Gate Driver For EGaN HEMTs In MHz Resonant Converters

With the continuous trend of miniaturization and integration of power supplies,the demand for extremely high power density and high efficiency is on the rise.However the rise is mostly limited by the passive components,especially the bulky inductance and high capacitance.Recently,the switching frequency has been pushed up to tens of MHz,directly reducing the energy storage requirements of the inductance and capacitance and shrinking the size and weight.Additionally,the emerging eGaN HEMTs with excellent figure of merits are very promising at high frequency converters.However,the application of eGaN HEMTs in high frequency converters is very challenging due to eGaN HEMT instinct characteristics.Therefore,the research on the gate driver densign for GaN based MHz converters is carried out in this thesis.Commercial gate drivers for eGaN HEMTs focus on high reliability to the strict gate driving voltage against parasitic components.However,they do not consider the high reverse conduction voltage due to the reverse conduction mechanism of eGaN HEMTs under ZVS condition with multi-MHz.To solve the problem,a three-level gate driver is proposed for eGaN control HEMTs.The mid-level voltage reduces the reverse conduction loss by reducing the reverse conduction voltage depending on the characteristic of the reverse conduction mechanism that the source-to-drain voltage decreases with the increase of the gate-to-source voltage.The proposed drivers are applied to the Class-E based isolated resonant converter with 7 MHz.The efficiency was improved from 72.7% using the conventional driver to 73.4%(an improvement of 0.7%)with 24 V input,5 V/ 10 W output.A gate driver with low count components for the eGaN SR is proposed to minimize the reverse conduction time.The driver uses a wave-shape circuit,designed by the mathematic model of the HEMT rectifier built with the MATLAB script,to compensate the propagation delay of the driving IC and obtain the desired driving signal timing.The proposed SR driver improves the efficiency from 79.9% without considering the delay of the driving IC to 84.7%(an improvement of 4.8%)with 18 V input,5 V/ 10 W output.VHF isolated converters with the conventional driving scheme suffers from serious efficiency drop over entire input voltage range due to gate drive timing mismatch of the eGaN control and SR HEMTs.A digital adaptive driving scheme is proposed to adjust the control and SR gate drive signals efficiently over input voltage range.A state-space model is derived to predict drive timing precisely according to different input voltage and adjust the gate drive signals correspondingly.A high time resolution circuit to improve time resolution by selecting cascaded high speed buffers is proposed.With the proposed driving scheme,high frequency switching loss and reverse conduction time of eGaN HEMTs are reduced significantly to maintain high efficiency over entire input voltage range.A 30-MHz VHF SR flyback with the PCB fabrication air-core transformer and digital ON/OFF control was built.With 18 V input and 5 V/ 10 W output,the measured efficiency realizes 80.0%.With 24 V input and 5 V/ 10 W output,the proposed solution improves the efficiency from 72.9% using the conventional driver to 75.1%(an improvement of 2.2%).