Adaptive Dead-time Regulation Method For GaN-based LLC Resonant Converters

In recent years,the new energy technology and industry are booming under the "double carbon" development strategy.LLC resonant converter is widely used in the field of new energy conversion because of its good soft switching characteristics and high reliability.High switching frequency,high power density and high efficiency(3H)are the pursuit of high performance power supplies,while traditional silicon(Si)based devices have gradually failed to meet the needs of some high performance applications.New wide-band gap semiconductor gallium nitride(GaN)devices have excellent performance such as high switching speed and zero reverse recovery charge,which has the potential to improve the performance of the converter.Therefore,LLC resonant converter based on GaN has become a research hotspot of power supply.Dead time is a key index for LLC resonant converter to realize soft switching.Due to the complexity of modeling of LLC resonant converter,traditional dead times are selected either based on engineering experience or on inaccurate frequency analysis methods(FDA)and are generally set to a fixed length of time.However,in the case of GaN HEMT devices,considering the unique reverse conduction mechanism,a long fixed dead time will generate large reverse conduction losses during the redundancy time,which will reduce the efficiency of the converter;and when the dead time is insufficient,the primary side bridge arm switch tube cannot achieve zero voltage switching(ZVS),or even a short circuit will occur.Therefore,the traditional fixed-dead-time strategy for GaN-based LLC converter is flawed.To solve the above problems,the main research of this thesis is as follows:(1)A time-domain analysis(TDA)-based segmental modelling analysis of the LLC resonant converter is investigated.The analytical formula for calculating the bridge arm off-current and the corresponding optimal dead time are derived in detail,and the current prediction results for both DCM and CCM modes of operation are fully considered.At the same time,the influence of parasitic parameters on the ZVS implementation process during the dead time is considered.And the correctness of the prediction formula is verified by building a Matlab simulation comparison.(2)A variable dead time regulation strategy based on mathematical modelling prediction is proposed.According to the different GaN switch-off currents of the LLC converter under different operating load conditions,resulting in different junction capacitor charging and discharging times,the dead time is adaptively adjusted to reduce the reverse conduction losses of GaN devices and improve the converter efficiency.At the same time,the switching process of GaN HEMTs is analysed,and the accuracy of the variable dead time adjustment strategy is further optimised by considering the influence of actual hardware transmission delay and GaN device switching delay.(3)Hardware implementation of LLC resonant converter based on GaN devices and verification of adjustment strategy.A 480 W hardware prototype is designed and manufactured,including main circuit parameters and peripheral circuit design,GaN device characteristic analysis and driving circuit design.The closed-loop control and dynamic dead-time regulation are realized by pre-storage of the optimal dead-time lookup table in the TMS320F28377 D digital control chip.Compared with the traditional fixed dead-time strategy,the proposed adaptive dead-time regulation strategy can achieve a maximum 0.9% efficiency improvement.