Abstract：In modern three-phase quasi-single-stage charging systems for electric vehicles (EVs), the replacement of large electrolytic capacitors with compact film capacitors and the adoption of two-phase clamped discontinuous pulse-width modulation (2/3DPWM) in the front-stage AC/DC converter have significantly improved power density and reliability. However, this architecture introduces a 300 Hz six-pulse voltage ripple on the DC link, challenging the subsequent LLC resonant converter to maintain stable operation across a wide range of output voltages (200 V to 800 V) and powers. The time-varying output voltage accelerates equipment aging and degrades efficiency due to suboptimal operating points. This paper addresses these challenges through an innovative strategy combining comprehensive time-domain analysis, hybrid frequency-phase modulation, and dynamic feedforward compensation. Time-domain characteristics of the LLC converter under operational conditions are analyzed, including pulse frequency modulation (PFM) and phase-shift modulation (PSM) modes. A novel analytical framework is developed to model seven resonant states, enabling precise characterization of voltage gain, resonant currents, and switching losses. The model identifies critical operational boundaries, such as zero-voltage switching (ZVS) conditions and mode transitions. The model integrates detailed loss calculations for MOSFETs, diodes, and magnetic components, thereby quantifying energy dissipation under real-world conditions and guiding the design of efficient control algorithms. To address the conflicting demands of wide voltage regulation and high efficiency, a dual-mode hybrid control strategy is proposed. For mid-to-high voltage ranges, PFM maintains ZVS and efficiency by operating above the resonant frequency. In instances where the gain falls below a certain threshold, the utilization of PSM becomes imperative. This approach entails dynamically adjusting the optimal frequency to achieve balanced management of switching and conduction losses. A hysteresis-based algorithm ensures seamless mode transitions without transient oscillations. A time-domain feedforward control is implemented to suppress output voltage ripple. Real-time estimation of voltage gain (M) and quality factor (Q) from input/output measurements enables the derivation of pre- computed frequency/phase shift commands using look-up tables (LUTs) generated from time-domain analysis. These commands are combined with PI controllers to compensate for voltage variations, resulting in a 64% reduction in transient response time for PFM and a 93% reduction for PSM compared to conventional methods. The effectiveness and reliability of the control are demonstrated at different operating points, as well as in arbitrary operating modes. Efficiency optimization is achieved through a loss-minimization algorithm that uses a 3D efficiency map integrating voltage gain, quality factor, and switching frequency. This map enables real-time selection of the most efficient mode and frequency, ensuring transitions occur where PSM efficiency exceeds PFM. In PSM mode, an optimized switching frequency is adjusted to minimize losses by reducing the RMS conducting current, resulting in a 0.72% efficiency improvement at 200 V compared to fixed-frequency PSM. A 20 kW Vienna+LLC prototype has been constructed. The system achieves >80% ripple rejection under a range of operating conditions, thereby reducing the output voltage ripple from 42 V (in conventional PI) to 7 V. Furthermore, dynamic performance is improved by more than 60% under different load conditions (constant resistance and constant current). Overshoots are kept within ±1% of the reference value. A peak efficiency of 96.2% is achieved for 400 V/5 kW, and a 0.72% improvement is observed for 200 V/5 kW compared to state-of-the-art hybrid control. The prototype exhibits seamless functionality in both parallel (200 V to 400 V) and series (400 V to 800 V) configurations. In this paper, an optimized control strategy for LLC converters is proposed to provide a compact, reliable, and efficient solution for three-phase quasi-single-stage charging systems. This solution integrates full-modal time-domain analysis, hybrid modulation, and dynamic feedforward compensation, combining wide-ranging voltage regulation, fast transient response, and real-time efficiency optimization.
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