High step-up isolated forward-flyback DC/DC converter based on resonance with pulse frequency modulation

A high step-up isolated forward–flyback DC–DC converter with pulse frequency modulation (PFM) is presented in this paper. The proposed converter has features of both a forward and a flyback converter and can deliver energy during the turn-on and turn-off intervals in a switching period. In addition, by utilizing resonance it can achieve zero-voltage switching (ZVS) for the primary switches and zero-current switching (ZCS) for the secondary rectifier diodes. All of the power semiconductors operate under soft switching conditions. The structure of the proposed converter is simple and easily implemented. It also provides a high step-up voltage gain. The current stresses of the power devices in PFM are lower than those in pulse width modulation (PWM). Therefore, the converter efficiency in PFM is higher than that in PWM. The operation principle and characteristic of the presented converter are described in detail, and the converter performances for PWM and PFM are compared. A 160 W hardware prototype has been built, and experimental results verify the validity of the theoretical analysis.