An Isolated Bi-directional Series Bridge DC Transformer Without Resonant Tank

The series resonant converters (SRC) such as LLC/CLLC are very popular in dc transformer (DCX) application due to the full load range ZVS, low circulating current and no voltage regulation requirement. In order to realize high rating power and increase the power density, high frequency transformers paralleling, and modularization techniques are necessary for DCX. However, paralleling HF transformers is challenging in SRC-DCX because a small tolerance on the resonant tank will lead to severe current-sharing issue. To avoid this issue, this paper proposes a series bridge DCX (SB-DCX), which replaces the resonant capacitor with a full bridge and keep the full load range ZVS. With this proposed SB-DCX, HF transformer paralleling is feasible because the voltage on series bridge capacitor will regulate automatically to guarantee the current sharing. The operation principle of SB-DCX was explained first including the load-independent unit voltage gain, ZVS mechanism and impedance compensation. And then, the automatic current sharing mechanism was highlighted. Finally, a 12 kW 300 kHz prototype of SB-DCX was built to verify the operation. The experimental results matched well with simulation results.