Efficiency Improvements of a Full Bridge Current-Fed DC-DC Power Converter for Fuel Cell Applications

The most important requirements expected from a DC-DC converter for fuel cell (FC) applications are a high voltage conversion ratio and a low current ripple, where the lower the current ripple, the longer is the FC life span. The isolated Full-Bridge Current-Fed Converter (FBCFC) is most suited for this application especially for grid connection, since it draws a low ripple unidirectional input current, which obviates the need for a reverse blocking series diode, has a large voltage conversion ratio, and a high efficiency. However, in its basic configuration this converter suffers from voltage overshoot across the bridge due to the leakage reactance of the isolation transformer. An active clamp circuit can be employed to overcome this problem, but investigations into the clamp circuit design have shown that the power semiconductors and transformer currents can have a high r.m.s. value and non-ideal shape, which affects converter efficiency. This paper describes an improved FBCFC, focusing on two modifications; one to the active clamp circuit and the other by replacing the output rectifier with a voltage-doubling rectifier. A modification to the active clamp maintains low r.m.s. currents in the bridge and transformer. The addition of a voltage-doubling rectifier allows the step-up transformer to be wound with half the number of secondary turns, resulting in significantly less leakage reactance, which has a positive influence on the design and operation of the active clamp. Hence the combined effect of these two modifications has been shown to improve the overall efficiency, reaching up to 98%. The final FBCFC design employs three step-up voltage conversion mechanisms, and a new improved zero current switching clamp, resulting in a considerable improvement in efficiency, which was validated in a prototype converter. A comparison is made, with simulations, for current-fed bridge converters with and without active clamps to demonstrate the achievement in performance with these improvements. Finally, a cascaded twin-loop controller was designed in Simulink to control the output voltage under varying load conditions, and implemented on the prototype using dSpace hardware