Research On The DC Bias Suppression Method For A Dual-Active-Bridge Converter

Among various bidirectional DC-DC power electronics topologies,the dual active bridge topology(DAB)has advantages including wide voltage gain range and the feature of soft switching,which makes it a promising topology in applications such as energy storage system,uninterrupted power supply(UPS),electric vehicles and solid-state transformers.In practice,DC current often arises in the transformer and inductor of a DAB converter,causing the magnetization bias in the core.The steady state DC bias is brought by unmatched parameters of the circuit and the asymmetrical driving signals,which will cause the loss of ZVS and the increase of RMS current,and these contribute to a larger switching loss and conduction loss.The transient DC bias is caused by the sudden change of phase shift ratio,a peak current will arise in this process,which may saturate magnetic cores of the transformer and inductor,leading to the failure of the converter in the end.In this paper,new methods of suppressing steady state and transient DC bias are proposed to increase the stability and reliability of DAB.For steady state DC bias,a suppression method based on duty cycle modulation is proposed.The conditions to achieve zero DC bias and the small-signal model from the duty cycle to the DC bias current are presented,and the closed loop control for suppressing steady state DC bias is designed.In this paper,a periodic integral circuit is adopted to precisely sample the DC current,a 3200 W DAB prototype is constructed,and tunnel magnetoresistance(TMR)is employed as the current sensor.The proposed method is verified through experimental results with all steady state DC bias removed.For transient DC bias,a suppression method based on peak current limit is proposed.By setting current thresholds,the corresponding H-bridge square wave voltage will flip in advance when current exceeds the thresholds,which in turn slows down the increasing speed of current and avoid saturating the magnetic cores.The simulation results shows that the proposed method has limited influence on dynamic response of DAB.The peak current limit circuit is realized with monostable triggers and logic gates,and the effectiveness of the transient DC bias suppression method is verified using experimental results.