Software control-based synchronous rectification current backflow prevention method and circuit

Abstract

The invention discloses a synchronous rectification current backflow prevention method based on software control, and the method comprises the steps: setting a comparison value to be smaller than a resonance point period value, a synchronous rectification control waveform to be the same as an LLC drive waveform in frequency, a secondary side switch conduction time to be smaller than a primary side switch conduction time, and enabling the secondary side switch and the primary side switch to be switched on at the same time when a switching frequency is smaller than a resonant frequency; the secondary side switch is turned off earlier than the primary side switch. According to the method, the upper limit of the conduction time of the secondary side switch is locked at the conduction time corresponding to the resonant frequency, so that the efficiency is improved through synchronous rectification, and the current is prevented from flowing backwards. In addition, the method is based on software control, additional hardware devices are not needed, and cost is reduced.

Description

Technical Field

[0001] The present invention relates to the technical field of battery management systems, and in particular to a synchronous rectification current backflow prevention method and circuit based on software control. Background Art

[0002] With the widespread popularity of energy storage devices, various technologies in battery packs have developed rapidly. LLC resonant circuit, as an important component of DC / DC converters, is indispensable in energy storage devices. In the over-resonance mode, the secondary diode of the bidirectional LLC resonant circuit operates in DCM mode. When the switching frequency is lower than the resonant frequency, the synchronous rectification will cause current backflow and reduce efficiency. The current solution is to use the secondary MOS as a diode or to use a hardware current detection solution. Both methods use hardware methods, which are costly and inefficient. Therefore, a new method is needed to control the current backflow problem caused by the synchronous rectification. Stop current backflow. Summary of the invention

[0003] In order to solve the above technical problems, the present invention provides a method and circuit for preventing current backflow of synchronous rectification based on software control. When the switching frequency is greater than or equal to the resonant frequency, the software-controlled synchronous rectification control waveform is the same as the LLC drive waveform, and the secondary switch on time is equal to the primary switch on time; when the switching frequency is less than the resonant frequency, the method sets the comparison value less than the resonance point period value, thereby controlling the secondary switch on time to be less than the primary switch on time, the secondary switch and the primary switch are turned on at the same time, and the secondary switch is turned off earlier than the primary switch. That is, the method locks the upper limit of the on time of the secondary switch at the on time corresponding to the resonant frequency, that is, the efficiency is improved through synchronous rectification, and current backflow is prevented.

[0004] The technical solution adopted by the present invention is specifically: a synchronous rectification current backflow prevention method based on software control according to an embodiment of the present invention, the method comprises the following steps: S1: When the switching frequency is greater than the resonant frequency, the synchronous rectification drive and the LLC drive are turned on and off at the same time, and the secondary switch on time is equal to the primary switch on time; S2: When the switching frequency is equal to the resonant frequency, the synchronous rectification drive and the LLC drive are turned on and off at the same time, and the secondary switch on time is equal to the primary switch on time; S3: When the switching frequency is lower than the resonant frequency, the synchronous rectification drive and the LLC drive are turned on at the same time, the LLC drive on-time is equal to half of the reciprocal of the resonant frequency, and the secondary switch on-time is lower than the primary switch on-time.

[0005] As an optional solution of the technical solution of the present invention, the resonant frequency expression is: , Among them, ƒ r is the resonant frequency, L r1 is the resonant inductance, L r2 is the transformer leakage inductance, C r is the resonant capacitor.

[0006] As an optional solution of the technical solution of the present invention, in step S3, the secondary switch and the primary switch are turned on at the same time, and the secondary switch is turned off earlier than the primary switch.

[0007] As an optional scheme of the technical scheme of the present invention, according to another embodiment of the synchronous rectification anti-current backflow circuit of the present invention, the circuit implements the above-mentioned synchronous rectification anti-current backflow method based on software control, including a primary H-bridge, a resonant cavity and a secondary H-bridge connected in series, wherein the input terminal of the primary H-bridge is connected to the voltage input, and the output terminal of the secondary H-bridge is connected to the voltage output.

[0008] As an optional solution of the technical solution of the present invention, the primary side H-bridge includes a first switch tube, a second switch tube, a third switch tube and a fourth switch tube, and a diode is connected between the source and the drain of each switch tube, the positive electrode of the diode is connected to the source of the switch tube, and the negative electrode is connected to the drain of the switch tube; wherein the source of the first switch tube is connected to the drain of the second switch tube, and is connected to the positive electrode of the input end of the resonant cavity; the source of the third switch tube is connected to the drain of the fourth switch tube, and is connected to the negative electrode of the input end of the resonant cavity; the drain of the first switch tube is connected to the drain of the third switch tube, and the source of the second switch tube is connected to the source of the fourth switch tube; the gates of the first switch tube and the fourth switch tube are connected to the first control end of the LLC controller, and the gates of the second switch tube and the third switch tube are connected to the second control end of the LLC controller.

[0009] As an optional solution of the technical solution of the present invention, the resonant cavity includes a resonant inductor, an excitation inductor, a first resistor and a resonant capacitor, the resonant inductor is connected to the positive output end of the primary H-bridge, the resonant capacitor is connected to the negative output end of the primary H-bridge, the resonant cavity also includes a transformer, the input end of the transformer is connected to the two ends of the excitation inductor, and the output end is connected to the secondary H-bridge.

[0010] As an optional solution of the technical solution of the present invention, the secondary side H bridge includes a fifth switch tube, a sixth switch tube, a seventh switch tube and an eighth switch tube, and a diode is connected between the source and the drain of each switch tube, the positive electrode of the diode is connected to the source of the switch tube, and the negative electrode is connected to the drain of the switch tube; wherein the source of the fifth switch tube is connected to the drain of the sixth switch tube, and is connected to the positive electrode of the output end of the transformer; the source of the seventh switch tube is connected to the drain of the eighth switch tube, and is connected to the negative electrode of the output end of the transformer; the drain of the fifth switch tube is connected to the drain of the seventh switch tube, and the source of the sixth switch tube is connected to the source of the eighth switch tube; the gates of the fifth switch tube and the eighth switch tube are connected to the third control end of the LLC controller, and the gates of the sixth switch tube and the seventh switch tube are connected to the fourth control end of the LLC controller.

[0011] The beneficial effects achieved by the present invention are as follows: the software-controlled synchronous rectification method for preventing current backflow, when the switching frequency is less than the resonant frequency, controls the conduction time of the secondary switch to be less than the conduction time of the primary switch by setting the comparison value less than the resonant point period value, and the secondary switch and the primary switch are turned on at the same time, and the secondary switch is turned off earlier than the primary switch. That is, the method locks the upper limit of the conduction time of the secondary switch at the conduction time corresponding to the resonant frequency, that is, the efficiency is improved through synchronous rectification, and current backflow is prevented. In addition, the method is based on software control, and no additional hardware devices are required, which reduces costs.

[0012] The effects of the present invention are not limited to the above effects, and those skilled in the art can derive effects not described above from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 4 is a synchronous rectification circuit diagram for preventing current backflow according to an embodiment of the present invention.

[0014] Figure 2 It is the resonant inductor current waveform and A1 test current waveform when the switching frequency is greater than the resonant frequency.

[0015] Figure 3 When the switching frequency is equal to the resonant frequency, the resonant inductor current waveform and the A1 test current waveform are shown.

[0016] Figure 4 It is the resonant inductor current waveform before optimization and the A1 test current waveform when the switching frequency is lower than the resonant frequency.

[0017] Figure 5 When the switching frequency is less than the resonant frequency, the optimized resonant inductor current waveform and A1 test current waveform are shown.

[0018] Figure 6 This is the LLC drive waveform and synchronous rectification control waveform before optimization.

[0019] Figure 7 This is the LLC drive waveform and synchronous rectification control waveform after optimization.

[0020] Figure 8 This is the schematic diagram of the LLC controller outputting the LLC drive waveform and the synchronous rectification control waveform. DETAILED DESCRIPTION

[0021] In order to make the technical problems, technical solutions and beneficial effects solved by the present invention clearer, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the following specific embodiments are only used to explain the present invention, but not to limit the present invention. Based on the following embodiments, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention.

[0022] It should be noted that in the description of the present invention, the terms "include" and "comprise" indicate the presence of the described features, wholes, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components and / or their collections. The orientation or positional relationship indicated by the terms "upper", "lower", "left", "right", "front", "back" and the like is based on the orientation or positional relationship of the accompanying drawings, and is only a simplified description for the convenience of describing the present invention, and therefore cannot be understood as a limitation of the present invention. In the description of the embodiments, unless otherwise clearly specified and limited, the terms "set", "connect" and the like should be understood in a broad sense. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0023] like Figure 1The figure shows a synchronous rectification anti-current backflow circuit diagram of an embodiment of the present invention. From left to right, they are the primary H-bridge, the resonant cavity and the secondary H-bridge, wherein the input terminal of the primary H-bridge is connected to the voltage input, and the output terminal of the secondary H-bridge is connected to the energy storage filter capacitor C1 and then to the voltage output. The primary H-bridge includes a first switch tube Q1, a second switch tube Q2, a third switch tube Q3 and a fourth switch tube Q4. The source of the first switch tube Q1 is connected to the drain of the second switch tube Q2 to form the positive electrode of the output terminal of the primary H-bridge, and is connected to the positive electrode of the input terminal of the resonant cavity; the source of the third switch tube Q3 is connected to the drain of the fourth switch tube Q4 to form the negative electrode of the output terminal of the primary H-bridge, and is connected to the negative electrode of the input terminal of the resonant cavity. The drain of the first switch tube Q1 is connected to the drain of the third switch tube Q3 to form the positive electrode of the input terminal of the primary H-bridge, and is connected to the positive electrode of the voltage input terminal; the source of the second switch tube Q2 is connected to the source of the fourth switch tube Q4 to form the negative electrode of the input terminal of the primary H-bridge, and is connected to the negative electrode of the voltage input terminal. The gates of the first switch tube Q1 and the fourth switch tube Q4 are connected to the first control terminal Dr1 of the LLC controller, and the gates of the second switch tube Q2 and the third switch tube Q3 are connected to the second control terminal Dr2 of the LLC controller (the LLC controller is not shown in the figure). In addition, a diode is connected between the source and drain of each switch tube. A first diode D1 is connected between the source and drain of the first switch tube Q1, the anode of the first diode D1 is connected to the source of the first switch tube Q1, and the cathode of the first diode D1 is connected to the drain of the first switch tube Q1; a second diode D2 is connected between the source and drain of the second switch tube Q2, the anode of the second diode D2 is connected to the source of the second switch tube Q2, and the cathode of the second diode D2 is connected to the drain of the second switch tube Q2; a third diode D3 is connected between the source and drain of the third switch tube Q3, the anode of the third diode D3 is connected to the source of the third switch tube Q3, and the cathode of the third diode D3 is connected to the drain of the third switch tube Q3; a fourth diode D4 is connected between the source and drain of the fourth switch tube Q4, the anode of the fourth diode D4 is connected to the source of the fourth switch tube Q4, and the cathode of the fourth diode D4 is connected to the drain of the fourth switch tube Q4.

[0024] like Figure 1 As shown, the resonant cavity includes a series resonant inductor Lr1, an excitation inductor Lm, a first resistor R1 and a primary resonant capacitor Cr, the resonant inductor Lr1 is connected to the positive output end of the primary H-bridge, and the primary resonant capacitor Cr is connected to the negative output end of the primary H-bridge; the resonant cavity also includes a transformer T, the input end of the transformer T is connected to the two ends of the excitation inductor Lm, and the output end is connected to the secondary H-bridge. In addition, Lr2 is the transformer leakage inductance.

[0025] like Figure 1As shown, the secondary side H bridge includes a fifth switch tube Q5, a sixth switch tube Q6, a seventh switch tube Q7 and an eighth switch tube Q8. The source of the fifth switch tube Q5 is connected to the drain of the sixth switch tube Q6 to form the positive input terminal of the secondary side H bridge, and is also connected to the positive output terminal of the transformer T; the source of the seventh switch tube Q7 is connected to the drain of the eighth switch tube Q8 to form the negative input terminal of the secondary side H bridge, and is also connected to the negative output terminal of the transformer T. The drain of the fifth switch tube Q5 is connected in series with the ammeter A1 and then connected to the drain of the seventh switch tube Q7 to form the positive output terminal of the secondary side H bridge, and then connected to the positive voltage output terminal after the positive energy storage filter capacitor C1; the source of the sixth switch tube Q6 is connected to the source of the eighth switch tube Q8 to form the negative output terminal of the secondary side H bridge, and then connected to the negative voltage output terminal after the negative energy storage filter capacitor C1. The gates of the fifth switch tube Q5 and the eighth switch tube Q8 are connected to the third control terminal Dr3 of the LLC controller, and the gates of the sixth switch tube Q6 and the seventh switch tube Q7 are connected to the fourth control terminal Dr4 of the LLC controller. In addition, a diode is connected between the source and drain of each switch tube. A fifth diode D5 is connected between the source and drain of the fifth switch tube Q5, the anode of the fifth diode D5 is connected to the source of the fifth switch tube Q5, and the cathode of the fifth diode D5 is connected to the drain of the fifth switch tube Q5; a sixth diode D6 is connected between the source and drain of the sixth switch tube Q6, the anode of the sixth diode D6 is connected to the source of the sixth switch tube Q6, and the cathode of the sixth diode D6 is connected to the drain of the sixth switch tube Q6; a seventh diode D7 is connected between the source and drain of the seventh switch tube Q7, the anode of the seventh diode D7 is connected to the source of the seventh switch tube Q7, and the cathode of the seventh diode D7 is connected to the drain of the seventh switch tube Q7; an eighth diode D8 is connected between the source and drain of the eighth switch tube Q8, the anode of the eighth diode D8 is connected to the source of the eighth switch tube Q8, and the cathode of the eighth diode D8 is connected to the drain of the eighth switch tube Q8.

[0026] like Figure 2 The figure shows the resonant inductor current waveform and A1 test current waveform when the switching frequency is greater than the resonant frequency. At this time, the LLC drive waveform and the synchronous rectification control waveform are as follows: Figure 6 As shown, the synchronous rectification control waveform is the same as the LLC driving waveform, that is, the first control terminal Dr1 and the third control terminal Dr3 of the LLC controller control the first switch tube Q1, the fourth switch tube Q4, the fifth switch tube Q5 and the eighth switch tube Q8 to be turned on and off at the same time, and the second control terminal Dr2 and the fourth control terminal Dr4 control the second switch tube Q2, the third switch tube Q3, the sixth switch tube Q6 and the seventh switch tube Q7 to be turned on and off at the same time.

[0027] like Figure 3 The figure shows the resonant inductor current waveform and A1 test current waveform when the switching frequency is equal to the resonant frequency. At this time, the LLC drive waveform and the synchronous rectification control waveform are as follows: Figure 6As shown, the synchronous rectification control waveform is the same as the LLC driving waveform, that is, the first control terminal Dr1 and the third control terminal Dr3 of the LLC controller control the first switch tube Q1, the fourth switch tube Q4, the fifth switch tube Q5 and the eighth switch tube Q8 to be turned on and off at the same time, and the second control terminal Dr2 and the fourth control terminal Dr4 control the second switch tube Q2, the third switch tube Q3, the sixth switch tube Q6 and the seventh switch tube Q7 to be turned on and off at the same time.

[0028] like Figure 4 The figure shows the resonant inductor current waveform and A1 test current waveform when the switching frequency is less than the resonant frequency. If the synchronous rectification control waveform and the LLC drive waveform are made the same at the same time, the following will appear: Figure 4 The current backflow shown in the A1 test current waveform in this embodiment is as follows Figure 7 The LLC drive waveform and the synchronous rectification control waveform shown in the figure have the same frequency as the LLC drive waveform, but different duty cycles. The LLC controller controls the fifth switch tube Q5 and the eighth switch tube Q8 to be turned on simultaneously with the first switch tube Q1 and the fourth switch tube Q4, and controls the fifth switch tube Q5 and the eighth switch tube Q8 to be turned off earlier than the first switch tube Q1 and the fourth switch tube Q4; the LLC controller controls the sixth switch tube Q6 and the seventh switch tube Q7 to be turned on simultaneously with the second switch tube Q2 and the third switch tube Q3, and controls the sixth switch tube Q6 and the seventh switch tube Q7 to be turned off earlier than the second switch tube Q2 and the third switch tube Q3. That is, the conduction time of the secondary switch is less than the conduction time of the primary switch. At this time, the resonant inductor current waveform and the A1 test current waveform are as shown in FIG. Figure 5 As shown, no current backflow occurs.

[0029] like Figure 8The figure shows the principle diagram of LLC controller outputting LLC driving waveform and synchronous rectification control waveform. Number ① is the LLC controller carrier waveform (triangle wave), in which the peak value is the resonance point period value (CNT), and CMP is the comparison value; number ② is the LLC driving waveform, corresponding to the on and off of the first control terminal Dr1 or the second control terminal Dr2 of the LLC controller in the circuit diagram; number ③ is the synchronous rectification control waveform, corresponding to the on and off of the third control terminal Dr3 or the fourth control terminal Dr4 of the LLC controller in the circuit diagram. When the count value increases upward and is less than or equal to the period value, the first control terminal Dr1 is turned on and the second control terminal Dr2 is turned off; when the count value decreases downward and is greater than 0, the first control terminal Dr1 is turned off and the second control terminal Dr2 is turned on; when the count value increases upward and is less than or equal to the comparison value, the third control terminal Dr3 is turned on and the fourth control terminal Dr4 is turned off; when the count value decreases downward and is greater than the period value minus the comparison value, the third control terminal Dr3 is turned off and the fourth control terminal Dr4 is turned on; by adjusting the comparison value, the third control terminal Dr3 can be controlled to be turned on at the same time as the first control terminal Dr1 and the third control terminal Dr3 can be turned off earlier than the first control terminal Dr1, and the fourth control terminal Dr4 can be turned on at the same time as the second control terminal Dr2 and the fourth control terminal Dr4 can be turned off earlier than the second control terminal Dr2 In summary, the software-controlled synchronous rectification current backflow prevention method includes the following steps: S1: When the switching frequency is greater than the resonant frequency, the comparison value is set equal to the resonant point period value, the synchronous rectification drive and the LLC drive are turned on and off at the same time, the first control terminal Dr1 and the third control terminal Dr3 are turned on and off at the same time, and the second control terminal Dr2 and the fourth control terminal Dr4 are turned on and off at the same time. At this time, the synchronous rectification control waveform is the same as the LLC drive waveform, and the secondary switch on time is equal to the primary switch on time.

[0030] S2: When the switching frequency is equal to the resonant frequency, the comparison value is set equal to the resonant point period value, the synchronous rectification drive and the LLC drive are turned on and off at the same time, the first control terminal Dr1 and the third control terminal Dr3 are turned on and off at the same time, and the second control terminal Dr2 and the fourth control terminal Dr4 are turned on and off at the same time. At this time, the synchronous rectification control waveform is the same as the LLC drive waveform, and the secondary switch on time is equal to the primary switch on time.

[0031] S3: When the switching frequency is lower than the resonant frequency, the set comparison value is lower than the resonant point period value, the synchronous rectification drive and the LLC drive are turned on at the same time, the LLC drive on time is equal to half of the inverse of the resonant frequency, the first control terminal Dr1 and the third control terminal Dr3 are turned on at the same time, the third control terminal Dr3 is turned off earlier than the first control terminal Dr1, the second control terminal Dr2 and the fourth control terminal Dr4 are turned on at the same time, the fourth control terminal Dr4 is turned off earlier than the second control terminal Dr2, at this time the synchronous rectification control waveform has the same frequency as the LLC drive waveform, and the secondary switch on time is less than the primary switch on time.

[0032] The above resonant frequency expression is: , Among them, ƒ r is the resonant frequency, L r1 is the resonant inductance, L r2 is the transformer leakage inductance, C r is the resonant capacitor.

[0033] In summary, the software-controlled synchronous rectification method for preventing current backflow, when the switching frequency is less than the resonant frequency, controls the conduction time of the secondary switch to be less than the conduction time of the primary switch by setting the comparison value less than the resonant point period value, and the secondary switch and the primary switch are turned on at the same time, and the secondary switch is turned off earlier than the primary switch. That is, the method locks the upper limit of the conduction time of the secondary switch at the conduction time corresponding to the resonant frequency, that is, the efficiency is improved through synchronous rectification, and the current backflow is prevented. In addition, this method is based on software control, and no additional hardware devices are required, which reduces costs.

[0034] The effects of the present invention are not limited to the effects listed above, and those skilled in the art can derive effects not described above from the description.

[0035] The above-described embodiments of the software-controlled synchronous rectification anti-current backflow method and circuit are merely illustrative preferred embodiments and are not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection scope of the present invention. In addition, various technical solutions can be combined with each other, but they must be based on the ability of ordinary technicians in the field to implement them; when the combination of technical solutions is contradictory or cannot be implemented, it should be considered that such a combination of technical solutions does not exist and is not within the protection scope required by the present invention.

Claims

1. A synchronous rectification anti-current backflow method based on software control, characterized in that: The method comprises the following steps: S1: When the switching frequency is greater than the resonant frequency, the synchronous rectification drive and the LLC drive are turned on and off at the same time, and the secondary switch on time is equal to the primary switch on time; S2: When the switching frequency is equal to the resonant frequency, the synchronous rectification drive and the LLC drive are turned on and off at the same time, and the secondary switch on time is equal to the primary switch on time; S3: When the switching frequency is lower than the resonant frequency, the synchronous rectification drive and the LLC drive are turned on at the same time, the LLC drive on-time is equal to half of the reciprocal of the resonant frequency, and the secondary switch on-time is lower than the primary switch on-time.

2. The software-controlled synchronous rectification current backflow prevention method according to claim 1, characterized in that: The resonant frequency expression is: ， Among them, ƒ r is the resonant frequency, L r1 is the resonant inductance, L r2 is the transformer leakage inductance, C r is the resonant capacitor.

3. The software-controlled synchronous rectification current backflow prevention method according to claim 1, characterized in that: In step S3, the secondary switch and the primary switch are turned on at the same time, and the secondary switch is turned off earlier than the primary switch.

4. A synchronous rectification circuit for preventing current backflow, characterized in that: The circuit is connected to an LLC controller to implement the software-controlled synchronous rectification anti-current backflow method as described in any one of claims 1 to 3, including a primary H-bridge, a resonant cavity and a secondary H-bridge connected in series, wherein the input terminal of the primary H-bridge is connected to a voltage input, and the output terminal of the secondary H-bridge is connected to a voltage output.

5. The synchronous rectification anti-current backflow circuit according to claim 4, characterized in that: The primary side H-bridge includes a first switch tube, a second switch tube, a third switch tube and a fourth switch tube, and a diode is connected between the source and the drain of each switch tube, the positive electrode of the diode is connected to the source of the switch tube, and the negative electrode is connected to the drain of the switch tube; wherein the source of the first switch tube is connected to the drain of the second switch tube, and is connected to the positive electrode of the input end of the resonant cavity; the source of the third switch tube is connected to the drain of the fourth switch tube, and is connected to the negative electrode of the input end of the resonant cavity; the drain of the first switch tube is connected to the drain of the third switch tube, and the source of the second switch tube is connected to the source of the fourth switch tube; the gates of the first switch tube and the fourth switch tube are connected to the first control end of the LLC controller, and the gates of the second switch tube and the third switch tube are connected to the second control end of the LLC controller.

6. The synchronous rectification circuit for preventing current backflow as claimed in claim 5, characterized in that: The resonant cavity includes a resonant inductor, an excitation inductor, a first resistor and a resonant capacitor. The resonant inductor is connected to the positive output end of the primary H-bridge, and the resonant capacitor is connected to the negative output end of the primary H-bridge. The resonant cavity also includes a transformer. The input end of the transformer is connected to the two ends of the excitation inductor, and the output end is connected to the secondary H-bridge.

7. The synchronous rectification circuit for preventing current backflow as claimed in claim 6, characterized in that: The secondary side H bridge includes a fifth switch tube, a sixth switch tube, a seventh switch tube and an eighth switch tube, and a diode is connected between the source and the drain of each switch tube, and the positive electrode of the diode is connected to the source of the switch tube, and the negative electrode is connected to the drain of the switch tube; The source of the fifth switch tube is connected to the drain of the sixth switch tube, and is also connected to the positive pole of the transformer output terminal; the source of the seventh switch tube is connected to the drain of the eighth switch tube, and is also connected to the negative pole of the transformer output terminal; the drain of the fifth switch tube is connected to the drain of the seventh switch tube, and the source of the sixth switch tube is connected to the source of the eighth switch tube; the gates of the fifth switch tube and the eighth switch tube are connected to the third control terminal of the LLC controller, and the gates of the sixth switch tube and the seventh switch tube are connected to the fourth control terminal of the LLC controller.

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