TWI672899B - Method for controlling resonant converter - Google Patents

Abstract

The invention provides a control method for 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 controller unit controls the internal of the resonant converter when the instantaneous value of the output voltage and the amount of change in the output current exceed the set value. The power switch unit performs on/off switching according to the full-load operating frequency (for example, 100 KHz), thereby greatly reducing the instantaneous drop value 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 the resonant converter switching from full load operation to no-load operation.

Description

Control method of resonant converter

The present invention relates to the related art of power converters, and more particularly to a method of controlling a resonant converter.

Switching-mode power supply (SMPS) technology has been widely used to make power supplies for various types of motors and electronics. Moreover, as the trend toward thinner and lighter electronic products, it is necessary to increase the power density of the switching power converter by increasing the switching frequency, so that the mechanism volume of the switching power converter can be effectively reduced. Thus, an LLC resonant converter featuring zero voltage switching (ZVS) and zero current switching (ZCS) has thus been proposed.

Referring to FIG. 1, a circuit architecture diagram of a conventional LLC series resonant converter is shown. As shown in FIG. 1 , a conventional LLC series resonant converter 2 ′ includes: a power switching unit 23 ′ coupled to a DC power source V _DC ′, a resonant unit 24 ′, a transformer unit 25 ′, and an output rectifying unit. 26', and an output filtering unit 27'; wherein a closed-loop control module 1' is connected between the output of the LLC series resonant converter 2' and the power switching unit 23'. Moreover, it can be seen from FIG. 1 that the closed loop control module 1 ′ mainly includes a signal detecting unit 11 ′, a controller unit 12 ′, and an isolated driving unit 13 ′. Under normal operation of the LLC series resonant converter 2', the operating frequency of the LLC series resonant converter 2' can be varied to control the adjustment of the output voltage range. In other words, the operating frequency must also be widened relative to a wide range of output voltages. 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 raised to account for the operating frequency, but this will result in an increase in volume, and if the efficiency is not relatively increased, thermal energy will also be generated.

In addition to the above problems, the wider the output voltage, the harder it is to be controlled by 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 modulation, in order to stabilize the output voltage within specifications. 2 is a graph showing the measurement data of a conventional LLC series resonant converter. As can be seen from Figure 2, when the load is full from no-load, the energy of the output capacitor is not enough and the operating frequency is too slow to change from high frequency to low frequency (as indicated by the dotted box), causing the output voltage to drop first and instantaneously. The drop is over 2V. In view of this, the manufacturer of the LLC series resonant converter 2' proposes a method of accelerating the modulation speed of the operating frequency by adjusting the compensation parameters of the controller unit 12'. However, it has been found that although this method can improve the output voltage transient, it will cause unstable jitter of the steady-state output voltage.

As can be seen from the above description, it is necessary to redesign and propose a solution for enabling the LLC series resonant converter 2' to output a wide range of voltage adjustments in a steady state. In view of this, the creator of the present invention tried his best to study the invention, and finally developed a control method of the resonant converter of the present invention.

In the architecture of the conventional LLC series resonant converter, the feedback circuit is mainly used to perform feedback compensation according to the (output voltage) error amount, and then the control signal is used 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 is fully loaded from no load, the operating frequency cannot be quickly modulated from high frequency to low frequency (as indicated by the dashed box in Figure 2), causing the output voltage to drop. Accordingly, it is a primary object of the present invention to provide a control method for a resonant converter that is applied to a controller unit of a feedback circuit. In this way, when the resonant converter jumps from the no-load (or light-load) operation to the full-load operation, the feedback circuit controls the power switch unit according to the instantaneous value of the output voltage and the change of the output current exceeding the set value. The full-load operating frequency (for example, 100 KHz) is switched on/off, which greatly reduces 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 the resonant converter switching from full load operation to no-load operation. In addition, the control method of the present invention can simultaneously avoid the occurrence of an overload (flow) phenomenon during the transition of the resonant converter to the no-load operation. The specific overload protection measures are to set the two set values of the output current change amount first. When the output current change exceeds the first set value (for example, 10A), the feedback circuit directly outputs the second control signal to control the power switch unit to perform on/off switching according to the full load operating frequency. Moreover, when the current change amount continues to increase and reaches or exceeds the second set value (for example, 13A), the feedback circuit outputs 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 have the following advantages: (1) The resonant converter can be equipped with a wide range of voltages at the lowest cost without changing the design of the feedback circuit. At the same time, it can also quickly change the operating frequency according to the large fluctuation of the load to maintain the output of the stable voltage; (2) quickly adjust the operating frequency in the PWM control mode; or, in the phase shift control (Phase-shift) mode Next, the variable shift amount is quickly adjusted; and (3) the control method of the present invention is integrated into the existing feedback circuit to obtain a feedback controller capable of simultaneously stabilizing the transient reaction and the steady state reaction.

In order to achieve the above-mentioned primary object of the present invention, the creator of the present invention 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 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 one of the power converter units 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 is configured according to the a user control signal controls the power factor converter and the power switch unit; (3) the power factor converter establishes a first output voltage, and the power switch unit controls a resonant unit of the resonant converter and a transformer unit a second output voltage is established; wherein the first output voltage is established earlier than the second output voltage; (4) the controller judges Disconnecting the output voltage control message as a high output voltage message or a low output voltage message; if the output voltage signal is increased, performing step (5); and if the output voltage signal is low, executing Step (6); (5) the first output voltage is increased, and at the same time, the second output voltage is increased; returning to the step (3); and (6) the first output voltage is lowered, and at the same time, the The two output voltages are reduced; return to step (3).

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

Referring to Fig. 3, there is shown an architectural diagram of a resonant converter to which a control method of a 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 three. As shown in FIG. 3, the resonant converter 2 includes at least: a power factor converter 21, a PFC switch 22, a power switch unit 23, a resonating unit 24, a transformer unit 25, an output rectifying unit 26, and a The output filtering unit 27 includes a signal detecting unit 11, a controller 12 and an isolated driving unit 13. An electronic engineer who has long been involved in the development and manufacture of the 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 the duty ratio of the PFC switch 22.

In general, the power switch unit 23 can be a half bridge architecture composed of two MOSFETs, the resonant unit 24 can be an LLC series resonant line, and the signal detecting unit 11 can be included by an amplifier. A circuit is formed for detecting an output voltage and an output current that the resonant converter 2 transmits to the load 3. Alternatively, the isolated drive unit 13 can be an optocoupler and the controller 12 can be a generally programmable digital control chip. In the present invention, as shown in FIG. 3, after detecting the output voltage and current, the signal detecting unit 11 returns to the controller 12 via the isolation driving unit 13, so that the controller 12 controls the power according to the result of the detection output. The switching unit 23 is turned on/off. It should be additionally noted that, in terms of the actual circuit arrangement, the controller 12 may also be disposed on the other side of the isolated driving unit 13; thus, after the signal detecting unit 11 detects the output voltage and current, the controller 12 performs detection according to the detection. 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 drive unit 13.

Therefore, the user can appropriately plan the architecture of the feedback circuit 1 according to the actual needs. For example, if the output signal requires more than two detection points, then the two isolation driving units 13 are required to be transmitted back to the controller 12. . Briefly, as the number of detection points increases, an equal number of isolated drive units 13 can be arranged correspondingly within the feedback line 1. Continuing to refer to FIG. 3, and referring also to FIGS. 4A and 4B, a flow chart of a method of controlling a resonant converter of the present invention is shown. The present invention is intended to provide a line having a wide range of voltage outputs, comprising the steps of: Step (S1): inputting 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 the power switching unit 23 controls the resonant conversion a resonant unit 24 of the device 2 and a transformer unit 25 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 determining that the output voltage control message is a high output voltage message or a low output voltage message; if the output voltage signal is increased, performing step (S5); and, if the output is low The message proceeds to step (S6); step (S5): the first output voltage V1 is increased, and at the same time, the second output voltage V2 is increased; back to the step (3); and the step (S6): the first The output voltage V1 is lowered while the second output voltage V2 is lowered; returning to the 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 can be converted into a duty cycle signal, and the controller 12 controls the power factor converter 21 according to the duty cycle signal, and the user control signal 4 is converted 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 switching 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 the output voltage. In the foregoing, the user control signal 4 is converted into the frequency signal. If the controller 12 determines that the frequency signal has exceeded the preset value, the user control signal 4 is converted into a current value in order to continue to adjust the output voltage. The duty cycle signal, the duty cycle signal is modulated within a certain range, and in the case of 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 The ratio is modulated between the lowest duty cycle d and the highest duty cycle Tr, so that the output voltage can be modulated over a wider range.

In general, for the resonant converter 2 to achieve a wide range of voltage outputs, the frequency range of the modulation is also widened, but this increases the volume of the hardware. In view of this, the control method of the present invention also provides a method that allows the resonant converter 2 to achieve a wide range of outputs. The specific method is to convert the user control signal 4 into a first duty cycle signal, so that the controller 12 controls the power factor converter 21 and the output of the resonant converter 2 according to the first duty signal. When the voltage is to be adjusted, the first duty 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; meanwhile, the user control signal 4 is also converted to a certain value. The frequency signal modulated in the range, in the case of a fixed duty ratio, adjusts the frequency of the power switching unit 23, when the frequency has exceeded the range, for example, as shown in FIG. 5, when the frequency has been high to H point, the hardware If the higher frequency is not reached, the user control signal 4 is converted into a second duty signal, so that the controller 12 adjusts the second duty signal to control the fixed frequency. The power switching unit 23, in turn, 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; thus, when the step (S4) determines that the voltage value converted by the user control signal 4 is higher than a preset value, then control The device 12 adjusts the duty ratio and the frequency signal upward at the same time, so that the overall operating interval of the first output voltage V1 and the second output voltage V2 is increased. Conversely, when the step (S4) determines that the voltage value converted by the user control signal 4 is lower than the preset value, the overall operating interval of the first output voltage V1 and the second output voltage V2 is lowered. In other words, the overall output voltage of the resonant converter 2 is controlled stepwise using the method of the present invention. Whenever the voltage value converted by the user control signal 4 is high (low) by a predetermined value, the first output voltage V1 and the second output voltage V2 are raised (lowered) upward. It should be noted that the control method of the present invention can control the wide range of output voltage of the resonant converter 2, and also makes the hardware design of the feedback circuit 1 relatively simple while also reducing the overall volume of the resonant converter 2.

Please refer to FIG. 6, which is a diagram showing the step of stabilizing the output voltage 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 performs step S11, and the feedback circuit 1 monitors the resonant conversion. A change in output current and an output voltage of the device 2. Continuing, the control method then performs 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 decision result in the step S12 is "NO", the method flow then proceeds to a step S13. Here, it must be additionally added that the voltage threshold referred to herein can be set to 200 mV; simply speaking, if the variation of the output voltage exceeds 200 mV, the system tends 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 has been 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 threshold and the current threshold in accordance with the characteristics of the resonance unit 24.

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

Please also refer to Figure 7, which shows the measurement data of the series resonant converter. If the result of the determination in step S12 is YES, the method flow proceeds to step S14, so that the feedback circuit 1 correspondingly outputs a second control signal to the resonant converter 2, so that the resonant converter 2 is based on the Operates at full load operating frequency. According to the measurement data of the model of the specific resonant converter 2 by the inventors, it can be known that the full-load operating frequency is 100 KHz. Briefly, when the feedback circuit 1 detects that the amount of change in the load exceeds the set condition (as shown in the measurement data (a) of Fig. 7), the feedback circuit 1 does not return according to the (output voltage) error amount. The compensation is directly applied, and the second control signal is directly output to control the power switching unit 23 to perform on/off switching in accordance with the full load operating frequency of 100 KHz. As shown in the measurement data (b) of FIG. 7, after directly controlling the power switch unit 23 to perform on/off switching according to the full-load operating frequency, the instantaneous voltage drop of the output voltage can be reduced to 0.9V without causing a steady-state output. Unstable jitter of the voltage.

It is particularly emphasized that the control method of the resonant converter of the present invention further has the function of overload (flow) protection. Please refer to FIG. 5 and FIG. 6 repeatedly, and please refer to FIG. 8 and FIG. 9 at the same time. FIG. 8 is a diagram showing the overload stability step of the present invention, and FIG. 9 is a measurement data diagram showing the series resonant converter. When an overload occurs, the present invention further provides an overload stabilization method comprising the steps of: (S21) the resonant converter 2 supplies energy to the load 3, and the feedback circuit 1 detects an output current; (S22) The controller 12 determines whether the change of the output current rises to a first predetermined value, and if so, performs the step (S23); if not, repeats the step (S21); (S23) the controller 12 adjusts the power switch unit 23 a frequency signal, and determining whether the change of the output current rises to a second predetermined value, and if so, performing the step (S24); if not, repeating the step (S21); (S24) the feedback circuit 1 correspondingly A third control signal is outputted to the resonant converter 2 for turning off the power switching unit 23 for a resonance period; then, the step (S21) is repeated. In the present invention, the second predetermined value is greater than the first predetermined value, and the resonant period refers to the time required for the energy of the resonant unit 24 to be completely released. The energy can be completely released in relation to the resonant tank of the resonant unit 24. In the present invention, the power switching unit 23 is controlled by a pulse width modulation value, that is, a duty cycle value, and a pulse width modulation value represents The time during which the power switching unit 23 is turned on plus the time that is turned off also represents a duty cycle ranging from one duty cycle to three duty cycles.

The present invention specifically designs two set values of the output current change amount, 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 10 A, the feedback circuit 1 directly outputs the second control signal to control the power switching unit 23 to perform on/off switching in accordance with the 100 KHz full-load operating frequency. 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 correspondingly output a third control signal to the resonant converter. 2. It is used to turn off the power switch unit 23 for two voltage loop execution times. The results of the implementation are shown in the measurement data (b) of Figure 7.

Thus, the above-described system has completely and clearly explained all the implementation steps of the control method of the resonant converter of the present invention; and, as described above, the present invention has the following advantages:

(1) In the architecture of the conventional LLC series resonant converter 2' (shown in Fig. 1), it is mainly that the closed loop control module 1' has difficulty controlling the LLC series resonant converter 2' to provide voltage reading. Therefore, the present invention particularly proposes a control method for a resonant converter. When the resonant converter 2 is jumped from a no-load (or light-load) operation to a full-load operation, the feedback circuit 1 will have a transient value of the output voltage and an output current. When the amount of change exceeds the set value, the control power switching unit 23 performs on/off switching in accordance with the full-load operating frequency (for example, 100 KHz), whereby the instantaneous drop value of the output voltage is greatly reduced. Therefore, the control method of the present invention can exert the effect of stabilizing the output voltage during the operation of the LLC resonant converter in the no-load operation to the full load operation.

(2) In addition to this, the control method of the present invention can simultaneously avoid the occurrence of an overload (flow) phenomenon during the transition of the resonant converter to the full load operation of the no-load operation. The specific overload protection measures are to set the two set values of the output current change amount first. When the output current change amount exceeds the first set value (for example, 10A), the feedback circuit 1 directly outputs the second control signal to control the power switch unit 23 to perform on/off switching according to the full load operation frequency. Moreover, when the current change amount continues to increase and reaches or exceeds the second set value (for example, 13A), the feedback circuit 1 correspondingly outputs a control signal to turn off the power switch unit 23 for two voltage loops to be executed. time.

It is to be understood that the foregoing detailed description of the embodiments of the present invention is not intended to Both should be included in the scope of the patent in this case.

<present 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 rectification unit 27 Output filter unit 11 Signal detection unit 12 Controller 13 Isolation drive unit 16a Second electrical latch S1-S6 Step S11-S14 Step S21-S24 Step V1 First output voltage V2 Second output voltage f2 Maximum frequency f1 Lowest frequency D Minimum duty ratio Tr Maximum duty ratio

<知知>   2' LLC Series Resonant Converter VDC' 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

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

Claims (10)

A control method for 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 resonant converter The control method comprises the following steps: (1) inputting a user control signal to a controller of the feedback circuit, wherein the controller is connected to one of the power converter units of the resonant converter, and is also connected with a power factor a PFC switch of the converter; (2) the user control signal is an output voltage control message, and the controller generates 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, and the power switching unit controls a resonant unit of the resonant converter and a transformer unit to establish a second output voltage; wherein the first output voltage is earlier than the second The output voltage is established; (4) the controller determines that the output voltage control message is a high output voltage message or a low output voltage message; Step (5) is performed to increase the output voltage message; and step (6) is performed if the output voltage message is turned down; (5) the first output voltage is increased, and the second output voltage is increased. Returning to the step (3); and (6) the first output voltage is lowered while the second output voltage is lowered; returning to the step (3). The control method of the resonant converter according to claim 1, wherein after the step (5) and the step (6), the user control signal is converted into a frequency signal, and the frequency signal is The modulation is controlled within a certain range; and, in the case of fixing a duty ratio of the frequency signal, the controller controls the power switching unit according to the frequency signal. The method for controlling a resonant converter according to claim 1, wherein after the step (5) and the step (6), the user control signal is converted into a duty cycle signal, and the The space ratio signal is modulated within a certain range; and, in the case of fixing a frequency of the duty cycle signal, the controller controls the power switch unit according to the duty ratio signal. The method for controlling a resonant converter according to claim 1, further comprising the steps of: (71) monitoring, by the feedback circuit, a change in an output current and an output voltage of the resonant converter; (72) determining 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 in a period of time; if yes, performing step (74); if not, performing 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 correspondingly outputs a first control signal to the resonant converter; and then repeats the step ( 71); (74) the feedback circuit correspondingly outputs a second control signal to the resonant converter, causing the resonant converter to operate based on a full load operating frequency; and then repeating the step (71). The control method of the resonant converter of claim 4, 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 duty cycles, and each duty 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 of controlling a resonant converter according to claim 4, wherein the full-load operating frequency is obtained according to the resonant unit and is pre-stored in a controller of the feedback circuit. The control method of the resonant converter according to claim 1, further comprising the following steps: (81) the resonant converter supplies 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 rises to a first predetermined value, and if so, performs step (83); if not, repeats the step (81); (83) the controller adjusts the power a frequency signal of the switching unit, and determining whether the change of the output current rises to a second predetermined value; if yes, performing step (84); if not, repeating step (81); (84) corresponding to the feedback circuit A third control signal is output to the resonant converter to turn off the power switching unit for a resonance period; then, step (81) is repeatedly performed. The control method of the resonant converter of claim 7, wherein the second predetermined value is greater than the first predetermined value. The control method of the resonant converter according to claim 7, wherein the resonant period refers to a time required for the energy of the resonant unit to be completely released. The control method of the resonant converter according to claim 7, wherein the power switching unit 23 is controlled by a pulse width modulation value, and the pulse width modulation value represents that the power switching unit 23 is turned on. The time plus the time that is turned off represents a duty cycle that ranges from one duty cycle to three duty cycles.