A dv/dt Filter Design Based on the Voltage Reflection Theory at SiC Converter

This paper analyzes a voltage surge generation at the end of cable in an inverter-cable-high impedance load system, and the analysis is based on the voltage reflection theory. Cable parameters are estimated by using the network analyzer, and the whole system is simulated for generation of voltage surge. A 150 V unit pulse from a SiC buck converter is injected to a high impedance load through several lengths of two cables. Voltage surge in terms of the ratio of rising time to the propagation time are analyzed. In addition, the excessive voltage surge at the cable end is mitigated by adding a dv/dt filter at the converter output to increase rise time of PWM pulses. An effective dv/dt filter is proposed and verified through the experiment. Using a cable with a characteristics impedance of $Z_{c}=80.2\Omega$ and a propagation delay of $t_{d}=44.2$ ns, the dv/dt filter is able to reduce a voltage surge of 300V to 173V which corresponds to percentage reduction of 42.3%.

I. Introduction

The advances in wide bandgap power switching devices such as SiC and GaN have led to high performance at the PWM converter due to the higher switching frequency and lower losses. While these high switching operations can enhance the performance of PWM converter-fed high impedance load, the sharp voltage rise can degrade the insulation of the load because of the increased leakage current. In addition, connection of a long cable between converter and high impedance load can results in the undamped high frequency voltage surges at the terminal of the load, which can stress the load insulation [1]. Therefore, to minimize the sharp voltage rise at the load terminal, a dv/dt filter is used at the converter output to increase the rise time or a shunt RC filter can be located at the load terminal to reduce a load impedance at high frequency [2] [3]. A relatively short length of cable connected between SiC converter and high impedance load is equivalent to a transmission line. In this paper, transmission line model theory and voltage reflection theory are used to explain a voltage surge phenomenon at a cable. In addition, voltage surge generation in terms of the ratio of rise time to the propagation time is analyzed and verified by simulation and experiment.