Modified Topology and PWM Modulation for Bidirectional LLC-DCX Converter with Center-Tapped Transformer

In low-voltage high-current applications, LLC-DCX converter with center-tapped circuit is attractive in volume, cost, and efficiency. However, under the parameter fluctuation of resonant components, if the synchronous rectification (SR) is not accurately achieved, the switches connecting with center-tapped circuit may suffer from high voltage spike due to the force interruption of current circulating path. To deal with this, lots of SR technology has been studied with hardware detection for unidirectional applications. However, in bidirectional applications, the SR with hardware detection becomes a challenge since SR bridge should be exchanged under different power direction. This paper proposes a topology modification and PWM modulation for bidirectional LLC-DCX converter with center-tapped transformer. The active-clamp circuits, composed of capacitors and auxiliary switches, are added in parallel with the low-voltage side power switches to avoid turn-off voltage spike. Under the proposed modulation, both high-voltage (HV) side and low-voltage (LV) side power switches as well as auxiliary switches can turn on with zero-voltage (ZVS), which can greatly reduce EMI and switching loss. Meanwhile, since the driving signals are consistent in bidirectional power transmission, both SR detection and logic exchanging are avoided and the naturally bidirectional power flow changing can be achieved. The experimental results verify the correctness of theoretical analysis.