A Pulse-Frequency-Modulated Full-Bridge DC/DC Converter With Series Boost Capacitor

The conventional zero-voltage switching phase-shift full-bridge (ZVS PSFB) converter has a large circulating energy during the freewheeling interval caused by the small duty cycle, which could increase the primary-side conduction losses, the turn-off switching losses of lagging-leg switches, and the current ripple through the output inductor. To overcome these problems, this paper proposes a new pulse-frequency-modulated full-bridge direct-current (dc)/dc converter with a series boost capacitor (SBC). The proposed converter controls the output voltage by varying the voltage across the SBC according to the switching frequency and has no freewheeling interval due to a 50% fixed duty operation. As a result, since its freewheeling current is eliminated, the conduction losses can be considerably reduced, as compared with those of the conventional ZVS PSFB converter. Moreover, the ZVS of all power switches cannot only be ensured along wide load ranges, but the current ripple through the output inductor can be also significantly reduced. Therefore, it has very desirable merits such as high efficiency, small output inductor, and improved heat generation. Operational principles, theoretical analysis, and design considerations are presented. To confirm the operation, the validity, and the features of the proposed converter, experimental results from a prototype that is 400-12 V/100 A are presented.