Research On A MHz RDCX Converter Based On Half Bridge LLC Topology

With the fast development of modern information technology,the scale and energy consumption of data centers are increasing rapidly.Using high voltage DC power supply architecture and integrating server power modules directly onto the motherboard can significantly increase the power efficiency and power density of the server system.In order to realize the on-board integration of the power supply module,the DC-DC module needs to meet the requirements of high efficiency and high power density,and also meet the requirement of output voltage regulation.The LLC resonant converter can operate in MHz frequency due to the soft switching characteristics of primary and secondary switches,which can meet the requirements of high power density and high efficiency.However,there exist problems with output voltage regulation of high frequency LLC,and the soft switching characteristics may be lost as the load increases.The traditional zero voltage(ZVS)analysis is based on the equivalent circuit during the dead time of the primary switches.The magnetic inductance acts as a constant current source to charge and discharge the junction capacitance of primary switches.However,the magnetic current will be bypassed by the output voltage source due to the forward conduction of the secondary rectifier.And as the load increases,the magnetic current is bypassed earlier.As a result,the ZVS of the primary switches is seriously affected.Therefore,this paper detailed analyzes the circuit modes when LLC operates at the resonant frequency in MHz condition.The ZVS power and resonance parameters boundary model is established and the accuracy of the model is verified by experimental results.Based on the ZVS boundary model,when the high frequency LLC adopts frequency modulation methods to adjust the output voltage,the frequency variation range is too wide.Th erefore,there exists potential problem for the magnetic size and efficiency optimization.When the LLC operates as a DC transformer(DCX),it has the advantages of high efficiency and high power density.In order to achieve output voltage regulation,a two-stage scheme of DCX cascaded regulated DC-DC is generally adopted.However,the two-stage scheme greatly reduces the efficiency and power density of the system.Therefore,this paper makes full use of the advantages of the DCX,and the output voltage regulation is achieved by a controllable voltage source in DCX.In the regulated DCX(RDCX)scheme,most of the input power is directly processed by DCX.Only a small part of the input power is processed by two-stage to achieve output voltage regulation,and as a result,high efficiency and high power density of RDCX are ensured.A RDCX prototype with an input voltage of 360V-400V,output voltag,ce of 12V and full load power of 750W is built in this paper,and the RDCX scheme is verified by experimental results.