Modeling of Switching Power Losses in Cascaded H-Bridges With Unipolar PWM

Abstract

This article studies the extraction of an equivalent resistance for calculation of the switching power losses in cascaded H-bridge (CHB) multilevel inverters with unipolar pulsewidth modulation. The concept of local and global root mean square (rms) currents and switching power losses is introduced in each H-bridge (HB) considering different frequencies of the inverter output and the switching network, i.e., transistor and diode. Energy conservation law has been used to show that the equivalent averaged local and global resistances for switching power losses are functions of load power factor angle, load rms current, and modulation index. This dependency is then reduced to only load rms current when the equivalent resistances are transformed to the inverter output branch based on the reflection rule. Consequently, a resistive model of switching power losses of the inverter is deduced, which depends neither on the knowledge of the physics of the switch nor on complicated nonlinear equations of the semiconductor devices and loop inductances. Double-pulse tests (DPTs) are conducted to acquire reliable data on the switching characteristics of the devices for different operating points.