High Frequency Resonant Converters with Optimized Circulating Energy for Ultra-Wide Voltage Range EV Charging Applications

Abstract

The rapid growth of electric vehicles (EVs) has created a strong demand for isolated dc–dc converters capable of operating over an ultra-wide output voltage range while maintaining high efficiency and high power-density. Among various isolated converter topologies, the LLC resonant converter is widely adopted due to its inherent soft-switching capability. However, in wide-output-voltage EV charging applications, conventional LLC converters rely on large switching frequency variation to regulate the output voltage. This operating characteristic inevitably leads to excessive circulating energy through the entire primary switching networks, higher voltage and current stress on resonant components, and significant degradation of full-range efficiency, especially as the operating point deviates from the resonant frequency. In universal EV charging systems, where the output voltage is required to cover a broad range of battery voltages (e.g., 50 V–1000 V), the fundamental challenge is no longer voltage regulation alone, but rather how to optimize the resonant circulating energy of LLC-based converters to an efficient operating region while supporting ultra-wide voltage gain. Conventional resonant converter topologies struggle to simultaneously satisfy these conflicting requirements, particularly under high-frequency operation where circulating energy directly impacts efficiency and power density. This dissertation investigates wide-output-range resonant dc–dc converter design for EV charging applications, with a focus on limiting the circulating energy of LLC resonant converters across an ultra-wide voltage range. Three approaches are explored, including modulation, topology, and resonant tank design methodology. To accurately capture resonant behavior over wide operating conditions, time-domain analysis (TDA) is employed in place of the conventional first-harmonic approximation (FHA), enabling precise resonant tank optimization. First, a novel pulse-width-modulated (PWM) LLC resonant converter with voltage multiplier rectifiers is proposed to decouple voltage regulation from switching frequency variation. By operating at a fixed resonant frequency and utilizing secondary-side PWM control, the proposed topology can operate at the resonant frequency which is considered as its optimal point, thereby limiting circulating energy while achieving an ultra-wide output voltage range. A time-domain-based optimization methodology is developed to accurately model the resonant behavior on the primary side and guide resonant tank parameter optimization for improved full-range efficiency while maintaining zero-voltage switching (ZVS) across the entire operating range. Second, to fundamentally restrict the resonant energy circulation through topology design, a LLC-T resonant converter topology is introduced to reshape the voltage gain characteristic at the resonant-tank level. By incorporating an inverse-coupled auxiliary transformer into the resonant tank, the equivalent resonant inductance and capacitance are load dependent. Thus, the LLC-T topology significantly compresses the required switching frequency range and reduces circulating energy and resonant component stress compared to conventional LLC converters with identical resonant tank designs. Load-independent ZVS is maintained over a wide voltage and load range, enabling efficient high-frequency operation. The operating principles and characteristics of the proposed topology are analyzed using a time-domain model under different operating conditions, including above-resonant, below-resonant (inductive and capacitive regions), and at-resonant operation. Finally, building upon the LLC-T resonant tank as a low-gain, high-efficiency operating mode, an LLC-T–based reconfigurable resonant converter architecture is developed to extend the achievable voltage regulation range for universal EV charging systems covering a 50 V–1000 V output range. An optimization strategy is proposed for resonant converters with wide gain range requirements to guide resonant tank design and limit circulating energy within each operating mode. By dividing the overall voltage gain into multiple operating modes, the proposed architecture replaces a wide continuous operating range with multiple narrow resonant operation regions, further reducing circulating energy while preserving soft-switching performance. Experimental results from a 3.7 kW prototype validate the proposed concepts and demonstrate high efficiency and a power density exceeding 5 kW/L across the entire output voltage range. This dissertation demonstrates that constraining circulating energy through modulation, topology design, and resonant tank optimization provides an effective approach toward high-efficiency, high power density, ultra-wide-range isolated dc–dc converters for next-generation EV charging applications.