Abstract
The new L-LLC resonant bidirectional DC-DC converter (L-LLC-BDC) will produce a large resonance current and voltage inrush during the startup, posing a threat to the safe operation of the power device. Although a very high starting frequency can effectively suppress the inrush, it will also increase the output current demand of the driving ICs. This paper proposes a phase-shifting soft-start control strategy based on the current-limiting curve. Using operating mode analysis, the peak value of the resonant current is limited according to the output voltage and the phase shift angle of the switch, with the limit current curve at the startup stage drawn. By this current curve, a one-to-one correspondence between the output voltage and the phase shift angle of the switch is obtained. The phase-shifted soft-start control strategy can quickly establish the output voltage on the basis of a resonant frequency and can effectively suppress the resonance current inrush. An experimental prototype with a power of 6 kW and an input of 760 V and an output of 380 V is built. The experimental results prove the correctness and effectiveness of the soft start control strategy proposed in this paper.
Highlights
Due to LLC resonant converter’s high efficiency, high power density, and wide gain and power, there are many researchers focusing on it [1,2,3]
In order to obtain the highest working efficiency, the converter generally works around the switching frequency f equal to the resonance frequency fr, i.e., the normalized frequency fn f/fr 1. e voltage gain M is the ratio of the output voltage to the input voltage of the converter, and M is 1
When the input voltage increases, the converter works in the fn > 1, so as to prevent the converter from entering the zero current (ZCS) region during startup process, and the switching frequency at the initial stage of startup must be greater than the resonance frequency [8, 9]
Summary
Due to LLC resonant converter’s high efficiency, high power density, and wide gain and power, there are many researchers focusing on it [1,2,3]. Twelve switches are used in the LLC converter in literature [4], the cost of the converter is increased, and it reduces the power density because of the two transformers. It can be used as the key equipment for interaction between ESSs and the DC microgrid for stabilization of the DC bus voltage. When the input voltage increases, the converter works in the fn > 1, so as to prevent the converter from entering the zero current (ZCS) region during startup process, and the switching frequency at the initial stage of startup must be greater than the resonance frequency [8, 9]. When the switching frequency in the startup phase is large enough, Mathematical Problems in Engineering
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