Method and circuit for avoiding freewheeling diode avalanche breakdown in AHB flyback converter

Abstract

The invention relates to a method and a circuit for avoiding avalanche breakdown of a fly-wheel diode in an AHB flyback converter. The method comprises the following steps: a detection system is powered on and started, a controller enters a resonant capacitor voltage self-balancing mode, a high-side resonant power tube is controlled to be kept switched off, and a low-side switching power tube is controlled to be switched on and switched off periodically; detecting the voltage on the primary side detection resistor during the conduction period of each period of the low-side switching power tube, and comparing the voltage with the threshold voltage; when the voltage is detected to be lower than the threshold voltage before the low-side switching power tube in two continuous periods reaches the maximum conduction time, the controller starts the current source to carry out constant-current charging on the resonant capacitor after the low-side switching power tube is turned off, and the current source is turned off after the set charging duration is reached; and when the voltage is not detected to be lower than the avalanche breakdown current detection threshold voltage before the low-side switching power tube reaches the maximum conduction time in two continuous periods, the controller quits the resonant capacitor voltage self-balancing mode and recovers the normal pulse width modulation control mode.

Description

Technical Field

[0001] The present invention relates to the field of power electronics technology, and in particular to a method and circuit for preventing avalanche breakdown of a freewheeling diode in an AHB flyback converter. Background Art

[0002] Flyback converters are widely used in low- and medium-power power systems, such as light-emitting diode (LED) drivers and battery chargers. To improve system energy efficiency and power density, flyback converters based on the asymmetrical half-bridge (AHB) topology have been developed in recent years. This topology significantly increases operating frequency, reduces device stress, and improves system efficiency and power density while achieving soft switching (such as zero voltage switch-on (ZVS) and zero current switch-off (ZCS)).

[0003] The maximum reverse voltage VSR(max) that the secondary diode of a conventional flyback circuit experiences during normal operation is: Where Vout is the output voltage, Vin(max) is the maximum input voltage, and n is the transformer primary-to-secondary turns ratio.

[0004] Compared with the traditional flyback topology, under the same input and output conditions, the maximum reverse voltage VSR(max) that the secondary diode of the AHB half-bridge resonant topology can withstand when operating stably is:

[0005] Wherein, Vcr is the resonant capacitor voltage.

[0006] In the stable operating state of an AHB flyback converter, the resonant capacitor voltage Vcr equals n times the output voltage (n is the transformer's primary-to-secondary turns ratio), i.e., Vcr = n × Vout. This shows that, during stable operation, the maximum reverse voltage experienced by the secondary diode in the AHB half-bridge resonant topology is one Vout lower than in conventional flyback circuits. Therefore, under the same conditions, especially in applications with high output voltages (Vout), the AHB topology offers advantages in selecting a secondary freewheeling diode or synchronous rectifier, and has a wider range of performance requirements, such as withstand voltage. Furthermore, since its primary power transistors can achieve zero voltage switching (ZVS) and its secondary power transistors can achieve zero current switching (ZCS), efficient power conversion is achieved. Furthermore, thanks to soft switching, the power supply can operate at higher switching frequencies, significantly improving power density. Therefore, AHB circuits are widely used in applications such as LED lighting and battery charging.

[0007] However, in practical applications, such as LED lighting and battery charging, the output voltage Vout remains high after input power is lost due to the output voltage clamping characteristics of these loads. The resonant capacitor Cr in the AHB topology slowly discharges through the parallel resistor Rcr, eventually approaching 0V. If the system is powered back on at this point, before the resonant capacitor voltage has recovered, the voltage across the primary inductor is approximately equal to the input voltage Vin. Under the action of the transformer, the reverse voltage applied by the secondary freewheeling diode is the sum of the output voltage and the primary-to-secondary voltage. Therefore, turning on the power transistor causes the transformer's secondary freewheeling diode to experience a reverse voltage of Vout + Vin / n, which can easily exceed its reverse withstand voltage threshold, causing avalanche breakdown. This results in excessive avalanche energy, thermal damage to the device, and even permanent damage to the freewheeling diode, impacting system reliability and lifespan. The secondary freewheeling diode does not exit avalanche breakdown until the resonant capacitor voltage Vcr reaches a certain value.

[0008] Existing solutions mainly include selecting diodes with higher voltage ratings or connecting a load in parallel with the output to reduce the output voltage. However, the former limits the choice of components and increases costs, while the latter has poor controllability and reduces system efficiency. Figure 1 Figure 66 shows a circuit that uses the two existing methods mentioned above. D_hbv in the figure is a freewheeling diode with a higher reverse withstand voltage value, that is, by increasing the reverse withstand voltage threshold, the diode is prevented from avalanche breakdown. However, this method not only places higher requirements on the selected diode, but also directly increases the system cost. R_load in the figure is a consumption load set at the output end. When the input power is off, the output voltage can be discharged to 0V through this small load to avoid the freewheeling diode avalanche breakdown caused by the imbalance between the high output voltage Vout and the resonant capacitor voltage Vcr (close to 0) when the power is turned on next time. However, this method is uncontrollable and will face the situation of unbalanced discharge between the resonant capacitor and the output capacitor. At the same time, increasing the load will bring additional consumption and reduce the conversion efficiency.

[0009] Therefore, a strategy that can effectively control the recovery of the resonant capacitor voltage during the system restart phase is urgently needed to avoid the diode avalanche breakdown problem, ensure stable system operation and control costs. Summary of the Invention

[0010] The purpose of the present invention is to address the defects of the prior art and provide a method and circuit for avoiding avalanche breakdown of the freewheeling diode in an AHB flyback converter. By detecting the avalanche breakdown current of the freewheeling diode during power-on, controlling the conduction behavior of the switching power tube and cooperating with a constant current source to quickly charge the resonant capacitor, self-balancing of the resonant capacitor voltage is achieved, effectively avoiding avalanche breakdown of the secondary freewheeling diode in the initial power-on stage, improving system reliability and reducing device costs.

[0011] To achieve the above objectives, in a first aspect, an embodiment of the present invention provides a method for preventing avalanche breakdown of a freewheeling diode in an AHB flyback converter, the method comprising:

[0012] Perform power-on detection on the system. When it is detected that the system is powered on, the controller enters the resonant capacitor voltage self-balancing mode;

[0013] In the resonant capacitor voltage self-balancing mode, the controller controls the high-side resonant power tube to remain turned off, and controls the low-side switching power tube to be periodically turned on and off;

[0014] Wherein, in each cycle when the low-side switch power tube is turned on, the controller detects the voltage Vcs on the primary-side detection resistor Rcs during the period when the low-side switch power tube is turned on, and compares it with the avalanche breakdown current detection threshold voltage Vcs (srbv);

[0015] When the voltage Vcs is not detected to be lower than the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum on-time Tonmax in two consecutive cycles, the controller starts the current source after the low-side switch power tube is turned off in the current cycle, charges the resonant capacitor with a constant current according to the set charging time to increase the resonant capacitor voltage Vcr, and turns off the current source after the set charging time is reached; wherein, when Vcs is higher than the avalanche breakdown current detection threshold voltage Vcs(srbv), it indicates that the resonant capacitor voltage Vcr is low during the conduction period of the low-side switch power tube, causing the freewheeling diode to avalanche breakdown and generate reverse current;

[0016] When the voltage Vcs is not detected to be greater than or equal to the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum on-time Tonmax for two consecutive cycles, the controller exits the resonant capacitor voltage self-balancing mode and resumes the normal pulse width modulation PWM control mode.

[0017] Preferably, the power-on detection of the system, when detecting that the system is powered on, the controller enters the resonant capacitor voltage self-balancing mode, includes:

[0018] The power-on detection module monitors the system input voltage Vin; when the input voltage Vin reaches a preset threshold, causing the controller's supply voltage Vcc to reach a start-up voltage Vcc(st), the module outputs a first level drive control input signal det_bv;

[0019] When the controller receives the driving control input signal det_bv of the first level, it enters the resonant capacitor voltage self-balancing mode.

[0020] Further preferably, when the controller exits the resonant capacitor voltage self-balancing mode, the drive control input signal det_bv is set to a second level.

[0021] Preferably, the step of increasing the voltage amplitude of the driving signal of the low-side switching power tube periodically specifically includes:

[0022] The controller dynamically adjusts to gradually increase the gate drive voltage of the low-side switch power tube according to the accumulated number of cycles after entering the resonant capacitor voltage self-balancing mode;

[0023] In which, the controller is configured as follows: in the resonant capacitor voltage self-balancing mode, periodically turning on the low-side switching power tube, and applying a driving current to the low-side switching power tube with the set capacitor voltage self-balancing mode; at the same time, the resonant capacitor Cr is gradually charged in each cycle in which the low-side switching power tube is turned on, so that the resonant capacitor voltage Vcr increases cycle by cycle, and the rising slope of the reverse current flowing through the low-side switching power tube decreases, thereby slowing down the rising speed of the voltage Vcs, and extending the time required to reach the avalanche breakdown current detection threshold voltage Vcs (srbv), so that the conduction time of the low-side switching power tube gradually increases; thereby, the driving voltage of the low-side switching power tube gradually rises in each cycle, thereby slowing down the reverse current impact and reducing the transient spikes caused by the avalanche current until the set gate drive voltage maximum value is reached, or until the self-balancing mode is exited.

[0024] Preferably, in each cycle in which the low-side switch power tube is turned on, the controller detects the voltage Vcs on the primary-side detection resistor Rcs during the conduction period of the low-side switch power tube, and compares it with the avalanche breakdown current detection threshold voltage Vcs (srbv), specifically including:

[0025] In each cycle when the low-side switch power tube is turned on, the controller detects the voltage Vcs on the primary-side detection resistor Rcs during the current conduction period, and compares it with the avalanche breakdown current detection threshold voltage Vcs (srbv), and generates a control signal srbv as a first level signal or a second level signal according to the comparison result;

[0026] When Vcs<Vcs(srbv) is always maintained before the low-side switch power tube reaches the maximum on-time Tonmax in a cycle, it indicates that the freewheeling diode avalanche breakdown does not occur in the current cycle, the current count value of the low-level counter is obtained, and the count value is increased by 1 as the updated current count value; wherein the initial count value of the low-level counter is 0;

[0027] When Vcs≥Vcs(srbv) occurs before the low-side switch power tube reaches the maximum on-time Tonmax in one cycle, it indicates that the freewheeling diode avalanche breakdown occurs in the current cycle, and the count value of the low-level counter is set to 0;

[0028] When the updated current count value reaches 2, it is determined that the voltage Vcs is not detected to be greater than or equal to the avalanche breakdown current detection threshold voltage Vcs (srbv) before the low-side switch power tube reaches the maximum conduction time Tonmax for two consecutive cycles; otherwise, the voltage Vcs is not detected to be lower than the avalanche breakdown current detection threshold voltage Vcs (srbv) before the low-side switch power tube reaches the maximum conduction time Tonmax for two consecutive cycles.

[0029] Further preferably, the controller starts the current source after the low-side switch power tube is turned off in the current cycle, and performs constant current charging to the resonant capacitor according to the set charging time, specifically including:

[0030] When Vcs≥Vcs(srbv), the control signal srbv generated is a control signal of the first level; after the low-side switch power tube is turned off in the current cycle, the current source is started according to the control signal of the first level, and the resonant capacitor is charged with a constant current according to the set charging time;

[0031] When Vcs<Vcs(srbv), the generated control signal srbv is a control signal of a second level; and the count value is increased by 1 according to the control signal of the second level.

[0032] In a second aspect, an embodiment of the present invention provides a flyback converter circuit, comprising:

[0033] A flyback converter includes a primary-side circuit and a secondary-side circuit; the primary-side circuit includes a high-side resonant power transistor, a low-side switching power transistor, a resonant capacitor, a primary winding, and a detection resistor; the high-side resonant power transistor, the resonant capacitor, and the primary winding constitute a resonant cavity loop; the detection resistor is disposed outside the resonant cavity loop; the secondary-side circuit is inductively coupled to the primary-side circuit via the secondary winding and outputs a DC voltage signal to a load;

[0034] Power-on detection module, used to monitor the system input voltage Vin and generate drive control input signals;

[0035] a controller, determining whether to enter a resonant capacitor voltage self-balancing mode according to the drive control input signal;

[0036] In the resonant capacitor voltage self-balancing mode, the controller controls the high-side resonant power tube to remain turned off, and controls the low-side switching power tube to be periodically turned on and off;

[0037] Wherein, in each cycle when the low-side switch power tube is turned on, the controller detects the voltage Vcs on the primary-side detection resistor Rcs during the period when the low-side switch power tube is turned on, and compares it with the avalanche breakdown current detection threshold voltage Vcs (srbv);

[0038] When the voltage Vcs is not detected to be lower than the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum on-time Tonmax in two consecutive cycles, the controller starts the current source after the low-side switch power tube is turned off in the current cycle, charges the resonant capacitor with a constant current according to the set charging time to increase the resonant capacitor voltage Vcr, and turns off the current source after the set charging time is reached; wherein, when Vcs is higher than the avalanche breakdown current detection threshold voltage Vcs(srbv), it indicates that the resonant capacitor voltage Vcr is too low during the conduction period of the low-side switch power tube, causing the freewheeling diode to avalanche breakdown and generate reverse current;

[0039] When the voltage Vcs is not detected to be greater than or equal to the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum on-time Tonmax for two consecutive cycles, the controller exits the resonant capacitor voltage self-balancing mode and resumes the normal pulse width modulation PWM control mode.

[0040] Preferably, the controller includes: an input overcurrent detection module, a control logic module, a driving module and the current source;

[0041] The first input terminal of the input overcurrent detection module is connected to a preset avalanche breakdown current detection threshold voltage Vcs(srbv), and the second input terminal is connected to the voltage Vcs on the primary-side detection resistor Rcs; when Vcs≥Vcs(srbv), the control signal srbv is generated as a control signal of a first level; when Vcs<Vcs(srbv), the control signal srbv is generated as a control signal of a second level;

[0042] The first input terminal of the control logic module is connected to the driving control input signal det_bv, and the second input terminal is connected to the control signal srbv; the control logic module outputs corresponding driving module control signals GH and GL according to the driving control input signal det_bv and the control signal srbv;

[0043] The driving module outputs switch tube driving signals GTH and GTL according to the driving module control signals GH and GL;

[0044] The current source receives the control signal srbv. When the control signal srbv is a control signal of the first level, after the low-side switch power tube is turned off in the current cycle, the current source performs constant current charging to the resonant capacitor according to the set charging time.

[0045] Further preferably, the control module includes a low-level counter, and an initial count value of the low-level counter is 0;

[0046] When the control signal srbv is a control signal of a second level, the control module adds 1 to the count value stored in the low-level counter according to the control signal of the second level.

[0047] Preferably, the flyback converter is a flyback converter based on a half-bridge resonant AHB topology.

[0048] The present invention provides a method for preventing avalanche breakdown of the freewheeling diode in an AHB flyback converter. During the system power-on startup phase, the controller automatically enters self-balancing mode. In this mode, only the low-side switching power tube is periodically turned on, and the driving voltage is slowly increased, reducing the conduction speed of the low-side switching power tube to reduce transient spikes and avalanche current during the conduction process. The controller detects the voltage Vcs across the primary-side detection resistor Rcs during each conduction cycle and compares it with the avalanche breakdown current detection threshold Vcs (srbv) to determine whether diode avalanche has occurred. If no avalanche breakdown signal is detected for two consecutive cycles, the controller determines that the state is safe, exits self-balancing mode, and resumes normal PWM control. If the exit condition is not met, the constant current source is activated after each cycle to charge the resonant capacitor for a set duration, increasing the resonant capacitor voltage Vcr and accelerating the achievement of a stable state. In addition, the charging time or charging rate of the resonant capacitor Cr in the present invention can be dynamically adjusted, which can effectively speed up the recovery speed of the resonant capacitor voltage, thereby reducing the reverse voltage borne by the freewheeling diode during the conduction period of the switch tube, avoiding long-term avalanche breakdown and accumulation of high avalanche energy. This method effectively reduces the diode reverse overvoltage problem caused by too low Vcr by automatically adjusting the resonant capacitor charging process, avoiding the problem of increased cost due to the use of high-voltage devices in traditional solutions, or the problem of reduced efficiency due to parallel energy-consuming loads. The use of the method of the present invention can not only avoid device damage caused by avalanche breakdown during the startup phase, improve system reliability, stability and life, but also allow the use of diodes with lower voltage resistance and lower cost, thereby achieving efficient and low-cost system design. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 A circuit diagram of an AHB flyback converter provided in the prior art;

[0050] Figure 2 A block diagram of the internal structure of a controller in a flyback converter circuit with a half-bridge resonant topology provided by an embodiment of the present invention;

[0051] Figure 3The circuit topology structure of the prior art half-bridge resonant topology flyback converter circuit (wherein the converter is an AHB flyback converter) provided in an embodiment of the present invention;

[0052] Figure 4 This is a schematic diagram of the internal circuit logic of the switch tube drive signal GTL;

[0053] Figure 5 A specific implementation circuit of the flyback converter circuit provided in an embodiment of the present invention;

[0054] Figure 6 A flow chart of a control method for self-balancing the resonant capacitor voltage provided by an embodiment of the present invention;

[0055] Figure 7 Schematic diagram of the circuit structure when the high-side power tube GH is turned off;

[0056] Figure 8 This is an example signal diagram of a conventional AHB flyback converter resonant capacitor voltage imbalance in the prior art;

[0057] Figure 9 Schematic diagram of the avalanche breakdown current detection principle using an AHB flyback converter as an example in an embodiment of the present invention;

[0058] Figure 10 Shown is a signal diagram of an example of the discharge principle of an AHB flyback converter provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0059] To make the objectives, technical solutions, and advantages of the present invention more apparent, the present invention will be further described in detail below with reference to the accompanying drawings. It is apparent that the embodiments described are only some, not all, of the present invention. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present invention without creative effort are intended to fall within the scope of protection of the present invention.

[0060] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments.

[0061] First, a flyback converter circuit for realizing the method of avoiding avalanche breakdown of the freewheeling diode in the AHB flyback converter of the present invention is described. Figure 2 As shown, it includes: a controller 1, a power-on detection module 2 and a flyback converter 3.

[0062] The power-on detection module 2 is used to monitor the system input voltage Vin and generate the drive control input signal det_bv. When the input voltage Vin rises to Vin(hv), the internal module power supply voltage Vcc is supplied, and after it rises to a certain value Vcc(st), the output det_bv signal is high.

[0063] Specifically, the power-on detection module can be implemented using a commonly used power-on detection solution. It uses a voltage comparator and a voltage-divider resistor network to monitor whether Vin reaches a preset threshold, Vin(hv). When Vin exceeds the set threshold, Vin(hv), the power-on detection module activates an internal startup circuit, such as a high-voltage startup current source or a low-dropout linear regulator (LDO) connected to the Vcc output pin. This internal startup circuit draws energy from Vin to provide the startup voltage, Vcc, for the subsequent controller. Vcc gradually increases as the startup circuit charges. The controller has a minimum operating voltage requirement, Vcc(st), such as 12V. The power-on detection module continuously monitors Vcc. When Vcc ≥ Vcc(st), the controller power supply is stable and startup conditions have been met. At this point, the power-on detection module outputs the det_bv signal, which in this example is a high-level signal. This signal serves as an input to the controller, notifying controller 1 that the system has been powered on and can begin the logic control process.

[0064] The controller 1 includes three input terminals, one input terminal is connected to the preset avalanche breakdown current detection threshold voltage Vcs (srbv), one input terminal is the detection voltage input terminal, which is connected to the voltage Vcs on the detection resistor of the flyback converter, and the other input terminal is connected to the drive control input signal det_bv.

[0065] Based on input signals and internal control logic, controller 1 determines whether flyback converter 3 operates in normal operating mode or enters resonant capacitor voltage self-balancing mode. In normal operating mode, controller 1 outputs switch tube drive signals GTH and GTL, which are pulse width modulation (PWM) signals in this mode, to control the high-side resonant power tube and low-side switching power tube of flyback converter 3 to alternately turn on and off, outputting a DC voltage signal Vout to the load. In resonant capacitor voltage self-balancing mode, controller 1 outputs switch tube drive signals GTH and GTL to dynamically adjust the periodic on and off of the low-side switching power tube while keeping the high-side resonant power tube off.

[0066] In each cycle when the low-side switch power tube is turned on, the controller 1 detects the voltage Vcs on the primary-side detection resistor Rcs during the conduction period of the low-side switch power tube, and compares it with the avalanche breakdown current detection threshold voltage Vcs (srbv); when the voltage Vcs is not detected to be lower than the avalanche breakdown current detection threshold voltage Vcs (srbv) before the low-side switch power tube reaches the maximum conduction time Tonmax in two consecutive cycles, the controller 1 starts the current source after the low-side switch power tube is turned off in the current cycle, and charges the resonant capacitor with a constant current through the HB port according to the set charging time to increase the resonant current. The resonant capacitor voltage Vcr is increased, and the current source is turned off after the set charging time is reached; wherein, when Vcs is higher than the avalanche breakdown current detection threshold voltage Vcs (srbv), it indicates that the resonant capacitor voltage Vcr is too low during the conduction period of the low-side switch power tube, causing the freewheeling diode to avalanche breakdown and generate reverse current; when the voltage Vcs is not detected to be greater than or equal to the avalanche breakdown current detection threshold voltage Vcs (srbv) before the low-side switch power tube reaches the maximum conduction time Tonmax for two consecutive cycles, the controller exits the resonant capacitor voltage self-balancing mode and resumes the normal pulse width modulation PWM control mode.

[0067] The maximum on-time, Tonmax, is set primarily based on the following considerations: During power-up, if the resonant capacitor voltage, Vcr, exceeds the minimum critical value that causes avalanche breakdown in the secondary freewheeling diode, then turning on the low-side power switch after this point will prevent the secondary diode from avalanche breakdown and generating reverse current. Therefore, a maximum on-time, Tonmax, must be set in the logic control module within controller 1. When the on-time reaches the maximum on-time, Tonmax, the low-side power switch is forced to shut down, preventing it from remaining on for extended periods and failing to exit the resonant capacitor self-balancing mode in a timely manner.

[0068] The internal structure diagram of controller 1 is as follows Figure 3 As shown, it includes: an input overcurrent detection module 11, a control logic module 12, a driving module 13 and a current source 14;

[0069] The first input terminal of the input overcurrent detection module 11 is connected to the avalanche breakdown current detection threshold voltage Vcs(srbv), and the second input terminal is connected to the voltage across the flyback converter's sense resistor, i.e., the feedback voltage Vcs. The corresponding control signal srbv is output based on the comparison result between the feedback voltage Vcs and the avalanche breakdown current detection threshold voltage Vcs(srbv). The control logic module 12 records the current operating mode, which is either the normal operating pulse width modulation (PWM) mode or the resonant capacitor self-balancing mode. In the corresponding operating mode, the control logic module 12 generates corresponding control signals GH and GL based on the received control signal srbv and the drive control input signal det_bv. The control driver module 13 outputs the switch tube drive signals GTH and GTL to drive the primary-side power tube of the AHB flyback converter to perform the corresponding turn-on and turn-off operations.

[0070] In a specific example, when Vcs≥Vcs(srbv), the control signal srbv is generated as a control signal of the first level; when Vcs<Vcs(srbv), the control signal srbv is generated as a control signal of the second level; the first input terminal of the control logic module 12 is connected to the drive control input signal det_bv, and the second input terminal is connected to the control signal srbv; the control logic module 12 outputs corresponding drive module control signals GH and GL according to the drive control input signal det_bv and the control signal srbv; the drive module 13 outputs the switch tube drive signals GTH and GTL according to the drive module control signals GH and GL.

[0071] The internal circuit logic of the switch tube drive signal GTL in the driving module 13 for the circuit working in the normal PWM mode and the capacitor voltage self-balancing module is as follows: Figure 4 As shown, it should be noted that Figure 4 The examples are only given for the convenience of understanding, and are not intended to limit the circuit implementation or control method to the one given, and the data shown are all for illustration.

[0072] like Figure 4 It can be seen that the output control logic of the low-side switch tube drive signal GTL is divided into two branches in the drive module 13, and different branch resistors are set in each branch to regulate the drive current output in different working modes.

[0073] The process of turning on the low-side switch is the process of charging the gate-source capacitance Cgs within the low-side power switch. When Vgs exceeds the low-side switch threshold voltage Vth, the low-side switch turns on. By controlling the current charging Cgs, the turn-on speed of the low-side switch can be controlled. The capacitor charging formula is: C × U = I × T, where C is the gate-source capacitance, U is the gate-source capacitor voltage, I is the gate-source capacitor charging current, and T is the charging time. The rise rate of the low-side power switch drive voltage GTL is determined by the charging current and charging time.

[0074] When the resonant capacitor voltage self-balancing mode is entered, that is, when det_bv = 1, the drive current I2 is one-tenth of the current I1 in normal PWM operation mode. Therefore, when det_bv = 1, under the same charging time conditions, the voltage amplitude rise rate of the low-side switch tube drive signal GTL is also one-tenth of that in normal operation mode.

[0075] The current source 14 receives a control signal srbv. When the control signal srbv is a control signal of the first level, after the low-side switch power tube is turned off in the current cycle, the current source charges the resonant capacitor with a constant current according to a set charging time.

[0076] In this example, the first level is a high level, and the second level is a low level.

[0077] The logic control of whether the voltage Vcs is detected to be lower than the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum on-time Tonmax in two consecutive cycles can be implemented by loading a low-level counter in the logic control module 12. The implementation using the low-level counter mentioned here is only one possible implementation method, and does not limit the above-mentioned logic control of the present invention to only being implemented in this way.

[0078] The initial count value of the low-level counter is set to 0; when the control signal srbv is a control signal of the second level, the control module adds 1 to the count value stored in the low-level counter according to the control signal of the second level.

[0079] That is to say, during the conduction period of the low-side switch power tube in one cycle, that is, before reaching the maximum conduction time Tonmax, the controller detects the voltage Vcs on the primary-side detection resistor Rcs during the current conduction period, and compares it with the avalanche breakdown current detection threshold voltage Vcs(srbv); when Vcs<Vcs(srbv) is always maintained before reaching the maximum conduction time, it indicates that the freewheeling diode avalanche breakdown has not occurred in the current cycle, the current count value of the low-level counter is obtained, and the count value is added by 1 as the updated current count value; when Vcs≥Vcs(srbv) occurs before reaching the maximum conduction time, it indicates that the freewheeling diode avalanche breakdown has occurred in the current cycle, and the count value of the low-level counter is set to 0.

[0080] When the updated current count value reaches 2, it is determined that the voltage Vcs is continuously detected to be lower than the avalanche breakdown current detection threshold voltage Vcs (srbv) before the low-side switch power tube reaches the maximum conduction time Tonmax in two consecutive cycles (that is, the voltage Vcs is not detected to be greater than or equal to the avalanche breakdown current detection threshold voltage Vcs (srbv)); otherwise, the voltage Vcs is not detected to be lower than the avalanche breakdown current detection threshold voltage Vcs (srbv) before the low-side switch power tube reaches the maximum conduction time Tonmax in two consecutive cycles.

[0081] A specific implementation structure of the flyback converter 3 is as follows Figure 5 The circuit shown in Figure 1 includes a primary-side circuit and a secondary-side circuit. The primary-side circuit comprises a high-side resonant power transistor QH, a low-side switching power transistor QL, a resonant capacitor Cr, a primary winding, and a sense resistor Rcs. The high-side resonant power transistor QH, the resonant capacitor Cr, the resonant inductor Lr, and the primary winding of the transformer Tr form a resonant cavity loop. The sense resistor Rcs is located outside the resonant cavity loop. The high-side resonant power transistor QH and the low-side switching power transistor QL are connected between the power input Vin and ground. They are controlled by the switch drive signals GTH and GTL output by the controller 1. The sense resistor Rcs is connected between the low-side switching power transistor QL and the second input terminal of the controller 1 (for receiving the Vcs voltage) and is located outside the resonant cavity loop. The secondary-side circuit is inductively coupled to the primary-side circuit via the secondary winding of the transformer Tr. The secondary circuit connected to the secondary winding is connected to the load and outputs a DC voltage signal Vout to the load. A freewheeling diode D is provided in the secondary-side circuit.

[0082] Below, Figure 5 The circuit is used as an example to quantitatively analyze the avalanche breakdown principle of the AHB flyback converter:

[0083] Because when working stably, the maximum reverse voltage V SR(max) for:

[0084]

[0085] Then let the diode breakdown voltage be V SR(BV) , leaving a certain margin (e.g. 10%), the breakdown voltage value is:

[0086]

[0087] When the power switch is turned on, if the diode is applied with a reverse voltage V SR Exceed V SR(BV) , it means the diode has avalanche breakdown. That is:

[0088]

[0089] Then consider the worst case: when the input voltage is the maximum value V in(max) According to Formula 3, it can be seen that the secondary freewheeling diode is most likely to avalanche breakdown at this time, so it is necessary to ensure the safety of the diode in the worst case. Substituting into Formula 3, in order to prevent the diode from avalanche breakdown, it is necessary to:

[0090]

[0091] Simplifying, we can get: V cr ≥n×V out -0.1×V in(max) (Formula 5)

[0092] That is, when the power switch is on, ensuring that the resonant capacitor voltage satisfies Equation 5 can ensure that the secondary freewheeling diode avoids avalanche breakdown under full input range.

[0093] For example: AHB topology is used to power LEDs. The transformer primary-to-secondary turns ratio is n=6. The output voltage Vout is 28V. The input voltage Vin(max) is 400V. The breakdown voltage of the freewheeling diode is V. SR Substituting (BV)=73V into the above formula, we can obtain that when Vcr≥128V, it can be guaranteed that the secondary freewheeling diode will not avalanche breakdown during the conduction period of the switching power tube.

[0094] Figure 6 For the present invention Figure 2-5 The control method flow of the resonant capacitor voltage self-balancing of the flyback conversion circuit is shown.

[0095] The system is powered on and detected. When it is detected that the system is powered on and started, the controller enters the resonant capacitor voltage self-balancing mode.

[0096] Specifically, the power-on detection module monitors the system input voltage Vin; when the input voltage Vin reaches a preset threshold, causing the controller's supply voltage Vcc to reach the start-up voltage Vcc(st), the first level drive control input signal det_bv is output;

[0097] When the controller receives the driving control input signal det_bv of the first level, it enters the resonant capacitor voltage self-balancing mode.

[0098] In the resonant capacitor voltage self-balancing mode, the controller controls the high-side resonant power tube to remain off and controls the low-side switching power tube to be periodically turned on and off.

[0099] Wherein, in each cycle when the low-side switch power tube is turned on, the controller detects the voltage Vcs on the primary-side detection resistor Rcs during the conduction period of the low-side switch power tube, and compares it with the avalanche breakdown current detection threshold voltage Vcs (srbv), and generates a control signal srbv as a first level signal or a second level signal according to the comparison result;

[0100] When the voltage Vcs is not detected to be lower than the avalanche breakdown current detection threshold voltage Vcs (srbv) before the low-side switching power tube reaches the maximum conduction time Tonmax for two consecutive cycles, the controller starts the current source after the low-side switching power tube is turned off in the current cycle, charges the resonant capacitor with a constant current according to the set charging time to increase the resonant capacitor voltage Vcr, and turns off the current source after the set charging time is reached; here, when Vcs is higher than the avalanche breakdown current detection threshold voltage Vcs (srbv), it indicates that the resonant capacitor voltage Vcr is too low during the conduction period of the low-side switching power tube, causing the freewheeling diode to avalanche breakdown and generate reverse current.

[0101] Specifically, when Vcs≥Vcs(srbv) occurs before the low-side switching power tube reaches the maximum conduction time Tonmax within a cycle, it indicates that the freewheeling diode avalanche breakdown occurs in the current cycle, and the control signal srbv generated by the input overcurrent detection module is a control signal of the first level; after the low-side switching power tube is turned off in the current cycle, the current source is started according to the control signal of the first level, and the resonant capacitor is charged with a constant current according to the set charging time; after the constant current charging is completed, the count value of the low-level counter is set to 0, and the control signal srbv is reset to the second level.

[0102] When Vcs<Vcs(srbv) is always maintained before the low-side switch power tube reaches the maximum conduction time Tonmax within a cycle, it indicates that no avalanche breakdown of the freewheeling diode occurs in the current cycle, and the control signal srbv generated by the input overcurrent detection module in the controller is a control signal of the second level. The controller obtains the current count value of the low-level counter, and adds 1 to the count value according to the control signal srbv of the second level as the updated current count value; when the updated current count value reaches 2, it is determined that the voltage Vcs is detected to be continuously lower than the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum conduction time Tonmax for two consecutive cycles (that is, the voltage Vcs is not detected to be greater than or equal to the avalanche breakdown current detection threshold voltage Vcs(srbv)); otherwise, it means that the voltage Vcs is not detected to be lower than the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum conduction time Tonmax for two consecutive cycles.

[0103] When the voltage Vcs is detected to be lower than the avalanche breakdown current detection threshold voltage Vcs(srbv) in two consecutive cycles, the controller exits the resonant capacitor voltage self-balancing mode and resumes the normal pulse width modulation (PWM) control mode.

[0104] When the controller exits the resonant capacitor voltage self-balancing mode, the drive control input signal det_bv is set to the second level.

[0105] In the specific implementation of this embodiment, the first level is defined as a high level signal and the second level is defined as a low level signal. However, those skilled in the art will appreciate that the first level and the second level may be set oppositely as long as the corresponding control logic is defined in the logic control module.

[0106] In other words, when the power-on process reaches the starting voltage, the controller's drive control input signal det_bv is high, controlling the flyback converter to stop normal operation and start the resonant capacitor voltage self-balancing mode. At this time, the flyback converter's switch tube drive signal GTH is always low, controlling the primary-side high-side resonant power tube QH to be normally off. The switch tube drive signal GTL controls the low-side switch power tube QL to periodically turn on or off, and the voltage amplitude of the drive signal GTL slowly increases during each conduction cycle.

[0107] In the present invention, the controller is configured to periodically turn on the low-side switching power tube in the resonant capacitor voltage self-balancing mode, and apply a drive current to the low-side switching power tube with the set capacitor voltage self-balancing mode; at the same time, the resonant capacitor Cr is gradually charged in each conduction cycle of the low-side switching power tube, so that the resonant capacitor voltage Vcr increases cycle by cycle, the voltage difference across the resonant inductor Lr at the moment of conduction of the low-side switching power tube gradually decreases, and the rising slope of the reverse current flowing through the low-side switching power tube decreases, thereby slowing down the rising speed of the voltage Vcs, and extending the time required to reach the avalanche breakdown current detection threshold voltage Vcs (srbv), so that the conduction time of the low-side switching power tube gradually increases; thereby, the driving voltage of the low-side switching power tube gradually increases in each cycle.

[0108] Further explanation is as follows. When the high-side power tube GH is turned off, the circuit is as follows Figure 7 As shown. During the power-on process, when the low-side switch power tube is turned on for the first time, the secondary freewheeling diode avalanche breakdown occurs. At this time, Vs is clamped, and the corresponding primary side Vlpn is also clamped. Assuming that the value of Vcr is 0 at this time, Vcrp=V HB When QL is turned on, the current flowing through the resonant inductor Lr is ILr, and ILr is equal to the reverse current Ics on the detection resistor. The slope of the reverse current is Where Lr is the value of the resonant inductor, and Ulr is Vlpn - Vcrp. As the resonant capacitor Cr charges cycle by cycle, when the switching power tube turns on and the diode avalanche breakdown occurs, Vcrp gradually increases, while Ulr gradually decreases, and therefore the rising slope of the reverse current also gradually decreases.

[0109] As the rising slope of the reverse current gradually decreases, the slope of the voltage Vcs also gradually decreases, and the time it takes to reach the threshold voltage Vcs(srbv) gradually increases. Once Vcs ≥ Vcs(srbv), the low-side switch power tube is immediately turned off. At this time, the low-side power tube drive current remains unchanged, the low-side power tube GL conduction time becomes longer, and the voltage amplitude of its drive signal GTL also increases cycle by cycle. This process is a closed-loop control and is determined by the reverse current.

[0110] Therefore, when the low-side power switch GL is on, if the resonant capacitor voltage Vcr is significantly less than n×Vout, causing avalanche breakdown of the secondary freewheeling diode, the secondary freewheeling diode generates a large reverse current due to the low impedance of the primary-side Cr in series with Lr. The current flowing through the primary-side resonant inductor under the action of the transformer generates a voltage Vcs across the sense resistor Rcs that exceeds the threshold voltage Vcs(srbv). Then, after the low-side power switch QL is turned off, the current source is turned on to charge the resonant capacitor for a certain period of time before turning off. Because the current source Ic is a DC source, the primary magnetizing inductor and the resonant inductor are short-circuited during the charging period, preventing the secondary freewheeling diode from experiencing avalanche breakdown. This method charges the resonant capacitor Cr, increasing the Vcr voltage. When the low-side power switch QL is turned on in the next cycle, the anode voltage Vcrp of the resonant capacitor is increased, reducing the voltage across the magnetizing inductor Vlp. The above process repeats in the next cycle. Until Vcs falls below the threshold voltage Vcs(srbv), the overcurrent detection module outputs a low-level srbv signal, exiting the resonant capacitor self-balancing mode. The present invention can also improve charging speed by adjusting the current source output algorithm in the controller to complete charging of the resonant capacitor in fewer cycles.

[0111] Figure 8 This is a signal diagram showing an example of voltage imbalance in the resonant capacitor of a conventional AHB flyback converter in an LED application. It can be seen that at initial power-up, the resonant capacitor voltage Vcr is close to zero, while n×Vout is relatively high. When the drive signal GTL is high, the low-side power switch turns on. At this point, the voltage Vlp across the primary inductor Lp is essentially equal to the input voltage. This voltage, converted to the secondary side, subjects the freewheeling diode to a reverse voltage of Vout + Vin / n. At this point, the secondary freewheeling diode experiences avalanche breakdown, generating a large reverse current ID. Under the influence of the transformer, a large current ILr also flows through the primary resonant inductor. The freewheeling diode exits avalanche breakdown only when the voltage Vcr on the resonant capacitor Cr reaches a critical value. Without a self-balancing resonant capacitor voltage mode, the resonant capacitor Cr would require several cycles to charge. Prior to this, the secondary freewheeling diode would experience avalanche breakdown during each on-time period of the power switch. If the heat generated exceeds the diode's maximum power dissipation, the probability of freewheeling diode damage increases significantly, shortening the system's operating life.

[0112] As can be seen, in a structure without a resonant capacitor voltage self-balancing mode, the resonant capacitor voltage takes a long time to balance, causing the freewheeling diode to avalanche breakdown for a long time and generate high avalanche energy. The heat generated exceeds the maximum power dissipation of the diode, which may cause permanent damage to the diode. The AHB flyback converter using the technical solution of the present invention can effectively solve the above problems. Figure 9Schematic diagram of the avalanche breakdown current detection principle taking an AHB flyback converter as an example according to an embodiment of the present invention. Figure 10 Shown is a signal diagram of an example of the discharge principle of an AHB flyback converter provided by an embodiment of the present invention. Figure 9 The arrow directions of ILp, Ics, ILr and ID are positive, and the voltages at both ends of the primary excitation inductor Lp, Cr and Rcs are positive and negative as shown in the following table: Figure 7 Indicated by plus and minus signs. Figure 9 and Figure 10 The detection principle and algorithm of the present invention are further explained.

[0113] Upon power-up, the system enters self-balancing mode, outputting a high-level drive control input signal det_bv. This controls the high-side resonant power transistor QH to be normally off, while periodically turning on the low-side switching power transistor QL. The resonant capacitor voltage Vcr approaches zero, clamping the output voltage Vout to a higher voltage. During the periodic on-state of the low-side switching power transistor QL, the high-level amplitude of the low-side transistor drive signal GTL increases slowly cycle by cycle to limit the switch's conduction speed and minimize current spikes. When the low-side switching power transistor QL turns on, the primary inductor begins to magnetize, and ILp increases linearly in the forward direction. At this point, the primary-side inductor voltage VLp is essentially equal to the input voltage Vin. Under the action of the transformer Tr, the freewheeling diode reverse voltage is Vout + Vin / n. If this voltage exceeds the diode's reverse withstand voltage threshold, the diode undergoes avalanche breakdown, generating a large reverse current ID. This current flows from the secondary MOSFET terminal, is converted to the primary MOSFET terminal, and flows through the resonant inductor Lr with a magnitude of ID / n. The resonant inductor current ILr flows through the primary-side loop, flowing through the sense resistor Rcs with ILr = Ics. This generates a negative voltage Vcs across Rcs, which is then compared with the set voltage threshold Vcs(srbv). If the voltage exceeds the set threshold, a high-level control signal srbv is output after the low-side power switch QL turns off, and the current source is turned on to provide a charging current Ic to charge the resonant capacitor Cr. This raises the resonant capacitor voltage Vcr, thereby reducing the reverse voltage on the secondary freewheeling diode when the low-side power switch QL turns on in the next cycle. After the preset charging time Tc is reached, the current source is turned off and a low-level control signal srbv is output. This operation is repeated until the next cycle arrives. If the voltage Vcs at the Rcs terminal is detected to be less than the set threshold voltage Vcsh(srbv) for two consecutive cycles, a low-level control signal det_bv is output, exiting the resonant capacitor voltage self-balancing mode and outputting a normal PWM signal.

[0114] By comparison Figure 8 and Figure 10It can be seen that the application of the AHB flyback converter circuit of the present invention can greatly reduce the avalanche breakdown time and avalanche energy of the freewheeling diode.

[0115] The technical solution of the present invention can effectively prevent the reverse voltage on the secondary freewheeling diode from exceeding its reverse withstand voltage threshold at the beginning of power-on, causing the diode to avalanche breakdown. In particular, when AHB is applied to LED scenarios, due to the imbalance between the resonant capacitor voltage and the output refracted voltage n×Vout after plugging in, the secondary freewheeling diode will avalanche breakdown when the switching power tube is turned on, resulting in the problem of high reverse current.

[0116] The technical solution of the present invention automatically enters the self-balancing mode during the system power-on startup phase. In this mode, only the low-side switch power tube is periodically turned on, and the driving voltage is slowly increased to reduce the transient spikes and avalanche currents during the conduction process; the controller detects the voltage Vcs on the primary side detection resistor Rcs in each conduction cycle, and compares it with the avalanche breakdown current detection threshold Vcs (srbv) to determine whether a diode avalanche occurs; if no avalanche breakdown signal is detected for two consecutive cycles, the controller determines that it is a safe state, exits the self-balancing mode, and resumes normal PWM control; if the exit condition is not met, the constant current source is started after the end of each cycle, and the resonant capacitor is charged according to the set time, thereby increasing the resonant capacitor voltage Vcr and accelerating the reaching of a stable state. In the present invention, the charging time or charging rate of the resonant capacitor Cr can be dynamically adjusted by the internal setting of the system, which can effectively speed up the recovery speed of the resonant capacitor voltage, thereby reducing the reverse voltage borne by the freewheeling diode during the conduction of the switch tube, avoiding long-term avalanche breakdown and accumulating high avalanche energy. This invention effectively reduces diode reverse overvoltage caused by a low Vcr by automatically adjusting the resonant capacitor charging process. This avoids the cost increases associated with using high-voltage components or the reduced efficiency associated with connecting energy-consuming loads in parallel, as is often the case with traditional solutions. The method and circuit of this invention not only prevent device damage caused by avalanche breakdown during startup, improving system reliability, stability, and lifespan, but also allows the use of diodes with lower voltage and cost, resulting in a highly efficient and cost-effective system design.

[0117] Professionals should also be further aware that the units and algorithm steps of each example described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, computer software, or a combination of the two. In order to clearly illustrate the interchangeability of hardware and software, the above description has generally described the components and steps of each example according to their functions. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professionals and technicians can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of the present invention.

[0118] The steps of the methods or algorithms described in conjunction with the embodiments disclosed herein may be implemented using hardware, a software module executed by a processor, or a combination of the two. The software module may be placed in a random access memory (RAM), a memory, a read-only memory (ROM), an electrically programmable ROM, an electrically erasable programmable ROM, a register, a hard disk, a removable disk, a CD-ROM, or any other form of storage medium known in the art.

[0119] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preventing avalanche breakdown of a freewheeling diode in an asymmetric half-bridge AHB flyback converter, characterized in that: The method comprises: Perform power-on detection on the system. When it is detected that the system is powered on, the controller enters the resonant capacitor voltage self-balancing mode; In the resonant capacitor voltage self-balancing mode, the controller controls the high-side resonant power tube to remain off, and controls the low-side switching power tube to be periodically turned on and off; Wherein, in each cycle when the low-side switch power tube is turned on, the controller detects the voltage Vcs on the primary-side detection resistor Rcs during the period when the low-side switch power tube is turned on, and compares it with the avalanche breakdown current detection threshold voltage Vcs (srbv); When the voltage Vcs is not detected to be lower than the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum conduction time in two consecutive cycles, the controller starts the current source after the low-side switch power tube is turned off in the current cycle, charges the resonant capacitor with a constant current according to the set charging time to increase the resonant capacitor voltage Vcr, and turns off the current source after the set charging time is reached; wherein, when Vcs is higher than the avalanche breakdown current detection threshold voltage Vcs(srbv), it indicates that the resonant capacitor voltage Vcr is low during the conduction period of the low-side switch power tube, causing the freewheeling diode to avalanche breakdown and generate reverse current; When the voltage Vcs is not detected to be greater than or equal to the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum on-time Tonmax for two consecutive cycles, the controller exits the resonant capacitor voltage self-balancing mode and resumes the normal pulse width modulation PWM control mode.

2. The method for preventing avalanche breakdown of a freewheeling diode in an asymmetric half-bridge AHB flyback converter according to claim 1, wherein: The power-on detection of the system, when detecting that the system is powered on and started, the controller enters the resonant capacitor voltage self-balancing mode, includes: The power-on detection module monitors the system input voltage Vin; when the input voltage Vin reaches a preset threshold, causing the controller's supply voltage Vcc to reach a start-up voltage Vcc(st), the module outputs a first level drive control input signal det_bv; When the controller receives the driving control input signal det_bv of the first level, it enters the resonant capacitor voltage self-balancing mode.

3. The method for preventing avalanche breakdown of a freewheeling diode in an asymmetric half-bridge AHB flyback converter according to claim 2, wherein: When the controller exits the resonant capacitor voltage self-balancing mode, the drive control input signal det_bv is set to a second level.

4. The method for preventing avalanche breakdown of a freewheeling diode in an asymmetric half-bridge AHB flyback converter according to claim 1, wherein: The controller is configured as follows: in the resonant capacitor voltage self-balancing mode, periodically turning on the low-side switch power tube, and applying a drive current set in the capacitor voltage self-balancing mode to the low-side switch power tube; at the same time, the resonant capacitor Cr is gradually charged in each cycle in which the low-side switch power tube is turned on, so that the resonant capacitor voltage Vcr increases cycle by cycle, and the rising slope of the reverse current flowing through the low-side switch power tube decreases, thereby slowing down the rising speed of the voltage Vcs, and extending the time required to reach the avalanche breakdown current detection threshold voltage Vcs (srbv), so that the conduction time of the low-side switch power tube gradually increases; thereby, the drive voltage of the low-side switch power tube gradually increases in each cycle, thereby slowing down the reverse current impact and reducing the transient spikes caused by the avalanche current until the set gate drive voltage maximum value is reached, or until the self-balancing mode is exited.

5. The method for preventing avalanche breakdown of a freewheeling diode in an asymmetric half-bridge AHB flyback converter according to claim 1, wherein: The controller detects the voltage Vcs on the primary side detection resistor Rcs during the conduction period of the low-side switch power tube during each cycle of the low-side switch power tube being turned on, and compares the voltage Vcs with the avalanche breakdown current detection threshold voltage Vcs (srbv) in detail including: In each cycle when the low-side switch power tube is turned on, the controller detects the voltage Vcs on the primary-side detection resistor Rcs during the current conduction period, and compares it with the avalanche breakdown current detection threshold voltage Vcs (srbv), and generates a control signal srbv as a first level signal or a second level signal according to the comparison result; When Vcs<Vcs(srbv) is always maintained before the low-side switch power tube reaches the maximum on-time Tonmax in a cycle, it indicates that the freewheeling diode avalanche breakdown does not occur in the current cycle, the current count value of the low-level counter is obtained, and the count value is increased by 1 as the updated current count value; wherein the initial count value of the low-level counter is 0; When Vcs≥Vcs(srbv) occurs before the low-side switch power tube reaches the maximum on-time Tonmax in one cycle, it indicates that the freewheeling diode avalanche breakdown occurs in the current cycle, and the count value of the low-level counter is set to 0; When the updated current count value reaches 2, it is determined that the voltage Vcs is not detected to be greater than or equal to the avalanche breakdown current detection threshold voltage Vcs (srbv) before the low-side switch power tube reaches the maximum conduction time Tonmax for two consecutive cycles; otherwise, the voltage Vcs is not detected to be lower than the avalanche breakdown current detection threshold voltage Vcs (srbv) before the low-side switch power tube reaches the maximum conduction time Tonmax for two consecutive cycles.

6. The method for preventing avalanche breakdown of a freewheeling diode in an asymmetric half-bridge AHB flyback converter according to claim 5, wherein: The controller starts the current source after the low-side switch power tube is turned off in the current cycle, and performs constant current charging to the resonant capacitor according to the set charging time, specifically including: When Vcs≥Vcs(srbv), the control signal srbv generated is a control signal of the first level; after the low-side switch power tube is turned off in the current cycle, the current source is started according to the control signal of the first level, and the resonant capacitor is charged with a constant current according to the set charging time; When Vcs<Vcs(srbv), the generated control signal srbv is a control signal of a second level; and the count value is increased by 1 according to the control signal of the second level.

7. A flyback converter circuit, characterized in that: The flyback conversion circuit includes: A flyback converter includes a primary-side circuit and a secondary-side circuit; the primary-side circuit includes a high-side resonant power transistor, a low-side switching power transistor, a resonant capacitor, a primary winding, and a detection resistor; the high-side resonant power transistor, the resonant capacitor, and the primary winding constitute a resonant cavity loop; the detection resistor is disposed outside the resonant cavity loop; the secondary-side circuit is inductively coupled to the primary-side circuit via the secondary winding and outputs a DC voltage signal to a load; Power-on detection module, used to monitor the system input voltage Vin and generate drive control input signals; a controller, determining whether to enter a resonant capacitor voltage self-balancing mode according to the drive control input signal; In the resonant capacitor voltage self-balancing mode, the controller controls the high-side resonant power tube to remain turned off, and controls the low-side switching power tube to be periodically turned on and off; Wherein, in each cycle when the low-side switch power tube is turned on, the controller detects the voltage Vcs on the primary-side detection resistor Rcs during the period when the low-side switch power tube is turned on, and compares it with the avalanche breakdown current detection threshold voltage Vcs (srbv); When the voltage Vcs is not detected to be lower than the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum on-time Tonmax in two consecutive cycles, the controller starts the current source after the low-side switch power tube is turned off in the current cycle, charges the resonant capacitor with a constant current according to the set charging time to increase the resonant capacitor voltage Vcr, and turns off the current source after the set charging time is reached; wherein, when Vcs is higher than the avalanche breakdown current detection threshold voltage Vcs(srbv), it indicates that the resonant capacitor voltage Vcr is too low during the conduction period of the low-side switch power tube, causing the freewheeling diode to avalanche breakdown and generate reverse current; When the voltage Vcs is not detected to be greater than or equal to the avalanche breakdown current detection threshold voltage Vcs(srbv) before the low-side switch power tube reaches the maximum on-time Tonmax for two consecutive cycles, the controller exits the resonant capacitor voltage self-balancing mode and resumes the normal pulse width modulation PWM control mode.

8. The flyback converter circuit according to claim 7, wherein: The controller includes: an input overcurrent detection module, a control logic module, a driving module and the current source; The first input terminal of the input overcurrent detection module is connected to a preset avalanche breakdown current detection threshold voltage Vcs(srbv), and the second input terminal is connected to the voltage Vcs on the primary-side detection resistor Rcs; when Vcs≥Vcs(srbv), the control signal srbv is generated as a control signal of a first level; when Vcs<Vcs(srbv), the control signal srbv is generated as a control signal of a second level; The first input terminal of the control logic module is connected to the driving control input signal det_bv, and the second input terminal is connected to the control signal srbv; the control logic module outputs corresponding driving module control signals GH and GL according to the driving control input signal det_bv and the control signal srbv; The driving module outputs switch tube driving signals GTH and GTL according to the driving module control signals GH and GL; The current source receives the control signal srbv. When the control signal srbv is a control signal of the first level, after the low-side switch power tube is turned off in the current cycle, the current source performs constant current charging to the resonant capacitor according to the set charging time.

9. The flyback converter circuit according to claim 8, wherein: The control module includes a low-level counter, wherein the initial count value of the low-level counter is 0; When the control signal srbv is a control signal of a second level, the control module adds 1 to the count value stored in the low-level counter according to the control signal of the second level.

10. The flyback converter circuit according to claim 7, wherein: The flyback converter is specifically a flyback converter based on a half-bridge resonant AHB topology.

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