Detailed explanation of a high-frequency output asymmetric half-bridge inverter circuit

　　introduction

　　In recent years, a new high-frequency power transmission and distribution system HFPDS (High frequency power distribution system) has been proposed. Unlike the traditional DC distribution system, HFPDS uses a high-frequency AC distribution system. It has the following advantages: (1) simple system; (2) high efficiency; (3) high reliability; (4) low cost. Due to the high output frequency, it is impossible to use control methods such as SPWM, so the current high-frequency output inverter is mostly square wave or quasi-square wave output, and then a high-frequency sine wave is obtained through a resonant filter network. This paper analyzes the working principle of the converter, the soft switching implementation conditions and the design of the resonant filter circuit.

　　Figure 1 High frequency output asymmetric half-bridge inverter

　　Figure 2 Key waveforms of an asymmetric half-bridge inverter

　　Figure 1 shows the topology of a high-frequency output asymmetric half-bridge inverter, which consists of an asymmetric half-bridge converter, a fourth-order resonant filter network and a high-frequency transformer. Figure 2 shows the key waveform of a high-frequency output asymmetric half-bridge inverter. The asymmetric half-bridge inverter can be divided into 6 different working periods in one switching cycle. When the asymmetric half-bridge inverter carries a resistive load, the resonant filter is designed to be inductive, so that the output voltage leads the current of the series resonant branch, realizing soft switching of all switch tubes. For the convenience of analysis, without affecting the analysis results, the following assumptions are made: ① All switch tubes, inductors, capacitors, and transformers are ideal devices; ② The resonant filter has sufficient filtering capacity, and the frequency of the output voltage is the same as the switch tube frequency. The working conditions of each switch state are described as follows:

　　(1) Working mode 1 [t0---t1]

　　At t0, the switch Q2 is turned off. Because of the negative current is, C1 discharges and C2 charges. Once the voltage at the C1 end is zero, the negative resonant current is turns D1 on. There must be enough energy to extract the energy in C1 during the dead time.

　　(2) Working mode 2 [t1---t2]

　　When Q1 is turned on at time t1, the switch tube Q1 is turned on at zero voltage. At this time, D1 and Q1 are turned on at the same time.

　　(3) Working mode 3 [t2---t3]

　　At time t2, the current is zero. At this time, the input voltage is applied to the input end of the resonant filter, causing the current is to flow in the forward direction to supply power to the load.

　　(4) Working mode 4 [t3---t4]

　　At t3, the switch tube Q1 is turned off. Due to the existence of parasitic capacitance, the switch tube Q1 is equivalent to soft shutdown. Since the current is positive, C1 is charged and C2 is discharged. Once the voltage at the C2 end is zero, the positive resonant current is turns on D2. During this period of time, there must be enough energy to extract the energy in C2.

　　(5) Working mode 5 [t4---t5]

　　At time t4, Q2 is turned on, and the switch tube Q2 is turned on with zero voltage. At this time, D2 and Q2 are turned on at the same time.

　　(6) Working mode 6 [t5---t6]

　　At t5, the current is zero. After this time, the resonant filter starts to release energy, causing the current is to flow in a negative direction to supply power to the load.

　　Soft switching conditions

　　The asymmetric half-bridge inverter must have enough energy during the dead time to draw away the charge on the switch tube junction capacitance or external parallel capacitance that is about to be turned on, and to charge the other switch tube junction capacitance or external parallel capacitance that has just been turned off. The resonant filter must be designed to be inductive, that is, the output voltage leads the current of the series resonant branch. This is a necessary condition for achieving ZVS of the power switch tube. If equation (1) is not satisfied, then ZVS cannot be achieved. The dead time between the switch tubes Q1 and Q2 must be long enough to allow the switch tube junction capacitance or external parallel capacitance to be fully charged and discharged.

(1)

　　Where Lr is the equivalent inductance of the resonant filter, I1 is the average current during the dead time, and Ci() is the junction capacitance of the switch tube or the external capacitance.

　　Resonant filter analysis

　　The output end vs(t) of the asymmetric half-bridge converter is a quasi-half square wave, and the output voltage waveform THD is to be less than 2%. Therefore, only a 4th-order or higher resonant filter can be used to filter out a sine wave with a THD less than 2%. At the same time, in order to reduce the size of the filter, a 4th-order resonant filter is selected. The functions of the resonant filter are: (1) filtering the output quasi-square wave, isolating the DC component, reducing the harmonic content of the output voltage, and making the waveform close to a sine wave; (2) in the case of a resistive load, making the output voltage lead the current of the series resonant branch, thereby achieving ZVS of the full-bridge switch tube. The resonant frequency of Ls and Cs of the resonant filter is designed to be equal to the switching frequency, and the resonant frequency of Lp and Cp is higher than the switching frequency. For the convenience of analysis, Lp is decomposed into Lp1 and Lp2, where Ls and Cs are in series resonance at the switching frequency, and Lp1 and Cp are in parallel resonance at the switching frequency. The equivalent circuit diagram is shown in Figure 3.

　　Figure 3 Equivalent circuit diagram of series-parallel resonant filter

　　The output of the asymmetric half-bridge converter vs(t) is decomposed by Fourier transform to obtain equation (2).

　　(2)

　　in

　　Vin is the DC bus voltage, D is the duty cycle of the switch tube Q1, and is the fundamental angular frequency.

　　The output end vs(t) of the asymmetric half-bridge converter is filtered by the resonant filter to obtain the fundamental voltage, that is, the effective value of the output voltage is:

　(3)

　　The relationship between the ratio of the output voltage to the input voltage and the duty cycle D of the switch tube Q1 is shown in the figure.

　　Figure 4 Relationship between the ratio of output voltage to input voltage and D

　　Relative to the fundamental wave, the resonant filter is inductive. According to the traditional series-parallel resonant filter, Ls, Cs and Lp1, Cp are designed to be completely resonant at the fundamental frequency; and the inductor Lp2 is designed through soft switching conditions.

　　Simulation Results

　　In order to verify the feasibility of this solution, the inverter was simulated. The simulation data are as follows: input voltage Vin = 510V, output voltage Vo = 500V, output voltage frequency is 118kHz, Ls = 88μH, Cs = 20nF, Ls = 20μH, Cp = 70nF, switching frequency fs = 118kHz, transformer primary-to-secondary ratio

　　Figure 5 Output voltage vo, filter series branch current is waveform

　　Figure 6 Driving voltage waveform vGS and vDS waveform

　　Figure 7 Driving voltage waveform vGS and vDS waveform

　　Figure 8 Output voltage THD distribution

　　Figure 5 shows the waveforms of the output voltage vo and the current is of the series branch of the resonant filter under resistive load. Figure 6 shows the driving voltage waveform vGS of Q1 and the voltage vDS waveform at its DS end under resistive load. When the driving voltage vGS becomes positive, the vDS of the MOS tube is already zero, and the switch tube is turned on with zero voltage. When the switch tube is turned off, its junction capacitance limits the rising rate of vDS, so the switch tube is turned off with approximately zero voltage, which shows that the switch tube has achieved ZVS. Figure 7 shows the driving voltage waveform vGS of Q2 and the voltage vDS waveform at its DS end under resistive load.

　　in conclusion

　　This paper studies the asymmetric half-bridge soft-switching high-frequency output inverter, and analyzes in detail the working principle of the asymmetric half-bridge inverter, the soft-switching implementation conditions and the design of the resonant filter. Experiments show that this scheme can output a high-frequency AC sine wave with a THD of less than 3%, and achieve zero voltage switching from no-load to full-load when the typical load is resistive, with high efficiency, and is suitable for small and medium power high-frequency AC output occasions.