Research On Resonant Converters For High Voltage Applications

With the rapid development of power electronics,high voltage power supply has been widely used in electrostatic precipitator,medical X-ray machine,radar transmitters,and so on.This thesis is dedicated to the resonant converters suitable for high voltage power supply.In this thesis,the operating principle of the LCC resonant converter is analyzed.With the extended fundamental harmonic approximation(FHA)approach,the equivalent circuit and the dc voltage gain function of the LCC resonant converter is derived.Based on that,the effect of?(the ratio of the parallel resonant capacitor to the series resonant capacitor)on LCC resonant converter is discussed,and it is pointed out that increasing?could reduce the variation range of the switching frequency.Meanwhile,the LCC resonant converter operated as a dc transformer(DCX)is discussed,and the normorlized switching frequency f_n is set to be slightly higher than 1 in order to achieve a better output voltage regulation and realize zero-voltage-switching(ZVS)for the power switches at heavy load.In order to achieve ZVS at light lowd for the DCX-LCC resonant converer,a resonant indoctor,which is realiazed by the transformer magnetizing inductor,is introduced to be connected in parallel with the parallel resonant capacitor,and thus forming the LCLC resonant converter.The operating principle of the LCLC resonant converter is analyzed,and the expression of the dc voltage gain is derived and its characteristics are discussed.It is found that,with the added parallel resonant indoctor,an additional inductive area arises around f_n=1.By properly designing the parallel resonant indoctor,the power switches could achive ZVS over the entire load range.For the variable frequency controlled LCC resonant converter,a five-output prototype with rated power of 550 W and maxmum output voultage of 8 kV is built and tested in the lab.The experimental results show that ZVS is achieved for the power switches and zero-current-switching(ZCS)is achieved for the secondary rectifiers over the entire load range,and the peak conversion efficiency is96.1%.In addition,the prototypes of the DCX-LCC resonant converter and DCX-LCLC resonant converter are also fabricated and tested in the lab.The experimental results show that the DCX-LCLC resonant converter can realize the ZVS for the power switches over the entire load range,and the conversion efficiency is improved significantly over the DCX-LCC resonant converter.