Efficiency Optimization Strategies For Bidirectional DC-DC Converters In Wide Operating Ranges

In recent years,with the fast development of dc smart/nano grid,electric vehicles,more electric aircrafts and so forth,bidirectional dc-dc converters,as the key interface circuit between storage elements and dc bus,play a significant role in system energy management and optimization.However,affected by voltage fluctuation of the storage element and power control of energy management system,bidirectional dc-dc converters are usually operated in wide voltage and power ranges,which brings severe challenges to the high efficiency operation of the converters.In order to overcome this issue,this thesis focuses on efficiency optimization for bidirectional dual active bridge(DAB)resonant converter operated in wide voltage and power ranges.Two types of converters – DAB LC series resonant dc-dc converters and DAB CLLLC resonant converters – are studied,which can be divided into two parts.The first part of this thesis discusses the DAB LC series resonant dc-dc converters.It is the key part of this thesis,in which advanced digital control techniques are employed to improve the efficiency by utilizing multiple control variables.Firstly,in order to address the drawback of traditional phase shift control related to excessive circulating current,which leads to low efficiency in the light load,a wide-range low conduction losses control strategy is proposed.The control method is based on piecewise linear approximation of the minimum current trajectory(MCT)using phase shift + PWM control scheme,and dynamically adjusts the operating point according to input/output voltage sampling,approximately making the rms inductor current minimum,and hence achieving near-to-lowest conduction losses.The proposed control algorithm merely involves addition and multiplication operation,very suitable for the implementation in low cost digital controllers.The result shows that,compared with the traditional phase shift modulation,the proposed control method effectively reduces the conduction and switching losses in light-to-medium load,and hence significantly improves the efficiency.With the increase of switching frequency,the effect of switching losses on efficiency becomes increasingly significant.Zero voltage switching(ZVS)analysis is of importance for evaluating the switching losses,and also is the basis for studying low switching losses control strategies.The traditional fundamental harmonic approximation(FHA)shows poor accuracy in ZVS analysis.To solve this problem,accurate ZVS boundary model and switched current closed-form expressions are proposed covering all control variables of the converter,which can be used for the ZVS analysis of any operating point.The proposed model is based on converter state-plane trajectory and shows higher accuracy relative to FHA method for the switching behavior analysis.On this basis,1)ZVS range of phase shift control and phase shift+ PWM control is analyzed and it is concluded that even using the latter,the converter still cannot achieve full range ZVS operation;2)from the perspective of reducing switching losses,a constraint condition related to the resonant and switching frequency is proposed for the MCT control,which provides a guideline for the selection of switching frequency;3)the effect of parasitic resistance on the ZVS boundary is investigated and a compensation method is proposed which further improves the accuracy of the ZVS boundaries.Furthermore,a wide-range ZVS control strategy is proposed which is built on the ZVS analysis results.The proposed control method is a combination of variable frequency control and phase shift control.The variable frequency part extends the ZVS range when the voltage conversion ratio is far away from unity gain,while the phase shift part boosts the power range when the ratio is close to unity.The result shows that,compared with MCT control,the proposed variable frequency + phase shift ZVS control method achieves lower switching losses and higher efficiency while maintains the conduction losses at near-to-minimum level,and also reveals lower dv/dt at switching instants.In practical applications,the efficiency optimization control needs to be combined with voltage/current closed-loop control.In order to guanrantee the stability of the converter under multiple control variables,loop design must be performed based on small-signal dynamics.In this thesis,a small-signal modeling technique for the multiple control variables system is proposed with respect to voltage closed-loop control mode.In order to precisely describe the dynamics of resonant tank,the average model and small-signal model of the converter is built using generalized average technique.On this basis,the small-signal model for variable frequency + phase shift ZVS control scheme is presented.The correctness of the small-signal model and its effectiveness in closed-loop design is verified via the simulation and experimental results.The second part of this thesis is focused on DAB CLLLC resonant dc-dc converters.The CLLLC resonant converter can be regarded as an extension of LC series resonant converter,by adding a LLC resonant network.In this part,the flexibility of design parameters combined with simple digital control method is employed to improve the steady-state performance and efficiency.With traditional variable frequency control,the steady-state performance of the converter is severely affected by the parasitic parameters of transistors.For this issue,an efficiency optimization method based on constant frequency phase shift control and parameter design is proposed.Firstly,the steady-state model under phase shift control is built using frequency domain method.Then,optimal design steps for resonant tank are proposed on premise of ensuring full-range ZVS operation while reducing conduction losses as much as possible.The result shows that compared with traditional variable frequency control,the proposed optimization method not only achieves ZVS in full operation range,but also lowers the effects of parasitic parameters on converter performance.With respect to a converter with poor body diode reverse recovery characteristics and large parasitic junction capacitances,steady-state performance and operating efficiency can be significantly enhanced.