Constant Frequency Duty Cycle Controlled Current Resonant Converter

This paper presents a constant frequency duty cycle control of a current resonant converter with secondly side resonance. Pulse frequency modulation (PFM) is usually used to control resonant converters, which have the disadvantage that the output filter must be designed for the lowest switching frequency, then the output filter cannot be made small. To solve this problem, we propose the use of the pulse width modulation (PWM). Switching frequency is set constant and it is above the resonant frequency to achieve zero voltage switching (ZVS) for the LC resonant converter. As for the LLC converter, the switching frequency is set above the second resonant frequency. The principle of circuit operation is described in detail. Proposed method is confirmed by experiments that are performed with a 12V to 300V voltage multiplier circuit. As results, it is found that ZVS is achieved for the light-load to rated-load by setting switching frequency at higher than resonant frequency.