Active Magnetizing Current Splitting ZVS Modulation of a 7 kV/400 V DC Transformer

The LLC series resonant converter (SRC) is one of the most popular galvanically isolated dc-dc converters since it provides zero voltage switching (ZVS), reduces rms currents, and tightly couples the input and output voltages, when it is operated at (or below) the resonance frequency, and, therefore, acts as a dc transformer (DCX) without requiring closed-loop voltage control. Hence, this topology is of particular importance for the dc-dc converter stage of high power medium voltage to low voltage solid-state transformers (SSTs). This paper first highlights the limitations of passive and synchronous rectification (e.g., oscillations, current distortion, load-dependent voltage transfer ratio) for bridges employing semiconductors with large output capacitances. Afterward, a magnetizing current splitting ZVS (MCS-ZVS) modulation scheme, which allows an active sharing of the magnetizing current between the primary side and secondary side metal oxide semiconductor field effect transistor ( mosfet )-based bridges, is analyzed. It is shown that the ZVS mechanism is acting equivalent to a controller, allowing for a robust open-loop operation of the converter. The proposed modulation scheme features a load-independent voltage transfer ratio, load-independent ZVS for both bridges, and quasi-sinusoidal currents. Finally, the phase shift modulation scheme is experimentally verified for the SiC mosfet -based dc-dc converter of a {\text{25}} kW ac-dc SST, which operates at {\text{48}} kHz between a {\text{7}} kV and a {\text{400}} V dc bus with an efficiency of {\text{99.0}}{\%}.