Flyback converter, overvoltage protection method and fast charging power supply

Abstract

The invention provides a flyback converter, an overvoltage protection method and a fast charging power supply. The overvoltage protection method comprises the following steps: judging whether a primary side controller receives a first control signal transmitted by a secondary side controller or not; if not, the primary side controller obtains a first feedback signal representing the output voltage and starts output overvoltage protection according to the first feedback signal and a preset protection threshold value; wherein the preset protection threshold value is set according to the transformer turn ratio of the flyback converter, the output voltage in the starting stage and different output voltage ranges, so that overvoltage protection in the different output voltage ranges is realized. In the invention, the primary side feedback control is adopted in the starting stage to carry out the output overvoltage protection, and the secondary side feedback control is adopted in the subsequent process to carry out the output overvoltage protection. Meanwhile, the set preset protection threshold is suitable for output overvoltage protection in different output ranges, so that the preset protection threshold does not need to be adjusted when the output ranges change, the control complexity is reduced, and the applicability of the method is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of power electronics, and in particular relates to a flyback converter, an overvoltage protection method, and a fast charging power supply. Background Art

[0002] In power supply design, the flyback converter generally uses primary-side feedback control or secondary-side feedback control for output overvoltage protection. Under primary-side feedback control, an auxiliary coil is used to extract the output voltage signal, and the control chip on the primary side compares the output voltage signal with the set threshold to determine whether to start the output overvoltage protection. Under secondary-side feedback control, the output voltage signal is sampled directly on the secondary side of the transformer, and the output voltage signal is coupled to the primary side. The control chip on the primary side compares the output voltage signal with the set threshold to determine whether to start the output overvoltage protection. Alternatively, the control chip on the secondary side compares the output voltage signal with the set threshold to obtain an overvoltage protection control signal, and then couples the overvoltage protection control signal to the primary side, and the control chip on the primary side starts the output overvoltage protection according to the overvoltage protection control signal.

[0003] In the existing design of the flyback converter, generally one of the primary feedback control and the secondary feedback control is selected. However, the above design has the following problems: (1) If the flyback converter adopts the primary feedback control, the turns ratio of the auxiliary winding generally changes with the output range. Therefore, when the output range changes, it is necessary to adjust the set threshold and the setting parameters of the peripheral circuit, which increases the complexity of the control; (2) If the flyback converter adopts the secondary feedback control, the primary side will run in an open loop for a period of time during the startup phase. During this period, the primary side cannot receive the output voltage information, which may cause the secondary side output to be too high and damage the components. Summary of the invention

[0004] In order to solve the technical problems existing in the prior art, the present invention provides a flyback converter and an overvoltage protection method and a fast charging power supply, comprising the steps of:

[0005] Determine whether the primary-side controller receives a first control signal transmitted by the secondary-side controller, and the secondary-side controller generates the first control signal according to the output feedback signal and the reference signal;

[0006] If not, the primary-side controller obtains a first feedback signal representing the output voltage, and determines whether the output of the flyback converter is over-voltage according to the first feedback signal and a preset protection threshold;

[0007] If the output is over-voltage, the output over-voltage protection is activated;

[0008] Among them, the preset protection threshold is set according to the transformer turns ratio, preset output voltage and different output voltage ranges of the flyback converter to achieve overvoltage protection within different output voltage ranges, and the preset output voltage represents the output voltage of the flyback converter during the startup phase.

[0009] Further, the different output voltage ranges include N different output voltage ranges, and the preset protection threshold satisfies: N_x*Vset<VT<N_x*Vmax_x;

[0010] Among them, N_x is the primary-to-secondary turns ratio when the output of the flyback converter is set in the xth output voltage range, Vset is the preset output voltage, Vmax_x is the upper limit value of the xth output voltage range, VT is the preset protection threshold, and x is 1-N.

[0011] Furthermore, it also includes: if the primary side controller receives the first control signal, the primary side controller determines whether to start the output overvoltage protection according to the first control signal.

[0012] Preferably, the primary-side controller obtains the first feedback signal according to the drain terminal voltage of the main switch tube, or obtains the first feedback signal according to the volt-second balance.

[0013] A flyback converter includes a primary side controller and a secondary side controller.

[0014] The secondary-side controller generates a first control signal according to the output feedback signal and the reference signal and couples the first control signal to the primary-side controller;

[0015] The primary side controller includes:

[0016] A first control module obtains a first feedback signal representing an output voltage, and starts output overvoltage protection according to the first feedback signal and a preset protection threshold;

[0017] A second control module, starting output overvoltage protection according to the first control signal;

[0018] The switching control module controls the first control module to work or the second control module to work according to the first control signal;

[0019] Among them, the preset protection threshold is set according to the transformer turns ratio, preset output voltage and different output voltage ranges of the flyback converter to achieve overvoltage protection within different output voltage ranges, and the preset output voltage represents the output voltage of the flyback converter during the startup phase.

[0020] Further, the different output voltage ranges include N different output voltage ranges, and the preset protection threshold satisfies: N_x*Vset<VT<N_x*Vmax_x;

[0021] Among them, N_x is the primary-to-secondary turns ratio when the output of the flyback converter is set in the xth output voltage range, Vset is the preset output voltage, Vmax_x is the upper limit value of the xth output voltage range, VT is the preset protection threshold, and x is 1-N.

[0022] Furthermore, the first control module includes:

[0023] A first feedback signal generating circuit, which obtains the first feedback signal according to the drain terminal voltage of the main switch tube, or obtains the first feedback signal according to the volt-second balance;

[0024] A comparison circuit, comparing the first feedback signal with the preset protection threshold to output an overvoltage judgment signal;

[0025] The driving circuit stops driving the primary side main power tube according to the overvoltage judgment signal to start output overvoltage protection.

[0026] Preferably, the first feedback signal generating circuit obtains the first feedback signal according to the volt-second balance, including:

[0027] A first conversion circuit converts an input voltage into a first current;

[0028] The first integration unit obtains the ampere-second product of the first current during the conduction period of the main switch tube, and the ampere-second product of the first current during the conduction period of the main switch tube is the first feedback signal.

[0029] Furthermore, the first control module also includes:

[0030] A second conversion circuit converts the preset protection threshold into a second current;

[0031] The second integration unit obtains the ampere-second product of the second current during the freewheeling period, where the ampere-second product of the second current during the freewheeling period represents the preset protection threshold.

[0032] A fast charging power source comprises the flyback converter described above.

[0033] The present invention proposes a design that combines primary-side feedback control and secondary-side feedback control in a flyback converter, so that primary-side feedback control is used in the startup phase to perform output overvoltage protection, and secondary-side feedback control is used in the subsequent process to perform output overvoltage protection. At the same time, the present invention also proposes setting a preset protection threshold value so that the preset protection threshold value is suitable for output overvoltage protection under different output ranges. Therefore, when the output range changes, there is no need to adjust the preset protection threshold value and the setting parameters of the peripheral circuit, which greatly reduces the complexity of the control and improves the applicability of the method. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] Figure 1A flow chart of the overvoltage protection method proposed by the present invention;

[0035] Figure 2 The waveform diagram of the voltage at the drain terminal of the main switch tube;

[0036] Figure 3 A structural block diagram of the flyback converter proposed by the present invention;

[0037] Figure 4 It is the detailed block diagram of the primary side controller;

[0038] Figure 5 FIG. 4 is a circuit structure diagram of a first control module in an embodiment. DETAILED DESCRIPTION

[0039] In order to facilitate the understanding of the present invention, the present invention will be described more fully below with reference to the relevant drawings. The preferred embodiments of the present invention are shown in the drawings. However, the present invention can be implemented in different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0040] In power supply design, the flyback converter generally uses primary-side feedback control or secondary-side feedback control for output overvoltage protection. Under primary-side feedback control, an auxiliary coil is used to extract the output voltage signal, and the control chip on the primary side compares the output voltage signal with the set threshold to determine whether to start the output overvoltage protection. Under secondary-side feedback control, the output voltage signal is sampled directly on the secondary side of the transformer, and the output voltage signal is coupled to the primary side. The control chip on the primary side compares the output voltage signal with the set threshold to determine whether to start the output overvoltage protection. Alternatively, the control chip on the secondary side compares the output voltage signal with the set threshold to obtain an overvoltage protection control signal, and then couples the overvoltage protection control signal to the primary side, and the control chip on the primary side starts the output overvoltage protection according to the overvoltage protection control signal.

[0041] In the existing design of the flyback converter, generally one of the primary feedback control and the secondary feedback control is selected. However, the above design has the following problems: (1) If the flyback converter adopts the primary feedback control, the turns ratio of the auxiliary winding generally changes with the output range. Therefore, when the output range changes, it is necessary to adjust the set threshold and the setting parameters of the peripheral circuit, which increases the complexity of the control; (2) If the flyback converter adopts the secondary feedback control, the primary side will run in an open loop for a period of time during the startup phase. During this period, the primary side cannot receive the output voltage information, which may cause the secondary side output to be too high and damage the components.

[0042] Therefore, considering the defects in the prior art, the present invention proposes an overvoltage protection method for a flyback converter, comprising the steps of:

[0043] Start the flyback converter;

[0044] Determine whether the primary-side controller receives a first control signal transmitted by the secondary-side controller, wherein the secondary-side controller generates the first control signal according to the output feedback signal and the reference signal;

[0045] If the primary side controller does not receive the first control signal, the primary side controller obtains a first feedback signal representing the output voltage, and determines whether the output of the flyback converter is over-voltage according to the first feedback signal and a preset protection threshold;

[0046] If the output is judged to be over-voltage, the output over-voltage protection is activated.

[0047] In particular, the preset protection threshold is set according to the primary-to-secondary turns ratio of the flyback converter, the preset output voltage and different output voltage ranges, so that the preset protection threshold is suitable for output overvoltage judgment within different output voltage ranges, and the preset output voltage represents the output voltage of the flyback converter during the startup phase.

[0048] In summary, the present invention proposes a design that combines primary-side feedback control and secondary-side feedback control for the flyback converter, so that primary-side feedback control is used in the startup phase to perform output overvoltage protection, and secondary-side feedback control is used in the subsequent process to perform output overvoltage protection. At the same time, the present invention also proposes setting a preset protection threshold so that the preset protection threshold is suitable for output overvoltage protection under different output ranges. Therefore, when the output range changes, there is no need to adjust the preset protection threshold and the setting parameters of the peripheral circuit, which greatly reduces the complexity of the control and improves the applicability of the method.

[0049] Specifically, Figure 1 As shown, the flyback converter is started, the secondary side controller directly samples the output feedback signal at the secondary side of the converter, the secondary side controller generates a first control signal according to the output feedback signal and the reference signal, and transmits the first control signal to the primary side controller through the isolation device;

[0050] If the primary controller does not receive the first control signal, the primary controller obtains a first feedback signal representing the output voltage, and determines whether the output is over-voltage by comparing the first feedback signal with a preset protection threshold, and starts output over-voltage protection if the output is over-voltage;

[0051] If the primary side controller receives the first control signal, the primary side controller determines whether the output is over-voltage according to the first control signal, and starts output over-voltage protection if the output is over-voltage.

[0052] Among them, the preset protection threshold is set according to the primary-secondary turns ratio of the flyback converter, the preset output voltage, and multiple different output voltage ranges (there are overlapping areas in the multiple different output voltage ranges), so that the set preset protection threshold can be applicable to overvoltage protection under multiple different output voltage ranges.

[0053] Specifically, the output of the flyback converter can be set in any one of the first output voltage range V0-Vmax_1, the second output voltage range V0-Vmax_2...Nth output voltage range V0-Vmax_N, and the preset protection threshold VT satisfies: N_1*Vset<VT<N_1*Vmax_1, N_2*Vset<VT<N_2*Vmax_2...N_N*Vset<VT<N_N*Vmax_N. Among them, N_1 is the primary-secondary turns ratio when the output of the flyback converter is set in the first output voltage range V0-Vmax_1, N_2 is the primary-secondary turns ratio when the output of the flyback converter is set in the second output voltage range V0-Vmax_2, N_N is the primary-secondary turns ratio when the output of the flyback converter is set in the Nth output voltage range V0-Vmax_N, and Vset is the preset output voltage. Therefore, the preset protection threshold VT can be applied to the output overvoltage protection in the first output voltage range V0-Vmax_1, the second output voltage range V0-Vmax_2, ... the Nth output voltage range V0-Vmax_N. That is, when the output of the flyback converter is set in the xth output voltage range V0-Vmax_x, the primary side controller can determine whether the output is overvoltage according to the preset protection threshold VT and the first feedback signal, and x is any value from 1 to N.

[0054] In one embodiment, the primary side controller obtains a first feedback signal representing the output voltage through volt-second balance. Figure 5 As shown, the first capacitor is charged with a first current during the conduction period of the main switch tube. The first current is proportional to the input voltage, and the charging charge (ampere-second product) of the first capacitor is I1*Ton=k*Vin*Ton=k*N*Vout*Toff, that is, the ampere-second product of the first capacitor can represent the first feedback signal k*Vin*Ton=k*N*Vout*Toff of the output voltage.

[0055] In another embodiment, the primary side controller obtains the first feedback signal representing the output voltage by sampling the drain voltage of the main switch tube. Figure 2 As shown, the drain voltage when the main switch is turned off is Vin+N*Vout. Therefore, a subtraction circuit is designed, the first input end of the subtraction circuit receives the input voltage Vin, and the other input end receives the drain voltage Vin+N*Vout when the main switch is turned off, so that the output end of the subtraction circuit generates a first feedback signal N*Vout representing the output voltage.

[0056] In other embodiments, the primary-side controller may obtain a first feedback signal representing the output voltage through an auxiliary winding.

[0057] Therefore, the flyback converter adopts primary feedback control for output overvoltage protection in the startup phase, and adopts secondary feedback control for output overvoltage protection in the subsequent process, so as to avoid the flyback converter being in a state without overvoltage protection due to open-loop operation in the startup phase. At the same time, the preset protection threshold is suitable for output overvoltage protection under different output voltage ranges, so that there is no need to adjust the preset protection threshold and the setting parameters of the peripheral circuit when the output voltage range changes, which greatly reduces the complexity of control and improves the applicability of the method.

[0058] It should be noted that the above overvoltage protection method is not limited to use in the open-loop operation of the primary side during the startup phase. In other situations, such as when a fault causes the secondary side controller to be unable to transmit the first control signal to the primary side controller, the above overvoltage protection method is also applicable.

[0059] like Figure 3 and Figure 4 As shown, the present invention also proposes a flyback converter, the flyback converter includes: a primary side controller and a secondary side controller, wherein:

[0060] The secondary-side controller samples the output feedback signal at the secondary side of the flyback converter, generates a first control signal according to the output feedback signal and a reference signal, and transmits the first control signal to the primary-side controller through an isolation device;

[0061] A first control module of the primary-side controller is used to obtain a first feedback signal representing the output voltage, and start output overvoltage protection according to the first feedback signal and a preset protection threshold;

[0062] A second control module of the primary side controller, configured to start output overvoltage protection according to the first control signal;

[0063] The switching control module of the primary side controller controls the second control module to work when receiving the first control signal, and controls the first control module to work when not receiving the first control signal;

[0064] In particular, the preset protection threshold is set according to the primary-to-secondary turns ratio of the flyback converter, the preset output voltage and different output voltage ranges, so that the preset protection threshold is suitable for output overvoltage judgment within different output voltage ranges, and the preset output voltage represents the output voltage of the flyback converter during the startup phase.

[0065] Specifically, the output of the flyback converter can be set in any one of N output voltage ranges, and the preset protection threshold VT satisfies: N_x*Vset<VT<N_x*Vmax_x, Vmax_x is the upper limit value of the x-th output voltage range V0-Vmax_x, N_x is the primary-secondary turns ratio when the output is set in the x-th output voltage range, Vset is the preset output voltage, and x is any value between 1-N. The preset protection threshold VT set in this way can be applicable to overvoltage protection under N output voltage ranges.

[0066] In one embodiment, the above-mentioned flyback converter is applied to a fast charging power supply. When the output voltage range of the fast charging power supply is 3.3V-12V, the primary-to-secondary turns ratio N of the transformer is usually 10-16, and the output voltage of the fast charging power supply is 5V by default during the startup stage. At this time, the preset protection threshold VT satisfies: N*Vset<VT≤N*Vmax. If N is 10, then 50<VT≤120; if N is 11, then 55<VT≤132; if N is 12, then 60<VT≤144; if N is 13, then 65<VT≤156; if N is 14, then 70<VT≤168; if N is 15, 75<VT≤180; if N is 16, then 80<VT≤192. When the output voltage range of the fast charging power supply is 3.3V-21V, the primary-to-secondary turns ratio N of the transformer is usually 6-10, and the output voltage of the fast charging power supply is 5V by default during the startup stage. At this time, the preset protection threshold VT satisfies: N*Vset<VT≤N*Vmax. If N is 6, then 30<VT≤126; if N is 7, then 35<VT≤147; if N is 8, then 40<VT≤168; if N is 9, then 45<VT≤189; if N is 10, then 50<VT≤210. Based on the above analysis, let the preset protection threshold VT be 120V, then the preset protection threshold VT is applicable to the output overvoltage protection in the output voltage range of 3.3V-12V, and is also applicable to the output overvoltage protection in the output voltage range of 3.3V-21V.

[0067] Furthermore, the secondary side controller directly samples the output feedback signal at the secondary side, and outputs the first control signal according to the output feedback signal and the reference signal. Then, the secondary side controller transmits the first control signal to the primary side controller through the isolation device. In one embodiment, when the output feedback signal is greater than the reference signal, the secondary side controller outputs the first control signal in a high level state, and the first control signal in a high level state indicates that the output of the flyback converter is overvoltage.

[0068] Furthermore, the first control module includes:

[0069] A first feedback signal generating circuit, wherein the first feedback signal generating circuit obtains a first feedback signal according to an auxiliary winding, or obtains the first feedback signal according to a drain terminal voltage of a main switch tube, or obtains the first feedback signal according to a volt-second balance;

[0070] A comparison circuit, comparing the first feedback signal with a preset protection threshold to output an overvoltage judgment signal;

[0071] The driving circuit stops driving the primary side main switch tube according to the overvoltage judgment signal to start the output overvoltage protection.

[0072] Preferably, Figure 5 As shown, the first feedback signal generating circuit obtains the first feedback signal according to the volt-second balance. Wherein, the first feedback signal generating circuit includes: a first conversion circuit, converting the input voltage into a first current at a first ratio; a first integration unit, obtaining the ampere-second product of the first current during the conduction period of the main switch tube, and the ampere-second product is the first feedback signal representing the output voltage. Correspondingly, the first control module also includes: a second conversion circuit, converting the preset protection threshold into a second current at a second ratio; a second integration unit, obtaining the ampere-second product of the second current during the freewheeling period, and the ampere-second product represents the preset protection threshold. The comparison circuit includes a comparator, the first input end of the comparator receives the ampere-second product of the first current during the conduction period of the main switch tube, the second input end receives the ampere-second product of the second current during the freewheeling period, and the output end generates an overvoltage judgment signal. When the overvoltage judgment signal represents the output overvoltage, the driving circuit stops driving the primary side main power tube according to the overvoltage judgment signal, and the primary side main power tube is turned off to perform output overvoltage protection.

[0073] Specifically, the working process is as follows: during the conduction period of the main switch tube, the switch K1 in the first integration unit is closed and the first capacitor C1 is charged with the first current I1; during the freewheeling period, the switch K5 in the second integration unit is closed and the second capacitor C2 is charged with the second current I2. At the end of the freewheeling (when the inductor current is detected to be zero), the switch K3 is closed, and the comparator U1 compares the ampere-second product I1*Ton and the ampere-second product I2*Toff. When the ampere-second product I1*Ton is greater than the ampere-second product I2*Toff, the comparator U1 outputs an overvoltage judgment signal signal in a high-level state, which indicates that the output of the flyback converter is overvoltage. At this time, the drive circuit receives the overvoltage judgment signal signal in a high-level state to stop driving the main power tube on the primary side.

[0074] Thereafter, if the switching control module receives the first control signal, the switching control module controls the switch K4 to turn off. At this time, the first control module stops working, and the second control module starts the output overvoltage protection according to the first control signal. Specifically, when the second control module receives the first control signal in a high-level state, the second control module stops driving the primary side main power tube to start the output overvoltage protection; when the second control module receives the first control signal in a low-level state, the second control module drives the primary side main power tube with a certain duty cycle, and the flyback converter operates normally.

[0075] It should be noted that in the above embodiment, when the first feedback signal includes information about the product of the output voltage and the primary-secondary turns ratio, the first feedback signal can be directly compared with the preset protection threshold to determine whether the output is over-voltage. In other embodiments, when the first feedback signal only includes information about the output voltage, the first feedback signal is compared with the preset protection threshold. Determine whether the output is overvoltage, VT is the preset protection threshold, and N is the primary-secondary turns ratio.

[0076] In summary, the present invention proposes a design that combines primary-side feedback control and secondary-side feedback control in a flyback converter, so that primary-side feedback control is used in the startup phase to perform output overvoltage protection, and secondary-side feedback control is used in the subsequent process to perform output overvoltage protection. At the same time, the present invention also proposes setting a preset protection threshold so that the preset protection threshold is applicable to different output ranges. Therefore, when the output range changes, there is no need to adjust the preset protection threshold, which greatly reduces the complexity of the control and improves the applicability of the method.

[0077] In addition, the present invention also proposes a fast charging power supply, including the flyback converter proposed above.

[0078] It should be noted that the specific implementation and corresponding illustrations given are merely a way of describing the implementation method of the present invention, and do not limit the specific structure of the implementation scheme of the present invention. Various changes or modifications can be made to these implementation schemes without departing from the principle and essence of the present invention, but these changes and modifications fall within the scope of protection of the present invention.

[0079] Although the embodiments are described and illustrated separately above, some common technologies are involved. It is the opinion of ordinary technicians in this field that the embodiments can be replaced and integrated. If the content is not clearly recorded in one of the embodiments, reference can be made to another recorded embodiment.

[0080] The above-described implementation methods do not constitute a limitation on the protection scope of the technical solution. Any modification, equivalent replacement and improvement made within the spirit and principle of the above-described implementation methods shall be included in the protection scope of the technical solution.

Claims

1. An overvoltage protection method for a flyback converter, It is characterized in that Includes steps: Determine whether the primary-side controller receives a first control signal transmitted by the secondary-side controller, and the secondary-side controller generates the first control signal according to the output feedback signal and the reference signal; If not, the primary-side controller obtains a first feedback signal representing the output voltage, and determines whether the output of the flyback converter is over-voltage according to the first feedback signal and a preset protection threshold; If the output is over-voltage, the output over-voltage protection is activated; Among them, the preset protection threshold is set according to the transformer turns ratio, preset output voltage and different output voltage ranges of the flyback converter to achieve overvoltage protection within different output voltage ranges, and the preset output voltage represents the output voltage of the flyback converter during the startup phase.

2. The overvoltage protection method according to claim 1, It is characterized in that The different output voltage ranges include N different output voltage ranges, and the preset protection threshold satisfies: N_x*Vset<VT<N_x*Vmax_x; Among them, N_x is the primary-to-secondary turns ratio when the output of the flyback converter is set in the xth output voltage range, Vset is the preset output voltage, Vmax_x is the upper limit value of the xth output voltage range, VT is the preset protection threshold, and x is 1-N.

3. The overvoltage protection method according to claim 1, It is characterized in that Also includes: If the primary-side controller receives the first control signal, the primary-side controller determines whether to start the output overvoltage protection according to the first control signal.

4. The overvoltage protection method according to claim 1, It is characterized in that The primary side controller obtains the first feedback signal according to the drain terminal voltage of the main switch tube, or obtains the first feedback signal according to the volt-second balance.

5. A flyback converter, comprising a primary side controller and a secondary side controller, It is characterized in that The secondary-side controller generates a first control signal according to the output feedback signal and the reference signal and couples the first control signal to the primary-side controller; The primary side controller includes: A first control module obtains a first feedback signal representing an output voltage, and starts output overvoltage protection according to the first feedback signal and a preset protection threshold; A second control module, starting output overvoltage protection according to the first control signal; The switching control module controls the first control module to work or the second control module to work according to the first control signal; Among them, the preset protection threshold is set according to the transformer turns ratio, preset output voltage and different output voltage ranges of the flyback converter to achieve overvoltage protection within different output voltage ranges, and the preset output voltage represents the output voltage of the flyback converter during the startup phase.

6. The flyback converter according to claim 5, It is characterized in that The different output voltage ranges include N different output voltage ranges, and the preset protection threshold satisfies: N_x*Vset<VT<N_x*Vmax_x; Among them, N_x is the primary-to-secondary turns ratio when the output of the flyback converter is set in the xth output voltage range, Vset is the preset output voltage, Vmax_x is the upper limit value of the xth output voltage range, VT is the preset protection threshold, and x is 1-N.

7. The flyback converter according to claim 5, It is characterized in that The first control module comprises: A first feedback signal generating circuit, which obtains the first feedback signal according to the drain terminal voltage of the main switch tube, or obtains the first feedback signal according to the volt-second balance; A comparison circuit, comparing the first feedback signal with the preset protection threshold to output an overvoltage judgment signal; The driving circuit stops driving the primary side main power tube according to the overvoltage judgment signal to start output overvoltage protection.

8. The flyback converter according to claim 7, It is characterized in that The first feedback signal generating circuit obtains the first feedback signal according to the volt-second balance, including: A first conversion circuit converts an input voltage into a first current; The first integration unit obtains the ampere-second product of the first current during the conduction period of the main switch tube, and the ampere-second product of the first current during the conduction period of the main switch tube is the first feedback signal.

9. The flyback converter according to claim 8, It is characterized in that The first control module also includes: A second conversion circuit converts the preset protection threshold into a second current; The second integration unit obtains the ampere-second product of the second current during the freewheeling period, where the ampere-second product of the second current during the freewheeling period represents the preset protection threshold.

10. A fast charging power source, It is characterized in that A flyback converter comprising any one of claims 5 to 9.

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