A Flexible Hold-Up Time Application for AC/DC Converters Based on Active Capacitor

This letter proposes a DC-side hold-up-time extension method for AC/DC converters using a low-voltage active capacitor. A low-voltage H-bridge with an auxiliary energy-storage capacitor is inserted in series with the DC-link capacitor and operated in three modes (charging, bypass during normal operation, and holdup during AC line-cycle dropout) without modifying the original converter topology or control loops. Closed-form hold-up-time analysis and benchmark evaluation are provided to guide the tradeoff between the volume and power loss of the active capacitor. Experiments on a commercial 3.3 kW/400 V server power supply demonstrate a 20.7 ms hold-up time at full load, while reducing capacitor volume by 34.1% compared with a passive solution and reducing measured power loss by 32% compared with a baby-boost solution (13.4 W). Other electrical and thermal performances are also verified by experiments in terms of operating temperature, cascaded stability, transient response to load variation, and response to output voltage variation. Cost and electrolytic-capacitor lifetime are also evaluated, showing comparable prototype cost and B5 lifetime to the passive solution.