Analytical analysis of the equivalent parasitic capacitance of the high-voltage cascade multiplier in medical application of X-ray power generator

Abstract

A major component of high voltage (HV) generators for medical X-ray machines is the HV multiplier. This is typically a Cockcroft-Walton cascade circuit, multiplying a moderately high voltage from the transformer to the final high voltage level at round 100kV. It is often equipped in a resonant converter as a part of capacitive output. Previous work shows that the parasitic capacitance of the cascade multiplier becomes highly relevant to the resonant tank. The equivalent parasitic capacitance of the multiplier has been numerically analyzed to discover its dependence on the material technologies of the HV module. However, the simplifications in the previous research, such as assumed equal voltage distribution across series connected diodes, absence of bulk capacitors etc., give limitations to validity of the equivalent capacitance characteristics. In this paper, an improved approximate capacitance network is proposed for the multiplier with parasitic capacitances to exhibit a more realistic parasitic model of the HV module. Based on the network, the equivalent capacitance of the multiplier is analytically expressed and analyzed. The relationship between the capacitance and parameters including the diode junction capacitance, the structural capacitance, the number of diodes per chain and the feeding voltage of the cascade is shown. According to the results, the capacitances of the cascade are compared for two semiconductor technologies that are silicon (Si) diode and silicon carbide (SiC) diode. Design guidelines are obtained for the minimization of the cascade capacitance.