A pulse frequency modulated full bridge DC/DC converter with series boost capacitor

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