Design of LCC series-parallel resonant charging high voltage pulse power supply

The circuit for obtaining steep-front high-voltage narrow pulses using a MARX generator is relatively complex, and there are many design and process difficulties in steepening the front. It is easy to obtain high-voltage pulse output by using an inductor circuit breaker, but the charging of the inductor must be rapid, and the energy storage time cannot be too long. The power supply needs to have a higher internal resistance and a larger power, and the circuit breaker is the bottleneck of its development. Compared with the inductor energy storage device, the stable and repeatable fast closing switch of the capacitor is much more popular. The energy retention time of the capacitor is much longer than that of the inductor energy storage device, and it can be charged with a small current to reduce the requirement for charging power. The high efficiency and miniaturization of the charging power supply are mainly determined by the charging circuit. The traditional high-voltage power pulse power supply generally uses an industrial frequency transformer to step up the voltage, and uses a magnetic compression switch or a rotating spark gap to obtain a high-voltage pulse. Therefore, most of them are relatively bulky, and the obtained pulse frequency range is limited. Its repetition frequency is difficult to adjust and control, the pulse waveform is unstable, the reliability is low, and the cost is high.

This paper uses the LCC series-parallel resonant converter as the charging power supply for the high-voltage pulse power supply. The LCC series-parallel resonant converter combines the advantages of the anti-short-circuit characteristics of the series resonant converter and the anti-open-circuit characteristics of the parallel resonant converter [1]. It has good characteristics and high conversion efficiency in situations where the output voltage and output current change strongly. This paper introduces the system structure and the principle of the LCC charging circuit, as well as the simulation analysis of the LCC charging process and the generator discharge output using the simulation software PSIM.

1 LCC resonant conversion charging high-voltage pulse power supply system structure

1.1 Power supply main circuit structure and working principle
The circuit consists of power frequency rectification and filtering, power factor correction circuit PFC (Power Factory Correction), LCC resonant converter, high-frequency rectification, capacitor charging energy storage, inductor buffer isolation, IGBT full-bridge inverter and pulse boost transformer. Circuit working process: 220 V AC is rectified, filtered and corrected by PFC to obtain a continuously adjustable DC output, which is charged to the energy storage capacitor C after high-frequency boost through the LCC series-parallel resonant inverter, and bipolar pulse output is achieved through the IGBT full-bridge inverter topology. The system structure is shown in Figure 1.

In the figure, the LCC series-parallel resonant converter consists of four power switching tubes and a resonant inductor Lr, a series resonant capacitor Cs, and a parallel resonant capacitor Cp. The working principle is: using the effects of resonant elements such as inductors and capacitors to convert the current or voltage waveform of the power switching tube into a sine wave, a quasi-sine wave, or a partial sine wave, so that the power switching tube can be turned on or off under zero voltage or zero current conditions, reducing the loss when the switching tube is turned on and off, while increasing the switching frequency, reducing switching noise, and reducing EMI interference and switching stress.

(4) Switching mode 4 [t3, t4]
In this switching mode, all switches and diodes are turned off, iLr is zero, and vCp remains unchanged. At t4, the switches Q2 and Q4 are turned on with zero current, starting the other half of the switching cycle, and the repeated working process begins. The circuit working waveform is shown in Figure 3. Assume that at t0, the initial current of the resonant inductor is

1.3 High voltage pulse forming circuit

The high-voltage pulse is formed by switching the high voltage (current) generated by the previous stage to output the pulse. In the design, when the switching speed meets the requirements, the IGBT series form is adopted, and the full-bridge inverter topology is used to realize the bipolar pulse output [4]. As shown in Figure 1, when switches Q5 and Q7 are closed and Q6 and Q8 are open, the output voltage is positive; when switches Q6 and Q8 are closed and Q5 and Q7 are open, the output voltage is negative, and a bipolar pulse output is obtained. By changing the switching frequency of the two sets of switches, the frequency of the output AC power can be changed. By controlling the on and off time of the switch tube, the duty cycle of the output pulse can be adjusted to obtain a bipolar high-voltage pulse wave with adjustable pulse width and frequency.
The control of the entire system is realized by the controller and the drive circuit, which mainly completes the output voltage regulation and control of the LCC resonant circuit, the full-bridge drive, the variable frequency and variable width output pulse control of the subsequent pulse forming circuit, and the IGBT synchronous triggering. The TMS320F2812 development board used integrates 16 12-bit A/D converters, 2 event manager modules, and 1 high-performance CPLD device XC95144XL. It can realize full digital control of the power system operation including overvoltage and overcurrent protection, and improve the accuracy and stability of the output voltage. The control algorithm is implemented by software programming, making the system upgrade and modification more flexible and convenient.
2 Selection and simulation analysis of circuit parameters
Let K = Cp/Cs, Figure 4 shows the charging voltage, charging current and resonant current waveforms under different k values. For k, take 1, 1/2, 1/4, and 0 respectively. From Figures 4 (a) and (b), it can be seen that the smaller the k value, the higher the charging voltage; and the charging current can be considered constant when the error is allowed, that is, constant current charging. As can be seen from Figure 4 (c), as the k value decreases, the modal time when iLr is zero increases. When iLr is zero, no energy is transmitted, resulting in a decrease in output power. Therefore, according to the above analysis, under the premise of satisfying resonant soft switching, a suitable k value should be selected to make the LCC resonant converter work in the best state, so as to reduce the resonance dead time and improve the power supply efficiency.

This paper designs a high voltage pulse power supply based on LCC series-parallel resonant inverter charging, analyzes the working mode of LCC circuit in DCM mode, and derives the formula to illustrate the importance of k value. The simulation results verify that LCC series-parallel resonant charging technology can realize constant current charging and improve the working efficiency of power supply; the design is easy to realize soft switching of switch tube, and can include the leakage inductance and distributed capacitance of transformer into the resonant parameters, thereby eliminating the influence of these parameters on inverter. In addition, the use of series-parallel resonant inverter charging as the structure for charging the intermediate energy storage capacitor is conducive to the miniaturization and fast charging of the device.
References
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[5] WANG Xuefei, FAN Peng. Analysis and design of series-parallel resonant high voltage converter[J]. Power Electronics Technology, 2008(9):55-57.