PFM-PWM Dual-Mode Control Strategy for LLC Resonant Ion Pump Power Supplies Under Ultralight Load Conditions

Ion pumps operate under drastically varying load conditions, where maintaining stable high-voltage output during the extremely light-load stage (ultra-high vacuum maintenance) is particularly challenging for conventional resonant converters due to frequency runaway and gain nonlinearity. To address this, a hybrid PFM-PWM control strategy is proposed to ensure precise regulation and robust stability across the entire wide load range. The proposed method employs Pulse Frequency Modulation (PFM) with a fixed conduction time under heavy loads to ensure sufficient energy delivery, and seamlessly transitions to Pulse Width Modulation (PWM) under light-load conditions. This dual-mode approach effectively eliminates the voltage drift and instability issues inherent to traditional PFM at light loads, enabling smooth, monotonic regulation down to near-open-circuit conditions. Experimental validation on a digitally controlled 7 kV, 200 W prototype demonstrates superior performance across the complete operational spectrum. The system achieves a stable 7 kV constant voltage output even under complete open-circuit conditions, maintains rated Voltage-Current (UI) curve characteristics through constant current (100 mA), constant power (200 W), and constant voltage (7 kV) regions, and exhibits excellent dynamic response during load step transitions. Arcing simulation tests confirm robust operation under periodic short-circuit conditions with 110 ms cycles, while actual ion pump driving experiments validate reliable vacuum pumping performance from initial pump-down through ultra-high vacuum maintenance, with small and large ion pumps requiring 6.7 and 16 minutes, respectively, to complete the pumping process.