A discrete-time controller for Phase Shift Controlled load-resonant inverter without PLL

Abstract

Traditional approach of controlling a Phase Shift Controlled (PSC) load-resonant inverter involves a Phase Locked Loop (PLL) to vary the inverter switching frequency over a narrow range to minimize the switching stresses through Zero Voltage Switching (ZVS) and to improve the output-side inverter power-factor at all operating conditions. The PLL circuit introduces inherent delay in frequency control due to filter and Proportional Integral (PI) control block. This delay increases switching stresses and occasional control system failure. In addition, a problem of non-linear phase-shift variation is often encountered due to frequency variation in such converter. The paper proposes a new discrete-time controller for a load resonant, Voltage Source Inverter (VSI) without the use of PLL. The controller ensures fast frequency control along-with the desired phase shift control, simultaneously. The problem of non-linear phase-shift variation is solved by newly proposed slope compensation technique. The proposed digital control system is implemented in real-time with DSP hardware and the results are presented. The experimental system with IGBT based VSI is tested for validation of the proposed digital controller for induction brazing applications. The controller has potential applications in induction heating, hybrid electric vehicle on-board battery charger and ion-generator power supply for vacuum sputtering.