Hybrid Phase-Frequency Control-enabled Global Loss Minimization of a Full-Bridge LLC Converter under Wide Gain and Load-range Operation

In this paper, a loss optimized hybrid phase-frequency control of LLC converter is proposed, investigated, and validated for aircraft auxiliary power unit (APU) onboard charging applications. The proposed control method emphasizes on achieving minimum power loss by adopting an optimum duty cycle of LLC tank input voltage based on the existing load and gain conditions while the output voltage regulation is realized by controlling the frequency sweep. In comparison to the conventional frequency-controlled LLC converter, the proposed methodology can achieve a wider gain range with a lower frequency sweep by including an optimized phase-shift control. A proof-of concept 500W LLC converter, generating 20–28 V output from the 270-V dc link is designed and tested to verify the claims of the proposed hybrid control technique. The experimental results demonstrate the effectiveness of the phase-frequency control by exhibiting a peak conversion efficiency of 96.7% at full load with an average efficiency improvement of 7.5% at 20% rated load over the conventional frequency-based control alone.