| With the miniaturization of data centers,EVs,and other applications,the high-frequency requirements of power supplies are increasing year by year.However,increasing the switching frequency will decrease efficiency and narrow voltage regulation range.At present,the single-stage resonant converters or PWM converters cannot achieve both features of"high-frequency and high-efficiency"and"wide voltage regulation capacity".Fortunately,the two-stage structure that combines the two converters,especially Four-switch Buck-Boost converter cascades with LLC resonant converter,can effectively achieve both features.Therefore,the cascaded Buck-Boost+LLC converter is a good candidate for isolated DC-DC power supply.However,there are still deficiencies in the research of high-frequency and high-efficiency control strategies for this two-stage structure.Especially soft-switching control strategies and efficiency improvement methods need to be studied.In this thesis,the soft-switching control of the FSBB converter（regulation stage）,the high-frequency synchronous rectification control of the LLC resonant converter（isolation stage）,and the power switching integration without the compromise of efficiency and frequency（cascaded structure）are studied.Then,a complete set of high-frequency and high-efficiency control strategies is proposed.The main research contents and innovations are as follows:（1）To solve the multi-objective control issues of soft-switching,efficiency optimization,and closed-loop voltage regulation in the FSBB converter,a multi-input multi-output architecture is proposed.The architecture is divided into three steps to realize the multi-objective control:Firstly,the monotonical relationship between the total loss and the control variable T1 is determined,and then the optimal control variable T1 can be calculated;Secondly,the parallel voltage loop and soft-switching current loops are built to calculate the control variables T2 and T3 that meet the requirements of MHz soft-switching and wide-range voltage regulation;Thirdly,a decoupling compensation method is proposed,which actively introduces the adjacent loop components to balance the coupling components,thereby improving the stability of the parallel loop architecture.The control architecture can regulate the three control variables to achieve MHz soft-switching,optimal efficiency,and stable closed-loop voltage regulation.Finally,the designed FSBB prototype realizes a high-frequency soft-switching of1MHz,a peak efficiency of 98.1%,and a wide voltage regulation range of 36-72Vin.（2）In LLC resonant converter,body diode conduction（BDC）of SR can result from both turning-off premature and late.And the latter will cause logic errors and large backflow loss in the conventional adaptive SR method.This paper is the first time to quantitatively analyze the mechanism of the turn-off-late type of BDC.Through the time-domain modeling of the resonance process after SR turning off late,the phase-leading effect of the reverse current on the drain-source voltage is clarified.Moreover,the phenomenon that different turn-off timing of the primary-side switches can enhance or weaken the trend of BDC is analyzed.Thereby,the mechanism of the turn-off-late type BDC is revealed.（3）To solve the backflow problem caused by the turning-off late type of BDC,an adaptive SR control strategy based on dual detection is proposed.The detection of the falling edge of the drain-source voltage during the resonance process in dead-time is added to cooperate with body diode detection.By comparing the two detection signals,the proposed SR method can effectively identify two different types of BDC and tune SR to be turned off exactly.Compared with the conventional adaptive SR method and commercial SR ICs,the proposed control strategy achieves high-frequency SR at the switching frequency of 0.6MHz,and the period of BDC is reduced from 96ns to zero,the efficiency is improved by 0.54%.（4）To solve the degradation of efficiency and soft-switching capacity in the conventional power switch integration,an integrated Buck-Boost LLC converter is proposed,where the lagging leg of the FSBB converter is merged into the half-bridge of the LLC resonant converter.Meanwhile,the optimized inductor and phase-shift control strategy are combined to improve efficiency and soft-switching capacity.The soft-switching capability can be enhanced due to the accumulation of the two currents of the buck-boost stage and the LLC stage during dead-time.Moreover,the current of the two-stage will commutate to cancel out with each other during the conduction period,reducing the conduction loss and improving the overall efficiency.Finally,a 1MHz 720W BBLLC prototype is built,with a peak efficiency of 96.4%and an input voltage regulation range of 250-425Vin,to verify the proposed model and control strategy. |
