Complete model of parasitic capacitances in a cascade voltage multiplier in the high voltage generator

摘要

Symmetry Cockcroft Walton multiplier is a typical cascade voltage multiplier. It is an attractive alternative to the high voltage (HV) transformer with high turn ratio and bridge rectifier in the HV generator in medical X-ray machine. It can reduce the stray capacitance of the HV transformer. However, previous work reports that the parasitic capacitances in the multiplier itself can be added to the stray capacitance of the transformer, which becomes a burden to the resonant capacitance of the generator. Thus, it is crucial to minimize the parasitic capacitances in the multiplier. In this paper, the complete model of the parasitic capacitances in the multiplier is exhibited. The model gives full description of parasitic capacitances in any spatial configuration of the multiplier module. Then, the equivalent capacitance of the model is obtained and analyzed to exhibit the role of parasitic capacitances in the system circuit. The dependence of the equivalent capacitance on different parameters, such as different groups of parasitic capacitances and the number of diodes per chain, is addressed. Besides, the impact of breakdown of the diodes on the equivalent capacitance is also exhibited. The complete capacitance model and the analysis of the equivalent parasitic capacitance are validated by the experimental measurements. In the end, guidelines are concluded for how to minimize the equivalent parasitic.