Optimal variable switching frequency scheme for grid connected full bridge inverters with bipolar modulation scheme

Abstract

This paper proposes a scheme to improve the system overall efficiency while still meeting a given THD requirement by implementing variable instantaneous switching frequency scheme in a full bridge inverter. Previous work has analyzed variable frequency scheme for unipolar PWM. This paper aims at minimizing the switching loss for inverters using bipolar PWM scheme by including characteristics of the loss and current ripple relative to instantaneous switching frequency in the optimization process. Compared with unipolar modulation, bipolar modulation is much more suitable to implement the variable switching frequency scheme due to its ripple distribution pattern. An additional motivation is that the common mode leakage current performance of bipolar PWM is superior to that of the unipolar PWM for grid connected converters. A 1 kW full bridge inverter using SiC MOSFETs and ferrite core has been built to validate the theoretical analysis. Both constant switching frequency scheme (200 kHz) and optimal variable frequency scheme with simple code modification have been implemented in the experimental prototype using DSP TMS320f28335. The optimal variable switching frequency scheme can increase the inverter efficiency at rated power (1 kW) from 95.5% to 96.8% and has a correspondingly significant 38.1% saving on the switching loss compared with constant switching frequency scheme.