Digital synchronous rectification method suitable for symmetric LLC resonant converter

Abstract

The invention discloses a digital synchronous rectification method suitable for a symmetric LLC resonant converter, the symmetric LLC resonant converter comprises a first split switch tube, a second split switch tube and a full-wave rectification circuit, and the full-wave rectification circuit comprises switch tubes SR1 and SR2. A switch tube SR2 in the full-wave rectification circuit measures a source-drain voltage Vds2 and a current ISR2 of the switch tube SR2 through a sensor, compares the source-drain voltage Vds2 and the current ISR2 with threshold values Vth and Ith respectively, and inputs comparison results into the DSP; the DSP outputs a signal to the driving chip according to a digital synchronous rectification algorithm; the digital synchronous rectification algorithm starts to output a driving signal which is the same as that of the second split switch tube when the current ISR2 exceeds a threshold value Ith, the driving signal is adjusted by detecting the conduction time and the working frequency of an SR2 body diode, and a synchronous rectification driving signal is determined. The synchronous rectification method of the switch tube SR1 is the same as that of the switch tube SR2. According to the invention, the synchronous rectification driving signal of the switch tube of the secondary rectification circuit can be determined in a small number of switching periods.

Description

Technical Field

[0001] The present invention relates to a digital synchronous rectification method applicable to a symmetrical LLC resonant converter, belonging to the technical field of drive control. Background Art

[0002] In recent years, power electronics technology integrating power semiconductor technology and automatic control has achieved efficient and substantial development, and power electronic devices have penetrated into various fields with their characteristics of small size and intelligence. The popularity of power electronic devices can be seen everywhere, from traditional power industry equipment to daily life.

[0003] As a secondary power supply, the DC switching power supply is a basic product of power electronic devices. Along with the progress of the latest generation of information technology, energy conservation and environmental protection measures, and new energy industries, the requirements for DC switching power supplies in terms of energy conversion efficiency, volume reduction, power density increase, and intelligence are increasing day by day. Among them, the LLC resonant converter stands out with its two major advantages of high efficiency and high power density and has become the main topology of medium and low voltage DC switching power supplies. However, in actual applications, due to the fact that the rectifier diodes inside the LLC resonant converter in a medium and low voltage environment work in a large current situation for a long time, the conduction loss of the rectifier diodes in the full-wave rectification circuit of the converter is relatively large, which will cause serious heating of the rectifier diodes. In the light case, the efficiency is reduced, and in the heavy case, it will lead to thermal breakdown of the rectifier diodes. Therefore, the synchronous rectification technology using MOS transistors with small on-resistance to replace diodes is adopted.

[0004] At present, there are many research results on the synchronous rectification technology for LLC resonant converters. Classified by software and hardware, there are hardware chip synchronous rectification and digital synchronous rectification based on DSP; classified by principle, there are detection of drain-source voltage type drive and current type drive. These methods have the following problems to be solved in applications.

[0005] First of all, the hardware chip synchronous rectification technology is based on a hardware chip, integrating a detection circuit, a logic conditioning circuit, and a power amplifier circuit. It has the advantages of small size and convenient circuit layout. However, the drain-source voltage of the MOS transistor is weak, which has high requirements for the detection accuracy of the detection of drain-source voltage type drive chips. In addition, regardless of the principle, the chip itself has an upper limit of applicable frequency and cannot meet the needs of some specific high frequencies.

[0006] In addition, the detection of drain-source voltage type drive has frequent amplitude fluctuations, resulting in frequent instability of the synchronous rectification signal in the algorithm output result of digital synchronous rectification, which is likely to cause unsuccessful synchronous rectification. And excessive switching off of the MOS transistor will lead to a decrease in service life and an increase in the device maintenance cost. Summary of the Invention

[0007] The technical problem to be solved by the present invention is to provide a digital synchronous rectification method applicable to a symmetric LLC resonant converter, which can determine the synchronous rectification drive signal of the switching tube in the secondary rectification circuit within a small number of switching cycles.

[0008] The present invention adopts the following technical solutions to solve the above technical problems:

[0009] A digital synchronous rectification method applicable to a symmetric LLC resonant converter, the symmetric LLC resonant converter includes a DC input, a first split switching tube, a second split switching tube, a first split capacitor, a second split capacitor, a three-port transformer, a full-wave rectification circuit, an output capacitor and a load resistor, wherein the full-wave rectification circuit includes a first switching tube and a second switching tube; after the first and second split switching tubes are connected in series, they are then connected in parallel with the DC input; after the first and second split capacitors are connected in series, they are then connected in parallel with the DC input; the midpoint of the first and second split switching tubes is connected to the same-named end of the primary side of the three-port transformer through a resonant inductor, and the midpoint of the first and second split capacitors is connected to the different-named end of the primary side of the three-port transformer; the exciting inductor is connected in parallel with the primary side of the three-port transformer; the drain of the first switching tube is connected to the same-named end of the first winding of the secondary side of the three-port transformer, the drain of the second switching tube is connected to the different-named end of the second winding of the secondary side of the three-port transformer, the sources of the first switching tube and the second switching tube are both connected to one end of the output capacitor, the other end of the output capacitor is connected to the midpoint of the first winding and the second winding, and the load resistor and the output capacitor are connected in parallel;

[0010] The specific digital synchronous rectification method is as follows:

[0011] The digital synchronous rectification methods of the first switching tube and the second switching tube are the same;

[0012] For the first switching tube, measure the current and the source-drain voltage of the first switching tube;

[0013] Current judgment link: If the current of the first switching tube exceeds a preset current threshold, the DSP outputs the initial value of the synchronous rectification signal and enters the body diode conduction judgment link, and the initial value of the synchronous rectification signal is the same as the drive signal of the first split switching tube; otherwise, no synchronous rectification signal is output;

[0014] Body diode conduction judgment link: If the source-drain voltage of the first switching tube is not greater than a preset voltage threshold, subtract the time ΔD from the drive time of the first switching tube in the current switching cycle as the drive time of the first switching tube in the next switching cycle. ΔD is in a proportional relationship with the switching frequency of the first split switching tube, and in the next switching cycle, repeat the above voltage judgment and drive time calculation process until in a certain switching cycle, the source-drain voltage of the first switching tube is greater than the preset voltage threshold, and record the body diode conduction time T of the first switching tube on And enter the switching frequency judgment link;

[0015] Switching frequency judgment section: If the switching frequency of the first split switch is not greater than the resonant frequency, then add the driving time of the first switch in the current switching period plus (T on -ΔT) as the driving time of the first switch in the next switching period, and the DSP outputs the corresponding synchronous rectification signal. ΔT is the correction time of the synchronous rectification signal. If the switching frequency of the first split switch is greater than the resonant frequency, then add the driving time of the first switch in the current switching period plus (T on +ΔT) as the driving time of the first switch in the next switching period, and the DSP outputs the corresponding synchronous rectification signal.

[0016] As a preferred embodiment of the present invention, ΔD is 10% of the switching period of the first split switch.

[0017] As a preferred embodiment of the present invention, the conduction time T of the body diode on is recorded by the internal eCap module of the DSP.

[0018] As a preferred embodiment of the present invention, during the operation of the digital synchronous rectification method, if the switching frequency of the first split switch remains unchanged, the DSP maintains the original output. If the switching frequency of the first split switch becomes smaller, the synchronous rectification drive signal is re-determined and output according to the digital synchronous rectification method. If the switching frequency of the first split switch becomes larger, subtract the time ΔD from the driving time of the first switch in the current switching period as the driving time of the first switch in the next switching period, and then re-determine and output the synchronous rectification drive signal according to the digital synchronous rectification method.

[0019] Compared with the prior art, the present invention adopts the above technical solutions and has the following technical effects:

[0020] 1. The present invention can output a stable synchronous rectification drive signal under wide frequency requirements through the digital synchronous rectification technology, reducing the rectification loss.

[0021] 2. The present invention can quickly determine the synchronous rectification drive signal within a limited switching period.

[0022] 3. The present invention effectively avoids the problem of loss of synchronous rectification signal caused by voltage fluctuation through dual detection of current and voltage.

[0023] 4. The present invention can realize the dynamic change of the synchronous rectification drive signal through transient and static control algorithms, with a fast response speed. Brief Description of the Drawings

[0024] Figure 1 is the circuit diagram of a symmetric half-bridge LLC converter applicable to the method of the present invention;

[0025] Figure 2 is the flowchart of a digital synchronous rectification method applicable to a symmetric LLC resonant converter according to the present invention;

[0026] Figure 3 is the flowchart of a digital synchronous rectification transient control algorithm applicable to a symmetric LLC converter according to the present invention. Specific embodiments

[0027] The following details the embodiments of the present invention, and examples of the embodiments are shown in the accompanying drawings. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain the present invention and should not be construed as limiting the present invention.

[0028] Figure 1 is the topology diagram of a symmetric LLC converter applicable to the method of the present invention. The symmetric LLC resonant converter includes a DC input V in , and a split capacitor and split switch tubes are connected in parallel at both ends of the DC input V in . The split capacitor includes a first split capacitor C r1 and a second split capacitor C r2 connected in series with each other. The split switch tubes include a first split switch tube Q 1 and a second split switch tube Q 2 connected in series with each other; a resonant inductor L r and an exciting inductor L m are connected in series between the midpoints of the split capacitor and the split switch tubes; the exciting inductor L m is connected in parallel with the primary side of a three-port transformer T r ; the secondary side of the three-port transformer T r is connected to a full-wave rectifier circuit; the full-wave rectifier circuit is connected to an output capacitor C o and a load resistor R load .

[0029] The full-wave rectifier circuit is formed by connecting the same-name ends and different-name ends of the secondary winding of the transformer and leading out wires; the drain of the switch tube SR 1 is connected to the same-name end of the secondary winding of the three-port transformer T r , and the drain of the switch tube SR 2 is connected to the different-name end of another winding and leads out a wire; an output capacitor C o and a load resistor R load are connected in parallel between the two lead-out wires.

[0030] The digital synchronous rectification technology algorithm includes transient control and static control; the static control includes a current judgment link, a body diode conduction judgment link, and a switching frequency judgment link; the transient control includes a switching frequency change judgment link and a drive signal processing link.

[0031] The switch tube SR1 and SR 2 The static control process is the same as that of the switch tube SR 2 For example, the execution process of static control is as follows: the switch tube SR in the full-wave rectifier circuit 2 The source-drain voltage V is measured by the sensor ds2 and current I SR2 , source-drain voltage V ds2 and current I SR2 Enter the current judgment link in sequence and input the comparison result into DSP; DSP outputs the signal to the driver chip after passing through the body diode conduction judgment link and the switching frequency link; DSP SR2 Exceeding the threshold I th The start output and the second split switch tube Q 2 The same driving signal, by detecting SR 2 Body diode conduction time T on And the operating frequency f s The driving signal is adjusted to determine the synchronous rectification driving signal.

[0032] like Figure 2 As shown, the switch tube SR 2 For example, the digital synchronous rectification static control is as follows:

[0033] The SR in the full-wave rectifier circuit is measured by a current comparator. 2 The current sampling value and threshold I th Make a comparison and obtain a comparison result;

[0034] The comparison result is no, that is, the current sampling value does not exceed the threshold I th When , the DSP output is low level and no driving signal is output; the comparison result is yes, that is, the current sampling value exceeds the threshold value I th , entering the body diode conduction judgment link.

[0035] The body diode conduction judgment link is composed of the initial value of the DSP output drive signal and the split switch tube Q 2 Corresponding; through the voltage comparator, the SR in the full-wave rectifier circuit 2 Drain-source voltage sampling value V ds2 With threshold V th Make a comparison and obtain a comparison result;

[0036] If the comparison result is no, that is, the body diode is not conducting when turned off, the next switch cycle SR 2 Driving time T SRSubtract a small time period ΔD (ΔD is 10% of the switching period of the first split switch) proportional to the switching frequency, and then enter the body diode conduction determination link again; if the comparison result is yes, that is, the body diode has conducted during turn-off, use the internal eCap module of the DSP to record the body diode conduction time T on , and then enter the switching frequency comparison link.

[0037] Compare the switching frequency with the resonant frequency to obtain a comparison result;

[0038] If the comparison result is yes, that is, when the switching frequency is not greater than the resonant frequency, the next switching period T SR plus (T on -ΔT) and the DSP outputs the corresponding synchronous rectification signal (ΔT is 5% of the switching period of the first split switch); if the comparison result is no, that is, when the switching frequency is greater than the resonant frequency, the next switching period T SR plus (T on +ΔT) and the DSP outputs the corresponding synchronous rectification signal. In this case, the circuit only needs two switching periods of synchronous rectification to work ideally.

[0039] The digital synchronous rectification static control of the switch SR 1 is the same as that of the switch SR 2 .

[0040] As Figure 3 shown, the digital synchronous rectification transient control is as follows:

[0041] If the switching frequency remains unchanged, the DSP maintains the original output; if the switching frequency decreases, re-determine the synchronous rectification drive signal according to the static control: if the switching frequency increases, the drive time T of the next switch tube cycle SR SR subtracts a small time period ΔD proportional to the switching frequency, and then re-determines the synchronous rectification drive signal according to the static control. The transient process of this control algorithm is short and can quickly follow the change of the switching frequency.

[0042] The circuit applicable to the technical solution of the present invention is not limited to the symmetric half-bridge LLC circuit, but also includes but is not limited to the full-bridge LLC resonant converter, the bidirectional LLC resonant converter, and the CLLC resonant converter.

[0043] The above embodiments are only used to illustrate the technical idea of the present invention, and the protection scope of the present invention cannot be limited by this. Any modification made on the basis of the technical solution according to the technical idea proposed by the present invention falls within the protection scope of the present invention.

Claims

1. A digital synchronous rectification method suitable for a symmetrical LLC resonant converter, the symmetrical LLC resonant converter comprising a DC input, a first split switch tube, a second split switch tube, a first split capacitor, a second split capacitor, a three-port transformer, a full-wave rectification circuit, an output capacitor and a load resistor, wherein the full-wave rectification circuit comprises a first switch tube and a second switch tube; the first and second split switch tubes are connected in series and then connected in parallel with the DC input; the first and second split capacitors are connected in series and then connected in parallel with the DC input; the midpoints of the first and second split switch tubes are connected to the same-name end of the primary side of the three-port transformer through a resonant inductor, and the midpoints of the first and second split capacitors are connected to the opposite-name end of the primary side of the three-port transformer; the excitation inductor is connected in parallel with the primary side of the three-port transformer; the drain of the first switch tube is connected to the same-name end of the first winding of the secondary side of the three-port transformer, the drain of the second switch tube is connected to the opposite-name end of the second winding of the secondary side of the three-port transformer, the source of the first switch tube and the second switch tube are both connected to one end of the output capacitor, the other end of the output capacitor is connected to the midpoint of the first winding and the second winding, and the load resistor and the output capacitor are connected in parallel; It is characterized in that The digital synchronous rectification method is specifically as follows: The digital synchronous rectification method of the first switch tube and the second switch tube is the same; For the first switch tube, measuring the current and source-drain voltage of the first switch tube; Current judgment link: If the current of the first switch tube exceeds the preset current threshold, the DSP is used to output the initial value of the synchronous rectification signal and enter the body diode conduction judgment link. The initial value of the synchronous rectification signal is consistent with the driving signal of the first split switch tube; Otherwise, no synchronous rectification signal is output; Body diode conduction judgment link: If the source-drain voltage of the first switch tube is not greater than the preset voltage threshold, the driving time of the first switch tube in the current switching cycle minus the time ΔD is used as the driving time of the first switch tube in the next switching cycle. ΔD is proportional to the switching frequency of the first split switch tube. In the next switching cycle, the above voltage judgment and driving time calculation process are repeated until the source-drain voltage of the first switch tube is greater than the preset voltage threshold in a certain switching cycle. The body diode conduction time T of the first switch tube is recorded. on And enter the switching frequency judgment link; Switching frequency judgment link: If the switching frequency of the first split switch is not greater than the resonant frequency, then add the driving time of the first switch in the current switching cycle to (T on -ΔT), as the driving time of the first switch tube in the next switching cycle, and the DSP outputs the corresponding synchronous rectification signal, ΔT is the correction time of the synchronous rectification signal; If the switching frequency of the first split switch is greater than the resonant frequency, then the driving time of the first switch in the current switching cycle plus (T on +ΔT) is used as the driving time of the first switch tube in the next switching cycle, and the DSP outputs the corresponding synchronous rectification signal.

2. The digital synchronous rectification method for a symmetrical LLC resonant converter according to claim 1, characterized in that: The ΔD is 10% of the switching period of the first split switch tube.

3. The digital synchronous rectification method for a symmetrical LLC resonant converter according to claim 1, characterized in that: The body diode conduction time T on Recorded by the eCap module inside the DSP.

4. The digital synchronous rectification method for a symmetrical LLC resonant converter according to claim 1, characterized in that: During the operation of the digital synchronous rectification method, if the switching frequency of the first split switch tube remains unchanged, the DSP maintains the original output; if the switching frequency of the first split switch tube decreases, the synchronous rectification drive signal is re-determined and output according to the digital synchronous rectification method; if the switching frequency of the first split switch tube increases, the driving time of the first switch tube in the current switching cycle minus the time ΔD is used as the driving time of the first switch tube in the next switching cycle, and the synchronous rectification drive signal is re-determined and output according to the digital synchronous rectification method.

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