Research On Asymmetric Operation Optimized Strategy Of Three-Phase Coupled Bidirectional Half-Bridge DC/DC Converter

The multi-phase bi-directional half-bridge DC/DC converter has the advantages of simple structure and control,it is widely used in EV on-board charger which doesn’t require electrical isolation.By integrating the DC inductors into a coupled inductor,the DC flux can be significantly reduced and the power density of the converter can be improved.However,when the multi-phase DC/DC converter adjusts the power of each phase or when a fault occurs,the system will enter the asymmetric operation state,and the transfer function will become more complicated,of which the coupling terms cannot be ignored,and the controller based on the symmetric operation model is no longer applicable.Also,the coupled inductor will be saturated due to the increase of DC flux,and the inductive voltage of the coupled inductor will lead to abnormal clamping of the bridge arm freewheel diodes.In this paper,the three-phase coupled bi-directional half-bridge DC/DC converter is the object of study,which contains the following parts of research.For obtaining the complicated mathematical model under asymmetric operation conditions,the small-signal model and steady-state operation point are derived and proposed,which consider the coupling relationship among duty cycle,inductor current,bus voltage and output load,then the current ripple of coupled inductor is calculated.Under the asymmetric operation conditions,the change of duty cycle will have different effects on different phases’ currents,and the transfer functions are complex.By deriving the analytical expressions of inductor current and bus voltage,characteristics of steady-state operation point and current ripple can be quantitatively analyzed to provide a theoretical basis for the design of inductor and controller under asymmetric operation.For the saturation problem of the three-phase coupled inductor under asymmetric operation conditions,an optimized design method for the power density of the three-phase coupled inductor considering asymmetric operation is proposed.To solve the saturation caused by the flux under asymmetric operation such as decrease-phase operation and fault-tolerant operation,the constraints of the three-phase coupled inductor design under asymmetric operation conditions are optimized,and the effects of the coupling coefficient and the maximum operating current on the flux distribution and the maximum flux density are analyzed.Finally,the comparative design of inductors with different core structures is carried out under uniform parameter requirements,and the effectiveness of the design method is verified by using simulations and actual inductors.An asymmetric feedforward control strategy based on the inverse solution calculation of mathematical model is proposed to address the challenges of operational stability and dynamic performance of the three-phase coupled converter under asymmetric operation conditions.Based on the three-phase coupled converter,the mathematical model of the multi-phase coupled converter can be derived.By analyzing the mathematical model,it is known that the amplitude-frequency characteristics of the transfer function can be affected by the changes of steady-state duty cycle,output load and operating phase,and the relative stability of the closed-loop performance of the controller can be modified by adding compensation correction.The traditional feedforward control based on the small signal model under asymmetric conditions is too complicated.To simplify the controller,a feedforward control strategy based on the inverse solution of the steady-state operating point is proposed,which improves the dynamic performance of the system and eliminates the effects of changes of voltage,duty cycle,output load and operating phase.To address the challenge of fault-tolerant operation in the three-phase coupled bidirectional half-bridge DC/DC converter,the fault modes and induced voltage clamping conditions under asymmetric operation conditions are analyzed,and a fault-tolerant operation strategy applicable to the three-phase coupled converter is proposed.In the asymmetric operations,the induced voltage clamping will lead to the abnormal conduction of the freewheel diodes.By analyzing the duty cycle range and coupling coefficient range of the induced voltage clamping condition,clamping operation can be avoided by designing a suitable coupling coefficient and selecting a suitable switching mode.Based on the analysis of the fault modes of the three-phase coupled converter,the dynamic performance of fault-tolerant operation is improved by adding duty cycle compensation and feedforward control strategy to reduce the overshoot and settling time during fault-tolerant operation.