Hybrid Modulation Scheme for a High-Frequency AC-Link Inverter

This paper describes a hybrid modulation scheme for a high frequency ac link (HFACL) multi stage inverter comprising a front end dc/ac converter, followed by isolation transformers, an ac/pulsating-dc converter, and a pulsating-dc/ac converter. The hybrid modulation scheme enables: a) removal of the dc link filter evident in conventional fixed dc link (FDCL) inverters placed after the ac/pulsating-dc converter stage and before an end stage voltage source inverter (VSI); and b) significant reduction in switching loss of the inverter by reducing the high frequency switching requirement of the pulsating-dc/ac converter by 2/3rd yielding higher efficiency, improved voltage utilization, and reduced current stress. Unlike the FDCL approach, in the HFACL approach, hybrid modulation enables the retention of the sine wave modulated switching information at the output of the ac/pulsating-dc converter rather than filtering it to yield a fixed dc thereby reducing the high frequency switching requirement for the pulsating-dc/ac converter. Overall, the following is outlined: i) hybrid modulation scheme and its uniqueness; ii) operation of the HFACL inverter using the hybrid modulation scheme; iii) comparison of the efficiency and losses, current stress, and harmonic distortion between the hybrid modulation based HFACL inverter and the FDCL inverter; and iv) scaled experimental validation. It is noted that, the term hybrid modulation has no similarity with the modulation scheme for a hybrid converter (which are conjugation of two types of converters based on a slow and fast device) reported in literature. The term hybrid modulation scheme is simply chosen because at any given time, only one leg of the inverter output stage (i.e., pulsating-dc/ac converter) switch under high frequency while the other two legs do not switch. The outlined hybrid modulation scheme is unlike all reported discontinuous modulation schemes where the input is a dc and not a pulsating modulated dc and at most only one leg stays on or off permanently in a 60o or 120o cycle.