Flyback converter control method and flyback converter

Abstract

The present application provides a control method for a flyback converter and a flyback converter, wherein the flyback converter includes a main power switch tube, a transformer, a second auxiliary winding coupled to the transformer, and an auxiliary switch tube and an auxiliary capacitor connected to the second auxiliary winding, and is characterized in that the control method includes: at a first moment before the main power switch tube is turned on, turning on the auxiliary switch tube for a first time length so that the main power switch tube is turned on at the lowest value of the drain-source voltage; at a second moment after the main power switch tube is turned off, turning on the auxiliary switch tube for a second time length to charge the auxiliary capacitor. The control method of the present application reduces the loss of the auxiliary switch tube during the shutdown period of the main power switch tube by allowing the auxiliary switch tube to turn on pulses twice before and after the main power switch tube is turned on, thereby improving the efficiency of the system.

Description

Technical Field

[0001] The present application relates to an electronic power technology, and more specifically, to a control method of a flyback converter and a flyback converter. Background Art

[0002] Flyback converters are commonly used for conversion between AC to DC and DC to DC with current isolation between input and one or more outputs. Due to its simple, reliable and efficient characteristics, it is widely used in adapters for mobile phones and notebooks. The flyback converter includes a main power switch tube, which converts energy by turning on and off the main power switch tube. In order to achieve zero voltage switching (Zero Voltage Switching, ZVS) of the main power switch tube, the flyback converter in the prior art also includes an auxiliary switch tube. Before the main power tube is turned on, the auxiliary switch tube will turn on an appropriate small pulse to additionally reversely excite the transformer excitation inductance. After turning off, the main power switch tube can achieve ZVS with the help of this energy. Therefore, this method reduces the switching loss of the main power tube to a certain extent by sacrificing the conduction loss of the auxiliary switch tube. For the auxiliary switch tube, it can be achieved by using a low-cost and low-voltage metal-oxide semiconductor field-effect transistor (MOSFET) through the transformer turns ratio design.

[0003] However, currently the auxiliary switch tube can only be turned on with a small pulse in an open-loop or closed-loop manner before the main power switch tube, thereby realizing the discharge of the auxiliary winding capacitor to the excitation inductor and the reverse charging of the excitation inductor. However, after the main power switch tube is turned off, the auxiliary winding capacitor needs to be replenished with energy. At this time, since the auxiliary switch tube is not turned on, it charges through the body diode and generates additional conduction losses. Especially in high-frequency situations, the average current passing through the body diode here is relatively increased, resulting in further increase in its losses.

[0004] Therefore, in order to further improve the efficiency of the flyback converter, it is necessary to appropriately reduce the conduction loss of the auxiliary switch tube, and now it is necessary to further optimize the control method of the flyback converter. Summary of the invention

[0005] This Summary is provided to introduce some concepts in a simplified form that are further described in the Detailed Description below. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to limit the scope of the claimed subject matter.

[0006] The purpose of the present application is to provide a control method for a flyback converter to meet the application occasions with strict requirements on the conduction loss of the whole system. The above and other purposes are achieved by the features of the independent claims. Further implementation forms are obvious from the dependent claims, the specification and the drawings. According to the first aspect of the present application, a control method for a flyback converter is provided, the flyback converter includes a main power switch tube, a transformer, a second auxiliary winding coupled to the transformer, and an auxiliary switch tube and an auxiliary capacitor connected to the second auxiliary winding, characterized in that the control method includes: at a first moment before the main power switch tube is turned on, turning on the auxiliary switch tube for a first time length so that the main power switch tube is turned on at the lowest value of the drain-source voltage; at a second moment after the main power switch tube is turned off, turning on the auxiliary switch tube for a second time length to charge the auxiliary capacitor.

[0007] Optionally, when the flyback converter is in a critical conduction mode BCM, the first moment is when the inductor current of the flyback converter is detected to pass through zero; when the flyback converter is in a discontinuous conduction mode DCM, the first moment is when the switching frequency of the main power switch tube starts.

[0008] Optionally, the first duration T GAC1 The input voltage and output voltage of the flyback converter meet: T GAC1 =k1×V in -k2×V o +T GAC1_bias , where V in is the input voltage, V o is the output voltage, T GAC1_bias is the first minimum offset time, k1 and k2 are constants, or the first time length T GAC1 It is a fixed time t1.

[0009] Optionally, the second moment is when a fixed time t2 is detected after the main power switch tube is turned off, or the second moment is when it is detected that the output voltage of the first auxiliary winding of the flyback converter is greater than a first threshold.

[0010] Optionally, the second duration is a fixed duration t3.

[0011] Optionally, the second duration T GAC2 The input voltage and output voltage of the flyback converter meet: T GAC2 =k3×V in -k4×V o +T GAC2_bias , where V in is the input voltage, V o is the output voltage, TGAC2_bias is the second minimum bias time, k3 and k4 are constants.

[0012] Optionally, the second duration and the first duration satisfy T GAC2 = k × T GAC1 , where k is a constant.

[0013] Optionally, the second moment is when it is detected that the drain-source voltage of the auxiliary switch tube of the flyback converter is less than a second threshold; the second time length is from the second moment to when it is detected that the drain-source voltage of the auxiliary switch tube is greater than a third threshold.

[0014] According to a second aspect of the present application, a flyback converter is provided, characterized in that it includes: a main power switch tube, used to control the flyback converter to generate an output voltage; an auxiliary switch tube, used to turn on the main power switch tube at the lowest value of the drain-source voltage; and an auxiliary switch tube control circuit, applying the control method described above to control the auxiliary switch tube.

[0015] The present application provides a control method for a flyback converter and a flyback converter, which reduces the loss of the auxiliary switch tube when the main power switch tube is turned off by allowing the auxiliary switch tube in the flyback converter to turn on pulses twice before and after the main power switch tube, thereby improving the efficiency of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, a brief introduction will be given below to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0017] Figure 1 A schematic diagram of the structure of a flyback converter in the prior art is shown;

[0018] Figure 2 A control waveform diagram of a flyback converter in the prior art is shown;

[0019] Figure 3 A control waveform diagram of a flyback converter according to an embodiment of the present application is shown;

[0020] Figure 4 A flow chart of a control method for a flyback converter according to an embodiment of the present application is shown;

[0021] Figure 5 A control schematic diagram of the first opening of the auxiliary switch tube according to an embodiment of the present application is shown;

[0022] Figure 6 A control flow chart of the second conduction of the auxiliary switch tube according to an embodiment of the present application is shown;

[0023] Figure 7 and Figure 8 Shown according to Figure 6 A schematic diagram for controlling the second turn-on time of the auxiliary switch tube;

[0024] Fig. 9 A control schematic diagram of the second opening of the auxiliary switch tube according to another embodiment of the present application is shown.

[0025] In the following, identical reference numerals denote identical or at least functionally identical features. DETAILED DESCRIPTION

[0026] The following will describe the technical solutions in the embodiments of the present application in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, not all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0027] For example, it should be understood that the disclosure in conjunction with the described method is also applicable to the corresponding device or system for performing the method, and vice versa. For example, if a specific method step is described, the corresponding device may include a unit for performing the described method step, even if such a unit is not described or shown in detail in the accompanying drawings. On the other hand, for example, if a specific device is described based on a functional unit, the corresponding method may include a step for performing the described function, even if the step is not explicitly described or illustrated in the accompanying drawings. In addition, it should be understood that the features of the various example aspects described herein may be combined with each other unless otherwise specifically stated.

[0028] It should be understood that the connection / coupling of A and B in the embodiment of the present application means that A and B can be connected in series or in parallel, or A and B are connected through other devices, and the embodiment of the present application is not limited to this.

[0029] The control method and flyback converter provided in the present application improve the efficiency of the system by allowing the auxiliary switch tube to turn on small pulses twice before and after the main power switch tube, thereby reducing the turn-on loss generated when the auxiliary switch tube body diode is turned on before the main power switch tube.

[0030] Figure 1 FIG. 1 shows a schematic diagram of the structure of a flyback converter in the prior art. Figure 2 FIG. 4 shows a schematic diagram of a control waveform of a flyback converter in the prior art. Figure 1As shown, the zero voltage turn-on flyback converter includes a main power switch tube Qp, an auxiliary switch tube Qzvs, a primary winding L1 of a transformer, a secondary winding L2, a first auxiliary winding Naux, a second auxiliary winding Nzvs and an auxiliary capacitor C1, wherein the auxiliary switch tube Qzvs, the second auxiliary winding Nzvs and the auxiliary capacitor C1 are connected in series, the main power switch tube and the auxiliary switch tube are field effect transistors, the first auxiliary winding Naux characterizes the input voltage Vin and the output voltage Vo of the flyback converter according to the turns ratio with the primary winding L1 and the secondary winding L2, the control end of the auxiliary switch tube Qzvs is controlled by the switch control unit 1, and the switch control unit 1 controls the auxiliary switch tube Qzvs to open or close the loop mode before the main power switch tube Qp is turned on to perform a small pulse opening, so that the auxiliary winding capacitor discharges to the excitation inductor, and the excitation inductor is reversely charged, and the opening mode of the auxiliary switch tube Qzvs in the discontinuous conduction mode DCM of the flyback converter is as shown in FIG. Figure 2 As shown in (a), the turn-on mode of the auxiliary switch Qzvs in the critical continuous mode CRM is as follows Figure 2 As shown in (b), this method can reduce the switching loss of the main power switch tube Qp to a certain extent. However, after the main power switch tube Qp is turned off, the auxiliary winding capacitor C1 needs to replenish energy. At this time, since the auxiliary switch tube Qzvs is not turned on, it charges through the body diode and generates additional conduction loss. Especially in high-frequency situations, the average current passing through the body diode is relatively increased, resulting in further increase in loss.

[0031] As an example, it can be understood that by turning on the auxiliary switch Qzvs for a second time with a small pulse after the main power switch Qp is turned off, the loss when the body diode of the auxiliary switch Qzvs is turned on can be reduced, thereby improving the efficiency of the entire system. Figure 3 FIG. 4 shows a schematic diagram of a control waveform of an auxiliary switch tube according to an embodiment of the present application, wherein Figure 3 (a) shows the turn-on mode of the auxiliary switch Qzvs in DCM mode. Figure 3 (b) shows the turn-on mode of the auxiliary switch tube Qzvs in the CRM.

[0032] Figure 4 FIG. 4 shows a flow chart of a control method of a flyback converter according to an embodiment of the present application. Figure 4 As shown, the control method of the flyback converter includes steps S41-S42. The control method of the flyback converter is used to control the auxiliary switch tube, and the auxiliary switch tube is coupled to the second auxiliary winding of the flyback converter. The control method of the flyback converter can be applied to, for example, Figure 1 The flyback converter shown.

[0033] In step S41, at a first moment before the main power switch tube of the flyback converter is turned on, the auxiliary switch tube is turned on for a first time length so that the main power switch tube is turned on at the lowest value of the drain-source voltage;

[0034] In step S42, at a second moment after the main power switch tube is turned on, the auxiliary switch tube is turned on for a second time period to charge the auxiliary capacitor.

[0035] As an example, in step S41, the first time and first duration of the auxiliary switch tube being turned on before the main power switch tube of the flyback converter is turned on are as follows: Figure 5 As shown, Figure 5 FIG. 1 shows a control schematic diagram of the first opening of the auxiliary switch tube according to an embodiment of the present application, wherein when the circuit is in the critical conduction mode, when it is detected that the inductor current is zero, a signal is sent to enable the auxiliary switch tube to be opened for the first time length T GAC1 When the circuit is in intermittent conduction mode, the frequency of the auxiliary switch is consistent with the frequency of the circuit itself. The first duration T GAC1 The first time duration T is related to the setting of the input voltage Vin and the output voltage Vo. GAC1 The input voltage and output voltage of the flyback converter meet: T GAC1 =k1×V in -k2×V o +T GAC1_bias , where V in is the input voltage, V o is the output voltage, T GAC1_bias is the first minimum bias time, k1 and k2 are constants, and the auxiliary switch tube is turned on for the first time T GAC1 Then turn off.

[0036] It is understandable that the above only describes an embodiment of the first turn-on time of the auxiliary switch tube, but the embodiment of the present application is not limited thereto, and there may be other expansions and deformations, for example, the first time length T GAC1 It can be set to a fixed time length t1.

[0037] As an example, in step S42, the auxiliary switch is turned on for the second time after the main power switch of the flyback converter is turned on. The second turn-on logic of the auxiliary switch is as follows: Figure 6-Figure 8 As shown, Figure 6A control flow chart of the second turn-on of the auxiliary switch tube according to an embodiment of the present application is shown, wherein when the auxiliary switch tube Qzvs starts to be turned on for the second time, the circuit can detect that the main power switch tube has been turned off for Td time, and send a signal to enable the auxiliary switch tube to perform the second turn-on action. Alternatively, when the circuit detects that the voltage Vs output by the first auxiliary winding exceeds zero, a signal is sent to enable the auxiliary switch tube to perform the second turn-on action. It can be understood that the output voltage Vs of the first auxiliary winding crossing zero is a theoretical value. In actual applications, the turn-on condition is triggered when the output voltage of the first auxiliary winding is detected to be greater than the first threshold. As an embodiment, the first threshold can be 0-200mV; alternatively, when the circuit detects that the drain-source voltage Vzvs of the auxiliary switch tube Qzvs is less than the second threshold, a signal is sent to enable the auxiliary switch tube to perform the second turn-on action. After the auxiliary switch tube Qzvs is turned on, the timing T GAC2 After that, a signal is sent to enable the auxiliary switch tube to perform a second shutdown action, or when the circuit detects that the drain-source voltage Vzvs of the auxiliary switch tube Qzvs is greater than the third threshold, a signal is output to enable the auxiliary switch tube to perform a second shutdown action.

[0038] It is understandable that the above describes the judgment method for sending the second turn-on signal and the second turn-off signal, and the condition detection for controlling the turn-on and turn-off moments of the auxiliary switch tube can be arbitrarily selected and combined without limiting a certain matching relationship.

[0039] Figure 7 and Figure 8 Shown according to Figure 6 The control diagram of the second turn-on time of the auxiliary switch tube is as follows: Figure 7 As shown, the auxiliary switch is turned on for the second time T GAC2 After shutting down, the second time length T GAC2 The input voltage and output voltage of the flyback converter meet the following conditions: T GAC2 =k3×V in -k4×V o +T GAC2_bias , where V in is the input voltage, V o is the output voltage, T GAC2_bias is the second minimum bias time, k3 and k4 are constants. Or, as another embodiment, Figure 8 As shown, the second duration can be directly proportional to the first duration. GAC2 With the first duration T GAC1 Meet T GAC2 = k × T GAC1 , where k is a constant.

[0040] It is understandable that the above only describes an embodiment of the second turn-on time of the auxiliary switch tube, but the embodiment of the present application is not limited thereto, and there may be other expansions and deformations, for example, the second time length T GAC2 It can be set to a fixed time length t2.

[0041] As an example, Fig. 9 The structure diagram of the control circuit for the second opening of the auxiliary switch tube according to an embodiment of the present application is shown. This method of setting the pulse width requires detecting the drain-source voltage Vzvs of the auxiliary switch tube Qzvs. The second moment of the second opening can be when it is detected that the drain-source voltage Vzvs of the auxiliary switch tube Qzvs is less than the second threshold, a signal is sent to enable the auxiliary switch tube Qzvs to be turned on. Since the conduction voltage drop of the body diode of the auxiliary switch tube Qzvs is usually -700mV, as an embodiment, the second threshold can be -200mV-0; when it is detected that the drain-source voltage Vzvs of the auxiliary switch tube Qzvs passes through zero, a signal is sent to enable the auxiliary switch tube Qzvs to be turned off. It can be understood that the drain-source voltage crossing zero is a theoretical value. In actual applications, the drain-source voltage of the auxiliary switch tube Qzvs is detected to be greater than the third threshold to trigger the shutdown condition. As an embodiment, the third threshold can be 0-20mV. Fig. 9 As shown, the control circuit of the auxiliary switch tube includes a first comparator, a second comparator and a trigger, wherein the first input terminal of the first comparator receives the second threshold value, the second input terminal receives the drain-source voltage of the auxiliary switch tube Qzvs, and the output terminal outputs a signal to the trigger as a second turn-on signal, the second input terminal of the second comparator receives the third threshold value, the first input terminal receives the drain-source voltage of the auxiliary switch tube Qzvs, and the output terminal outputs a signal to the trigger as a second turn-off signal.

[0042] It can be understood that the above describes the control method for the first opening of the auxiliary switch tube and the control method for the second opening. The two control methods for controlling the opening and closing of the auxiliary switch tube can be arbitrarily selected and combined without limiting a certain matching relationship.

[0043] According to the second aspect of the present application, a flyback converter is also provided, which includes: a main power switch tube, used to control the flyback converter to generate an output voltage; an auxiliary switch tube, used to achieve zero voltage turn-on of the main power switch tube; and an auxiliary switch tube control circuit, which applies the control method described above to control the auxiliary switch tube.

[0044] As an example, the auxiliary switch tube control circuit includes a first turn-on control circuit and a second turn-on control circuit, wherein the first turn-on control circuit includes a first turn-on signal generating circuit, configured to generate a first turn-on signal, a first turn-off signal generating circuit, configured to generate a first turn-off signal, and a first control signal generating circuit, configured to receive the first turn-on signal and the first turn-off signal to generate a first control signal for controlling the auxiliary switch tube; the second turn-on control circuit includes a second turn-on signal generating circuit, configured to generate a second turn-on signal, a second turn-off signal generating circuit, configured to generate a second turn-off signal, and a second control signal generating circuit, configured to receive the second turn-on signal and the second turn-off signal to generate a second control signal for controlling the auxiliary switch tube. It can be understood that the first turn-on signal can be generated based on the detection of the inductor current, and the generation of the first turn-off signal can receive the input voltage and the output voltage, and be generated by using a timer according to the bias time, or directly set a fixed time to generate it; similarly, the second turn-on signal can be generated based on the detection of the auxiliary winding voltage, or be generated after detecting that the main power switch tube is turned off for a certain period of time, or the second turn-on signal can also be generated by detecting that the drain-source voltage of the auxiliary switch tube is less than the second threshold. The second turn-off signal can be in direct proportion to the timing time of the first turn-off signal, and can also be generated by accepting the input voltage and the output voltage. The principle is the same as that of the first turn-off signal generation, which will not be repeated here. The second turn-off signal can also be generated by detecting that the drain-source voltage of the auxiliary switch tube passes through zero.

[0045] It can be understood that the above only describes one embodiment of the auxiliary switch tube control circuit of the flyback converter, but the embodiments of the present application are not limited thereto, and there may be other expansions and deformations.

[0046] Any range or device value given herein may be expanded or modified without losing the effect sought. In addition, any embodiment may be combined with another embodiment not expressly prohibited.

[0047] Although the subject matter has been described in language specific to structural features and / or acts, it should be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Instead, the specific features and acts described above are disclosed as examples of implementing the claims, and other equivalent features and acts are intended to fall within the scope of the claims.

[0048] It should be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments. The embodiments are not limited to embodiments that solve any or all of the problems described, or embodiments that have any or all of the benefits and advantages described. It should also be understood that reference to "an" item may refer to one or more of those items.

[0049] The steps of the methods described herein may be performed in any suitable order, or simultaneously where appropriate. In addition, individual blocks may be deleted from any method without departing from the spirit and scope of the subject matter described herein. Aspects of any of the above embodiments may be combined with aspects of any other embodiment described to form further embodiments without losing the effects sought.

[0050] The terms "comprises," "comprising," or any other variation thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0051] It should be understood that the above description is given as an example only, and various modifications may be made by those skilled in the art. The above description, examples and data provide a complete description of the structure and use of the exemplary embodiments. Although various embodiments with a certain degree of particularity have been described above, or with reference to one or more separate embodiments, those skilled in the art may make various changes to the disclosed embodiments without departing from the spirit or scope of this specification.

Claims

1. A control method for a flyback converter, wherein the flyback converter comprises a main power switch tube, a transformer, a second auxiliary winding coupled to the transformer, and an auxiliary switch tube and an auxiliary capacitor connected to the second auxiliary winding, wherein: The control method comprises: At a first moment before the main power switch tube is turned on, turning on the auxiliary switch tube for a first time period, so that the main power switch tube is turned on at the lowest value of the drain-source voltage; At a second moment after the main power switch tube is turned off, the auxiliary switch tube is turned on for a second period of time to charge the auxiliary capacitor; the second moment is when a fixed time t2 is detected after the main power switch tube is turned off, or the second moment is when it is detected that the output voltage of the first auxiliary winding of the flyback converter is greater than the first threshold.

2. The control method according to claim 1, characterized in that: When the flyback converter is in a critical conduction mode BCM, the first moment is when the inductor current of the flyback converter is detected to be zero-crossing; When the flyback converter is in the discontinuous conduction mode DCM, the first moment is the beginning of a switching cycle of the main power switch tube.

3. The control method according to claim 1, characterized in that: The first time duration T GAC1 The input voltage and output voltage of the flyback converter meet: in, is the input voltage, is the output voltage, is the first minimum bias time, , is a constant, or, The first time duration T GAC1 It is a fixed time t1.

4. The control method according to claim 1, characterized in that: The second duration is a fixed duration t3.

5. The control method according to claim 1, characterized in that: The second duration T GAC2 The input voltage and output voltage of the flyback converter meet: in, is the input voltage, is the output voltage, is the second minimum offset time, , is a constant.

6. The control method according to claim 1, characterized in that: The second duration satisfies the first duration ,in is a constant.

7. The control method according to claim 1, characterized in that: The second moment is when it is detected that the drain-source voltage of the auxiliary switch tube of the flyback converter is less than a second threshold; The second time duration is from the second moment to when it is detected that the drain-source voltage of the auxiliary switch tube is greater than a third threshold.

8. A flyback converter, characterized in that: include: A main power switch tube, used for controlling the flyback converter to generate an output voltage; An auxiliary switch tube, used to turn on the main power switch tube at the lowest value of the drain-source voltage; An auxiliary switch tube control circuit applies the control method as described in any one of claims 1 to 7 to control the auxiliary switch tube.

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