A control method and control circuit of an LLC converter

Abstract

The application discloses a control method and circuit for a clamping LLC converter, which is applied to an LLC resonant converter composed of an inverter circuit, a resonant circuit, a clamping branch, a transformer and a rectification filter circuit. The method is charge type control based on LLC. By detecting the driving pulse width of the inverter circuit switch tube and comparing the driving pulse width with a pulse width threshold, adaptive switching of two modes of the converter is realized. When the driving pulse width is greater than the threshold, variable frequency PFM control is adopted to change the output voltage gain by changing the working frequency. When the driving pulse width is less than the pulse width threshold, fixed frequency duty cycle shift PWM control is adopted to change the output voltage gain by changing the duty cycle. The application realizes the switching of variable frequency PFM and fixed frequency PWM control by detecting the driving pulse width, realizes multi-mode control of the charge type control LLC converter, widens the voltage gain range of the charge type control LLC converter, and all the switch tubes can realize ZVS, so that the overall efficiency of the circuit is high.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of switching converter control, in particular to a multi-mode control method and circuit for a clamped LLC converter. BACKGROUND

[0002] The clamped LLC converter can not only realize ZVS of the primary side switch tube and quasi-resonant of the secondary side rectifier tube, but also realize ZCS.

[0003] Through the control of PFM / PWM two modes, the working frequency range is narrower than that of the traditional LLC in the same input voltage range, so the LLC converter with a clamping branch overcomes the problem of narrow voltage adjustable range of the LLC converter, and is more suitable for products with wide input / output voltage range.

[0004] The traditional LLC converter with single voltage loop multi-mode control, as shown in FIG. 1, includes an inverter circuit 101, a resonant circuit 102, a clamping branch 103 composed of a switch tube S3 and a switch tube S4, a transformer 104 and a rectifier circuit 105, a filter circuit 106 and a controller 200. Figure 1 The controller 200 samples the secondary side output voltage and compares it with the reference voltage to generate an error voltage signal to the voltage frequency controller 204, so as to realize the stable output of the converter by adjusting the switching frequency of the LLC converter. The switching of PWM / PFM modes is realized by setting the frequency threshold, that is, when the switching frequency of the clamped LLC converter reaches the frequency threshold, the switching frequency no longer rises, and the switching from the PFM mode to the PWM mode is realized, and the stable output is realized by PWM adjustment. The traditional LLC converter with charge type control samples the resonant current or resonant capacitor voltage, and uses a comparator with relatively fast response to control the switch tube to be turned off when the transformer primary side energy is charged to the target value, and the next state is switched. Compared with the traditional single voltage loop control, this control method has the advantage of faster response, and greatly improves the dynamic performance of the converter. However, for the converter with charge type control, the comparator is directly used to control the driving of the switch tube to realize stable output, and there is no voltage frequency controller (VCO) in the control circuit, so the switching frequency cannot be measured and controlled. Therefore, the traditional method of setting the frequency threshold cannot be used to realize the switching and control of PFM / PWM modes, and the scheme of separately adding a frequency detection circuit not only increases the control cost, but also increases the control difficulty, and the reliability of the scheme needs to be explored. SUMMARY

[0005] Therefore, the present application solves the technical problem of providing a PFM / PWM mode switching and control scheme for a clamped LLC converter with charge type control, which realizes adaptive switching and control of PFM and PWM control modes according to the change of the driving pulse width of the switch tube.

[0006] To solve the above problems, the technical scheme adopted by the present application is:

[0007] The present application provides a control method of LLC converter, LLC converter includes inverter circuit, clamping branch and control circuit, inverter circuit has first bridge arm switch tube and second bridge arm switch tube;The first clamping branch has first clamping switch tube and second clamping switch tube, and the control method comprises:

[0008] Set pulse width threshold, and convert pulse width threshold into pulse width threshold voltage signal;

[0009] Obtain the drive pulse width voltage signal representing the drive pulse width of the first bridge arm switch tube or the second bridge arm switch tube;

[0010] The drive pulse width voltage signal is compared with the pulse width threshold voltage signal;When the drive pulse width voltage signal is greater than the pulse width threshold voltage signal, control the control circuit to work in PFM mode;When the drive pulse width voltage signal is less than the pulse width threshold voltage signal, control the control circuit to work in PWM mode.

[0011] In an embodiment, when the control circuit works in PFM mode, the drive pulse of the first bridge arm switch tube and the drive pulse of the second bridge arm switch tube are complementary, the duty cycle of the drive pulse of the first bridge arm switch tube and the drive pulse of the second bridge arm switch tube is 50%, and the first clamping switch tube and the second clamping switch tube remain closed;When the control circuit works in PWM mode, in a switching cycle, the drive pulse of the clamping switch tube is complementary to the drive pulse of the first bridge arm switch tube, the drive pulse of the second clamping switch tube is complementary to the drive pulse of the second bridge arm switch tube, and the switching cycle of the first bridge arm switch tube and the second bridge arm switch tube is 2 times of the pulse width threshold.

[0012] In an embodiment, in PFM mode, the falling edge of the drive pulse width voltage signal is used as the drive set signal, and the drive set signal is used to realize the set control of the drive signal of the first bridge arm switch tube and the drive signal of the second bridge arm switch tube;In PWM mode, the falling edge of the pulse width threshold voltage signal is used as the drive set signal, and the drive set signal is used to realize the set control of the drive signal of the first bridge arm switch tube and the drive signal of the second bridge arm switch tube.

[0013] The present application also provides a control circuit for LLC converter, the LLC converter includes inverter circuit, clamping branch and the control circuit, the inverter circuit has first bridge arm switch tube (S1) and second bridge arm switch tube (S2);The clamping branch has first clamping switch tube (S3) and second clamping switch tube (S4), and the control circuit comprises: pulse width threshold sampling circuit, mode control comparison circuit, drive pulse width sampling circuit and control module;

[0014] The pulse width threshold sampling circuit is used for converting the pulse width threshold signal input from the outside into a pulse width threshold voltage signal and delivering the pulse width threshold voltage signal to the mode control comparison circuit;

[0015] The drive pulse width sampling circuit obtains a drive pulse width voltage signal representing the drive pulse width of the first bridge arm switch tube or the second bridge arm switch tube, and delivers the drive pulse width voltage signal to the mode control comparison circuit;

[0016] The mode control comparison circuit compares the pulse width threshold voltage signal and the drive pulse width voltage signal, and outputs a mode control signal to the control module according to the comparison result;

[0017] The control module receives the mode control signal, and controls the control module to work in the PWM mode or the PFM mode according to the mode control signal, specifically:

[0018] When the drive pulse width voltage signal is greater than the pulse width threshold voltage signal, the control circuit works in the PFM mode; when the drive pulse width voltage signal is less than the pulse width threshold voltage signal, the control circuit works in the PWM mode.

[0019] In an embodiment, the control circuit further comprises a logic circuit, a first input end of the logic circuit is connected with the drive pulse width signal of the first bridge arm switch tube or the second bridge arm switch tube, a second input end of the logic circuit is connected with the pulse width threshold signal input from the outside, and an output end of the logic circuit is connected with the control module; when the drive pulse width signal is greater than the pulse width threshold signal, the logic circuit takes the falling edge of the drive pulse width signal as a trigger signal, outputs a drive set signal, and the drive set signal is used for realizing the set control of the drive signals of the first bridge arm switch tube and the second bridge arm switch tube; when the drive pulse width signal is less than the pulse width threshold signal, the logic circuit takes the falling edge of the pulse width threshold signal as a trigger signal, outputs the drive set signal, and the drive set signal is used for realizing the set control of the drive signals of the first bridge arm switch tube and the second bridge arm switch tube.

[0020] The application further provides a control method of an LLC converter for charge type control, comprising:

[0021] Before the LLC converter starts to work, a pulse width threshold is set, which is usually half of a resonance period;

[0022] When the LLC converter starts to work, a PFM mode is started, the clamping branch does not work in this mode, the drive signals of the first bridge arm switch tube and the second bridge arm switch tube of the switching circuit are both fixed at 50% duty cycle, that is, the pulse widths are equal and complementary in the switching period, the LLC converter starts to work with the pulse width threshold as the initial pulse width of the drive of the first bridge arm switch tube and the second bridge arm switch tube, and converts the initial pulse width into a voltage signal as a comparison threshold.

[0023] When the driving of the switch tube is turned off, the pulse width of the current switching cycle is converted into a voltage signal, which is compared with a comparison threshold.

[0024] If the voltage signal is greater than the comparison threshold, it indicates that the switching frequency of the LLC converter is less than the resonant frequency at this time, and the PFM mode is continued to maintain.

[0025] If the voltage signal is greater than the comparison threshold, it indicates that the switching frequency of the LLC converter is greater than the resonant frequency at this time, and the LLC converter switches to the PWM mode. The first and second clamping tubes in the clamping branch start to work, and the driving signals of the first and second bridge arm switch tubes are fixed to twice the threshold pulse width, i.e. equal to the resonant period. The duty cycle of the driving signal is adaptively adjusted according to the actual off time of the first and second bridge arm switch tubes. The first and second clamping switch tubes are complementary to the first and second bridge arm switch tubes in the switching cycle, respectively, to form a freewheeling circuit in the PWM mode.

[0026] Compared with the prior art, the present application realizes the multi-mode switching of the clamped LLC converter according to the pulse width change and the setting of the pulse width threshold, without frequency detection. The pain point that the charge type LLC control cannot be applied to the clamped LLC converter is solved, and the combination of the two advantages of high dynamic performance and wide range of application is realized. On the other hand, the control method of the present application is accurate, reliable, low in cost and easy to realize. The mode switching is judged at the off time of the lower tube, ensuring the cycle integrity and improving the stability of the clamped LLC converter. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 It is a schematic diagram of a traditional single-voltage loop control type multi-mode clamped half-bridge LLC converter;

[0028] Figure 2 It is a schematic diagram of a charge control type multi-mode clamped half-bridge LLC converter of the present application;

[0029] Figure 3 It is a schematic diagram of a specific embodiment of the logic circuit 308 of the present application;

[0030] Figure 4 It is a working state waveform diagram of the PFM-PWM mode switching of the clamped half-bridge LLC converter of the present application;

[0031] Figure 5 It is a working state waveform diagram of the PWM-PFM mode switching of the clamped half-bridge LLC converter of the present application. DETAILED DESCRIPTION

[0032] The following will describe in detail specific embodiments of the multi-mode control method and circuit for a charge-controlled clamped LLC converter according to the present invention. It should be noted that the embodiments described herein are for illustrative purposes only and are not intended to limit the invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0033] Specifically, such as Figure 2 The diagram shown is a circuit schematic of the clamping LLC converter in this embodiment. The multi-mode control method and control circuit of this embodiment are applied to a clamping LLC resonant converter composed of an inverter circuit 101, a resonant circuit 102, a clamping branch 103, a transformer 104, a rectifier circuit 105, and a filter circuit 106. The inverter circuit 101 can be a full-bridge topology or a half-bridge topology, which is not limited here. Figure 2 The bridge arm switches S1 and S2 form an inverter circuit 101; the capacitor Cr and the inductor Lr form a resonant circuit; the clamping switches S3 and S4 form a clamping branch 103; and the diodes D1 and D2 form a rectifier circuit.

[0034] In this embodiment, a control circuit 300 for clamping LLC converter is provided, including an output voltage feedback circuit 301, an error amplifier circuit 302, a drive reset comparator circuit 303, a pulse width threshold sampling circuit 304, a resonant current sampling circuit 305, a mode control comparator circuit 306, a drive pulse width sampling circuit 307, a logic circuit 308, a control module 309, a bridge arm transistor gate drive circuit 310, and a clamping transistor gate drive circuit 311.

[0035] The first input terminal of the output voltage feedback circuit 301 is used to connect to the secondary output terminal of the clamping LLC converter and to sample the output voltage signal Vout of the clamping LLC converter; the first output terminal of the output voltage feedback circuit 301 is used to output the output voltage feedback signal V_FB, which represents the magnitude of the output voltage signal Vout, and is connected to the first input terminal of the error amplifier circuit 302.

[0036] The second input terminal of the error amplifier circuit 302 is connected to the reference voltage signal Vref, and the first output terminal of the error amplifier circuit 302 is used to output the voltage error signal Vcomp and is connected to the second input terminal of the drive reset comparator circuit 303.

[0037] The first input terminal of the drive reset comparator circuit 303 is connected to the first output terminal of the resonant current sampling circuit 305 to receive the resonant current feedback signal VCS. The first output terminal of the drive reset comparator circuit 303 outputs the drive reset signal turn_off and is connected to the first input terminal of the control module 309 to realize the reset of the bridge arm switch drive signal.

[0038] The first input end of the pulse width threshold sampling circuit 304 is connected with the pulse width threshold signal T-set and the first input end of the logic circuit 308, the first output end of the pulse width threshold sampling circuit 304 outputs the pulse width threshold voltage signal V-Tset and is connected with the first input end of the mode control comparison circuit 306, for converting the pulse width threshold signal T-set into the pulse width threshold voltage signal V-Tset.

[0039] The first input end of the resonant current sampling circuit 305 is coupled with the resonant circuit 102, for sampling the resonant current signal and processing the signal, to generate the resonant current feedback signal VCS.

[0040] The second input end of the mode control comparison circuit 306 is connected with the first output end of the drive pulse width sampling circuit 307, for receiving the drive pulse width voltage signal V_Ts, the first output end of the mode control comparison circuit 306 is connected with the second input end of the control module 309, for outputting the mode control signal Mode_C, to realize the switching of the PFM mode or the PWM mode.

[0041] The first input end of the drive pulse width sampling circuit 307 is connected with the second output end of the bridge arm tube gate drive circuit 310 and the second input end of the logic circuit 308, for receiving the drive pulse width signal Ts of the arm switch tube S1 or the arm switch tube S2 and converting the drive pulse width signal Ts into the drive pulse width voltage signal V_Ts, wherein the drive pulse width signal Ts can be the drive pulse width signal Ts of the bridge arm switch tube S1 or the drive pulse width signal Ts of the bridge arm switch tube S2.

[0042] The first output end of the logic circuit 308 is connected with the third input end of the control module 309, for outputting the drive set signal T_rest, to realize the setting of the drive signals of the bridge arm switch tube S1 and the bridge arm switch tube S2.

[0043] The first output end of the control module 309 is connected with the first input end of the bridge arm tube gate drive circuit 310, the second output end of the control module 309 is connected with the first input end of the clamping tube gate drive circuit 311, the working mode of the control module 309 has the PWM mode and the PFM mode, wherein in the PWM mode, the control module 309 is used for controlling the bridge arm tube gate drive circuit 310 and the clamping tube gate drive circuit 311; the first input end of the bridge arm tube gate drive circuit 310 is connected with the inverter circuit 101, to realize the control of the opening and the closing of the bridge arm switch tube S1 and S2; the first input end of the clamping tube gate drive circuit 311 is connected with the clamping branch 103, to realize the control of the opening and the closing of the clamping switch tube S3 and S4.

[0044] In the embodiment, a control method for a charge type control clamped LLC converter is provided, the clamped LLC converter has a multi-mode control circuit 300, comprising the following steps:

[0045] The control circuit 300 resets itself while starting up, and then detects a pulse width threshold signal T-set as the initial driving pulse width of the bridge arm switch S1 and S2, and enters a standby state.

[0046] Specifically, when the control circuit 300 starts to work, the control module 309 enters the PFM mode, and controls the bridge arm gate drive circuit 310, and the bridge arm switch S1 and S2 start to work with the pulse width threshold T-set as the initial driving pulse width. In the PFM mode, the control circuit only controls the bridge arm gate drive circuit 310 to drive the bridge arm switch S1 and S2 to work alternately in the same pulse width and complementary manner, and the duty cycle of the bridge arm switch S1 and S2 is 50%. At this time, the driving pulse width signal Ts is basically equal to the pulse width threshold T-set, and the logic circuit 308 outputs the driving set signal T_rest with the falling edge of the driving pulse width Ts as the trigger signal, to realize the set control of the driving signal of the bridge arm switch S1 and S2. The driving reset comparison circuit 303 realizes the reset control of the driving signal of the bridge arm switch S1 and S2 by comparing the resonant current feedback signal VCS and the voltage error signal Vcomp.

[0047] When the driving pulse width signal Ts is less than the pulse width threshold signal T-set, the logic circuit 308 outputs a high level at the falling edge of the pulse width threshold signal T-set as the set signal V-rest.

[0048] Further, in the PFM mode, the control circuit 300 realizes the steady state regulation of the output voltage by adjusting the switching frequency of the bridge arm switch S1 and S2, and the bridge arm gate drive circuit 310 detects the driving pulse width signal Ts in real time.

[0049] If the driving pulse width voltage signal V_Ts is greater than the pulse width threshold voltage signal V-Tset, the mode control signal Mode_C output by the mode control comparison circuit 306 is a low level signal, so that the control module 309 remains working in the PFM mode. At this time, the driving pulse width Ts is greater than the pulse width threshold T-set, and the logic circuit 308 outputs the driving set signal T_rest with the falling edge of the driving pulse width Ts as the trigger signal. Specifically, Figure 3For a specific implementation case of the logic circuit 308, a logic AND gate 3081, a first logic NOT gate 3082, and a second logic NOT gate 3083 are included. The input end of the first logic NOT gate 3082 is the first input end of the logic circuit 308, connected to the driving pulse width signal Ts. The input end of the second logic NOT gate 3083 is the second input end of the logic circuit 308, connected to the pulse width threshold signal T-set. The output ends of the first logic NOT gate 3082 and the second logic NOT gate 3083 are respectively connected to the first output end and the second output end of the logic AND gate 3081. The output end of the logic AND gate 3081 is the output end of the logic circuit 308, outputting the driving set signal V-rest. When the driving pulse width signal Ts is greater than the pulse width threshold signal T-set, the logic circuit 308 will jump from a low-level signal to a high-level signal at the falling edge of the driving pulse width signal Ts, as the set signal V-rest. The control module 309 takes the change of the driving set signal V-rest from a low-level signal to a high-level signal as a trigger signal, to realize the set control of the driving signal. The driving reset comparison circuit 303 compares the resonant current feedback signal VCS and the voltage error signal Vcomp. When the resonant current feedback signal VCS is greater than the voltage error signal Vcomp, the driving reset comparison circuit 303 outputs a jump from a low-level signal to a high-level signal, as the driving reset signal turn-off. The control module 309 takes the change of the driving reset signal turn-off from a low-level signal to a high-level signal as a trigger signal, to realize the reset control of the driving signal. The control circuit 300 adjusts the switching frequency of the bridge arm switch S1 and S2 to realize the steady-state regulation of the output voltage.

[0050] If the driving pulse width voltage signal V_Ts is less than the pulse width threshold voltage signal V-Tset, the mode control signal Mode_C output by the mode control comparison circuit 306 is a high level signal, so that the control module 309 enters the PWM mode. At this time, the driving pulse width Ts is less than the pulse width threshold T-set, and the logic circuit 308 takes the falling edge of the pulse width threshold T-set as a trigger signal to output the driving set signal T_rest. Specifically, when the driving pulse width signal Ts is less than the pulse width threshold signal T-set, the logic circuit 308 will jump from a low level signal to a high level signal at the falling edge of the pulse width threshold signal T-set, as the set signal V-rest. The control module 309 takes the change of the driving set signal V-rest from a low level signal to a high level signal as a trigger signal to realize the set control of the driving signal. The driving reset comparison circuit 303 compares the resonant current feedback signal VCS and the voltage error signal Vcomp. When the resonant current feedback signal VCS is greater than the voltage error signal Vcomp, the driving reset comparison circuit 303 outputs a high level signal from a low level signal, as the driving reset signal turn-off. The control module 309 takes the change of the driving reset signal turn-off from a low level signal to a high level signal as a trigger signal to realize the reset control of the driving signal. In addition, the control module 309 controls the clamping gate drive circuit 311 to start working, wherein the clamping switch S3 and the clamping switch S4 are respectively complementary turned on with the bridge arm switch S1 and the bridge arm switch S2 in the period of the current pulse width threshold T-set.

[0051] Specifically, the simulation circuit is used to test the control circuit and method in the embodiment. In order to illustrate the beneficial effects of the multi-mode control method and the control circuit of the embodiment, only the input voltage is debugged to realize the change of the switch driving pulse width, so as to realize the smooth switching and steady state control of the PFM / PWM mode. The same circuit parameters are used in the test process. Figure 4 PFM mode switches to the working state waveform of the PWM mode, Figure 5 PFM mode switches to the working state waveform of the PWM mode, from the working state waveform, it can be illustrated that the multi-mode control circuit and method for the charge type clamping LLC converter provided by the application can realize the smooth switching and steady state control of the PFM / PWM mode.

[0052] The above embodiments are only used to help understand the inventive concept of the application, and do not limit the application. Any modification, equivalent replacement, improvement, etc. made by those skilled in the art without departing from the principle of the application shall be included in the protection scope of the application.

Claims

1. A control method of an LLC converter, the LLC converter comprising an inverter circuit, a clamping branch and a control circuit, the inverter circuit having a first bridge arm switch (S1) and a second bridge arm switch (S2); the clamping branch having a first clamping switch (S3) and a second clamping switch (S4), characterized in that, The control method comprises: setting a pulse width threshold (T_set) and converting the pulse width threshold (T_set) into a pulse width threshold voltage signal (V_Tset); obtaining a drive pulse width voltage signal (V_Ts) representing the drive pulse width of the first bridge arm switch tube (S1) or the second bridge arm switch tube (S2); comparing the drive pulse width voltage signal (V_Ts) with the pulse width threshold voltage signal (V-Tset); when the drive pulse width voltage signal (V_Ts) is greater than the pulse width threshold voltage signal (V-Tset), controlling the control circuit to work in a PFM mode; and when the drive pulse width voltage signal (V_Ts) is less than the pulse width threshold voltage signal (V-Tset), controlling the control circuit to work in a PWM mode.

2. The control method according to claim 1, characterized by, When the control circuit works in the PFM mode, the drive pulse of the first bridge arm switch tube (S1) and the drive pulse of the second bridge arm switch tube (S2) are complementary, the duty cycle of the drive pulse of the first bridge arm switch tube (S1) and the drive pulse of the second bridge arm switch tube (S2) is 50%, and the first clamping switch tube (S3) and the second clamping switch tube (S4) remain closed; when the control circuit works in the PWM mode, in a switching cycle, the drive pulse of the first clamping switch tube (S3) is complementary to the drive pulse of the first bridge arm switch tube (S1), the drive pulse of the second clamping switch tube (S4) is complementary to the drive pulse of the second bridge arm switch tube (S2), and the switching cycle of the first bridge arm switch tube (S1) and the second bridge arm switch tube (S2) is twice the pulse width threshold (T_set).

3. The control method according to claim 1, characterized by, In the PFM mode, the falling edge of the drive pulse width voltage signal (V_Ts) is used as a drive set signal for setting the drive signals of the first bridge arm switch tube (S1) and the second bridge arm switch tube (S2); in the PWM mode, the falling edge of the pulse width threshold voltage signal (V-Tset) is used as a drive set signal for setting the drive signals of the first bridge arm switch tube (S1) and the second bridge arm switch tube (S2).

4. A control circuit for an LLC converter, the LLC converter comprising an inverter circuit, a clamping branch, and the control circuit, the inverter circuit having a first bridge arm switch (S1) and a second bridge arm switch (S2); the clamping branch having a first clamping switch (S3) and a second clamping switch (S4), the control circuit comprising: A pulse width threshold sampling circuit (304), a mode control comparison circuit (306), a drive pulse width sampling circuit (307), and a control module (309); The pulse width threshold sampling circuit (304) is used for converting a pulse width threshold signal (T-set) input from the outside into a pulse width threshold voltage signal (V-Tset) and delivering the pulse width threshold voltage signal (V-Tset) to the mode control comparison circuit (306); The drive pulse width sampling circuit (307) obtains a drive pulse width voltage signal (V_Ts) representing the drive pulse width of the first bridge arm switch tube (S1) or the second bridge arm switch tube (S2) and delivers the drive pulse width voltage signal (V_Ts) to the mode control comparison circuit (306); The pulse width threshold sampling circuit (304) is used for converting a pulse width threshold signal (T-set) input from the outside into a pulse width threshold voltage signal (V-Tset) and delivering the pulse width threshold voltage signal (V-Tset) to the mode control comparison circuit (306); The mode control comparison circuit (306) compares the pulse width threshold voltage signal (V-Tset) and the drive pulse width voltage signal (V_Ts), and outputs a mode control signal (Mode_C) to the control module (309) according to the comparison result; The control module (309) receives the mode control signal (Mode_C), and controls the control module (309) to work in a PWM mode or a PFM mode according to the mode control signal (Mode_C), specifically: When the drive pulse width voltage signal (V_Ts) is greater than the pulse width threshold voltage signal (V-Tset), the control circuit works in a PFM mode; when the drive pulse width voltage signal (V_Ts) is less than the pulse width threshold voltage signal (V-Tset), the control circuit works in a PWM mode.

5. The control circuit for an LLC converter of claim 4, wherein, The control circuit (300) further comprises a logic circuit (308), a first input end of the logic circuit (308) is connected with a drive pulse width signal (Ts) of the first bridge arm switch tube (S1) or the second bridge arm switch tube (S2), a second input end of the logic circuit (308) is connected with a pulse width threshold signal (T-set) input from outside, and an output end of the logic circuit (308) is connected with the control module (309); when the drive pulse width signal (Ts) is greater than the pulse width threshold signal (T-set), the logic circuit (308) takes a falling edge of the drive pulse width signal (Ts) as a trigger signal to output a drive set signal (V-rest), and the drive set signal is used to realize set control of drive signals of the first bridge arm switch tube (S1) and the second bridge arm switch tube (S2); when the drive pulse width signal (Ts) is less than the pulse width threshold signal (T-set), the logic circuit (308) takes a falling edge of the pulse width threshold signal (T-set) as a trigger signal to output a drive set signal (V-rest), and the drive set signal (V-rest) is used to realize set control of drive signals of the first bridge arm switch tube (S1) and the second bridge arm switch tube (S2).

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