a secondary winding Ns. The on-time control circuit of the present application is mainly used to control the on-time of the auxiliary switch tube. Specifically as follows:

[0029] refer to Figure 2 and Figure 3 , Figure 4 The on-time control circuit of the flyback converter of the present application includes an on-signal circuit and an off-signal circuit. The on-signal circuit receives a compensation signal Vcomp and an inductor current sampling signal Vcs to generate an on-signal V for controlling the on-state of the auxiliary switch. 开 Specifically, refer to Figure 3 , the turn-on signal circuit includes a frequency detection circuit and a current zero point detection circuit. The frequency detection circuit detects the operating frequency of the main switch tube, which is also the operating frequency of the system. When the operating frequency reaches the maximum frequency, an effective first detection signal is generated, wherein the maximum frequency is obtained according to the compensation signal Vcomp. Generally, the control circuit can pre-set the curve of the maximum frequency according to the value of Vcomp, which can be set to a linear relationship or other curve relationship according to the switch working mode. The current zero point detection circuit detects whether the excitation current passes through zero. When the excitation current passes through zero, an effective second detection signal is generated. When the first detection signal and the second detection signal are both valid, the turn-on signal is generated to control the auxiliary switch tube to turn on. Here, the detection of the excitation current can be obtained through the voltage of the pin Vs, and the time point of reaching zero is identified by the voltage curve of Vs, thereby indirectly obtaining the zero-crossing moment of the excitation current. For example, the first detection signal and the second detection signal can be obtained by performing an AND logic operation through a logic circuit to obtain the turn-on signal V 开 Here, it is supplemented that the compensation signal is obtained according to the output feedback signal of the flyback converter. For example, the flyback converter includes an optocoupler, and the optocoupler includes a light-emitting diode and a photoresistor. The high-voltage side of the secondary winding of the transformer is connected to the positive electrode of the light-emitting diode via a resistor. The output voltage feedback signal is connected to the negative electrode of the light-emitting diode after the signal of the voltage-dividing resistor is compared with the reference voltage, so as to send a control signal to the light-emitting diode to adjust the size of the optocoupler current of the light-emitting diode, thereby adjusting the conduction state of the photoresistor. One end of the photoresistor is connected to the reference ground, and the other end is connected to the compensation (V COMP ) pin.

[0030] Through the above-mentioned auxiliary switch on-off control, through the frequency limit and the current zero-crossing logic operation, when the frequency reaches the maximum value earlier than the current zero-crossing time, it indicates that the system works in the critical continuous conduction mode, that is, BCM mode, and the auxiliary switch is turned on at the current zero-crossing time; when the frequency reaches the maximum value later than the current zero-crossing time, it indicates that the system works in the discontinuous mode, that is, DCM mode, and the auxiliary switch is turned on at the next main switch drain-source voltage VDS It is turned on at the peak of the wave; through this control method, it can be ensured that the initial state of the excitation inductance is uniformly zero, and the system control is accurate.

[0031] refer to Figure 4 The shutdown signal circuit includes an integration circuit, a preset value generation circuit and a comparison circuit. The integration circuit includes a voltage-controlled current source, which receives a voltage Vs representing an auxiliary capacitor to obtain a current I associated therewith. VT The first switch S1 is connected in parallel with the first capacitor. When the auxiliary switch is turned on, the first switch is turned off. The current I VT Charge the first capacitor to obtain a capacitor voltage C VT , the capacitor voltage C VT As an integral signal; the preset value generating circuit includes a first voltage source and a first resistor, through which the voltage source is applied to the first resistor R T The voltage drop is generated to obtain the preset value V RT The comparison circuit compares the integrated signal C VT and the preset value V RT When the integrated signal reaches the preset value, the comparator outputs a valid high-level shutdown signal, and the shutdown signal controls the auxiliary switch tube to shut down.

[0032] It should be supplemented that the on-time control circuit also includes a trigger, and the on-signal and the off-signal are transmitted to the set end and the reset end of the trigger respectively, and the trigger outputs a switch signal GAC to control the switch state of the auxiliary switch tube.

[0033] According to the above solution, the on-time control circuit controls the on-time of the auxiliary switch tube to satisfy:

[0034] L P *I neg =K*V ZVS *t on (1)

[0035] Among them, Lp is the primary excitation inductance of the transformer, Ineg is the negative current of the excitation, V ZVS is the voltage of the auxiliary capacitor, K is the turns ratio of the primary / secondary winding and the zero-crossing winding of the transformer, and ton is the conduction time of the auxiliary switch tube.

[0036] According to the energy relationship equation, for the flyback converter,

[0037] 0.5*L P *I neg 2 = 0.5*C eq *V DC 2 (2)

[0038] Among them, Ceq is the junction capacitance of the main switch Qm, V DC is the DC value of the input bus voltage of the flyback converter. According to the above formulas (1) and (2), when V ZVS *t on When V is the preset value Vrt, ZVS *t on When the voltage of the junction capacitance of the main switch tube is constant, the energy of the junction capacitance voltage of the main switch tube is just consumed by the negative current generated by the excitation inductance during the conduction period of the auxiliary switch tube. Therefore, when the main switch tube is turned on in the next cycle, the main switch tube is turned on at zero voltage.

[0039] Preferably, the preset value is set according to the energy consumed by the flyback converter to achieve zero voltage conduction. Here, it can be seen from the above formula that for the main switch tube of the flyback converter, if zero voltage conduction is required, the energy consumed is the energy on the right side of formula (2), which can be reversed to V in formula (1). ZVS *t on If the most appropriate preset value is set, the accurate voltage zero crossing point can be obtained to ensure that the main switch tube is turned on at zero voltage. Figure 4 It can be seen that the size of the preset value is determined by the resistance value of the first resistor. Therefore, the resistance value of the first resistor is determined according to the energy consumed by the main switch tube of the flyback converter to achieve zero voltage conduction. Once determined, the preset value is determined, so that the required ton size can be obtained.

[0040] refer to Figure 5 , according to the present invention Figure 2 The second implementation circuit diagram of the shutdown signal circuit in the embodiment of the present invention is that the preset value is adjustable according to the input voltage of the flyback converter. Figure 4 A further improvement of the embodiment, in the previous embodiment, the preset value is designed according to the maximum input voltage to ensure that it can be applied to all occasions, but this will waste energy. Therefore, in this embodiment, the shutdown signal circuit also includes a volt-second reference conversion module, and the volt-second reference conversion module is connected to the first resistor R T The volt-second reference conversion module receives the input voltage of the flyback converter, where the input voltage is represented by a voltage signal Vs. For example, when the main switch tube Qm is turned on, the pin of the first winding performs current sampling to obtain a signal Vs representing the input voltage, and adjusts the preset value V according to the value of the input voltage. RT, output the adjusted preset value such as Vref to the comparison circuit. In this way, the problem that the volt-second integration threshold is not applicable under different input voltages can be compensated, so that the size of the preset value can be adjusted in time under different input voltages, so that the zero voltage turn-on of the main switch tube can be achieved corresponding to different input voltages.

[0041] The on-time control circuit of the flyback converter of the present application can decouple the most suitable on-time of the auxiliary switch tube under the predetermined output voltage through the volt-second integration method, accurately control the zero-voltage turn-on of the main switch tube, and can flexibly set the auxiliary switch turn-on pulse width in combination with the input voltage compensation, which can maximize the zero-voltage turn-on under the full input voltage range and improve the efficiency of the system. For different predetermined output voltages, the on-time of the auxiliary switch tube can be adjusted by adjusting the resistance value to accurately control the zero-voltage turn-on of the main switch tube for different predetermined output voltages.

[0042] Finally, the present application provides a flyback converter, including a main switch tube, an auxiliary switch tube and a control unit, and also includes the above-mentioned on-time control circuit, the control unit includes the on-time control circuit, and the control unit controls the switching action of the main switch tube and the auxiliary switch tube.

[0043] The control unit is integrated in a chip, which includes a resistance pin, a sampling pin and a compensation pin, wherein the resistance pin is connected to an external resistor, and the preset value is obtained according to the resistance value of the external resistor, the sampling pin detects the voltage across the sampling winding to obtain a sampling signal of the inductor current, and the compensation pin receives a feedback signal transmitted by the secondary side to obtain the compensation signal.

[0044] The flyback converter of the present application can accurately achieve zero-voltage conduction of the main switch tube under any input voltage and output voltage, thereby improving the overall efficiency of the system.

[0045] It should be noted that the specific implementation and corresponding illustrations given are merely a way of describing the implementation method of the present invention, and do not limit the specific structure of the implementation scheme of the present invention. Various changes or modifications can be made to these implementation schemes without departing from the principle and essence of the present invention, but these changes and modifications fall within the scope of protection of the present invention.

[0046] Although the embodiments are described and illustrated separately above, some common technologies are involved. It is the opinion of ordinary technicians in this field that the embodiments can be replaced and integrated. If the content is not clearly recorded in one of the embodiments, reference can be made to another recorded embodiment.

[0047] The above-described implementation methods do not constitute a limitation on the protection scope of the technical solution. Any modification, equivalent replacement and improvement made within the spirit and principle of the above-described implementation methods shall be included in the protection scope of the technical solution.

Claims

1. A flyback converter on-time control circuit, the flyback converter comprising a main switch tube, a transformer and an auxiliary switch tube, the auxiliary switch tube is connected in series with a zero-crossing winding and an auxiliary capacitor to form a loop, the zero-crossing winding is coupled with the transformer winding of the flyback converter, characterized in that: The conduction time control circuit is used to control the conduction time of the auxiliary switch tube, including: An on-signal circuit receives the compensation signal and the current sampling signal to generate an on-signal for controlling the auxiliary switch tube to be turned on. The shutdown signal circuit performs a volt-second integration on the voltage signal of the auxiliary capacitor after the auxiliary switch tube is turned on, and generates a shutdown signal for controlling the auxiliary switch tube to be turned off when the volt-second integration value reaches a preset value. Wherein, the compensation signal is obtained according to the output feedback signal of the flyback converter.

2. The on-time control circuit of the flyback converter according to claim 1, characterized in that: The preset value is set according to the energy consumed by the flyback converter to achieve zero voltage conduction.

3. The on-time control circuit of the flyback converter according to claim 1, characterized in that: The preset value is adjustable according to the input voltage of the flyback converter.

4. The on-time control circuit of the flyback converter according to claim 1, characterized in that: The opening signal circuit includes a frequency detection circuit and a current zero point detection circuit. The frequency detection circuit detects the operating frequency of the main switch tube, and generates a valid first detection signal when the operating frequency reaches a maximum frequency, wherein the maximum frequency is obtained according to the compensation signal. The current zero point detection circuit detects whether the excitation current passes through zero, and generates an effective second detection signal when the current passes through zero. When both the first detection signal and the second detection signal are valid, the turn-on signal is generated to control the auxiliary switch tube to turn on.

5. The on-time control circuit of the flyback converter according to claim 1, characterized in that: The shutdown signal circuit includes an integration circuit, a preset value generation circuit and a comparison circuit. After the auxiliary switch tube is turned on, the integration circuit charges the first capacitor with the voltage signal of the auxiliary capacitor to obtain an integration signal. The preset value generating circuit generates a voltage drop on the first resistor through a voltage source to obtain the preset value. The comparison circuit compares the integrated signal with the preset value, and when the integrated signal reaches the preset value, the shutdown signal controls the auxiliary switch tube to be turned off.

6. The on-time control circuit of the flyback converter according to claim 5, characterized in that: The resistance value of the first resistor is set according to the energy consumed by the flyback converter to achieve zero voltage conduction.

7. The on-time control circuit of the flyback converter according to claim 5, characterized in that: The shutdown signal circuit also includes a volt-second reference conversion module, The volt-second reference conversion module is connected between the first resistor and the comparison circuit, receives the input voltage of the flyback converter, adjusts the preset value according to the value of the input voltage, and outputs the adjusted preset value to the comparison circuit.

8. The on-time control circuit of the flyback converter according to claim 1, characterized in that: The conduction time control circuit controls the conduction time of the auxiliary switch tube to meet the following requirements: L P *I neg =K*V ZVS *t on Among them, Lp is the primary excitation inductance of the transformer, Ineg is the negative current of the excitation, VZVS is the voltage of the auxiliary capacitor, and K is the turns ratio of the primary / secondary winding and the zero-crossing winding of the transformer.

9. A flyback converter, comprising a main switch tube, an auxiliary switch tube and a control unit, characterized in that: It also includes the on-time control circuit according to any one of claims 1 to 8, The control unit includes the conduction time control circuit, and the control unit controls the switching actions of the main switch tube and the auxiliary switch tube.

10. The flyback converter according to claim 9, characterized in that: The control unit is integrated into a chip, which includes a resistance pin, a sampling pin and a compensation pin. The resistor pin is connected to an external resistor, and the preset value is obtained according to the resistance value of the external resistor. The sampling pin detects the voltage at both ends of the sampling winding to obtain a current sampling signal. The compensation pin receives a feedback signal transmitted by the secondary side to obtain the compensation signal.

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