A series resonant converter and control method thereof

Abstract

A series resonant converter, comprising an input voltage source Vin, a first power switch S1, a second power switch S2, a third power switch S3, a fourth power switch S4, a transformer, a first diode D1, a second diode D2, a resonant inductor L R , a resonant capacitor C R , a DC-blocking capacitor C B , an output capacitor C O and an output load R O ; The steps of the control method are as follows: 1) Assemble the series resonant converter; 2) The driving signals of the first power switch and the fourth power switch are complementary to those of the second power switch and the third power switch; The switching tubes on the secondary side of the transformer are in the synchronous rectification state; 3) Adopt conventional frequency modulation control, and the switching frequency f S increases as the output voltage V OUT decreases; 4) The output voltage V OUT is adjusted by controlling the boost duty cycle D B ; 5) The output voltage V OUT is adjusted by controlling the duty cycle of the fourth power switch; It has the characteristics of simple operation and high efficiency.

Description

Technical Field

[0001] The present invention belongs to the technical field of isolated direct current power conversion, and in particular relates to a series resonant converter and a control method thereof. Background Art

[0002] In recent years, electric vehicles have become more and more common, and charging of electric vehicles has become more and more important. At present, the two mainstream protocols for electric vehicle chargers are CHArge deMOve and Combined Charging System, and these two protocols have different voltage ranges for charging batteries. Generally speaking, CHArge deMOve covers relatively low voltage batteries, up to 500V, while Combined Charging System covers relatively high voltage batteries, up to 950V. Therefore, in order to be compatible with most electric vehicles that use these two mainstream protocols, an electric vehicle charger that covers a very large battery voltage range is required, so that higher efficiency can be achieved over the entire output voltage range.

[0003] Series resonant converter (SRC) and LLC converter have the advantages of soft switching and a small number of components, and are widely used in various applications. Both SRC and LLC converters use series resonant inductors and capacitors as the main resonant elements. The main difference between SRC and LLC converters lies in the size of the transformer's excitation inductance. The transformer excitation inductance of the SRC converter is larger than that of the LLC converter, the circulation loss of the SRC is smaller, and the efficiency is higher at the resonant frequency. SRC only provides a step-down conversion ratio, and the LLC converter will gain gain when the switching frequency becomes smaller. The circulating current will be stored in the resonant capacitor, and the energy will be delivered to the output in the next switching cycle. Therefore, SRC has a smaller circulating current, but the gain range is limited. If a larger gain range can be achieved in SRC, a small circulating current and a large gain range can be achieved at the same time.

[0004] In order to solve the shortcomings of the series resonant converter, some people use pulse width modulation (PWM) adaptive resonant converter in the existing technology. In this method, the PWM signal will increase the resonant current, so that the resonant converter can achieve increased gain. A narrower switching frequency range can cover a wider voltage conversion ratio range, reduce the switching frequency range, and reduce the size of the magnetic components. However, when a high boost gain is required, it is necessary to consider the large peak resonant current, which will cause huge root mean square current and turn-off loss; some people control a certain switching device to achieve the reconfiguration of the inverter or rectifier structure. For example, by fully opening a switch, the full-bridge inverter can also be used as a half-bridge inverter. When the full-bridge or half-bridge inverter is working, the converter gain can be reduced by half, so that the converter can have a wider gain range. However, the sudden change of the converter structure will cause problems such as output voltage sag and expansion. Summary of the invention

[0005] In order to overcome the above-mentioned deficiencies of the prior art, an object of the present invention is to provide a series resonant converter and a control method thereof, which have the characteristics of simple operation and high efficiency.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A series resonant converter includes an input voltage source Vin, a first power switch tube S 1 , the second power switch tube S 2 , the third power switch tube S 3 , the fourth power switch tube S 4 , turns ratio is N P :N S =n:1 transformer, first diode D 1 , the second diode D 2 , resonant inductor L R , resonant capacitor C R , DC blocking capacitor C B , output capacitor C O And output load R O ;

[0008] The positive electrode of the input voltage source Vin is connected to the first power switch tube S 1 The drain is connected to the cathode of the second power switch tube S 2 The source and resonant capacitor C R The second ends of the first power switch tube S 1 The source of the second power switch tube S 2 The drain and resonant inductance L R The first end of the resonant inductor L R The second end and the transformer primary N PThe first end of the transformer is connected to the primary N P The second end of the resonant capacitor C R The first end of the transformer is connected to the secondary N S The first end of the first diode D 1 The anode and the third power switch tube S 3 The drain of the transformer is connected; the secondary N S The second end of the DC blocking capacitor C B The first end of the DC blocking capacitor C B The second end of the fourth power switch tube S 4 The drain of the second diode D 2 The anodes of the first diode D 1 The cathode of the second diode D 2 The cathode and output capacitor C O The first end and output load R O The first ends of the third power switch tube S 3 The source and the fourth power switch tube S 4 The source and output capacitor C O The second end and output load R O The second ends of are connected.

[0009] The first power switch tube, the second power switch tube, the third power switch tube, and the fourth power switch tube are all SiC MOSFET switch tubes.

[0010] The first power switch tube, the second power switch tube, the third power switch tube, and the fourth power switch tube are all provided with diodes connected in parallel.

[0011] The duty cycles of the first power switch tube and the second power switch tube are both equal to 0.5 and are driven by complementary pulses.

[0012] The third power switch tube, the fourth power switch tube, the first diode and the second diode form a full-bridge synchronous rectifier on the secondary side of the transformer.

[0013] When the fourth power switch tube is fully turned on, the full-bridge rectifier is transformed into a voltage doubler rectifier.

[0014] A control method for a series resonant converter is provided, comprising the following steps:

[0015] Step 1: Connect the positive electrode of the input voltage source Vin to the first power switch tube S 1 The drain is connected to the cathode of the second power switch tube S 2 The source and resonant capacitor C R The second ends of the first power switch tube S are connected; 1 The source of the second power switch tube S2 The drain and resonant inductance L R The first end of the resonant inductor L R The second end and the transformer primary N P The first end of the transformer is connected to the primary N P The second end of the resonant capacitor C R The first end of the transformer is connected to the secondary N S The first end of the first diode D 1 The anode and the third power switch tube S 3 The drain of the transformer is connected; the secondary N S The second end of the DC blocking capacitor C B The first end of the DC blocking capacitor C B The second end of the fourth power switch tube S 4 The drain of the second diode D 2 The anodes of the first diode D 1 The cathode of the second diode D 2 The cathode and output capacitor C O The first end and output load R O The first ends of the third power switch tube S 3 The source and the fourth power switch tube S 4 The source and output capacitor C O The second end and output load R O The second ends of are connected;

[0016] Step 2: When the switching frequency f of the series resonant converter S Equal to the resonant frequency f R When the drive signals of the first power switch tube and the fourth power switch tube are the same and the duty cycle is 0.5, the drive signals of the second power switch tube and the third power switch tube are also the same and the duty cycle is 0.5, and at the same time, the drive signals of the first power switch tube and the fourth power switch tube are complementary to the drive signals of the second power switch tube and the third power switch tube; at this time, the switch tubes on the secondary side of the transformer are all in the synchronous rectification state, and the output voltage V OUT Vin / 2n, where n is the turns ratio of the transformer;

[0017] Step 3: When the output voltage V OUT When the output voltage V is less than Vin / 2n, the series resonant converter is in the step-down region. OUT When it is greater than Vin / 2n, the series resonant converter is in the boost region; when the series resonant converter is in the buck region, conventional frequency modulation control is adopted, and the switching frequency f S As the output voltage V OUT decreases and increases;

[0018] Step 4: When the output voltage VOUT When it is equal to Vin / 2n, the switching frequency f S is the resonant frequency f R , the series resonant converter reaches the first resonance point; when the series resonant converter is in the boost region, the output voltage V OUT By controlling the boost duty cycle D B The boost duty cycle D B The duty ratio of the entire switching cycle is the time obtained by subtracting the first switching tube turn-off time from the fourth switching tube turn-off time;

[0019] Step 5: When the output voltage V OUT Greater than Vin / 2n and the output voltage V OUT When it is less than Vin / n, the boost duty cycle D B The value range is 0-0.5, the switching frequency f S Maintain the resonant frequency f R , increase the duty cycle of the fourth power switch tube, and adjust the output voltage V by controlling the duty cycle of the fourth power switch tube OUT , define this area as PWM1 area.

[0020] Furthermore, when the output voltage V OUT When it is equal to Vin / n, the boost duty cycle D B is equal to 0.5, the fourth power switch tube remains in the on state, at this time the full-bridge synchronous rectifier is converted into a voltage doubler, and the series resonant converter reaches the second resonance point; when the output voltage V OUT When it is greater than Vin / n, the boost duty cycle D B If the duty cycle of the third power switch tube is greater than 0.5, the output voltage V is adjusted by controlling the duty cycle of the third power switch tube. OUT , define this area as PWM2 area; in the boost area, the switching frequency f S Always keep the resonant frequency f R , the series resonant converter has an output voltage V OUT There are two resonant operating points when it is equal to Vin / 2n and Vin / n. The series resonant converter has higher efficiency when operating at the resonant point. Therefore, the series resonant converter has two highest efficiency points in a wider gain range.

[0021] The resonant frequency is f R for

[0022] Among them, L R is the resonant inductor, C R is the resonant capacitor.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The efficiency of the traditional PWM resonant converter decreases as the output voltage increases. When a larger voltage gain is required, the efficiency of the system will drop significantly. The traditional PWM resonant converter will work away from the resonance point as the voltage gain increases. When a higher voltage gain is required, the resonant current waveform will present a sawtooth wave with large peak and root mean square, resulting in large conduction loss and core loss, and increasing the turn-off loss caused by the switch tube closing and the large peak current. The series resonant converter of the present invention has two resonant points. When the output voltage increases from Vin / 2n to Vin / n, the series resonant converter works close to the second resonant point, thereby limiting the large peak and root mean square current caused by the PWM gain increase. At the same time, the switching frequency of the series resonant converter in the boost region is constant, making the control simpler to operate and easier to achieve efficiency optimization. The series resonant converter can achieve higher efficiency in the entire output voltage range. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 A topological structure diagram of a series resonant converter of the present invention.

[0026] Figure 2 A control concept diagram of a series resonant converter of the present invention.

[0027] Figure 3 When the output voltage V OUT Key waveform diagram in PWM1 region when equal to Vin / 2n.

[0028] Figure 4 When the output voltage V OUT Key waveforms in the PWM1 region when greater than Vin / 2n and less than Vin / n.

[0029] Figure 5 It is a schematic diagram of the working mode 1 of the series resonant converter of the present invention in the PWM1 region.

[0030] Figure 6 It is a schematic diagram of the working mode 2 of the series resonant converter of the present invention in the PWM1 region.

[0031] Figure 7 It is a schematic diagram of the working mode 3 of the series resonant converter of the present invention in the PWM1 region.

[0032] Figure 8 When the output voltage V OUT Key waveforms in the PWM2 region when equal to Vin / n.

[0033] Fig. 9 When the output voltage V OUTKey waveforms in the PWM2 region when greater than Vin / n.

[0034] Fig.10 Schematic diagram of the operation mode 1 of the series resonant converter of the present invention in the PWM2 region. Fig.10 It is working mode 1; Fig.11 It is working mode 2; Fig.12 It is working mode 3;

[0035] Fig.11 It is a schematic diagram of the working mode 2 of the series resonant converter of the present invention in the PWM2 area.

[0036] Fig.12 It is a schematic diagram of the working mode 3 of the series resonant converter of the present invention in the PWM2 area.

[0037] In the figure: Vin is the input voltage source; S1 is the first power switch tube; S2 is the second power switch tube; S3 is the third power switch tube; S4 is the fourth power switch tube; LR is the resonant inductor; CR is the resonant capacitor; NP is the number of turns of the primary side coil of the transformer; NS is the number of turns of the secondary side coil of the transformer; CB is the DC blocking capacitor; CO is the output capacitor; D1 is the first diode; D2 is the second diode; RO is the load; VOUT is the output voltage. DETAILED DESCRIPTION

[0038] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings and embodiments. Obviously, the described embodiments are only embodiments of a part of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0039] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0040] The present invention is further described in detail below in conjunction with the accompanying drawings:

[0041] See also Figure 1The present invention provides a series resonant converter, comprising an input voltage source Vin; a first power switch tube S1; a second power switch tube S2; a third power switch tube S3; a fourth power switch tube S4; a turns ratio of N P :N S =n:1 transformer; first diode D1; second diode D2; resonant inductor LR; resonant capacitor C R ; DC blocking capacitor C B ; Output capacitor C O And the output load R O ;

[0042] The positive electrode of the input voltage source Vin is connected to the first power switch tube S 1 The drain is connected to the cathode of the second power switch tube S 2 The source and resonant capacitor C R The second ends of the first power switch tube S are connected; 1 The source of the second power switch tube S 2 The drain and resonant inductance L R The first end of the resonant inductor L R The second end and the transformer primary N P The first end of the transformer is connected to the primary N P The second end of the resonant capacitor C R The first end of the transformer is connected to the secondary N S The first end of the first diode D 1 The anode and the third power switch tube S 3 The drain of the transformer is connected; the secondary N S The second end of the DC blocking capacitor C B The first end of the DC blocking capacitor C B The second end of the fourth power switch tube S 4 The drain of the second diode D 2 The anodes of the first diode D 1 The cathode of the second diode D 2 The cathode and output capacitor C O The first end and output load R O The first ends of the third power switch tube S 3 The source and the fourth power switch tube S 4 The source and output capacitor C O The second end and output load R O The second ends of are connected.

[0043] See also Figure 2 The control method of the series resonant converter of the present invention, when the switching frequency f of the series resonant converter S is the resonant frequency f RWhen the first power switch tube S 1 and the fourth power switch tube S 4 The driving signal of the second power switch tube S is the same and the duty cycle is 0.5. 2 and the third power switch tube S 3 The driving signal is also the same and the duty cycle is 0.5. At the same time, the first power switch tube S 1 and the fourth power switch tube S 4 The second power switch tube S 2 and the third power switch tube S 3 The drive signals are complementary. At this time, the switches on the secondary side of the transformer are all in synchronous rectification state, and the output voltage V OUT = Vin / 2n, where n is the turns ratio of the transformer. OUT When the output voltage V OUT When the series resonant converter is in the step-down region, conventional frequency modulation control is adopted, and the switching frequency f S As the output voltage V OUT When the output voltage V OUT When it is equal to Vin / 2n, the switching frequency f S is the resonant frequency f R , the series resonant converter reaches the first resonance point. When the series resonant converter is in the boost region, the output voltage V OUT By controlling the boost duty cycle D B The boost duty cycle D B The time obtained by subtracting the first switch tube turn-off time from the fourth switch tube turn-off time, which accounts for the duty ratio of the entire switching cycle. OUT Greater than Vin / 2n and the output voltage V OUT When it is less than Vin / n, the boost duty cycle D B The value range is 0-0.5, the switching frequency f S Maintain the resonant frequency f R , increase the duty cycle of the fourth power switch tube, and adjust the output voltage V by controlling the duty cycle of the fourth power switch tube OUT , define this area as PWM1 area. When the output voltage V OUT When V is equal to Vin / n, the boost duty cycle D B is equal to 0.5, the fourth power switch tube remains in the on state, at this time the full-bridge synchronous rectifier is converted into a voltage doubler, and the series resonant converter reaches the second resonance point. OUTWhen V is greater than Vin / n, the boost duty cycle D B is greater than 0.5, the duty cycle of the third power switch tube is increased, and the output voltage V is adjusted by controlling the duty cycle of the third power switch tube OUT , define this area as PWM2 area. In the boost area, the switching frequency f S Always keep the resonant frequency f R , the series resonant converter has an output voltage V OUT There are two resonant operating points when Vin / 2n and Vin / n are equal. The series resonant converter has higher efficiency when operating at the resonant point. Therefore, the series resonant converter can have two highest efficiency points within a wider gain range.

[0044] A series resonant converter of the present invention mainly works in the boost region. As the output voltage increases, the rectifier on the secondary side of the series resonant converter transformer gradually switches from a full-bridge rectifier to a voltage doubler rectifier through simple PWM control. When working in the boost region, the switching frequency f S Always keep a constant resonant frequency f R The series resonant converter realizes two resonant operating points through a full-bridge rectifier and a voltage doubler rectifier. Since the two resonant operating points limit the efficiency drop of the series resonant converter in a wider gain range, the series resonant converter can achieve higher efficiency in a wider gain range, while using fewer devices and simpler control.

[0045] See also Figure 3 , when the output voltage V OUT Key waveform diagram in PWM1 area when equal to Vin / 2n;

[0046] See also Figure 4 , when the output voltage V OUT Key waveform diagram in PWM1 region when greater than Vin / 2n and less than Vin / n;

[0047] The working process of the present invention is described below:

[0048] In order to obtain maximum gain and minimum loop current, the boost region is mainly considered. OUT When the output voltage V OUT When V is greater than Vin / 2n and less than Vin / n, the secondary side of the series resonant converter transformer acts as a boost duty cycle D B of full bridge rectifier.

[0049] like Figures 5 to 7The three different working modes in the PWM1 region are shown. To simplify the circuit analysis of the converter, the following assumptions are made: 1) All semiconductor components in the converter are considered ideal; 2) All capacitors are large enough, and the voltage of all capacitors remains unchanged within a switching cycle, and the voltage ripple of all capacitors is ignored;

[0050] Working mode 1: Figure 5 As shown, at this time, the first power switch tube S 1 At this moment, the current i on the resonant inductor is LR The resonant inductor starts to have current, and the resonant element is the resonant inductor L. R and resonant capacitor C R , resonant current i LR Through the transformer and the first diode D on the secondary side of the transformer 1 and the fourth power switch tube S 4 Delivered to the output capacitor C O The fourth power switch tube S 4 The synchronous rectifier is turned on. Operation mode 1 lasts for half a resonant cycle. At the resonant frequency, the first power switch tube S 1 and the second power switch tube S 2 Working under the driving signal with duty cycle of 0.5. The current i on the resonant inductor is LR is zero again, the first power switch tube S 1 Disconnect, working mode 1 ends.

[0051] Working mode 2: Figure 6 As shown, the current on the resonant inductor is 0 at this time. The fourth power switch is in the on state. Due to the boost duty cycle D B The current of the resonant inductor increases rapidly, and the duration of working mode 2 is D B T S When the current on the resonant inductor increases to a certain level, the fourth power switch tube is disconnected and the working mode 2 ends.

[0052] Working mode 3: Figure 7 As shown, at this time, the second diode and the third power switch tube are turned on, the current on the resonant inductor is sent to the output end, and the current on the resonant inductor begins to decrease until the current on the resonant inductor is reduced to zero, then the second diode and the third power switch tube are disconnected, and working mode 3 ends.

[0053] See also Figure 8 , when the output voltage V OUT Key waveform diagram in PWM2 area when equal to Vin / n;

[0054] See also Fig. 9 , when the output voltage V OUT Key waveform diagram in PWM2 area when greater than Vin / n;

[0055] When the output voltage V OUT When D is equal to Vin / n, B is 0.5, the secondary side of the series resonant converter transformer gradually changes from a full-bridge rectifier to a voltage doubler rectifier. When the output voltage VOUT is greater than Vin / n, the secondary rectifier works as a voltage doubler rectifier and the resonant current increases. Figures 10 to 12 As shown, the three different working modes in the PWM2 area are similar to the three working modes in the PWM1 area. Example

[0056] A control method for a series resonant converter is provided, comprising the following steps:

[0057] Step 1: Connect the positive electrode of the input voltage source Vin to the first power switch tube S 1 The drain is connected to the cathode of the second power switch tube S 2 The source and resonant capacitor C R The second ends of the first power switch tube S 1 The source of the second power switch tube S 2 The drain and resonant inductance L R The first end of the resonant inductor L R The second end and the transformer primary N P The first end of the transformer is connected to the primary N P The second end of the resonant capacitor C R The first end of the transformer is connected to the secondary N S The first end of the first diode D 1 The anode and the third power switch tube S 3 The drain of the transformer is connected; the secondary N S The second end of the DC blocking capacitor C B The first end of the DC blocking capacitor C B The second end of the fourth power switch tube S 4 The drain of the second diode D 2 The anodes of the first diode D 1 The cathode of the second diode D 2 The cathode and output capacitor C O The first end and output load R O The first ends of the third power switch tube S 3 The source and the fourth power switch tube S 4 The source and output capacitor C O The second end and output load R OThe second ends of are connected;

[0058] Step 2: When the switching frequency f of the series resonant converter S Equal to the resonant frequency f R When the drive signals of the first power switch tube and the fourth power switch tube are the same and the duty cycle is 0.5, the drive signals of the second power switch tube and the third power switch tube are also the same and the duty cycle is 0.5, and at the same time, the drive signals of the first power switch tube and the fourth power switch tube are complementary to the drive signals of the second power switch tube and the third power switch tube; at this time, the switch tubes on the secondary side of the transformer are all in the synchronous rectification state, and the output voltage V OUT Vin / 2n, where n is the turns ratio of the transformer;

[0059] Step 3: When the output voltage V OUT When the output voltage V is less than Vin / 2n, the series resonant converter is in the step-down region. OUT When it is greater than Vin / 2n, the series resonant converter is in the boost region; when the series resonant converter is in the buck region, conventional frequency modulation control is adopted, and the switching frequency f S As the output voltage V OUT decreases and increases;

[0060] Step 4: When the output voltage V OUT When it is equal to Vin / 2n, the switching frequency f S is the resonant frequency f R , the series resonant converter reaches the first resonance point; when the series resonant converter is in the boost region, the output voltage V OUT By controlling the boost duty cycle D B The boost duty cycle D B The duty ratio of the entire switching cycle is the time obtained by subtracting the first switching tube turn-off time from the fourth switching tube turn-off time;

[0061] Step 5: When the output voltage V OUT Greater than Vin / 2n and the output voltage V OUT When it is less than Vin / n, the boost duty cycle D B The value range is 0-0.5, the switching frequency f S Maintain the resonant frequency f R , increase the duty cycle of the fourth power switch tube, and adjust the output voltage V by controlling the duty cycle of the fourth power switch tube OUT , define this area as PWM1 area.

[0062] Furthermore, when the output voltage V OUT When it is equal to Vin / n, the boost duty cycle D Bis equal to 0.5, the fourth power switch tube remains in the on state, at this time the full-bridge synchronous rectifier is converted into a voltage doubler, and the series resonant converter reaches the second resonance point; when the output voltage V OUT When it is greater than Vin / n, the boost duty cycle D B If the duty cycle of the third power switch tube is greater than 0.5, the output voltage V is adjusted by controlling the duty cycle of the third power switch tube. OUT , define this area as PWM2 area; in the boost area, the switching frequency f S Always keep the resonant frequency f R , the series resonant converter has an output voltage V OUT There are two resonant operating points when it is equal to Vin / 2n and Vin / n. The series resonant converter has higher efficiency when operating at the resonant point. Therefore, the series resonant converter has two highest efficiency points in a wider gain range.

[0063] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A control method for a series resonant converter, characterized in that: The following steps are involved: Step 1: Connect the positive electrode of the input voltage source Vin to the drain of the first power switch tube S1, and the negative electrode to the source of the second power switch tube S2 and the resonant capacitor C R The second ends of the first power switch tube S1 are connected to the drain of the second power switch tube S2 and the resonant inductor L R The first end of the resonant inductor L R The second end and the transformer primary N P The first end of the transformer is connected to the primary N P The second end of the resonant capacitor C R The first end of the transformer is connected to the secondary N S The first end of the transformer is connected to the anode of the first diode D1 and the drain of the third power switch tube S3; the transformer secondary N S The second end of the DC blocking capacitor C B The first end of the DC blocking capacitor C B The second end of is connected to the drain of the fourth power switch tube S4 and the anode of the second diode D2; the cathode of the first diode D1 and the cathode of the second diode D2 are connected to the output capacitor C O The first end and output load R O The source of the third power switch tube S3 and the source of the fourth power switch tube S4 are connected to the output capacitor C O The second end and output load R O The second ends of are connected; Step 2: When the switching frequency f of the series resonant converter S Equal to the resonant frequency f R When the drive signals of the first power switch tube and the fourth power switch tube are the same and the duty cycle is 0.5, the drive signals of the second power switch tube and the third power switch tube are also the same and the duty cycle is 0.5, and at the same time, the drive signals of the first power switch tube and the fourth power switch tube are complementary to the drive signals of the second power switch tube and the third power switch tube; at this time, the switch tubes on the secondary side of the transformer are all in the synchronous rectification state, and the output voltage V OUT Vin / 2n, where n is the turns ratio of the transformer; Step 3: When the output voltage V OUT When the output voltage V is less than Vin / 2n, the series resonant converter is in the step-down region. OUT When it is greater than Vin / 2n, the series resonant converter is in the boost region; when the series resonant converter is in the buck region, conventional frequency modulation control is adopted, and the switching frequency f S As the output voltage V OUT decreases and increases; Step 4: When the output voltage V OUT When it is equal to Vin / 2n, the switching frequency f S is the resonant frequency f R , the series resonant converter reaches the first resonance point; when the series resonant converter is in the boost region, the output voltage V OUT By controlling the boost duty cycle D B The boost duty cycle D B The time obtained by subtracting the turn-off time of the first switch tube from the turn-off time of the fourth switch tube, which accounts for the duty ratio of the entire switching cycle; Step 5: When the output voltage V OUT Greater than Vin / 2n and the output voltage V OUT When it is less than Vin / n, the boost duty cycle D B The value range is 0-0.5, the switching frequency f S Maintain the resonant frequency f R , increase the duty cycle of the fourth power switch tube, and adjust the output voltage V by controlling the duty cycle of the fourth power switch tube OUT , define this area as PWM1 area.

2. The control method of a series resonant converter according to claim 1, characterized in that: When the output voltage V OUT When it is equal to Vin / n, the boost duty cycle D B is equal to 0.5, the fourth power switch tube remains in the on state, at this time the full-bridge synchronous rectifier is converted into a voltage doubler, and the series resonant converter reaches the second resonance point; when the output voltage V OUT When it is greater than Vin / n, the boost duty cycle D B If the duty cycle of the third power switch tube is greater than 0.5, the output voltage V is adjusted by controlling the duty cycle of the third power switch tube. OUT , define this area as PWM2 area; in the boost area, the switching frequency f S Always keep the resonant frequency f R , the series resonant converter has an output voltage V OUT There are two resonant operating points when it is equal to Vin / 2n and Vin / n. The series resonant converter has higher efficiency when operating at the resonant point. Therefore, the series resonant converter has two highest efficiency points in a wider gain range.

3. The control method of a series resonant converter according to claim 1, characterized in that: The series resonant converter includes an input voltage source Vin, a first power switch tube S1, a second power switch tube S2, a third power switch tube S3, a fourth power switch tube S4, and a turns ratio of N. P :N S =n:1 transformer, first diode D1, second diode D2, resonant inductor L R , resonant capacitor C R , DC blocking capacitor C B , output capacitor C O And output load R O ; The positive electrode of the input voltage source Vin is connected to the drain of the first power switch tube S1, and the negative electrode is connected to the source of the second power switch tube S2 and the resonant capacitor C R The second ends of the first power switch tube S1 are connected to the drain of the second power switch tube S2 and the resonant inductor L R The first end of the resonant inductor L R The second end and the transformer primary N P The first end of the transformer is connected to the primary N P The second end of the resonant capacitor C R The first end of the transformer is connected to the secondary N S The first end of the transformer is connected to the anode of the first diode D1 and the drain of the third power switch tube S3; the transformer secondary N S The second end of the DC blocking capacitor C B The first end of the DC blocking capacitor C B The second end of is connected to the drain of the fourth power switch tube S4 and the anode of the second diode D2; the cathode of the first diode D1 and the cathode of the second diode D2 are connected to the output capacitor C O The first end and output load R O The source of the third power switch tube S3 and the source of the fourth power switch tube S4 are connected to the output capacitor C O The second end and output load R O The second ends of are connected.

4. The control method of a series resonant converter according to claim 3, characterized in that: The first power switch tube, the second power switch tube, the third power switch tube, and the fourth power switch tube are all SiC MOSFET switch tubes.

5. The control method of a series resonant converter according to claim 3, characterized in that: The first power switch tube, the second power switch tube, the third power switch tube, and the fourth power switch tube are all provided with diodes connected in parallel.

6. The control method of a series resonant converter according to claim 3, characterized in that: The duty cycles of the first power switch tube and the second power switch tube are both equal to 0.5 and are driven by complementary pulses.

7. The control method of a series resonant converter according to claim 3, characterized in that: The third power switch tube, the fourth power switch tube, the first diode and the second diode form a full-bridge synchronous rectifier on the secondary side of the transformer.

8. The control method of a series resonant converter according to claim 3, characterized in that: When the fourth power switch tube is fully turned on, the full-bridge rectifier is transformed into a voltage doubler rectifier.

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