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| Resonant Converter With Wide Voltage Gain in Narrow Frequency Range for Bipolar Low Voltage DC Distribution Networks |
| Zhang Ke1, Chen Zhijun1, Yin Wei1, Pi Hongwei2, Ning Guangfu2 |
1. State Grid Suzhou Power Supply Company Suzhou 215000 China;
2. School of Automation Central South University Changsha 410083 China |
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Abstract To meet the charging demands of a wide range of vehicles, the reliability and voltage range requirements for charging stations are constantly rising. Bipolar DC distribution networks offer high reliability, but the voltage gain range of existing bipolar DC-DC converters is relatively narrow. Expanding the voltage range of bipolar DC-DC converters has become a significant challenge. Some methods in unipolar DC-DC converters can be helpful. Among the unipolar DC-DC converters, resonant converters have been extensively studied.
This paper proposes a resonant converter for bipolar low-voltage DC distribution networks to achieve wide voltage gain over a narrow switching-frequency range. The primary side of the proposed converter consists of two cascaded half-bridge resonant circuits connected to the bipolar DC buses, with the input voltage constant. The input capacitors of half-bridges serve as resonant capacitors, and the resonant inductors and magnetizing inductors are integrated into the transformers. The secondary side is integrated with two voltage-doubling rectifiers, enabling multiplexing of voltage-doubling capacitors. The secondary sides of two transformers are connected in series directly, and an LC branch is inserted in the connection line between the midpoint of the two secondary windings and the midpoint of the two voltage-doubling capacitors.
By modulating the phase of the driving signal to two cascade half-bridges, the proposed converter can operate in in-phase boost mode or out-of-phase buck mode. In the in-phase boost mode, the resonant currents in the two resonant tanks are equal in magnitude and direction, and the currents flowing through the secondary LC branch cancel out in reverse. Hence, the secondary LC branch does not participate in resonance. In this mode, the proposed converter is equivalent to an LLC converter. In the out-of-phase buck mode, the resonant currents in the two resonant tanks are equal in magnitude but opposite in direction, and the currents flowing through the secondary LC branch are in phase. Hence, the secondary LC branch participates in resonance. In this mode, the proposed converter is equivalent to a CLLC converter. A wide voltage gain can be achieved within a narrow switching-frequency range. The operating mode is selected by sampling the output voltage vo and subtracting Vin/n, setting the resonant frequencies of the two operating modes equal. When the result is greater than zero, the converter operates in the in-phase boost mode; when it is less than zero, the converter operates in the out- of-phase Buck mode. When the parameters of the resonant components in the two half-bridge resonant circuits differ, the resonant currents differ. To minimize the impact of parameter inconsistencies, the accuracy of the resonant components should be maximized during the design. A 500 W experimental prototype is built, with a switching frequency range of 70 kHz to 120 kHz (1.71 times) and an output voltage range of 50 V to 160 V (3.2 times). The steady-state experimental waveforms are consistent with the theoretical analysis. The switching between the two modes is smooth, with only slight disturbances, and the prototype achieves a maximum efficiency of 96.9%.
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Received: 30 July 2025
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2026, Vol. 41 
