Variable DC-Link Control Loop Design for an Integrated Two-Stage AC/DC Converter

This paper presents an innovative technique of synthesizing variable dc-link voltage control loop for a two-stage integrated ac-dc converter, which is a cascaded combination of a single-phase continuous conduction mode interleaved totem-pole power factor correction and a half-bridge CLLC resonant converter. Although the variable dc-link control exhibits a better overall efficiency than the fixed dc-link control, it faces issues of: 1) loss of output voltage regulation and 2) low-frequency oscillations in the dc voltages and inductor current. In order to address and analyze these issues, both time-domain and frequency-domain studies of the control loop are performed, which led to two different voltage loop controller configurations, resolving the oscillation and regulation issues. As verification to the proof-of-concept, a hardware prototype of an integrated ac-dc converter is developed and tested up to 2 kW. The experimental results exhibit a power factor of 0.992, peak efficiency of >94% and a total harmonic distortion of <5%, with the elimination of low-frequency oscillation of the dc voltage waveforms.