Half-Bridge Diode-Clamped Four-Level LLC Resonant Converter with Pulse-Amplitude-Modulation for Railway Applications

In this paper, we propose a half-bridge (HB) diode-clamped four-level LLC resonant converter based on multi-neighboring reference vector discontinuous pulse-width modulation (MNRV DPWM). While maintaining the characteristics of the full-bridge MNRV DPWM, output voltage is controlled using the pulse-amplitude-modulation method with fixed switching frequency at the resonant point. Depending on the location of the command voltage, multiple reference vectors with different charging/discharging characteristics of the capacitors are selected. The duties of selected multiple switches are adjusted to effectively remove the voltage deviations among the input DC-link capacitors while satisfying the magnitude of the command voltage on average. By designing duty compensators proportional to the voltage deviations of the DC-link capacitors and correcting PWM pulse duties, the output voltage is easily controlled in a closed-loop manner. For the HB topology, selection of multiple reference vectors according to the command reference voltage was modified for enhanced symmetry. The control algorithm, steady state circuit operation, and voltage transfer gain were analyzed in detail, and design guidelines were also included. The effectiveness of the proposed converter was verified through simulations and experiments on the prototype circuit.