LLC resonant converter, control unit and control method thereof

Abstract

An LLC resonant converter includes a square wave generator having a first switch and a second switch, a resonant tank, a transformer, a synchronous rectification unit having a first synchronous rectification switch and a second synchronous rectification switch, and a control unit. The control unit provides a first control signal to control the first switch, provides a second control signal to control the second switch, provides a first rectification control signal to control the first synchronous rectification switch, and provides a second rectification control signal to control the second synchronous rectification switch. When the frequency control command is lower than the phase shift frequency, the first control signal and the first rectification control signal are variable frequency signals with different phases, and the second control signal and the second rectification control signal are variable frequency signals with different phases. The present disclosure also relates to a control unit of an LLC resonant converter and a control method of an LLC resonant converter.

Description

Technical Field

[0001] The present invention relates to an LLC resonant converter, a control unit and a control method thereof, and more particularly to an LLC resonant converter with extended hold time and a control method thereof. Background Art

[0002] The LLC resonant converter (LLC resonant converter) is a DC-to-DC (DC) power converter that offers numerous advantages over other DC converters. However, due to its complex design and control methods, it has previously received less attention and research. However, recent advances in circuit design and control technology have led to the development of numerous control techniques related to LLC resonant converters, making their design easier and attracting increased attention. The LLC resonant converter offers the advantages of zero voltage turn-on (ZVS) of the primary-side switch, zero current turn-off (ZCS) of synchronous rectification on the secondary side, and high efficiency.

[0003] Taking a full-bridge LLC resonant converter as an example, its primary side includes a first bridge arm consisting of a first switch Q1 and a second switch Q2, and a second bridge arm consisting of a third switch Q3 and a fourth switch Q4, wherein the first bridge arm and the second bridge arm are connected in parallel. Its secondary side includes a synchronous rectifier switch bridge arm consisting of a first synchronous rectifier switch SR1 and a second synchronous rectifier switch SR2. The current control method provides the first switch Q1 and the fourth switch Q4 on the primary side, and the first synchronous rectifier switch SR1 on the secondary side with the same phase PWM signal, while the second switch Q2 and the third switch Q3 on the primary side, and the second synchronous rectifier switch SR2 on the secondary side provide the same phase PWM control. However, this control method has the following disadvantages when the input voltage is insufficient:

[0004] 1. High-efficiency applications: To achieve high efficiency, the voltage gain value is generally designed to be relatively low. However, this may result in insufficient voltage gain, resulting in a shorter hold-up time.

[0005] 2. High hold-time applications: In order to achieve a higher hold-time, the voltage gain is usually designed to be higher, but this can easily lead to lower efficiency.

[0006] To address the above issues, existing technologies use secondary-side synchronous rectification to perform phase shifting to achieve a higher voltage gain, thereby increasing the holdover time. This method operates the first to fourth switches Q1 through Q4, as well as the first and second synchronous rectifier switches SR1 and SR2, at a required minimum fixed frequency. The first and second synchronous rectifier switches SR1 and SR2 then begin performing phase shifting, increasing the voltage gain and extending the holdover time when the input voltage is insufficient. However, this technology has a disadvantage: operating at a relatively low switching frequency, the first and second synchronous rectifier switches SR1 and SR2 begin phase shifting, resulting in significant instantaneous changes in the voltage gain during control, making the output voltage difficult to control.

[0007] Therefore, how to design an LLC resonant converter, a control unit and a control method thereof to solve the problems existing in the prior art is an important topic studied by the inventors of the present disclosure. Summary of the Invention

[0008] The object of the present invention is to provide an LLC resonant converter to solve the problems existing in the prior art.

[0009] To achieve the aforementioned objectives, the LLC resonant converter proposed in the present invention includes a square wave generator, a resonant tank, a transformer, a synchronous rectification unit, and a control unit. The square wave generator includes a first switch and a second switch connected in series with the first switch. The resonant tank is coupled to the square wave generator. The transformer includes a primary side and a secondary side, with the primary side coupled to the resonant tank. The synchronous rectification unit is coupled to the secondary side and includes a first synchronous rectification switch and a second synchronous rectification switch. The control unit receives the output voltage of the resonant converter, obtains a frequency control command based on the output voltage, provides a first control signal to control the first switch, provides a second control signal to control the second switch, provides a first rectification control signal to control the first synchronous rectification switch, and provides a second rectification control signal to control the second synchronous rectification switch. When the frequency control command is higher than the phase-shift frequency, the first control signal and the first rectification control signal are variable frequency signals with the same phase, and the second control signal and the second rectification control signal are variable frequency signals with the same phase. When the frequency control command is lower than the phase-shift frequency, the first control signal and the first rectification control signal are variable frequency signals with different phases, and the second control signal and the second rectification control signal are variable frequency signals with different phases.

[0010] The proposed LLC resonant converter can achieve the technical effects of both high voltage gain value and improved (extended) hold-up time.

[0011] Another object of the present invention is to provide a control unit of an LLC resonant converter to solve the problems existing in the prior art.

[0012] To achieve the aforementioned objectives, the control unit of the LLC resonant converter proposed in the present invention provides a control signal to control the square wave generator on the primary side of the resonant converter and a rectifier control signal to control the synchronous rectifier unit on the secondary side. The control unit also generates a frequency control command based on the output voltage of the resonant converter. When the frequency control command is higher than the phase-shift frequency, the control signal and the rectifier control signal are variable-frequency signals with the same phase. When the frequency control command is lower than the phase-shift frequency, the control signal and the rectifier control signal are variable-frequency signals with different phases.

[0013] The proposed LLC resonant converter control unit can achieve the technical effects of both a high voltage gain value and an improved (extended) hold-up time.

[0014] Yet another object of the present invention is to provide a control method for an LLC resonant converter to solve the problems existing in the prior art.

[0015] To achieve the aforementioned objectives, the present invention proposes a control method for an LLC resonant converter, wherein the LLC resonant converter includes a switching bridge arm arranged on the primary side of a transformer and a synchronous rectification unit arranged on the secondary side of the transformer, and the switching bridge arm includes a first switch controlled by a first control signal and a second switch controlled by a second control signal, and the synchronous rectification unit includes a first synchronous rectification switch controlled by the first rectification control signal and a second synchronous rectification switch controlled by the second rectification control signal; the control method includes: obtaining an operating frequency according to an output voltage feedback value; judging whether the operating frequency is higher than a phase-shift frequency; if the operating frequency is higher than the phase-shift frequency, controlling the first control signal and the first rectification control signal to be variable frequency signals with the same phase, and controlling the second control signal and the second rectification control signal to be variable frequency signals with the same phase; and if the operating frequency is lower than the phase-shift frequency, controlling the first control signal and the first rectification control signal to be variable frequency signals with different phases, and controlling the second control signal and the second rectification control signal to be variable frequency signals with different phases.

[0016] The proposed control method for the LLC resonant converter can achieve the technical effects of both a high voltage gain value and an improved (extended) hold-up time.

[0017] In order to further understand the technologies, means and technical effects adopted by the present invention to achieve the intended objectives, please refer to the following detailed description of the present invention and the accompanying drawings. It is believed that the objectives, features and characteristics of the present invention can be understood in depth and in detail. However, the accompanying drawings are provided for reference and illustration only and are not intended to limit the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 : is a circuit diagram of the first embodiment of the LLC resonant converter of the present invention.

[0019] Figure 2 : is a circuit diagram of a second embodiment of the LLC resonant converter of the present invention.

[0020] Figure 3 : is a circuit diagram of a third embodiment of the LLC resonant converter of the present invention.

[0021] Figure 4 : is a circuit diagram of a fourth embodiment of the LLC resonant converter of the present invention.

[0022] Figure 5 : is a block diagram of a control circuit of the present invention.

[0023] Figure 6 : Schematic diagram of the frequency and phase of the LLC resonant converter of the present invention operating in different modes.

[0024] Figures 7A to 7C : Schematic diagram of control signals of the LLC resonant converter of the present invention operating in different modes.

[0025] Figure 8 : is a flow chart of the control method of the LLC resonant converter of the present invention.

[0026] Description of reference numerals:

[0027] 10: LLC resonant converter

[0028] T: Transformer

[0029] 12: Primary side circuit

[0030] 14: Secondary side circuit

[0031] 16: Controller

[0032] 122: First switch bridge arm

[0033] 124: Second switch bridge arm

[0034] 126: Resonance Tank

[0035] 142: First synchronous rectifier bridge arm

[0036] 144: Second synchronous rectifier bridge arm

[0037] Q1: First switch

[0038] Q2: Second switch

[0039] Q3: The third switch

[0040] Q4: The fourth switch

[0041] SR1: First synchronous rectification switch

[0042] SR2: Second synchronous rectification switch

[0043] SR3: The third synchronous rectification switch

[0044] SR4: Fourth synchronous rectification switch

[0045] Lr: resonant inductance

[0046] Cr: resonant capacitor

[0047] S Q1 : First control signal

[0048] S Q2 : Second control signal

[0049] S Q3 : The third control signal

[0050] S Q4 : The fourth control signal

[0051] S SR1 : First rectifier control signal

[0052] S SR2 : Second rectifier control signal

[0053] S SR3 : The third rectifier control signal

[0054] S SR4 : The fourth rectifier control signal

[0055] 161: Comparison unit

[0056] 162: Voltage Controller

[0057] 163: Frequency Limiter

[0058] 164: Frequency Controller

[0059] 165: Frequency and Phase Controller

[0060] V OUT_FB : Output voltage feedback value

[0061] V OUT_REF : Output voltage reference value

[0062] V ERR : Voltage error value

[0063] f CMD : Frequency control command

[0064] α: Phase difference

[0065] f R : Resonant frequency

[0066] f PS : Phase shift frequency

[0067] S11~S15:Steps DETAILED DESCRIPTION

[0068] The technical content and detailed description of the present invention are described as follows with reference to the accompanying drawings.

[0069] The disclosed LLC resonant converter includes a square wave generator, a resonant tank, a transformer, a synchronous rectification unit, and a control unit. The LLC resonant converter includes a hold-up time extension function. The square wave generator is coupled to the resonant tank and the primary side of the transformer and includes a first switch and a second switch connected in series with the first switch. The synchronous rectification unit is coupled to the secondary side of the transformer and includes a first synchronous rectification switch and a second synchronous rectification switch.

[0070] The control unit receives an output voltage signal from the LLC resonant converter, where the output voltage signal provides information about the output voltage of the LLC resonant converter. The control unit obtains a frequency control command based on the output voltage signal, provides a first control signal to control the first switch, provides a second control signal to control the second switch, provides a first rectifier control signal to control the first synchronous rectifier switch, and provides a second rectifier control signal to control the second synchronous rectifier switch. It is worth noting that the frequency control command is the operating frequency of the first control signal, the second control signal, the first rectifier control signal, and the second rectifier control signal.

[0071] The following describes the differences between different circuit topologies. Figures 1 to 4 , which are circuit diagrams of the first to fourth embodiments of the LLC resonant converter according to the present invention, respectively. The LLC resonant converter 10 comprises a transformer T, a primary-side circuit 12 , a secondary-side circuit 14 and a control unit 16 .

[0072] Figure 1 and Figure 2 The primary-side circuit 12 is a full-bridge structure and includes a first switching arm 122 , a second switching arm 124 , and a resonant tank 126 , which form a square wave generator.

[0073] The first switch bridge arm 122 is coupled to the primary side of the transformer T and includes a first switch Q1 and a second switch Q2 connected in series with the first switch Q1. The control unit 16 provides a first control signal S Q1 Control the first switch Q1 and provide the second control signal S Q2The second switch bridge arm 124 is connected in parallel with the first switch bridge arm 122, and has a third switch Q3 and a fourth switch Q4 connected in series with the third switch Q3. The control unit 16 provides a third control signal S Q3 Control the third switch Q3 to provide a fourth control signal S Q4 Control the fourth switch Q4. In the application of the present invention, the first control signal S Q1 With the fourth control signal S Q4 is the same control signal; the second control signal S Q2 and the third control signal S Q3 For the same control signal (see Figure 7A As shown). In addition, the first control signal S Q1 With the second control signal S Q2 are complementary control signals.

[0074] The resonant tank 126 is coupled between the first switching bridge arm 122 and the second switching bridge arm 124 , and is an LLC resonant tank mainly composed of a resonant inductor Lr, a magnetizing inductor (not shown) of the transformer T, and a resonant capacitor Cr.

[0075] Figure 3 and Figure 4 The primary-side circuit 12 shown is a half-bridge configuration. The primary-side circuit 12 includes a first switching arm 122 and a resonant tank 126. The first switching arm 122 is coupled to the primary side of the transformer T and includes a first switch Q1 and a second switch Q2 connected in series with the first switch Q1. The control unit 16 provides a first control signal S Q1 Control the first switch Q1 and provide the second control signal S Q2 Control the second switch Q2. In the application of the present invention, the first control signal S Q1 With the second control signal S Q2 are complementary control signals.

[0076] The resonant tank 126 couples the first switch Q1 and the second switch Q2 of the first switching bridge arm 122 and primarily comprises an LLC resonant tank consisting of a resonant inductor Lr, the magnetizing inductor of the transformer T (not shown), and a resonant capacitor Cr. The resonant tank in the present invention is not limited to the connection shown; any structure that utilizes LC to generate two resonant frequencies is also encompassed within the scope of the present invention.

[0077] Figure 1 and Figure 3The secondary-side circuit 14 shown is a center-taped architecture. The secondary-side circuit 14 includes a first synchronous rectifier bridge arm 142 coupled to the secondary side of the transformer T, having a first synchronous rectifier switch SR1 and a second synchronous rectifier switch SR2 connected in series with the first synchronous rectifier switch SR1. The output side of the LLC resonant converter 10 is formed by the center tap of the transformer T and the common point of the first synchronous rectifier switch SR1 and the second synchronous rectifier switch SR2. The control unit 16 provides a first rectifier control signal S SR1 Controls the first synchronous rectification switch SR1 and provides a second rectification control signal S SR2 Control the second synchronous rectification switch SR2. In the application of the present invention, the first rectification control signal S SR1 and the second rectifier control signal S SR2 Complementary control signals (see Figure 7A shown).

[0078] Figure 2 and Figure 4 The secondary-side circuit 14 shown is a full-bridge architecture. The secondary-side circuit 14 includes a first synchronous rectifier bridge arm 142 and a second synchronous rectifier bridge arm 144. The first synchronous rectifier bridge arm 142 is coupled to the secondary side of the transformer T and includes a first synchronous rectifier switch SR1 and a second synchronous rectifier switch SR2 connected in series with the first synchronous rectifier switch SR1. The second synchronous rectifier bridge arm 144 is connected in parallel with the first synchronous rectifier bridge arm 142 and includes a third synchronous rectifier switch SR3 and a fourth synchronous rectifier switch SR4 connected in series with the third synchronous rectifier switch SR3. The control unit 16 provides a first rectifier control signal S SR1 Controls the first synchronous rectification switch SR1 and provides a second rectification control signal S SR2 Controls the second synchronous rectification switch SR2 and provides a third rectification control signal S SR3 Control the third synchronous rectification switch SR3 to provide a fourth rectification control signal S SR4 Control the fourth synchronous rectification switch SR4. The first rectification control signal S SR1 and the fourth rectifier control signal S SR4 is the same control signal; the second rectifier control signal S SR2 and the third rectifier control signal S SR3 is the same control signal. And, the first rectifier control signal S SR1 and the second rectifier control signal S SR2 are complementary control signals.

[0079] In the following, to explain the operation and control principle of LLC resonant converter, Figure 1 The circuit topology shown is used as an example. Figure 5 and Figure 6 , which are respectively a block diagram of the control circuit of the present invention and a frequency and phase diagram of the LLC resonant converter of the present invention operating in different modes.

[0080] The control unit 16 receives the output voltage signal output by the LLC resonant converter 10 and obtains the frequency control command f according to the output voltage signal. CMD Specifically, the control unit 16 receives the output voltage feedback value V of the LLC resonant converter 10 through the comparison unit 161. OUT_FB With the output voltage reference value V OUT_REF The comparison unit 161 outputs the voltage reference value V OUT_REF and the output voltage feedback value V OUT_FB After subtraction and comparison, the voltage error value V is obtained. ERR .

[0081] The voltage controller 162 of the control unit 16 receives the voltage error value V ERR , and the voltage error value V ERR The frequency control command f can be obtained by calculation CMD The voltage controller 162 is taken as a proportional-integral controller (PI controller) as an example, but the present invention is not limited thereto. The voltage controller 162 controls the voltage error value V ERR Perform linear combination operation of proportion and integral to obtain the control quantity, that is, the frequency control command f CMD Furthermore, in order to ensure the frequency control command f CMD Will not exceed the maximum value f of the control command MAX Or it is lower than the minimum value f of the control command MIN Therefore, the frequency control command f is limited by the frequency limiter 163. CMD The upper and lower limits of .

[0082] See 7A to 7C FIG. 1 is a schematic diagram of the control signal of the LLC resonant converter of the present invention operating in different modes, and FIG. 2 is a schematic diagram of the control signal of the LLC resonant converter of the present invention operating in different modes, and FIG. 3 is a schematic diagram of the control signal of the LLC resonant converter of the present invention operating in different modes, and FIG. 4 is a schematic diagram of the control signal of the LLC resonant converter of the present invention operating in different modes, and FIG. 5 is a schematic diagram of the control Figure 6 When the frequency control command f CMD Higher than the phase shift frequency f set by the control unit 16 PS and higher than the resonant frequency f of the LLC resonant converter 10 R When the LLC resonant converter 10 operates in the first mode M1 (see Figure 7A ), in this mode, the first control signal S provided by the control unit 16 Q1 (with the fourth control signal S Q4 is the same signal) and the first rectifier control signal S SR1 is a frequency conversion signal with the same phase. At the same time, the second control signal S Q2(with the third control signal S Q3 is the same signal) and the second rectifier control signal (S SR2 ) is a frequency conversion signal with the same phase. Therefore, the first control signal S Q1 and the first rectifier control signal S SR1 The phase difference α of the conduction signal rising edge is 0 degrees, and the second control signal S Q2 and the second rectifier control signal S SR2 The phase difference α of the conduction signal rising edge is also 0 degrees. In this mode, the first switch Q1, the second switch Q2, the third switch Q3, the fourth switch Q4 on the primary side and the first synchronous rectifier switch SR1 and the second synchronous rectifier switch SR2 on the secondary side all operate in variable frequency mode. In this mode, the variable frequency control is achieved by Figure 5 This is implemented by the frequency controller 164 shown.

[0083] When the frequency control command f CMD Higher than the phase shift frequency f set by the control unit 16 PS But it is lower than the resonant frequency f of the LLC resonant converter 10 R When the LLC resonant converter 10 operates in the second mode M2 (see Figure 7B ), in this mode, the first control signal S provided by the control unit 16 Q1 (with the fourth control signal S Q4 is the same signal) and the first rectifier control signal S SR1 is a frequency conversion signal with the same phase. At the same time, the second control signal S Q2 (with the third control signal S Q3 is the same signal) and the second rectifier control signal (S SR2 ) is a frequency conversion signal with the same phase. That is, the first control signal S Q1 and the first rectifier control signal S SR1 The phase difference α of the conduction signal rising edge is 0 degrees, and the second control signal S Q2 and the second rectifier control signal S SR2 The phase difference α of the conduction signal rising edge is also 0 degrees. In this mode, the first rectifier control signal S SR1 and the second rectifier control signal S SR2 The duty cycle (duty cycle) is the resonant period, where the resonant period is the resonant frequency f R The reciprocal of , that is, 1 / f R. In this mode, the first switch Q1, the second switch Q2, the third switch Q3, the fourth switch Q4 on the primary side and the first synchronous rectifier switch SR1 and the second synchronous rectifier switch SR2 on the secondary side all operate in variable frequency mode, and the duty cycle of the first synchronous rectifier switch SR1 and the second synchronous rectifier switch SR2 on the secondary side will be limited to the resonant period. That is, the on-time of the first synchronous rectifier switch SR1 is limited from the original time 0 to time t1 to time 0 to time t1'; the on-time of the second synchronous rectifier switch SR2 is limited from the original time t1 to time t2 to time t1 to time t2'. The variable frequency control in this mode is achieved through Figure 5 This is implemented by the frequency controller 164 shown.

[0084] When the frequency control command f CMD Lower than the phase shift frequency f set by the control unit 16 PS When the LLC resonant converter 10 operates in the third mode M3 (see Figure 7C ), in this mode, the first control signal S provided by the control unit 16 Q1 (with the fourth control signal S Q4 is the same signal) and the first rectifier control signal S SR1 is a frequency conversion signal with different phases. At the same time, the second control signal S Q2 (with the third control signal S Q3 is the same signal) and the second rectifier control signal S SR2 is a frequency conversion signal with different phases. That is, the first control signal S Q1 and the first rectifier control signal S SR1 The phase difference α of the conduction signal rising edge becomes larger as the frequency becomes lower. For example, the phase difference Φ L is any number between 0 and 180 and greater than 0. It is worth mentioning that the control unit 16 controls the first rectifier control signal S SR1 The phase of the first control signal S Q1 The phase is Φ L The specific control method is to increase the conduction period of the first synchronous rectification switch SR1 to achieve the first rectification control signal S SR1 The phase of the rising edge of the turn-on signal is advanced (from the original time 0 to time t1", that is, the lead Φ L Similarly, the second control signal S Q2 and the second rectifier control signal S SR2 The phase difference α of the conduction signal rising edge becomes larger as the frequency becomes lower. It also increases the conduction period of the second synchronous rectifier switch SR2 to achieve the second rectifier control signal S SR2 The phase of the conduction signal rising edge is advanced (from the original time t1 to time t2", that is, the lead ΦL Through this control method, when the output voltage of the LLC resonant converter 10 begins to drop due to insufficient input voltage, its output voltage can still be maintained within a voltage range for a maintenance period, allowing the electronic products coupled to the back end to have sufficient time to react and completely store or back up the data before the power outage.

[0085] In this mode, the first switch Q1, the second switch Q2, the third switch Q3, and the fourth switch Q4 on the primary side maintain operation in the variable frequency mode, while the first synchronous rectifier switch SR1 and the second synchronous rectifier switch SR2 on the secondary side operate in the variable frequency mode and the phase shift (phase advance) mode. In this mode, the variable frequency control and phase advance control are achieved through Figure 5 The frequency and phase controller 165 is shown.

[0086] See Figure 8 , which is a flow chart of a control method for an LLC resonant converter according to the present invention. The LLC resonant converter includes a square wave generator disposed on the primary side of a transformer and a synchronous rectification unit disposed on the secondary side of the transformer. The square wave generator includes at least a first switch controlled by a first control signal and a second switch controlled by a second control signal. The synchronous rectification unit includes at least a first synchronous rectification switch controlled by a first rectification control signal and a second synchronous rectification switch controlled by a second rectification control signal.

[0087] The control method includes: first, receiving an output voltage feedback value and an output voltage reference value, and comparing the output voltage feedback value and the output voltage reference value to generate a voltage error value (S11). Then, calculating the voltage error value to obtain a frequency control command (S12). Then, determining whether the frequency control command is higher than the phase shift frequency (S13). If so (i.e., the frequency control command is higher than the phase shift frequency), controlling the first control signal and the first rectifier control signal to be variable frequency signals with the same phase, and controlling the second control signal and the second rectifier control signal to be variable frequency signals with the same phase (S14). If not (i.e., the frequency control command is lower than the phase shift frequency), controlling the first control signal and the first rectifier control signal to be variable frequency signals with different phases, and controlling the second control signal and the second rectifier control signal to be variable frequency signals with different phases (S15).

[0088] Step (S14) further includes: when the frequency control command is higher than the phase shift frequency but lower than the resonant frequency, controlling the duty cycle of the first rectifier control signal and the second rectifier control signal to be the resonant period.

[0089] Step (S15) further includes: when the frequency control command is lower than the phase shift frequency, controlling the phase of the first rectifier control signal to lead the phase of the first control signal, and controlling the phase of the second rectifier control signal to lead the phase of the second control signal.

[0090] Therefore, when the input voltage of the LLC resonant converter is insufficient, the output voltage can still be maintained within a voltage range for a period of time, so that the electronic products coupled to the back end have enough time to react and completely store or back up the data before the power failure.

[0091] The above description is only a detailed description and drawings of preferred specific embodiments of the present invention, and the features of the present invention are not limited thereto and are not intended to limit the present invention. The full scope of the present invention shall be based on the following claims. All embodiments that are consistent with the concepts of the claims of the present invention and similar variations thereof shall be included in the scope of the present invention. Any changes or modifications that can be easily conceived by any person skilled in the art within the field of the present invention shall be covered by the following claims of the present disclosure.

Claims

1. An LLC resonant converter, comprising: A square wave generator comprising a first switch and a second switch connected in series with the first switch; a resonant tank coupled to the square wave generator; a transformer having a primary side and a secondary side, the primary side being coupled to the resonant tank; a synchronous rectification unit coupled to the secondary side and having a first synchronous rectification switch and a second synchronous rectification switch; and a control unit that receives an output voltage of the resonant converter, obtains a frequency control command based on the output voltage, provides a first control signal to control the first switch, provides a second control signal to control the second switch, provides a first rectification control signal to control the first synchronous rectification switch, and provides a second rectification control signal to control the second synchronous rectification switch; When the frequency control command is higher than a phase shift frequency, the first control signal and the first rectifier control signal are variable frequency signals with the same phase, and the second control signal and the second rectifier control signal are variable frequency signals with the same phase; when the frequency control command is lower than the phase shift frequency, the first control signal and the first rectifier control signal are variable frequency signals with different phases, and the second control signal and the second rectifier control signal are variable frequency signals with different phases, wherein the phase shift frequency is lower than the resonant frequency of the LLC resonant converter. When the frequency control command is lower than the phase shift frequency, the phase of the first rectifier control signal leads the phase of the first control signal, and the phase of the second rectifier control signal leads the phase of the second control signal. Among them, by controlling the conduction period of the first synchronous rectification switch to increase, the phase of the first rectification control signal leads the phase of the first control signal; by controlling the conduction period of the second synchronous rectification switch to increase, the phase of the second rectification control signal leads the phase of the second control signal.

2. The LLC resonant converter according to claim 1, wherein: When the frequency control command is higher than the phase shift frequency but lower than the resonant frequency, the duty cycle of the first rectifier control signal and the second rectifier control signal is a resonant period.

3. The LLC resonant converter according to claim 1, wherein: The control unit contains: a comparison unit receiving an output voltage feedback value and an output voltage reference value corresponding to the output voltage, and comparing the output voltage feedback value with the output voltage reference value to generate a voltage error value; and A voltage controller receives the voltage error value and performs calculation on the voltage error value to obtain the frequency control command.

4. The LLC resonant converter according to claim 1, wherein: The square wave generator includes a first switch bridge arm composed of the first switch and the second switch to form a half-bridge circuit structure.

5. The LLC resonant converter according to claim 1, wherein: The square wave generator includes a first switch bridge arm composed of the first switch and the second switch, and a second switch bridge arm composed of a third switch and a fourth switch, so as to form a full-bridge circuit structure.

6. The LLC resonant converter according to claim 1, wherein: The transformer is a center-tapped structure, and the first synchronous rectification switch and the second synchronous rectification switch are respectively coupled to two ends of the transformer.

7. The LLC resonant converter according to claim 1, wherein: The transformer is coupled to a first synchronous rectification bridge arm consisting of the first synchronous rectification switch and the second synchronous rectification switch, and a second synchronous rectification bridge arm consisting of a third synchronous rectification switch and a fourth synchronous rectification switch to form a full-bridge rectification circuit architecture.

8. A control unit of an LLC resonant converter, the control unit providing a control signal to control a square wave generator on a primary side of the resonant converter, providing a rectification control signal to control a synchronous rectification unit on a secondary side, and the control unit obtaining a frequency control command based on an output voltage of the resonant converter; in, When the frequency control command is higher than a phase shift frequency, the control signal and the rectifier control signal are variable frequency signals with the same phase; when the frequency control command is lower than the phase shift frequency, the control signal and the rectifier control signal are variable frequency signals with different phases, wherein the phase shift frequency is lower than the resonant frequency of the LLC resonant converter. When the frequency control command is lower than the phase shift frequency, the phase of the rectifier control signal leads the phase of the control signal, wherein the phase of the rectifier control signal leads the phase of the control signal by increasing the conduction period of a synchronous rectifier switch controlled by the rectifier control signal.

9. The control unit of the LLC resonant converter according to claim 8, wherein: When the frequency control command is higher than the phase shift frequency but lower than the resonant frequency, the duty cycle of the rectifier control signal is a resonant period.

10. A control method for an LLC resonant converter, wherein the resonant converter comprises a switching arm disposed on a primary side of a transformer and a synchronous rectifier unit disposed on a secondary side of the transformer, wherein the switching arm comprises a first switch controlled by a first control signal and a second switch controlled by a second control signal, and the synchronous rectifier unit comprises a first synchronous rectifier switch controlled by a first rectifier control signal and a second synchronous rectifier switch controlled by a second rectifier control signal; the control method comprising: Obtaining an operating frequency according to an output voltage feedback value; Determine whether the operating frequency is higher than a phase shift frequency, wherein: The phase shift frequency is lower than the resonant frequency of the LLC resonant converter; If the operating frequency is higher than the phase shift frequency, controlling the first control signal and the first rectifier control signal to be variable frequency signals with the same phase, and controlling the second control signal and the second rectifier control signal to be variable frequency signals with the same phase; If the operating frequency is lower than the phase shift frequency, the first control signal and the first rectifier control signal are controlled to be variable frequency signals with different phases, and the second control signal and the second rectifier control signal are controlled to be variable frequency signals with different phases. When the operating frequency is lower than the phase shift frequency, the phase of the first rectifier control signal is controlled to lead the phase of the first control signal, and the phase of the second rectifier control signal is controlled to lead the phase of the second control signal. The conduction period of the first synchronous rectification switch is controlled to increase, so that the phase of the first rectification control signal leads the phase of the first control signal; the conduction period of the second synchronous rectification switch is controlled to increase, so that the phase of the second rectification control signal leads the phase of the second control signal.

11. The control method of the LLC resonant converter according to claim 10, further comprising: When the operating frequency is higher than the phase shift frequency but lower than the resonant frequency, the duty ratios of the first rectifier control signal and the second rectifier control signal are controlled to be a resonant period.

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