Research Of Long Pulse Modulator Based On Parallel Resonant Converter

In X-ray tomography, accelerator, surface modification of materials and other areas of RF power sources, high voltage pulse has been applied widely. Traditional high voltage pulse modulation methods are varied, from the general principle of them are in down for high voltage pulse capacitor energy storage conditions. In some special applications such as X-ray tomography and part of the RF power source, it is often required a long pulse repetition frequency voltage of milliseconds to seconds duration as well as meet the lower stored energy in modern power electronics. The conventional pulse modulation methods mean various restrictions. Someone proposed a improved scheme based on the modulation of high-voltage power supply, which will directly connect load to the output of the high voltage power supply. By controlling the starting and stopping the power converter of the power supply, it is achieved the direct modulation of the long-pulse voltage.Under the background, this paper describes the significance of soft-switching and classification of resonant converter, derivates the voltage type load parallel resonant converter and use the converter as a research subject to long pulse voltage modulation. Then the basic working principles and control strategies of the converter circuit are analyzed and the steady-state model of the converter circuit is derived with the method of fundamental analysis and further conditions are calculated to achieve soft switching. And for three different control strategy of frequency control, phase shift control and frequency&phase shift control, it is analysised the soft-switch and deduced the gain of each control strategy implementation.Through theoretical analysis and calculation the relationship between the DC bus voltage fluctuations and DC-link capacitor capacity is achieved. On the basis of the simulation realization of long pulse voltage feedforward control output, and aiming at the shortcomings of the feedforward control, the solution of feedforward and feedback control is proposed. Using the method of DQ transformation, the DQ equivalent circuit model of the voltage-load parallel resonant converter is derived. And the transfer function of the equivalent circuit is solved. Meanwhile the accuracy of the transfer function is proved with means of simulation. Then the closed-loop PI controller was designed using the SISO tool of Matlab, and the pulse modulation of two different ways are compared. Combines the advantages of the two modulation methods, the feedforward plus feedback closed-loop controller is designed. In condition of380V three-phase input, the output of2ms duration,50%duty cycle5kV/20kW long pulse voltage is realized in simulation with rising time323μs,ripple1.8%,falling time83μs.Finally, based on Texas Instrument TMS320F28027DSP and experiment platform we write the corresponding control program and complete the work of the experimental section. Through the experimental validity, the correctness of the frequency and phase shift gain controlling principle and correctness of the soft switching principle are proved.