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| Pulse Width-Frequency Modulation Hybrid-Controlled Component-Reused Quasi-Single-Stage Buck-LLC Converter |
| Li Hongzhu, Yin Yandong, Li Chao, Geng Zhaoxin |
| Faculty of Electrical and Control Engineering Liaoning Technical University Huludao 125105 China |
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Abstract With the advancement of carbon peaking and carbon neutrality goals, there is a growing demand for wide-voltage-gain DC-DC converters for applications such as electric vehicles and data centers. LLC resonant converters, widely adopted for their structural simplicity and high efficiency, exhibit significant efficiency degradation under wide-voltage-range operation. This degradation occurs when the switching frequency deviates substantially from the resonant point, leading to increased circulating currents and reduced efficiency.
Existing improvement for LLC converters exhibits notable limitations. Rectifier-bridge reconfiguration and reconfigurable-transformer approaches achieve gain extension by adding switching devices, but increase structural complexity and cause significant resonant current spikes during transitions. Two-stage converters (e.g., Buck/LLC cascaded) offer a wide gain range and electrical isolation. However, the hard-switched PWM stage, which processes the full energy flow, incurs high losses, thereby reducing the overall system efficiency. Single-stage Boost-LLC combinations allow the LLC stage to operate near resonance but impose high bus voltage, limiting component selection. Quasi-single-stage Buck-LC solutions face challenges with complex control strategies, fixed sub-resonant operation of the LC stage, and underutilization of the Buck output capacitor.
This paper proposes a Pulse Width-Frequency Modulation hybrid-controlled component-reused quasi- single-stage Buck-LLC converter. The converter constructs the Buck stage by reusing one bridge leg of the LLC stage. Furthermore, it integrates the Buck output capacitor in series with the converter's input capacitor. The bridge-leg reuse and the series-connected capacitor configuration offers dual advantages. (1) It leverages a capacitive voltage division mechanism to enhance the voltage conversion ratio, enabling the use of lower-voltage-rated capacitors in high-input-voltage applications while simultaneously improving capacitor utilization. (2) It facilitates zero-voltage switching for power switches, thereby significantly reducing switching losses. Control is implemented via a hybrid PWM-PFM strategy. During variations in the input voltage, the switching frequency is adjusted to regulate the resonant tank gain. At the same time, the duty cycle is modulated to control the input voltage applied to the resonant tank. This coordinated control strategy enables the converter to maintain high efficiency and stable operation across a wide range of input voltages. Meanwhile, the Buck-stage inductor, LLC resonant inductor, and transformer are integrated onto a single magnetic core using a decoupled magnetic integration approach with flux cancellation.
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Received: 08 July 2025
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2026, Vol. 41 
