Optimal Duty Ratio Assisted PFM Control for VHF Isolated Class E DC-DC Converter

This article presents an optimal duty ratio assisted pulse frequency modulation control for very high frequency isolated class E dc–dc converter to achieve both continuous power flow and optimal switch timing. The continuous power flow significantly reduces output ripple of the converter, while the optimal switch timing guarantees consistent zero-voltage-switching (ZVS) and minimized reverse conduction loss. To pave the way for the proposed control, a resonant fitting model is established for deriving the optimal duty ratio and output power under dynamic switching frequency. The optimal duty ratio is calculated according to zero-crossing points of the switching node voltage (
v_ds
), and it aims to turn
on
the power switch once
v_ds
reaches zero. This ensures consistent ZVS while preventing reverse conduction of gallium nitride (GaN) high electron mobility transistor (HEMT) when the switching frequency changes. Furthermore, the proposed control is integrated with
on/off
modulation to extend load range of the converter. A 10–18 MHz prototype is built to verify effectiveness of the proposed control. Compared with conventional PFM control, the proposed control reduces the power loss by 35.9% at 40% load while maintains continuous power flow.