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| Cascaded Four-Switch Buck-Boost LLC Converter with Wide Gain Range and High Efficiency |
| Zhou Guohua, Qiu Senlin, Zhang Xiaobing |
| School of Electrical Engineering Southwest Jiaotong University Chengdu 611756 China |
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Abstract In new energy resource and electric vehicle fields, DC-DC converters are required to have more advantages, such as wide gain range, isolation, and high efficiency. Therefore, a cascaded four-switch Buck- Boost LLC converter with power switch integration is selected by discussing the advantages and disadvantages of various DC-DC converters. The cascaded converter has a four-switch Buck-Boost converter in the pre-stage and a half-bridge LLC resonant converter in the post-stage. Since designing and optimizing the magnetic components of the cascaded converter controlled by variable frequency is difficult, a special full bridge rectifier structure on the transformer’s secondary side is proposed. The cascaded converter with a special full bridge rectifier structure can widen the input voltage range under fixed frequency control and realize the wide gain range, isolation, high efficiency, and parameter optimization.
The low-side switching devices of the special full bridge rectifier structure on the transformer’s secondary side are switches. The simultaneous on-time of the low-side switches is called the overlapping turn-on time. The gain of the cascaded converter can be improved by controlling the overlapping turn-on time. Therefore, the gain range can be widened, and the efficiency is optimized under fixed frequency control. The gain of the converter that the parameters are unchanged is increased significantly so that the cascaded converter could be adapted to a wider input voltage range and maintain higher efficiency simultaneously. When the input voltage is less than or equal to the threshold input voltage, the cascaded converter adjusts the overlapping turn-on time to stabilize the output voltage. This mode is called overlapping mode. When the input voltage exceeds the threshold input voltage, the overlapping turn-on time is zero, and the cascaded converter performs pulse width modulation on the input leg on the primary side to stabilize the output voltage. This mode is called rectification mode. Based on the two modes, the phase shift control technique is applied to the two-leg on the primary side of the cascaded converter. The waveform shape of the middle inductor current of the pre-stage four-switch Buck-Boost converter is changed by the phase shift control technique, which can achieve zero voltage switching (ZVS) of switches and zero current switching (ZCS) of diodes. The modal analysis of the cascaded four-switch Buck-Boost LLC converter with fixed frequency control was carried out using the state plane trajectory analysis method, and the input-output expressions and the soft switching conditions of the switches in two modes were derived.
In order to verify the correctness of the theoretical analysis, an experimental prototype with 70~280 V input voltage and 240 W/28 V output was designed. The experimental tests included the wide input range, operating mode, phase shift control technique, soft switching, and efficiency of the converter. The peak efficiency was 93.6% in overlapping mode and 94.4% in rectification mode. In addition, it was experimentally verified that both operating modes could achieve soft switching, and there was a large soft switching time margin in the overlapping mode. The cascaded four-switch Buck-Boost LLC converter with a special full bridge rectifier structure on the secondary side had wide gain and high efficiency compared with the same type of converter.
The following conclusions can be drawn from the theoretical analysis and experiments. The proposed cascaded converter works under fixed frequency control, solving the difficulty of designing and optimizing the magnetic components controlled by variable frequency. The cascaded converter has a wide gain range and high efficiency, and can realize soft switching of all switches and diodes. In addition, the converter can be designed for more than four times of input voltage multiplier. In future work, the phase shift control technique of the converter will be optimized to make it smoother and simpler.
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Received: 05 December 2022
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2024, Vol. 39 
