An asymmetrical flyback converter control circuit and a control method thereof

Abstract

The application provides an asymmetric flyback converter control circuit and a control method thereof. The control circuit comprises a switch control module, a high-voltage starting charging module, a first capacitor charging module and a negative excitation module. After the system is powered on, the alternating current or direct current passes through the high-voltage starting charging module to generate a supply voltage, the first capacitor charging module receives the supply voltage to charge the first capacitor, and after a first voltage is generated on the first capacitor, the switch control module controls the negative excitation module to be turned on, so that a negative current is generated on the primary side of the transformer. After the negative excitation module is turned off, the charge on the junction capacitor of the first switch tube is removed before the first switch tube is turned on based on the negative current, so as to realize the zero-voltage turn-on of the first switch tube in the first switching cycle after power-on. By realizing the zero-voltage turn-on of the first switch tube in the first switching cycle after power-on, not only the switching loss can be reduced, but also the SR stress of the secondary side device can be reduced, and the selection of the secondary side device is facilitated.

Description

Technical Field

[0001] This invention relates to the field of flyback converters, and more specifically to an asymmetric flyback converter control circuit and its control method. Background Technology

[0002] Flyback converters include the asymmetric half-bridge flyback converter (AHB), such as... Figure 1a The diagram shown is a circuit diagram of an asymmetric half-bridge flyback converter, which includes a transformer T with a primary winding Np and a secondary winding Ns, a first switch Q1, a second switch Q2, an inductor Lk and a capacitor Cr located on the primary side of the transformer T, and a diode D1 and an output capacitor Co located on the secondary side of the transformer T. Figure 1b This is another structure for an asymmetric half-bridge flyback converter. Asymmetric half-bridge flyback converters are more efficient than conventional PWM or quasi-resonant flyback converters. Under heavy load, they typically operate in a complementary state between the two switches, while under light load, they generally reduce the switching frequency to decrease turn-on losses, thus entering DCM mode. DCM mode incurs additional turn-on losses, which increase with the higher the switching frequency. Existing technologies typically employ zero-voltage switching (ZVS) to reduce losses, such as... Figure 2 The image shows an embodiment of achieving zero-voltage turn-on of the first switch in the prior art, as shown below. Figure 2 As shown, a negative excitation module is connected in parallel across the second switch Q2. When the first and second switches are off, and the control signal Vctrl controls the negative excitation module to turn on, a negative excitation current can be generated in the circuit containing the inductor Lk. When the negative excitation module is off, this negative excitation current removes the charge from the capacitor of the first switch Q1 before it turns on, thus achieving zero-voltage turn-on of the first switch. Figure 3 As shown, this is another embodiment of achieving zero-voltage turn-on of the first switch in the prior art, such as... Figure 3 As shown, the negative excitation module is located at both ends of the auxiliary winding. When the control signal Vctrl controls the negative excitation module to be turned on, a negative excitation current can be induced on the primary side. When the negative excitation module is turned off, the charge on the capacitor of the first switching transistor Q1 is removed through this negative excitation current, so as to achieve zero-voltage conduction of the first switching transistor. Although Figure 2-3The method described above can achieve zero-voltage turn-on of the first switching transistor, but its negative excitation current is achieved by the flyback converter in a stable operating state. In steady state, there is a voltage across capacitor Cr, and the zero-voltage turn-on of the first switching transistor is achieved based on this voltage. However, in the first few switching cycles after the system is powered on, the voltage across capacitor Cr has not yet been established. Therefore, even if the negative excitation module is turned on at this time, a negative excitation current cannot be generated, and zero-voltage turn-on cannot be achieved in the first few switching cycles. This leads to higher stress on the secondary-side SR, which places higher demands on the secondary-side components, making the selection of secondary-side components inconvenient, and also introduces certain switching losses. Therefore, it is necessary to improve the existing technology to address these problems. Summary of the Invention

[0003] To address the aforementioned technical problems, this invention provides an asymmetric flyback converter control circuit and its control method to resolve the issues existing in the prior art.

[0004] According to a first aspect of the present invention, a control circuit for an asymmetric half-bridge flyback converter is provided. The flyback converter includes a transformer, a first switch and a second switch located on the primary side of the transformer and connected between the input voltage and a reference ground of the flyback converter, and a first capacitor and a first inductor located in the conduction loop of the second switch when the second switch is turned on. The control circuit includes:

[0005] A switch control module is connected to the first switch and the second switch, and outputs a first control signal and a second control signal to control the first switch and the second switch to be turned on or off respectively;

[0006] The high-voltage start-up charging module receives AC or DC power after being powered on, and outputs a supply voltage based on the AC or DC power.

[0007] The first capacitor charging module receives the power supply voltage and charges the first capacitor based on the power supply voltage, so that a first voltage is generated on the first capacitor before the first switch is turned on.

[0008] A negative excitation module is connected to the primary side and / or auxiliary winding side of the transformer, and the switching control module controls the conduction or cutoff of the negative excitation module.

[0009] When the negative excitation module is turned on, a negative current is generated on the primary side of the transformer based on the first voltage; when the negative excitation module is turned off, the charge on the junction capacitance of the first switch transistor is removed before the first switch transistor is turned on based on the negative current, so as to achieve zero voltage conduction of the first switch transistor in the first switching cycle after power-on.

[0010] Wherein, when the first switch is turned on under normal operating conditions, the direction of the current generated on the transformer is positive, and the negative current is the current opposite to the positive direction.

[0011] Optionally, the first switch and the second switch are connected in series between the input voltage and the reference ground of the flyback converter; the common node connecting the first switch and the second switch is the first node;

[0012] The output terminal of the first capacitor charging module is connected to the first node, or any node between the first node and the reference ground that enables the first voltage to be generated on the first capacitor.

[0013] Optionally, the first switch and the second switch are connected in series between the input voltage and the reference ground of the flyback converter; the common node connecting the first switch and the second switch is the first node;

[0014] The control circuit further includes an isolation circuit module; the input terminal of the isolation circuit module is connected to the output terminal of the first capacitor charging module or the output terminal of the high-voltage start-up charging module; the two output terminals of the isolation circuit module are connected to any two nodes between the first node and the reference ground that can generate the first voltage on the first capacitor.

[0015] The isolation circuit module is used to convert the voltage at the output terminal of the first capacitor charging module or the voltage between the supply voltage and the reference ground into a voltage proportional to a certain coefficient and apply it to any two nodes to generate the first voltage on the first capacitor.

[0016] Optionally, the second switch and the first switch are connected in series between the input voltage and the reference ground of the flyback converter; the common node connecting the first switch and the second switch is the first node;

[0017] The control circuit also includes an isolation circuit module. The input terminal of the isolation circuit module is connected to the output terminal of the first capacitor charging module or the output terminal of the high-voltage start-up charging module. The two output terminals of the isolation circuit module are connected to any two nodes between the input voltage and the first node that can generate the first voltage on the first capacitor.

[0018] The isolation circuit module is used to convert the voltage at the output terminal of the first capacitor charging module or the voltage between the supply voltage and the reference ground into a voltage proportional to a certain coefficient and apply it to any two nodes to generate the first voltage on the first capacitor.

[0019] Optionally, the second switch also serves as the negative excitation module;

[0020] After the first voltage is generated, the switch control module controls the second switch to turn on, generating a negative current on the primary side of the transformer based on the first voltage.

[0021] Optionally, the negative excitation module is connected in parallel across the two ends of the second switching transistor;

[0022] Before the first switch is turned on, when the switch control module controls the negative excitation module to turn on, the negative current is generated on the primary side of the transformer based on the first voltage.

[0023] Optionally, before the first switch is turned on, the switch control module controls the second switch to also be turned on, generating the negative current on the primary side of the transformer based on the first voltage.

[0024] Optionally, the negative excitation module includes a third switching transistor or a current source.

[0025] Optionally, the negative excitation module is connected in parallel across the two ends of the auxiliary winding; its conduction or deactivation is controlled by the switch control module.

[0026] Before the first switch is turned on, when the switch control module controls the negative excitation module to turn on, the negative current is generated on the primary side of the transformer based on the first voltage.

[0027] Optionally, the negative excitation module includes a fourth switching transistor or current source, and also includes a second capacitor connected between the supply voltage or the voltage generated by the supply voltage and a reference ground.

[0028] According to a second aspect of the present invention, a control method for an asymmetric half-bridge flyback converter is provided. The flyback converter includes a transformer, a first switch and a second switch located on the primary side of the transformer and connected between the input terminal of the flyback converter and a reference ground, and a first capacitor and a first inductor located in the conduction circuit of the second switch when the second switch is turned on. The control method includes:

[0029] The switch control module outputs a first control signal and a second control signal to control the first switch and the second switch to turn on or off, respectively.

[0030] The high-voltage start-up charging module outputs a power supply voltage based on AC or DC power after being powered on.

[0031] The first capacitor charging module receives the power supply voltage and charges the first capacitor based on the power supply voltage, so that a first voltage is generated on the first capacitor before the first switch is turned on.

[0032] The negative excitation module is connected to the primary side or the auxiliary winding side of the transformer; the switching control module controls the on or off of the negative excitation module.

[0033] When the negative excitation module is turned on, a negative current is generated on the primary side of the transformer based on the first voltage; when the negative excitation module is turned off, the first switching transistor is turned on with zero voltage in the first switching cycle after power-on based on the negative current.

[0034] Wherein, when the first switch is turned on under normal operating conditions, the direction of the current generated on the transformer is positive, and the negative current is the current opposite to the positive direction.

[0035] Optionally, when the first switch and the second switch are connected in series between the input voltage and the reference ground of the flyback converter, the common node connecting the first switch and the second switch is the first node;

[0036] Connect the output terminal of the first capacitor charging module to the first node, or to any node between the first node and the reference ground that enables the first voltage to be generated on the first capacitor.

[0037] Optionally, when the second switch and the first switch are connected in series between the input voltage and the reference ground of the flyback converter, the common node connecting the first switch and the second switch is the first node;

[0038] The input terminal of the isolation circuit module is connected to the output terminal of the first capacitor charging module or the output terminal of the high-voltage start-up charging module. The two output terminals of the isolation circuit module are connected to any two nodes between the input voltage and the first node that can generate the first voltage on the first capacitor.

[0039] The isolation circuit module is used to convert the voltage at the output terminal of the first capacitor charging module or the voltage between the supply voltage and the reference ground into a voltage proportional to a certain coefficient and apply it to any two nodes to generate the first voltage on the first capacitor.

[0040] Optionally, the second switch can also serve as the negative excitation module; after the first voltage is generated, the switch control module controls the second switch to turn on, generating a negative current on the primary side of the transformer based on the first voltage.

[0041] The beneficial effects of the present invention include at least the following:

[0042] This invention provides a control circuit and method for an asymmetric half-bridge flyback converter. The control circuit includes a switch control module, a high-voltage start-up charging module, a first capacitor charging module, and a negative excitation module. After the system is powered on, AC or DC power passes through the high-voltage start-up charging module to generate a supply voltage. The first capacitor charging module receives the supply voltage and charges the first capacitor Cr. After a first voltage VCr is generated on the first capacitor Cr, the switch control module controls the negative excitation module to turn on, thereby generating a negative current on the primary side of the transformer. When the negative excitation module is turned off, the charge on the junction capacitance of the first switch transistor is removed before the first switch transistor turns on, based on the negative current, to achieve zero-voltage turn-on of the first switch transistor in the first switching cycle after power-on. By achieving zero-voltage turn-on of the first switch transistor in the first switching cycle after power-on, not only can switching losses be reduced, but also the SR stress of the secondary-side devices can be reduced, facilitating the selection of secondary-side devices.

[0043] It should be noted that the above general description and the following detailed description are merely exemplary and explanatory, and do not limit the present invention. Attached Figure Description

[0044] Figure 1a This diagram illustrates a schematic of an asymmetric flyback converter in the prior art.

[0045] Figure 1b This diagram illustrates another schematic of an asymmetric flyback converter in the prior art.

[0046] Figure 2 This diagram illustrates a principle of achieving zero-voltage turn-on of the first switch in an asymmetric flyback converter in the prior art.

[0047] Figure 3 This diagram illustrates another principle for achieving zero-voltage turn-on of the first switch in an asymmetric flyback converter in the prior art;

[0048] Figure 4 The diagram illustrates the principle of the asymmetric flyback converter according to the first embodiment of the present invention, which enables the first switching transistor to conduct at zero voltage during the first switching cycle after power-on.

[0049] Figure 5 The diagram illustrates the principle of the asymmetric flyback converter according to the second embodiment of the present invention, which enables the first switching transistor to conduct at zero voltage during the first switching cycle after power-on.

[0050] Figure 6 The present invention is shown. Figure 5 A schematic diagram of one embodiment of the negative excitation module in the examples;

[0051] Figure 7 The diagram illustrates the principle of the asymmetric flyback converter according to the third embodiment of the present invention, which enables the first switching transistor to conduct at zero voltage during the first switching cycle after power-on.

[0052] Figure 8 The present invention is shown. Figure 7 A schematic diagram of one embodiment of the negative excitation module in the examples;

[0053] Figure 9 The diagram illustrates the principle of the asymmetric flyback converter according to the fourth embodiment of the present invention, which enables the first switching transistor to conduct at zero voltage during the first switching cycle after power-on.

[0054] Figure 10 The diagram illustrates the principle of the asymmetric flyback converter according to the fifth embodiment of the present invention, which enables the first switching transistor to conduct at zero voltage during the first switching cycle after power-on.

[0055] Figure 11 The diagram illustrates the principle of the asymmetric flyback converter according to the sixth embodiment of the present invention, which enables the first switching transistor to conduct at zero voltage during the first switching cycle after power-on. Detailed Implementation

[0056] To facilitate understanding of the present invention, a more complete description will be given below with reference to the accompanying drawings. Preferred embodiments of the invention are shown in the drawings. However, the invention can be implemented in various forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a thorough and complete understanding of the disclosure of the invention.

[0057] like Figure 4 The diagram shown illustrates the principle of zero-voltage conduction of the first switch in the first switching cycle after power-on in the asymmetric flyback converter of the first embodiment of the present invention. The flyback converter includes a transformer T, a first switch Q1 and a second switch Q2 located on the primary side of the transformer and connected between the input voltage Vin and the reference ground, and a first capacitor Cr and a first inductor Lk located in the conduction circuit of the second switch Q2 when it is on. Junction capacitances C1 and C2 exist on the first switch Q1 and the second switch Q2, respectively. The first inductor Lk can be the leakage inductance of the transformer or a separately configured inductor. The transformer T includes a primary winding Np and a secondary winding Ns, with the equivalent inductance of the primary winding Np being the magnetizing inductance Lm. The secondary side of the transformer T also includes a first diode D1 and an output capacitor Co; the first diode can be replaced by a synchronous rectifier switch.

[0058] like Figure 4As shown, the control circuit 100 includes a switch control module, a high-voltage start-up charging module, a first capacitor charging module, and a negative excitation module. The switch control module is connected to the first switch Q1 and the second switch Q2, and outputs a first control signal Vg1 and a second control signal Vg2 to control the conduction or cutoff of the first switch Q1 and the second switch Q2, respectively. The switch control module includes logic control and drive circuitry, etc. After power-on, the high-voltage start-up charging module receives AC or DC power and outputs a supply voltage VCC based on this AC or DC power. As an embodiment of the invention, it can receive a bus voltage HV and output a supply voltage VCC based on this bus voltage. The bus voltage HV can be obtained by rectifying AC power. The first capacitor charging module receives the supply voltage VCC and charges the first capacitor Cr based on the supply voltage VCC, so that a first voltage VCr is generated on the first capacitor Cr before the first switch Q1 and the second switch Q2 are turned on. Figure 4 As shown, the supply voltage VCC can also power the switch control module. The negative excitation module is connected to the primary side of transformer T, and its on / off state is controlled by the switch control module; as shown... Figure 4 As shown, the negative excitation module is composed of a second switch Q2. The switch control module outputs a second control signal Vg2 to control the conduction or turn-off of the second switch Q2. After the system is powered on, the bus voltage HV gradually charges the supply voltage VCC to the corresponding operating voltage through the high-voltage start-up power supply module. During the rise of the supply voltage VCC, the first capacitor Cr can be charged synchronously through the first capacitor charging module, and the first capacitor Cr can be charged to the first voltage VCr, so that there is sufficient initial voltage on the first capacitor Cr before the switch control module outputs the corresponding control signal. After the first voltage Cr exists on the first capacitor Cr, the switch control module controls the negative excitation module to conduct. Figure 4In this process, when the second switch Q2 is turned on, a negative current is generated on the primary side of the transformer due to the presence of the first voltage Vcr. When the second switch Q2 is turned off, this negative current removes the charge from the junction capacitance C1 of the first switch Q1 before it is turned on. When the first switch Q1 is turned on, zero-voltage conduction of the first switch Q1 can be achieved in the first switching cycle after power-on. In normal operation, the current generated on the transformer when the first switch is turned on is positive, and the negative current is the current in the opposite direction to the positive current. Initially, the system generally operates in discontinuous cycle mode (DCM) after power-on. In another embodiment of the invention, after power-on and the generation of the first voltage VCr on the first capacitor Cr, the system can also be forced to operate in critical cycle mode (BCM) for the first few switching cycles. In this operating mode, zero-voltage conduction of the first switch is achieved by first turning on the second switch. After the system is operating normally, it switches to normal operating mode.

[0059] The common node connecting the first switch Q1 and the second switch Q2 is the first node N1; the output terminal of the first capacitor charging module can be connected to the first node N1, or any node between the first node N1 and the reference ground that can generate the first voltage VCr on the first capacitor Cr. Figure 4 The diagram illustrates an embodiment where the first capacitor charging module charges the common node N2 of the first capacitor Cr and the magnetizing inductor Lm to generate a first voltage VCr. In other embodiments, the first capacitor charging module can also charge the first node N1 to generate the first voltage VCr; when the first inductor Lk is a separate inductor, the output terminal of the first capacitor charging module can be connected to the common node of the first inductor Lk and the magnetizing inductor Lm to generate the first voltage VCr. Therefore, the position of the output terminal of the first capacitor charging module can be set according to actual conditions, as long as the first voltage VCr can be generated on the first capacitor Cr. Based on this first voltage VCr, a negative current is generated when the negative excitation module is turned on. This negative current then draws the charge from the junction capacitance of the first switching transistor before it is turned on, achieving zero-voltage turn-on of the first switching transistor.

[0060] like Figure 5 The image shows a second embodiment of the control circuit provided by the present invention that enables the first switching transistor to conduct at zero voltage. Figure 4The difference lies in the setting of the negative excitation module. In this control circuit 200, the negative excitation module is a separate module, which is connected in parallel across the two ends of the second switch Q2. The switch control module outputs a control signal Vctrl to control the conduction or cutoff of the negative excitation module. After the first voltage VCr is generated on the first capacitor Cr, before the first switch is turned on, when the switch control module controls the negative excitation module to be turned on, a negative current is generated in the circuit where the first inductor Lk is located based on the first voltage VCr. After the negative excitation module is turned off, before the first switch Q1 is turned on, the first switch Q1 is turned on with zero voltage based on this negative current.

[0061] like Figure 6 As shown, Figure 5 In one embodiment of the negative excitation module, the negative excitation module includes a third switch Q3. In other embodiments of the invention, the negative excitation module may include a resistor R connected in series with the third switch Q3, in addition to the third switch Q3. The equivalent impedance of the negative excitation module should not be set too large. If its equivalent impedance is too large, the negative current generated based on the first voltage VCr will be small or even non-existent. Before the first switch Q1 is turned on, it will be insufficient to completely remove the charge from the junction capacitance of the first switch Q1, i.e., zero-voltage turn-on of the first switch Q1 cannot be achieved. Furthermore, in other embodiments of the invention, in... Figure 5-6 In the embodiment shown, when the negative excitation module is turned on, the second switch Q2 can also be turned on by the switch control module. Since the impedance of the second switch Q2 is usually small, the first switch Q1 can also be turned on at zero voltage by turning on the second switch Q2 and the negative excitation module at the same time.

[0062] Figure 6 The middle part only indicates Figure 5 One embodiment of the negative excitation module in the invention, as another embodiment of the invention, Figure 5 The switching transistor in the negative excitation module can also be a transistor; the negative excitation module can also be a current source, as long as it can generate a negative current in the circuit based on the first voltage Vcr when the negative excitation module is turned on.

[0063] Figure 7 The schematic diagram of the asymmetric half-bridge flyback converter in the third embodiment of the present invention, which achieves zero-voltage conduction of the first switching transistor after power-on, is shown below. Figure 4-6 The difference lies in the fact that in the control circuit 300, the negative excitation module is located on the auxiliary winding Na side of the transformer T, such as... Figure 7As shown, it is connected in parallel across the two ends of the auxiliary winding Na, and its conduction or cutoff is controlled by the control signal Vctrl output by the switch control module; the negative excitation module also receives the supply voltage VCC, or the voltage generated by the supply voltage VCC. Figure 7 The diagram illustrates the connection to the supply voltage VCC. After the first voltage VCr is generated and before the first switch Q1 is turned on, when the switch control module controls the negative excitation module to turn on, the negative excitation module and the auxiliary winding form a current loop, and a negative current can be induced on the primary side. When the negative excitation module is turned off, the charge on the junction capacitance of the first switch Q1 is removed based on this negative current, so as to achieve zero-voltage turn-on of the first switch Q1.

[0064] Figure 8 for Figure 7 One embodiment of the negative excitation module includes a fourth switch Q4 and a second capacitor Ca connected between the supply voltage VCC and reference ground. Other embodiments of the invention... Figure 7 The negative excitation module can also include a current source, as long as a negative current can be induced on the primary side based on the first voltage Vcr when the negative excitation inductor is turned on.

[0065] Figure 9 The diagram shown is a schematic of the asymmetric flyback converter according to the fourth embodiment of the present invention, which achieves zero-voltage conduction of the first switching transistor after power-on. It is consistent with... Figure 5 The difference lies in the position of the first capacitor Cr. In this embodiment, the first capacitor Cr is positioned between the first node N1 and the common node N3 of the first capacitor Cr and the first inductor Lk. The output terminal of the first capacitor charging module can be located on the first node N1, thereby charging the first capacitor Cr to generate the first voltage Vcr. Furthermore, this positioning of the first capacitor also applies to… Figure 4 , 6 Examples from -8.

[0066] Figure 10 The schematic diagram of the fifth embodiment of the asymmetric flyback converter provided by the present invention to achieve zero-voltage conduction of the first switch after power-on, and the positional relationship between the first switch Q1 and the second switch Q2 are as follows: Figure 5 The difference lies in the connection between the second switch Q2 and the first switch Q1, which are connected in series between the input voltage Vin and the reference ground. The common node of the second switch Q2 and the first switch Q1 is the first node N1, and the common node of the first capacitor Cr and the magnetizing inductor Lm is N2. The control circuit 400 and... Figure 5In contrast, the control circuit 400 also includes an isolation circuit module connected to the output terminal of the first capacitor charging module. In another embodiment of the invention, this isolation circuit module can also be connected to the supply voltage VCC. The isolation circuit module has two output terminals, such as... Figure 10 As shown, the positive output terminal of the isolation circuit module is connected to node N2, and the negative output terminal is connected to the first node N1. The isolation circuit module generates a first voltage VCr on the first capacitor Cr. Because the voltage of the first capacitor charging module or the supply voltage VCC is relative to a reference ground, and the first capacitor Cr is essentially floating before the first switch Q1 is turned on, the output voltage of the first capacitor charging module or the supply voltage VCC cannot be directly used to charge the first capacitor Cr. The isolation circuit module converts the voltage of the first capacitor charging module or the supply voltage VCC relative to the reference ground into a voltage proportional to this voltage, which is applied to the first capacitor Cr to generate the first voltage VCr. As another embodiment of the invention, the two output terminals of the isolation circuit module can be connected to any two nodes between the input voltage Vin and the first node N1 (including the input voltage Vin and the first node N1) that can generate the first voltage VCr on the first capacitor Cr. For example, besides… Figure 10 In addition to the connection method described above, the two output terminals of the isolation circuit module can be connected to the input voltage Vin and the first node N1 respectively. Alternatively, when the first inductor Lk is a separately configured inductor, the two output terminals of the isolation circuit module can also be connected to the common node of the first inductor Lk and the magnetizing inductor Lm and the first node N1 respectively to charge the first capacitor Cr. Furthermore, Figure 10 The connection method of the first switch Q1 and the second switch Q2, as well as the setting of the isolation circuit module, can also be applied to... Figure 4 , 6 Examples from -8. Figure 10 The connection method of the first switch Q1 and the second switch Q2 is also applicable to Figure 9 ,for Figure 9 In the embodiment, the capacitor connection method can also be configured with an isolation circuit module, which can be used to charge the first capacitor Cr.

[0067] Figure 11 The schematic diagram of the asymmetric flyback converter in the sixth embodiment of the present invention, which enables the first switching transistor to conduct at zero voltage after power-on, is shown below. Figure 10The difference lies in the location of the first capacitor Cr. The first capacitor Cr is positioned between the input voltage Vin and the common node N3 of the first capacitor Cr and the first inductor Lk. In this case, the two output terminals of the isolation circuit module can be connected to the input voltage Vin and node N3, or to the input voltage Vin and the first node N1 respectively, or to any two nodes between the input voltage Vin and the first node N1 that can generate the first voltage VCr on the first capacitor Cr. Furthermore... Figure 10 The connection method of the first switch Q1 and the second switch Q2, the position of the first capacitor Cr, and the setting of the isolation circuit module can also be applied to... Figure 4 , 6 The embodiment in -8. For Figure 9 In the embodiments described, the connection method of the first switch Q1 and the second switch Q2 can also be adopted. Figure 11 In the connection method described above, an isolation circuit module can also be set up to charge the first capacitor Cr.

[0068] The present invention also provides a control method for achieving zero-voltage conduction of the first switch in an asymmetric flyback converter after power-on. The flyback converter includes a transformer, a first switch and a second switch located on the primary side of the transformer and connected between the input voltage and the reference ground of the flyback converter, and a first capacitor and a first inductor located on the conduction circuit of the second switch when the second switch is on.

[0069] The control method controls the first and second switching transistors to turn on or off based on the first and second control signals, respectively. The high-voltage starting charging module outputs a supply voltage based on AC or DC power after power-on. The first capacitor charging module receives this supply voltage and charges the first capacitor based on it, resulting in a first voltage across the first capacitor before the first switching transistor turns on. The negative excitation module is connected to the primary side or auxiliary winding side of the transformer. The switching control module controls the turning on or off of the negative excitation module. When the negative excitation module is on, a negative current is generated on the primary side of the transformer based on the first voltage. When the negative excitation module is off, the first switching transistor achieves zero-voltage conduction in the first switching cycle after power-on based on the negative current. Under normal operating conditions, the current generated on the transformer when the first switching transistor is on is positive, and the negative current is the current opposite to the positive direction.

[0070] Furthermore, when the first switch and the second switch are connected in series between the input voltage and the reference ground of the flyback converter, the common node connecting the first switch and the second switch is the first node; the output terminal of the first capacitor charging module is connected to the first node, or any node between the first node and the reference ground that can generate the first voltage on the first capacitor.

[0071] Furthermore, when the second switch and the first switch are connected in series between the input voltage and the reference ground of the flyback converter, the common node connecting the first switch and the second switch is the first node; the input terminal of the isolation circuit module is connected to the output terminal of the first capacitor charging module, or to the output terminal of the high-voltage start-up charging module, and the two output terminals of the isolation circuit module are connected to any two nodes between the input voltage and the first node that can generate the first voltage on the first capacitor; wherein, the isolation circuit module is used to convert the voltage between the output voltage of the first capacitor charging module or the supply voltage and the reference ground into a voltage that is proportional to it and applies it to the arbitrary two nodes to generate the first voltage on the first capacitor.

[0072] Furthermore, the second switch is also used as a negative excitation module; after the first voltage is generated, the switch control module controls the second switch to turn on, generating a negative current on the primary side of the device based on the first voltage.

[0073] In summary, this invention provides a control circuit and method for an asymmetric half-bridge flyback converter. The control circuit includes a switch control module, a high-voltage start-up charging module, a first capacitor charging module, and a negative excitation module. After the system is powered on, AC or DC power passes through the high-voltage start-up charging module to generate a supply voltage. The first capacitor charging module receives the supply voltage and charges the first capacitor Cr. After a first voltage VCr is generated on the first capacitor Cr, the switch control module controls the negative excitation module to turn on, thereby generating a negative current on the primary side of the transformer. When the negative excitation module is turned off, the charge on the junction capacitance of the first switch transistor is removed before the first switch transistor turns on, based on this negative current, to achieve zero-voltage turn-on of the first switch transistor in the first switching cycle after power-on. By achieving zero-voltage turn-on of the first switch transistor in the first switching cycle after power-on, not only can switching losses be reduced, but also the SR stress of the secondary-side devices can be reduced, facilitating the selection of secondary-side devices.

[0074] Finally, it should be noted that the above embodiments are merely examples for clearly illustrating the present invention and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A control circuit for an asymmetric half-bridge flyback converter, the flyback converter comprising a transformer, a first switch and a second switch located on the primary side of the transformer and connected between the input voltage and a reference ground of the flyback converter, and a first capacitor and a first inductor located in the conduction loop of the second switch when the second switch is turned on; the control circuit comprising: A switch control module is connected to the first switch and the second switch, and outputs a first control signal and a second control signal to control the first switch and the second switch to be turned on or off respectively; The high-voltage start-up charging module receives AC or DC power after being powered on, and outputs a supply voltage based on the AC or DC power. The first capacitor charging module receives the power supply voltage and charges the first capacitor based on the power supply voltage, so that a first voltage is generated on the first capacitor before the first switch is turned on. A negative excitation module is connected to the primary side and / or auxiliary winding side of the transformer, and the switching control module controls the conduction or cutoff of the negative excitation module. When the negative excitation module is turned on, a negative current is generated on the primary side of the transformer based on the first voltage; when the negative excitation module is turned off, the charge on the junction capacitance of the first switch transistor is removed before the first switch transistor is turned on based on the negative current, so as to achieve zero voltage conduction of the first switch transistor in the first switching cycle after power-on. Wherein, when the first switch is turned on under normal operating conditions, the direction of the current generated on the transformer is positive, and the negative current is the current opposite to the positive direction.

2. The control circuit according to claim 1, characterized in that: The first switch and the second switch are connected in series between the input voltage and the reference ground of the flyback converter; the common node connecting the first switch and the second switch is the first node; The output terminal of the first capacitor charging module is connected to the first node or any node between the first node and the reference ground that enables the first voltage to be generated on the first capacitor.

3. The control circuit according to claim 1, characterized in that: The first switch and the second switch are connected in series between the input voltage and the reference ground of the flyback converter; the common node connecting the first switch and the second switch is the first node; The control circuit further includes an isolation circuit module; the input terminal of the isolation circuit module is connected to the output terminal of the first capacitor charging module or the output terminal of the high-voltage start-up charging module; the two output terminals of the isolation circuit module are connected to any two nodes between the first node and the reference ground that can generate the first voltage on the first capacitor. The isolation circuit module is used to convert the voltage at the output terminal of the first capacitor charging module or the voltage between the supply voltage and the reference ground into a voltage proportional to a certain coefficient and apply it to any two nodes to generate the first voltage on the first capacitor.

4. The control circuit according to claim 1, characterized in that: The second switch and the first switch are connected in series between the input voltage and the reference ground of the flyback converter; the common node connecting the first switch and the second switch is the first node; The control circuit also includes an isolation circuit module. The input terminal of the isolation circuit module is connected to the output terminal of the first capacitor charging module or the output terminal of the high-voltage start-up charging module. The two output terminals of the isolation circuit module are connected to any two nodes between the input voltage and the first node that can generate the first voltage on the first capacitor. The isolation circuit module is used to convert the voltage at the output terminal of the first capacitor charging module or the voltage between the supply voltage and the reference ground into a voltage proportional to a certain coefficient and apply it to any two nodes to generate the first voltage on the first capacitor.

5. The control circuit according to any one of claims 1-4, characterized in that: The second switch also serves as the negative excitation module; After the first voltage is generated, the switch control module controls the second switch to turn on, generating a negative current on the primary side of the transformer based on the first voltage.

6. The control circuit according to any one of claims 1-4, characterized in that: The negative excitation module is connected in parallel across the two ends of the second switching transistor; Before the first switch is turned on, when the switch control module controls the negative excitation module to turn on, the negative current is generated on the primary side of the transformer based on the first voltage.

7. The control circuit according to claim 6, characterized in that: Before the first switch is turned on, the switch control module controls the second switch to also be turned on, and generates the negative current on the primary side of the transformer based on the first voltage.

8. The control circuit according to claim 6, characterized in that: The negative excitation module includes a third switching transistor or a current source.

9. The control circuit according to any one of claims 1-4, characterized in that: The negative excitation module is connected in parallel at both ends of the auxiliary winding; its conduction or cutoff is controlled by the switch control module. Before the first switch is turned on, when the switch control module controls the negative excitation module to turn on, the negative current is generated on the primary side of the transformer based on the first voltage.

10. The control circuit according to claim 9, characterized in that: The negative excitation module includes a fourth switching transistor or current source, and also includes a second capacitor connected between the supply voltage or the voltage generated by the supply voltage and a reference ground.

11. A control method for an asymmetric half-bridge flyback converter, the flyback converter comprising a transformer, a first switch and a second switch located on the primary side of the transformer and connected between the input terminal of the flyback converter and a reference ground, and a first capacitor and a first inductor located in the conduction circuit of the second switch when the second switch is turned on; the control method comprising: The switch control module outputs a first control signal and a second control signal to control the first switch and the second switch to turn on or off, respectively. The high-voltage start-up charging module outputs a power supply voltage based on AC or DC power after being powered on. The first capacitor charging module receives the power supply voltage and charges the first capacitor based on the power supply voltage, so that a first voltage is generated on the first capacitor before the first switch is turned on. The negative excitation module is connected to the primary side or the auxiliary winding side of the transformer; the switching control module controls the on or off of the negative excitation module. When the negative excitation module is turned on, a negative current is generated on the primary side of the transformer based on the first voltage; when the negative excitation module is turned off, the first switching transistor is turned on with zero voltage in the first switching cycle after power-on based on the negative current. Wherein, when the first switch is turned on under normal operating conditions, the direction of the current generated on the transformer is positive, and the negative current is the current opposite to the positive direction.

12. The control method according to claim 11, characterized in that: When the first switch and the second switch are connected in series between the input voltage and the reference ground of the flyback converter, the common node connecting the first switch and the second switch is the first node; Connect the output terminal of the first capacitor charging module to the first node or any node between the first node and the reference ground that enables the first voltage to be generated on the first capacitor.

13. The control method according to claim 11, characterized in that: When the second switch and the first switch are connected in series between the input voltage and the reference ground of the flyback converter, the common node connecting the first switch and the second switch is the first node; The input terminal of the isolation circuit module is connected to the output terminal of the first capacitor charging module or the output terminal of the high-voltage start-up charging module. The two output terminals of the isolation circuit module are connected to any two nodes between the input voltage and the first node that can generate the first voltage on the first capacitor. The isolation circuit module is used to convert the voltage at the output terminal of the first capacitor charging module or the voltage between the supply voltage and the reference ground into a voltage proportional to a certain coefficient and apply it to any two nodes to generate the first voltage on the first capacitor.

14. The control method according to any one of claims 11-13, characterized in that: The second switch is also used as the negative excitation module; after the first voltage is generated, the switch control module controls the second switch to turn on, and generates a negative current on the primary side of the transformer based on the first voltage.

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