Analysis of Calculation Models for Device Resonance in Critical Mode Converters

Critical Mode (CRM) converters, superior for high density applications at high frequencies, rely on precision gating signals for best timing of device resonance. Previously, two calculation models were used, one based on device time related output capacitance, C_o(tr), and another on device output charge, Qoss. This work applies them to GaN and Si power devices, and analyzes the differences. Results show that the Qoss model is better for both devices in predicting resonant period. However, for prediction current/voltage waveforms the C_o(tr)model is better for GaN while the Qoss model better for Si devices. The Qoss model is then simplified and applied to a 1 MHz and 1.4 kW CRM boost converter built with GaN devices. The prototype achieves >99% efficiency from 600% to 100% load at 240V input and 400V output.