Research On Modulation Strategy Of High-Efficiency And High-power-density Integrated Intermediate Bus Converters

In recent years,with the rapid development of the internet,the power consumption of Internet Data Centers(IDC)has increased dramatically.The IDC employs Distributed Power System(DPS)to improve efficiency,reduce costs,and increase capacity.DPS requires Intermediate Bus Converters(IBC)that can achieve high-efficiency,high-power-density,isolation and voltage regulation.Increasing the switching frequency is an effective way to improve the power density,but it will increase the switching loss,the winding eddy current loss and the difficulty of Synchronous Rectification(SR),which is not conducive to improving the efficiency.An effective way to improve the efficiency of the IBC under high frequency is optimizing the modulation strategy to achieve soft switching,non-gap magnetic components,and open-loop SR.The LLC resonant converter is easy to achieve soft switching,but its voltage regulation needs variable frequency.Variable frequency increases the winding loss and the difficulty of SR,which is not conducive to improving efficiency and frequency.The fixed-frequency LLC can achieve lower winding loss and simpler SR,so it is easy to achieve high efficiency and high power density.Because of the fixed frequency,the voltage gain of LLC is constant.The LLC with constant voltage gain is called LLC DC Transformer(DCT or DCX).LLC DCX requires an additional circuit for voltage regulation,which reduces overall efficiency and power density.The integrated two-stage converter based on LLC DCX can reduce the number of switches and driving circuits,and is a better way to achieve high efficiency and high power density.Integrated two-stage converters are currently a research hotspot,and the research goal is to improve efficiency and power density by optimizing modulation strategies.In this paper,new modulation strategies are proposed for several integrated two-stage converters suitable for different bus architectures.For the integrated interleaved Boost-LLC converter suitable for low-voltage bus conversion,a bipolar variable-frequency quasi-resonant modulation strategy based on compensating commutation time is proposed.The proposed modulation strategy changes the realization method of soft switching,eliminates the air gap of the transformer,and solves the problem of high winding eddy current loss caused by air gap leakage flux at high frequency.By changing the switching frequency and compensating for the commutation time,the LLC can work accurately in a quasi-resonant state at high frequency and at different duty ratios.Without the problems of high resonant current and difficulty of achieving open-loop SR under the existing modulation strategies,the efficiency and power density are improved.For the integrated Buck-Boost-LLC converter suitable for high-voltage bus conversion,a bipolar symmetrical phase-shift modulation strategy is proposed.This modulation strategy achieves the symmetrical commutation process of LLC by adjusting the phase shift angle between the two half bridges.This method solves the problem of asymmetric secondary current caused by the asymmetric commutation process.Without the large resonant inductor required by the high quality factor and the difficulty of SR caused by the asymmetrical current,the power density is improved.With the help of the front-stage inductor current,the current stress of the LLC primary side switches is reduced,and the realization method of soft switching is changed.Therefore,the conduction loss of switches and eddy current loss of the windings are reduced,and the efficiency is improved.Combined with the advantage of the natural symmetry of the resonant tank input voltage of the integrated interleaved Boost-LLC converter and the advantage of the fixed frequency and constant duty cycle in the integrated Buck-Boost-LLC converter,a new integrated interleaved Buck-Boost-LLC converter is proposed.The proposed converter can naturally realize the symmetrical secondary current,thereby releasing the restriction on the phase shift angle.A new phase-shift modulation strategy is proposed under higher design freedom of phase-shift angle.The designed optimal phase-shift angle achieves the smallest inductance volume,realizes the soft switching of all switches,and minimizes all losses related to the phase-shift angle.Based on this phase-shift angle,the proposed converter achieves high efficiency and high power density.