基于双模式变频移相调制的单级式双有源桥型DC-AC变换器

A Single-Stage Dual Active Bridge DC-AC Converter Based on Dual Mode Modulation of Variable Frequency and Phase Shift

针对单级式双有源桥(DAB)型DC-AC变换器关断电流较大和软开关范围有限的问题,该文提出一种双模式变频移相调制策略。首先,对DAB变换器的单移相(SPS)调制和扩展移相(EPS)调制进行时域分析,得到其功率传输特性;然后,分析变换器的关断电流与软开关条件,得到移相比和开关频率约束方程;在此基础上,分析双模式间的移相比边界条件,提出变换器运行模式的切换机制,从而在实现变换器单级功率变换的同时,保证全范围软开关并降低低压侧的关断电流;最后,通过仿真和实验验证了所提调制策略的有效性与可行性。

In electric vehicle charging systems,energy storage systems,and solid-state transformers,isolated DC-AC converters serve as crucial energy conversion nodes.The single-stage dual active bridge(DAB)DC-AC converter structure achieve power transmission functions in a single stage,which effectively reducing volume and improving reliability.In reality,turn-off current is crucial for achieving zero-voltage switching(ZVS),however,high turn-off current can result in additional turn-off losses.Moreover,the low-voltage side usually bears a greater turn-off current,especially under high turns ratio conditions.This paper proposes a frequency-variable dual-mode switching modulation strategy based on single-phase-shift(SPS)and extended-phase-shift(EPS)control,which can reduce the turn-off current of the low-voltage side for the converter and achieve ZVS in the ac line voltage range.Firstly,the time-domain analysis is conducted on the SPS modulation and EPS modulation strategies for the DAB converter,and obtain their power transmission characteristics.Secondly,in order to achieve full-range ZVS,the turn-off currents of some switches are fixed at a constant value,the three phase-shift-ratios(D,D1 and D2)and switching frequency constraint equations are derived.Thirdly,the phase-shift-ratio and switching frequency trajectories of the converter under different turn-off current are analyzed in detail.When the turn-off current of DC and AC sides are set different low values,the switching frequency will increase seriously and the value of phase-shift-ratio D2 will exceed 1 near the voltage zero-crossing.This will cause a distortion of power transmission in the converter,leading to grid current distortion.Therefore,a frequency-shifted dual-mode-switching modulation strategy is proposed and the switching boundaries for the two operating modes are derived.During the time period of phase-shift-ratio D2 is lower than 1,the variable-frequency EPS modulation strategy is employed.When period of phase-shift ratio D2 is more than 1,the internal phase-shift-ratio D1 is set zero,and the external phase-shift ratio D2 is set to equal SPS modulation.The modulation strategy combines SPS,EPS and variable-switching-frequency control,which reduce the turn-off current of the switches compared with the EPS strategy,while avoiding a dramatically increase for switch frequency near the voltage zero-crossing based on the full-range ZVS.Lastly,the simulation results show that the phase shift ratio D2 exceeds 1 near the voltage zero-crossing,resulting in distorted output current.The phase-shift-ratio D2 will change and D1 will become 0 near the ac voltage zero-crossing under the proposed modulation scheme,the output current has no spikes.Experimental results demonstrate that the converter operates stably under the proposed modulation strategy.Smooth transitions between single-phase shift and extended phase shift modulation can be observed,and there is no significant distortion in the output current,ZVS of the AC and DC sides at different phases of the line frequency is also achieved.The following conclusions can be drawn from the simulation and experimental results:(1)The back-stage full-bridge adopts synchronous inverter control.Thus,the compact film capacitors can be placed on the dc-link.(2)The proposed modulation strategy can reduce turn-off current while achieving full-range soft switching,and eliminating AC current distortion through reasonable switching of operating modes.