The Investigation Of High Frequency And High Efficiency DC/DC Conversion Based On Current-feed ZVZCS Resonant Topologies

The integrated circuit(IC)chips and other equipment iin information and communication technology(ICT)systems require low voltage power from the high voltage grid.And to power various low-voltage digital and analog loads from high-voltage DC bus,the intermediate bus architecture(IBA)is widely used due to its high reliability,high flexibility and good extendibility.The IBA is a two-stage architecture.The first stage is an isolated dc/dc bus converter,which has voltage specifications of 48V/12V(4:1)and 24V/12V(2:1).The second stage is the point of load(PoL)converter which has regulating ability.In this architecture,high power density and high efficiency bus converter is very crucial to reduce weight and volume of relevant equipment.This dissertation targets at improving the power density and efficiency of bus converter in IBA,and focuses on high frequency single-switch Zero-Voltage and Zero-Current-Switching(ZVZCS)resonant topologies and their control schemes.Firstly,the intrinsic limit of existing voltage feed soft-switching resonant tank that leads to the complex structure of available isolated resonant topologies is revealed.Based on this,current-feed and current-sink single-switch ZVZCS resonant cells are proposed,from which current-feed and current-sink single-switch ZVZCS resonant forward topologies can be directly obtained.The proposed single-switch resonant forward topologies can realize ZVZCS,and they can always operate at this optimal ZVZCS point with switches operating at fixed frequency and fixed duty cycle,therefore,the work presented here makes high-efficiency single-switch resonant topology become available for low-power applications.Besides,more compound resonant cells can be constructed from the basic single-switch ZVZCS resonant cells,and a family of ZVZCS resonant topologies for IBA applications with various power and voltage levels is derived.The electrical characteristics and design considerations of the proposed ZVZCS resonant cells are analyzed in detail,and simulation and experimental results are provided to verify their high frequency and low loss features.The basic single-switch resonant forward converters are simple and have good ZVZCS features,but they utilize the drain-source voltage of switch to reset the transformer and this introduces high voltage stress issues.Therefore,they are more suitable for low voltage applications,and for high voltage applications,switches with high voltage rating must be adopted.To alleviate the voltage stress issue,the passive clamping technique using transformer clamping winding and clamping diode is adopted,and a current-feed single-switch resonant forward converter with secondary diode-clamping is proposed which is suitable for step-down applications.The proposed converter makes use of the output voltage to clamp the switch voltage stress,and maintains the ZVZCS features of the basic single-switch resonant forward converter,thus it can achieve high efficiency at high frequency.Applying the passive clamping technique to the basic single-switch resonant converter can reduce the switch voltage stress,but with this method the switch duty cycle is limited to 0.5 and thus the efficiency improvement is limited.Besides,the resonance between leakage inductance and parasitic capacitance weakens the clamping effect.In order to further improve the efficiency of single-switch resonant topologies and eliminate the high frequency ringing in passive clamping solution,a current-feed active-clamp resonant forward converter is proposed,which utilize an auxiliary switch and a clamping capacitor to clamp the switch voltage stress.This converter has better ZVZCS feature and less clamping loss than the passive clamping solution.Meanwhile,this converter can achieve duty cycle larger than 0.5 which can reduce the RMS current and improve the efficiency at heavy load.What’s more,by usage of the unique flexibility of the clamping capacitor,a load adaptive ON-time control that can significantly reduce core loss at light load is proposed,and this scheme greatly improves the light-load efficiency.Finally,experimental results verify that the proposed converter can effectively improve the whole-load-range efficiency.Because of the single-switch structure,the voltage stress of the primary or secondary switches of single-switch resonant converters is not well clamped.Therefore,not only the current stress issue in resonant tank which is common in traditional resonant converters but also voltage stress issue of switches exist in the start-up process of the proposed single-switch resonant topologies.In order to realize soft start-up of this kind of converters,a method using the constant current region of MOSFETs is proposed,which can slowly establish the output voltage and at the same time reduce the stress issues aforementioned.Besides,this soft start-up method can be extended to the start-up of double-ended resonant converters.