TW201935834A - Method for controlling resonant converter - Google Patents

Abstract

The present invention discloses a method applied in a controller unit of a feedback circuit for controlling a resonant converter. When the operation mode of the resonant converter turns from no load (or light load) to full load, the controller unit is configured to control a power switch unit of the resonant converter to switch ON and OFF according to an operation frequency of full load (such as 100 KHz) in the case of the fact that an instantaneous drop of the output voltage and a variation of the output current of the resonant converter are over predicted, so as to largely reduce the instantaneous drop of the output voltage. Therefore, this method is able to make the resonant converter provide the output voltage stably during the switch between the no-load operation mode and the full-load operation mode.

Description

Control method of resonant converter

The present invention relates to the related technical field of power converters, and in particular, to a method for controlling a resonant converter.

Switching-mode power supply (SMPS) technology has been widely used in the manufacture of power supplies for various motors and electronic products. In addition, as electronic products develop toward the trend of thinness, lightness, and shortness, the power density of the switching power converter must be increased by increasing the switching frequency to effectively reduce the mechanism size of the switching power converter. Therefore, an LLC resonant converter with the characteristics of Zero Voltage Switching (ZVS) and Zero Current Switching (ZCS) was proposed.

Please refer to FIG. 1, which is a circuit diagram of a conventional LLC series resonant converter. As shown in FIG. 1, the conventional LLC series resonant converter 2 ′ includes: a power switch unit 23 ′, a resonance unit 24 ′, a transformer unit 25 ′, and an output rectification unit coupled to a DC power source V _DC ′. 26 'and an output filtering unit 27'; wherein a closed-loop control module 1 'is connected between the output terminal of the LLC series resonant converter 2' and the power switching unit 23 '. Moreover, it can be known from FIG. 1 that the closed-loop control module 1 ′ mainly includes a signal detection unit 11 ′, a controller unit 12 ′, and an isolated driving unit 13 ′. Under the normal operation of the LLC series resonant converter 2 ', the operating frequency can be adjusted to change the voltage gain of the LLC series resonant converter 2' to control the adjustment of the output voltage range. In other words, with respect to a wide range of output voltages, the operating frequency must also be widened accordingly. The wider the range of output voltage modulation, the wider the operating frequency, and the hardware of the resonant line also needs to be relatively increased to correspond to the operating frequency, but this will cause an increase in volume, and if the efficiency is not relatively improved, thermal energy will also be generated.

In addition to the above problems, the wider the output voltage, the more difficult it is to control the converter. For example, when the load 3 'is switched from no load or light load to full load, the operating frequency must be fast and large. Ground regulation, so that the output voltage can be stabilized within specifications. FIG. 2 is a measurement data chart showing a conventional LLC series resonant converter. It can be seen from Figure 2 that when the load jumps from no-load to full-load, the output capacitor energy is not enough and the operating frequency is adjusted from high frequency to low frequency too slowly (as indicated by the dashed box). Depreciation exceeds 2V. In view of this, the manufacturer of the LLC series resonant converter 2 'has proposed a method of accelerating the modulation speed of the operating frequency by adjusting the compensation parameter of the controller unit 12'. However, it has been found in practice that although this method can improve the problem of instantaneous output voltage drop, it can cause unstable jumps in the steady-state output voltage.

It can be known from the above description that it is really necessary to redesign and propose a solution to enable the LLC series resonant converter 2 'to output a wide range of voltages in steady state. In view of this, the creators of this case made great efforts to research and invent, and finally developed a control method for a resonant converter of the present invention.

In the conventional LLC series resonant converter architecture, the feedback circuit is mainly used to perform feedback compensation according to the (output voltage) error amount, and then the output is used to control the signal to control the operating frequency of the power switching unit to stabilize the output voltage; however, The actual execution surface shows that when the load jumps from no-load to full-load, the operating frequency cannot be adjusted from high frequency to low frequency quickly (as indicated by the dashed box in Figure 2), resulting in a sudden drop in output voltage. Therefore, the main purpose of the present invention is to provide a control method of a resonant converter, which is applied to a controller unit of a feedback circuit. In this way, when the resonant converter jumps from no-load (or light-load) operation to full-load operation, the feedback circuit will control the power switch unit in accordance with the instantaneous drop in the output voltage and the change in the output current to exceed the set value. Full load operating frequency (for example: 100 KHz) is switched on / off to greatly reduce the instantaneous drop of output voltage. Therefore, the control method of the present invention can exert the effect of stabilizing the output voltage during the process of the resonant converter from no-load operation to full-load operation. In addition, the control method of the present invention can also avoid the occurrence of an overload (flow) phenomenon during the process of the resonant converter turning from a no-load operation to a full-load operation. Specific overload protection measures are to set up two set values of output current change. When the output current change exceeds the first set value (for example: 10A), the feedback circuit directly outputs a second control signal to control the power switch unit to switch on / off according to the full-load operating frequency. In addition, when the current variation continuously increases and reaches or exceeds the second set value (eg, 13A), the feedback circuit will output a control signal correspondingly to turn off the power switch unit for two voltage loop execution times.

It is worth emphasizing that the control method of the present invention can make the resonant converter also have the following advantages: (1) Without changing the design of the feedback circuit, the resonant converter can be provided with the ability to output a wide range of voltages at the lowest cost. At the same time, the operating frequency can be quickly changed according to the large fluctuation of the load to maintain a stable voltage output; (2) In the PWM control mode, the operating frequency can be quickly adjusted; or, in the phase-shift mode Next, quickly adjust the variable displacement; and (3) integrate the control method of the present invention into an existing feedback circuit to obtain a feedback controller capable of simultaneously stabilizing the transient response and the steady-state response.

In order to achieve the above-mentioned main purpose of the present invention, the creator of this case provides an embodiment of the control method of the resonant converter, which is applied to a feedback circuit; wherein the feedback circuit is connected to a resonant converter And at least one load connected to the resonant converter; the control method of the resonant converter includes the following steps: (1) inputting a user control signal to a controller of the feedback circuit, wherein the control The device is connected to a power switching unit of the resonant converter, and is also connected to a PFC switch of a power factor converter; (2) the user control signal can be converted into an output voltage control message, and the controller according to the The user control signal controls the power factor converter and the power switch unit; (3) The power factor converter establishes a first output voltage, and at the same time, the power switch unit controls a resonance unit and a transformer unit of the resonance converter To establish a second output voltage; wherein the first output voltage is established earlier than the second output voltage; (4) the controller judges If the output voltage control message is a high output voltage message or a low output voltage message; if it is the high output voltage message, step (5) is executed; and if it is the low output voltage message, then execute Step (6); (5) the first output voltage is increased, and at the same time, the second output voltage is increased; return to step (3); and (6) the first output voltage is decreased, and at the same time, the first The two output voltages are reduced; return to step (3).

In order to more clearly describe a control method of the resonant converter provided by the present invention, the preferred embodiments of the present invention will be described in detail below with reference to the drawings.

Please refer to FIG. 3, which is a structural diagram of a resonant converter to which the control method of the resonant converter of the present invention is applied. As shown in FIG. 3, the control method of the resonant converter of the present invention is applied to a feedback circuit 1; wherein the feedback circuit 1 is connected to a resonant converter 2 and connected to the resonant converter 2. At least one load between 3. As can be seen from FIG. 3, the resonant converter 2 includes at least: a power factor converter 21, a PFC switch 22, a power switching unit 23, a resonance unit 24, a transformer unit 25, an output rectification unit 26, and a The output filtering unit 27 includes a signal detection unit 11, a controller 12 and an isolated driving unit 13. Electronic engineers who have long been involved in the development and manufacture of resonant converter 2 should know that the power factor converter 21 inside the resonant converter 2 can be a Buck line, a Boost line, or a Buck-Boost line, and the PFC switch 22 is the power factor converter. The working switch of 21 can adjust the output voltage of the power factor converter 21 by changing and controlling the duty cycle of the PFC switch 22.

Generally speaking, the power switching unit 23 may be a half-bridge structure composed of two metal-oxide-semiconductor field-effect transistors, the resonance unit 24 may be an LLC series resonance circuit, and the signal detection unit 11 may include an amplifier circuit. A circuit is formed to detect the output voltage and output current transmitted from the resonant converter 2 to the load 3. On the other hand, the isolated driving unit 13 may be an optical coupler, and the controller 12 may be a general programmable digital control chip. In the present invention, as shown in FIG. 3, after the signal detection unit 11 detects the output voltage and current, it is transmitted back to the controller 12 through the isolation driving unit 13, so that the controller 12 controls the power according to the result of the detection output. On / off of the switching unit 23. It must be added that in terms of the actual circuit arrangement, the controller 12 can also be arranged on the other side of the isolation drive unit 13; in this way, after the signal detection unit 11 detects the output voltage and current, the controller 12 As a result, a control signal for controlling the on / off of the power switch unit 23 is generated, and the control signal is transmitted to the power switch unit 23 through the isolation driving unit 13.

It can be known that the user can appropriately plan the structure of the feedback circuit 1 according to practical requirements. For example, if the output signal requires more than two detection points, then two isolated drive units 13 will be required to transmit back to the controller 12 . In short, as the number of detection points increases, an equivalent number of isolation drive units 13 can be arranged in the feedback line 1 accordingly. Continue to refer to FIG. 3, and also refer to FIG. 4A and FIG. 4B, which are flowcharts showing a control method of a resonant converter according to the present invention. The present invention intends to provide a circuit with a wide range of voltage output, including the following steps: Step (S1): input a user control signal 4 to a controller 12 of the feedback circuit 1, wherein the controller 12 is connected to the controller 12 A power switching unit 23 of the resonant converter 2 is also connected to a PFC switch 22 of a power factor converter 21; Step (S2): The user control signal 4 is an output voltage control message, and the controller 12 is based on The user control signal 4 controls the power factor converter 21 and the power switching unit 23; Step (S3): The power factor converter 21 establishes a first output voltage V1, and at the same time, the power switching unit 23 controls the resonance conversion A resonance unit 24 and a transformer unit 25 of the converter 2 to establish a second output voltage V2; wherein the first output voltage V1 is established earlier than the second output voltage V2; step (S4): the controller 12 It is judged that the output voltage control message is a high output voltage message or a low output voltage message; if it is the high output voltage message, step (S5) is performed; and if the low output voltage is The message executes step (S6); step (S5): the first output voltage V1 is increased, and at the same time, the second output voltage V2 is increased; return to step (3); and step (S6): the first The output voltage V1 is reduced, and at the same time, the second output voltage V2 is reduced; return to step (3).

It should be particularly noted that, in the present invention, the first output voltage V1 must be established earlier than the second output voltage V2, otherwise the system will be unstable. The user control signal 4 may be converted into a duty cycle signal, and the controller 12 controls the power factor converter 21 according to the duty cycle signal, and at the same time, the user control signal 4 is transformed into a frequency signal The frequency signal is modulated within a certain range, and in the case of a fixed duty cycle, the controller 12 controls the power switch unit 23 according to the frequency signal; at this time, as shown in FIG. 5, the operating frequency is fixed at Between the highest frequency f1 and the lowest frequency f2, the duty cycle can be fixed at 0.5, and the frequency is adjusted with different output voltages. Continuing from the foregoing, the user control signal 4 is converted into the frequency signal. If the controller 12 determines that the frequency signal has exceeded a preset value, at this time, in order to continue to adjust the output voltage, the user control signal 4 is converted into Duty cycle signal, the duty cycle signal is modulated within a certain range, and at a fixed frequency, the controller 12 controls the power switch unit 23 according to the duty cycle signal; as shown in FIG. 5, the duty cycle The ratio is adjusted between the lowest duty cycle d and the highest duty cycle Tr. In this way, the modulation range of the output voltage can be made wider.

Generally speaking, in order for the resonant converter 2 to achieve a wide range of voltage output, the frequency range of the modulation must also be widened, but this will increase the hardware volume. In view of this, the control method of the present invention also provides a method that enables the resonant converter 2 to achieve a wide range of output. The specific method is to convert the user control signal 4 into a first duty cycle signal, so that the controller 12 controls the output of the power factor converter 21 and the resonant converter 2 according to the first duty cycle signal. When the voltage needs to be adjusted, the first duty cycle signal can be adjusted to change the output voltage of the power factor converter 21, thereby changing the output voltage of the resonant converter 2. At the same time, the user control signal 4 is also converted to a certain value. The frequency signal modulated in the range, under the condition of fixed duty cycle, adjust and control the frequency of the power switch unit 23, when the frequency has exceeded the range, for example, as shown in Figure 5, when the frequency has reached the H point, the hardware It is no longer possible to reach a higher frequency, the user control signal 4 is converted into a second duty cycle signal, so that the controller 12 adjusts the second duty cycle signal to control the signal at a fixed frequency. The power switching unit 23 further changes the output voltage of the resonant converter 2. .

In addition, in the present invention, the user control signal 4 may include a voltage value; so set, when it is determined in this step (S4) that the voltage value converted by the user control signal 4 is higher than a preset value, the control The controller 12 adjusts the duty cycle and the frequency signal upward at the same time, so that the overall working interval of the first output voltage V1 and the second output voltage V2 is increased. Conversely, when it is determined in this step (S4) that the voltage value converted by the user control signal 4 is lower than a preset value, the overall working interval of the first output voltage V1 and the second output voltage V2 is reduced. In other words, the method of the present invention is used to control the overall output voltage of the resonant converter 2 in a stepwise manner. The first output voltage V1 and the second output voltage V2 are increased (decreased) every time the voltage value converted by the user control signal 4 passes through a preset value. It is worth noting that the control method of the present invention can not only control the wide-range output voltage of the resonant converter 2, but also make the hardware design of the feedback circuit 1 simpler and reduce the overall volume of the resonant converter 2.

Please refer to FIG. 6, which shows a step diagram of output voltage stabilization of the present invention. When the load changes, the entire resonant converter may be unstable. At this time, as shown in FIG. 6, the control method of the resonant converter of the present invention first executes step S11, and the feedback conversion circuit 1 monitors the resonant conversion. The change of an output current and an output voltage of the device 2. Continuing, the control method then executes step S12 to determine whether the change in the output voltage is greater than a voltage threshold and the change in the output current is greater than a current threshold. If the determination result of step S12 is "No", the method flow then proceeds to step S13. Here, it must be added that the voltage threshold referred to here can be set to 200mV; simply, if the output voltage changes more than 200mV, the system will tend to be unstable; on the other hand, the current threshold can be Set to 10A, and when the output current value exceeds 10A, the system is already operating at heavy load. When the control method of the present invention is applied to the feedback circuit 1, the user can adjust the voltage critical value and the current critical value according to the characteristics of the resonance unit 24.

Conversely, if the feedback circuit 1 detects that the output voltage transient drop value of the resonant converter 2 does not exceed 200 mV, and also detects that the output current change amount per unit time of the resonant converter 2 does not exceed 10 A, the present invention The control method executes step S13, so that the feedback circuit 1 calculates a frequency signal of the resonant converter 2 according to the measured output current and output voltage, and outputs a first control signal to the resonance correspondingly. Converter 2. In step S11, the power switching unit 23 is controlled with a pulse width modulation value. A pulse width modulation value represents the time when the power switch unit 23 is turned on plus the time that it is turned off, and represents a duty cycle. The period ranges from two working cycles to nineteen working cycles. In addition, the full-load operating frequency is obtained according to the resonance slot of the resonance unit 24 and is stored in the controller 12 of the feedback circuit 1 in advance. In short, when the load change amount does not reach the set condition, the feedback circuit 1 still performs feedback compensation according to the (output voltage) error amount, and then outputs a signal that controls the power switch unit 23 with the first control signal. Operating frequency.

Please refer to FIG. 7 at the same time, which shows the measurement data of the series resonant converter. If the judgment result of step S12 is "YES", the method flow will execute step S14, so that the feedback circuit 1 correspondingly outputs a second control signal to the resonance converter 2, so that the resonance converter 2 is based on a Runs at full load operating frequency. According to the inventor's measurement data for a specific model of the resonant converter 2, it can be known that the full-load operating frequency is 100 KHz. In short, when the feedback circuit 1 detects that the load change amount exceeds the set condition (as shown in the measurement data (a) of Fig. 7), the feedback circuit 1 does not perform the feedback based on the (output voltage) error amount. Compensation is given, but a second control signal is directly output to control the power switch unit 23 to switch on / off according to a full load operating frequency of 100 KHz. As shown in the measurement data (b) of Fig. 7, after the power switch unit 23 is directly controlled to switch on / off according to the full-load operating frequency, the instantaneous drop in output voltage can be reduced to 0.9V without causing steady-state output. Unstable jump in voltage.

It is worth emphasizing that the control method of the resonant converter of the present invention further has the function of overload (current) protection. Please refer to FIG. 5 and FIG. 6 repeatedly, and refer to FIG. 8 and FIG. 9 at the same time. Among them, FIG. 8 shows a diagram of the overload stabilization procedure of the present invention, and FIG. When an overload occurs, the present invention further provides an overload stabilization method, including the following steps: (S21) The resonant converter 2 provides energy to the load 3, and the feedback circuit 1 detects an output current; (S22) the The controller 12 determines whether the change in the output current has risen to a first predetermined value, and if so, executes step (S23); if not, repeats step (S21); (S23) the controller 12 adjusts the power switching unit 23 A frequency signal, and determine whether the change in the output current has risen to a second predetermined value. If so, perform step (S24); if not, repeat step (S21); (S24) The feedback circuit 1 correspondingly A third control signal is output to the resonant converter 2 to turn off the power switch unit 23 for a resonant period. Then, the step (S21) is repeatedly performed. In the present invention, the second predetermined value is greater than the first predetermined value, and the resonance period refers to a time required for the energy of the resonance unit 24 to be completely released. The energy can be completely released related to the resonance slot of the resonance unit 24. In the present invention, the power switch unit 23 is controlled with a pulse width modulation value, that is, a duty cycle value, and a pulse width modulation value represents The time when the power switch unit 23 is turned on plus the time that it is turned off also represents a working cycle, and the resonance cycle ranges from one working cycle to three working cycles.

The present invention specifically designs two set values of output current variation, for example, the first set value and the second set value are 10A and 13A, respectively. As shown in the measurement data (a) of FIG. 9, when the current variation exceeds 10A, the feedback circuit 1 directly outputs a second control signal to control the power switch unit 23 to switch on / off according to a full-load operating frequency of 100 KHz. Further, if the amount of current change continues to increase and reaches or exceeds the second set value (ie, 13A), the control method of the present invention causes the feedback circuit 1 to output a third control signal to the resonant converter correspondingly. 2. It is used to turn off the power switch unit 23 for two voltage loop execution times. The performance of the implementation is shown in the measurement data (b) of Figure 7.

In this way, the above-mentioned system has completely and clearly explained all the implementation steps of the control method of the resonant converter of the present invention; and according to the above-mentioned, the present invention has the following advantages:

(1) In the conventional LLC series resonant converter 2 '(as shown in FIG. 1) architecture, it is mainly the closed-loop control module 1' that is difficult to control the LLC series resonant converter 2 'to provide the voltage. Therefore, the present invention particularly proposes a control method for a resonant converter. When the resonant converter 2 jumps from a no-load (or light-load) operation to a full-load operation, the feedback circuit 1 will respond to the instantaneous drop in output voltage and output current. When the amount of change exceeds the set value, the power switch unit 23 is controlled to switch on / off according to the full-load operating frequency (for example: 100 KHz), thereby greatly reducing the instantaneous drop of the output voltage. Therefore, the control method of the present invention can exert the effect of stabilizing the output voltage during the process of transferring the LLC resonant converter from no-load operation to full-load operation.

(2) In addition, the control method of the present invention can also avoid the occurrence of an overload (current) phenomenon during the process of the resonant converter transferring from a no-load operation to a full-load operation. Specific overload protection measures are to set up two set values of output current change. When the output current variation exceeds the first set value (for example: 10A), the feedback circuit 1 directly outputs a second control signal to control the power switch unit 23 to switch on / off according to the full-load operating frequency. And, when the amount of current change continues to increase and reaches or exceeds the second set value (for example: 13A), the feedback circuit 1 will output a corresponding control signal to turn off the power switch unit 23 for two voltage loop executions. time.

It must be emphasized that the above detailed description is a specific description of the feasible embodiment of the present invention, but this embodiment is not intended to limit the patent scope of the present invention, and any equivalent implementation or change without departing from the technical spirit of the present invention, All should be included in the patent scope of this case.

<Invention> 1‧‧‧ feedback circuit

2‧‧‧ resonant converter

3‧‧‧ load

4‧‧‧user control signal

21‧‧‧Power Factor Converter

22‧‧‧PFC switch

23‧‧‧Power Switch Unit

24‧‧‧Resonant unit

25‧‧‧Transformer unit

26‧‧‧Output rectifier unit

27‧‧‧output filter unit

11‧‧‧Signal detection unit

12‧‧‧ Controller

13‧‧‧Isolated drive unit

16a‧‧‧Second electrical bolt

S1-S6‧‧‧ steps

S11-S14‧‧‧step

S21-S24‧‧‧step

V1‧‧‧ the first output voltage

V2‧‧‧Second output voltage

f2‧‧‧ maximum frequency

f1‧‧‧lowest frequency

D‧‧‧Minimum Duty Cycle

Tr‧‧‧ Maximum Duty Cycle

＜ Learning ＞ 2’‧‧‧LLC series resonant converter

V _DC '‧‧‧ DC Power Supply

23’‧‧‧Power Switch Unit

24’‧‧‧ resonant unit

25’‧‧‧Transformer unit

26’‧‧‧ output rectifier unit

27’‧‧‧ output filter unit

1’‧‧‧closed-loop control module

11’‧‧‧Signal detection unit

12’‧‧‧controller unit

13’‧‧‧ Isolated Drive Unit

3’‧‧‧ load

Fig. 1 is a circuit structure diagram of a conventional LLC series resonant converter; Fig. 2 is a measurement data diagram of a conventional LLC series resonant converter; Fig. 3 is a circuit diagram of a resonant converter to which the present invention is applied; 4A and 4B are flowcharts showing a control method of a resonant converter according to the present invention; FIG. 5 is a graph showing a control method of a resonant converter according to the present invention; Series 6 shows the step of stabilizing the output voltage of the present invention; Figure 7 shows the measurement data chart of the series resonant converter; Figure 8 shows the step of stabilizing the overload of the present invention; and Figure 9 shows the amount of series resonant converter Test data map.

Claims (10)

A control method of a resonant converter is applied to a feedback circuit; wherein the feedback circuit is connected between a resonant converter and at least one load connected to the resonant converter; The control method includes the following steps: (1) inputting a user control signal into a controller of the feedback circuit, wherein the controller is connected to a power switching unit of the resonant converter and also connected to a power factor One of the converters' PFC switch; (2) the user control signal is an output voltage control message, and the controller controls the power factor converter and the power switch unit according to the output voltage control message; (3) the power factor The converter establishes a first output voltage. At the same time, the power switching unit controls a resonant unit and a transformer unit of the resonant converter to establish a second output voltage. The first output voltage is earlier than the second output voltage. The output voltage is established; (4) The controller judges that the output voltage control message is a higher output voltage message or a lower output voltage message; if it is If the output voltage is increased, step (5) is performed. If the output voltage is decreased, step (6) is performed. (5) The first output voltage is increased, and the second output voltage is increased. ; Return to step (3); and (6) the first output voltage is reduced, and at the same time, the second output voltage is reduced; return to step (3). The control method of the resonant converter according to item 1 of the scope of patent application, wherein after the step (5) and the step (6), the user control signal is converted into a frequency signal, and the frequency signal is in The modulation is performed within a certain range; and when a duty cycle of the frequency signal is fixed, the controller controls the power switch unit according to the frequency signal. The control method of the resonant converter according to item 1 of the scope of patent application, wherein after the step (5) and the step (6), the user control signal is converted into a duty cycle signal, and the The air-to-air signal is modulated within a certain range; and when a frequency of the duty cycle signal is fixed, the controller controls the power switch unit according to the duty cycle signal. The control method of the resonant converter according to item 1 of the scope of patent application, further comprising the following steps: (71) monitoring the change of an output current and an output voltage of the resonant converter with the feedback circuit; (72) judgment In a period, whether the change of the output voltage is greater than a voltage threshold and the change of the output current is greater than a current threshold; if yes, go to step (74); if not, go to step (73); (73) The feedback circuit 1 calculates a frequency signal of the resonant converter according to the measured output current and the output voltage, and outputs a first control signal to the resonant converter correspondingly; then, repeats this step ( 71); (74) The feedback circuit correspondingly outputs a second control signal to the resonant converter, so that the resonant converter operates based on a full-load operating frequency; then, the step (71) is repeatedly performed. The control method of the resonant converter according to item 4 of the scope of patent application, wherein in the step (71), the power switching unit is controlled by a pulse width modulation signal; and the pulse width modulation signal It includes 2-19 working cycles, and each working cycle includes the time when the power switch unit is turned on and the time when the power switch unit is turned off. The method for controlling a resonant converter according to item 4 of the scope of patent application, wherein the full-load operating frequency is obtained according to the resonant unit and is stored in the controller of the feedback circuit in advance. The method for controlling a resonant converter according to item 1 of the patent application scope further includes the following steps: (81) The resonant converter provides energy to the load, and the feedback circuit detects an output current of the resonant converter; (82) The controller determines whether the change in the output current has risen to a first predetermined value, and if so, executes step (83); if not, repeats this step (81); (83) the controller adjusts the power Switch on a frequency signal and determine whether the change in the output current has risen to a second predetermined value. If yes, go to step (84); if not, repeat step (81); (84) the feedback circuit corresponds Ground outputs a third control signal to the resonant converter to turn off the power switch unit for a resonant period; then, step (81) is repeatedly performed. The method for controlling a resonant converter according to item 7 of the scope of patent application, wherein the second predetermined value is greater than the first predetermined value. The control method of the resonant converter according to item 7 of the scope of the patent application, wherein the resonant period refers to the time required for the energy of the resonant unit to be completely released. The control method of the resonant converter according to item 7 of the scope of patent application, wherein the power switch unit 23 is controlled by a pulse width modulation value, and a pulse width modulation value represents that the power switch unit 23 is turned on. The time plus the time to be turned off represents a duty cycle, and the resonance period ranges from one duty cycle to three duty cycles.