Stacked-Switched Electrolytic Capacitor-less AC/DC Power Converters

Abstract

The growing adoption of Electric Vehicles (EVs) has increased the demand for compact, efficient, and reliable On-Board Chargers (OBCs) capable of interfacing directly with the AC grid. Conventional single-phase AC/DC converters often depend on bulky electrolytic capacitors, diode bridges, and hard-switching circuits, leading to reduced reliability, increased losses, and limited power density. Additionally, the need for bidirectional energy flow—such as Vehicle-to-Grid (V2G) and Vehicle-to-Home (V2H)—requires grid-compliant converters with enhanced functionality and robust control strategies.

This dissertation presents novel single-phase AC/DC converter topologies and control systems designed to eliminate electrolytic capacitors while enabling efficient bidirectional power transfer and robust grid-forming capabilities. The work is organized around three primary contributions.

First, a unidirectional bridgeless AC/DC converter is proposed, utilizing a stacked-switch configuration to reduce voltage stress and operating in Discontinuous Conduction Mode (DCM) to achieve inherent power factor correction. Closed-loop control based on Variable Frequency Modulation (VFM) is used for voltage regulation, and a secondary-side duty modulation strategy minimizes low-frequency ripple, enabling the use of compact, long-life capacitors. Experimental validation on a 1.1kW prototype confirms high power factor (>0.99), low THD (<2.5%), and soft-switching operation.

Second, a bidirectional converter is introduced, combining a half-bridge dual stacked-switch AC/DC stage and a CLLC resonant DC/DC stage. A dq-frame-based controller ensures low harmonic injection and effective grid synchronization in both power flow directions. Ripple reduction is again achieved through duty modulation, validated on a 1kW prototype.

Finally, a robust grid-forming control scheme is developed using H_infinity synthesis with Linear Matrix Inequalities (LMIs), ensuring voltage stability under uncertainty and distortion. Experimental results confirm dynamic performance and low output distortion.

Together, these contributions support next-generation EV charging systems with high reliability, bidirectional capability, and electrolytic capacitor-less design.