A new mode changeable full bridge dc/dc converter for wide input voltage range

Generally, a zero voltage switching (ZVS) phase shift full bridge (PSFB) converter is suitable for high power density/high efficiency application and its efficiency is maximized at 50% duty cycle. However, the steady state duty cycle is as small as approximately 30% at full load and nominal input voltage conditions which is because the PSFB must be designed to be ensure the stable operation for a wide input voltage range. Therefore, the efficiency cannot be optimized with this small duty cycle. To cope with this problem, a new mode changeable full bridge dc/dc converter is proposed in this paper. The proposed converter is operated as an asymmetrical full bridge converter during normal operation period with the duty cycle is less than 50%. But, its operation mode is changed to the active clamp full bridge converter during hold up time with the duty cycle is greater than 50%. Therefore, it can produce the stable output voltage along wide input voltage range even though the steady state duty cycle is 50% at full load condition. Moreover, ZVS of all power switches can be ensured along a wide load ranges and output current ripple is very small. Especially, since the freewheeling interval does not exist(the conduction loss is lower than that of the conventional ZVS PSFB dc/dc converter), high efficiency of the proposed converter can be achieved. The operational principles, theoretical analysis and design considerations are presented. To confirm the operation, validity and features of the proposed converter, experimental results from a 1.2kW(400Vdc/12Vdc) prototype are presented.