Research on capacitor charging power supply based on high frequency AC link technology

1 Introduction
In current pulse power systems, high-voltage capacitors are usually charged by linear power supply resonance or constant current of traditional resonant high-frequency switching power supply. Linear power supply works under power frequency conditions, and the transformer is large and bulky. In addition, to meet the needs of the application, the output needs to be fully filtered, which usually requires a large-capacity high-voltage capacitor, and high energy storage requires an additional protection system for the power supply. The traditional resonant high-frequency switching power supply uses a resonant circuit, and there is a DC-link part inside the power supply, which is usually a large-capacity electrolytic capacitor, and the volume and weight account for a large proportion of the entire power supply. With the development of new mobile concept weapons, higher requirements are placed on the volume and weight of the power supply. The series resonant capacitor charging power supply based on high-frequency AC link technology has no linear rectification and DC-link parts, and the power density is greatly improved. The series resonant technology is used to make the switch work under zero current conditions, and the operating frequency is further increased. The novel control method enables the three-phase input current to follow the three-phase input phase voltage to achieve higher power quality.

2 Working principle
The converter structure is shown in Figure 1. It consists of a three-phase input filter, a matrix switch composed of IGBTs, an LC series resonant circuit, a high-frequency high-voltage transformer, and a full-bridge high-voltage rectifier circuit. The three-phase input filter is a second-order low-pass filter composed of an inductor L and a capacitor C. The filter capacitor adopts a Y-type structure, and can also adopt a △ type, but the parameter design is slightly different. In addition to reducing the current harmonics in the circuit, the filter capacitor mainly plays the role of energy storage, supplies the series resonant circuit and the load, and reduces the distortion of the three-phase AC voltage. The matrix switch is composed of 12 ICBTs, and every two IGBTs form a bidirectional switch. The current can flow in both directions. The connection method can be that the c poles of the two IGBTs are connected, or the e poles are connected. 6 groups of bidirectional switches form a bridge rectifier structure. The matrix switch is connected to the series resonant circuit, which can realize zero current switching and bidirectional flow of energy. During the working process, by detecting the three-phase AC voltage and load voltage, the switching timing and switching time of the IGBT in the matrix switch are controlled, and the amount of charge provided by each phase to the resonant circuit is controlled, so that the three-phase input current follows the three-phase input phase voltage.

3 Working process analysis
The power supply works in three processes: the charging process of the resonant capacitor Cr is divided into two processes, which are recorded as mode 1 and mode 2 respectively; the discharging process of Cr is only one process, which is recorded as mode 3. The three processes form a resonant cycle, and the waveform of the resonant inductor current iLr and the resonant capacitor voltage uCr(t) are shown in Figure 2. In order to better understand the three working processes, the three-phase input phase voltage is introduced:

For the convenience of analysis, the phase of the three-phase input phase voltage is examined from 0 to 6/π. At this time, the three-phase input phase voltage satisfies |ua|≥|ub|≥|uc|, and UM=|ua-ub| and UN=|ua-uc| are defined. Since the resonant frequency (60 kHz) of the series resonant circuit is much higher than the power frequency (50 Hz), the phase voltage changes very little in one cycle, so it is assumed that the voltage loaded into the resonant circuit is constant during analysis. The load capacitance CL is equivalent to the primary capacitance value much larger than CL, so in one resonant cycle, the voltage rise of CL is very small, and it is regarded as a DC source during the analysis process.

At t0, VS2 and VS12 are driven first, UN is loaded onto the resonant circuit, phase a and phase c form a current loop, iLr increases, the current characteristics are determined by the LC series resonant circuit, Cr and load capacitor CL begin to charge, and the voltage Uo of UCr and CL begins to rise. The equivalent circuit is shown in Figure 3. Assume that the initial working conditions of mode 1 are: ILr(t0)=0, UCr(t0)=-2Uo. The resonant inductor current and resonant capacitor voltage are:

The amount of charge flowing out of phase a and phase c during the time period t0~t1 is:

At t1, VS10 is driven, UM is loaded onto the resonant circuit, and the current of phase c is naturally commutated. Phase a and phase b form a current loop, and iLr continues to change according to the series resonant current characteristics until the current is zero, at which time UCr reaches the peak. The equivalent circuit is similar to Figure 3a, only UN is replaced by UM. The initial conditions of mode 2 are:

At t2, VS1, VS9 are driven, iLr flows in the opposite direction, Cr begins to discharge, and the current is zero at t3. The equivalent circuit is the same as that of mode 2, but iLr is reversed. Mode 3 initial conditions: ILr(t2)=0, UCr(t2)=UM-Uo+IMZ.
Then the expressions of iLr(t) and uCr(t) are:


4 Control strategy
In one resonant cycle, the current direction of mode 1 and mode 2 is defined as positive, the charge of the forward current is defined as the charge flowing out of the three phases, and the charge of the reverse current is the charge flowing back to the three phases. Then the net charge flowing out of the three phases is:

Q can be regarded as flowing out of phase a, while the charge flowing out of phase c and phase b is Q1 and Q2-Q3 respectively. Using the charge control theory, the charge flowing out of phase a and phase c is proportional to their respective phase voltages, with a proportional coefficient of k, and its expression is:

The curve of θ changing with the three-phase AC phase voltage (0~π/6) and Uo is shown in Figure 4a. As can be seen from the figure, with the increase of Uo and the change of three-phase AC phase voltage, θ increases monotonically, and the maximum value is half a charging cycle. The change of θ shows from another perspective that with the increase of Uo, the output energy increases.

The curve of the resonant current period fs changing with the three-phase AC phase voltage and Uo is shown in Figure 4b. It can be seen that fs first increases and then decreases with the increase of Uo. With the change of the three-phase AC phase voltage, the resonant current period also increases first and then decreases. The maximum value of the period is about 6.47 rad. Compared with the DC-link technology, the current period of the series resonant converter increases by 0.19 rad. The maximum period appears at 0.15 rad.
The curve of the residual voltage on the resonant capacitor changing with the three-phase AC phase voltage and Uo is shown in Figure 4c.
The residual voltage on the resonant capacitor increases with the increase of uo, and first increases and then decreases with the change of the three-phase AC voltage. The voltages at the first and last points are the same, and the maximum voltage appears at the phase point of 3/π.

5 Experimental results
Based on the above principle analysis, an experimental prototype of a capacitor charging power supply is designed. The main parameters are: AC input 380 V, power supply output voltage 50 kV, charging rate 60 kJ/s, resonant capacitor 1.98μF, resonant inductor 2.25μH, switching frequency 30 kHz.

Figure 5a shows the waveforms of the three-phase AC input line current iac and the AC phase voltage uac. uac and iac maintain a proportional relationship and are in phase. The power factor was measured using an energy analyzer, and the measured value was 0.99. The switch current at the beginning and end of charging is shown in Figure 5b. As can be seen from the figure, as uo increases, the switching time increases from 1μs to 2μs, the first half cycle of the current increases from 6μs to 7μs, and the second half cycle becomes smaller due to the influence of distributed capacitance.

6 Conclusion
The current characteristics of the power supply in each working mode under the condition of discontinuous current are derived, and the influence of the three-phase grid voltage and output voltage on the switch switching time and the resonant current period is studied. A prototype of a capacitor charging power supply based on high-frequency AC link technology is designed, and experimental research is carried out. The experimental results show that: by applying the charge control method, the grid input end can achieve a very high power factor, and the switch switching time (angle) and the period of the resonant current change with the three-phase grid voltage and output voltage.