Application of natural commutation technique to an improved cycloconverter type high-frequency link inverter with center-tapped transformer

Abstract

This thesis presents a new isolated cycloconverter type high frequency (HF) link dc to ac power converter (inverter) that consists of three arms of bidirectional switches at transformer secondary. This topology has the advantage of light weight and reduced switch count, compared to other types of HF link inverter. However, like all HF link inverter topologies, this topology also suffer high voltage surge problem and power device switching loss. In order to overcome these problems, two sets of switching technique, namely the asymmetric bipolar PWM control and edgealigned unipolar PWM control, are introduced. These switching techniques utilize natural commutation technique that enables total soft switching operation and voltage surge reduction. Furthermore, the proposed switching techniques are able to reduce the transistor conduction loss due to the existence of freewheeling period. The research work verifies of the proposed topology and switching techniques by means of SPICE simulations. It also describes the natural commutation mechanism for HF link inverter in detail. An open loop laboratory prototype based on the Infineon C167 fixed-point microcontroller was constructed. The results obtained from the experimental rig were found to be in very close agreement with the theoretical prediction and simulation. The laboratory prototype was able to supply output voltage 340Vrms as well as 120Vrms, with THD less than 2.5%. The total power conversion efficiency reaches 86% when the output power was beyond 300W