A 10MHz Isolated Synchronous Class ?₂ DC-DC Converter

With the continuous trend of miniaturization and integration of the power supplies, the demand for extremely high power density and very fast dynamic response is on the rise, which 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, shrinking the size and weight and allowing faster transient response. However, most of the previous research on the high frequency resonant power conversion focuses on the non-isolated topologies with low output current, which limits the application of the high frequency resonant converters. Therefore, the research on the isolated low-voltage high-current high frequency resonant converter is carried out in this thesis.This thesis focuses on the research on a 10 MHz isolated class ?₂ resonant converter, in which the rectifier has great impact on the performance of the converter. Because of the forward recovery phenomenon, the performance of the diodes degrades seriously at high frequency operation, causing extremely high power loss in the diodes. Therefore, the synchronous rectification?SR? technique is strongly desired in the high frequency resonant converters, especially in high current applications. To realize the synchronous driving, a self-driven level-shifted resonant gate drive?RGD? is proposed for the SR FET in the isolated class ?₂ resonant converter. The proposed RGD introduces a level-shift stage to the auxiliary winding and the resonant tank of the RGD, which can generate a DC bias voltage to the gate drive voltage. The bias voltage increases the peak gate voltage to extend the conduction time with the optimal on-state resistance, so that the average on-state resistance of the MOSFET and the associated conduction loss can be reduced significantly. Moreover, by tuning the DC bias voltage, the proposed RGD provides precise switching timing for the SR FET, so that the MOSFET can realize ZVS and ZCS, and the body diode conduction time can be minimized at the same time. More importantly, to realize ON-OFFclose-loop operation of the converter, a control stage comprised of a shutdown branch and an auxiliary switch is introduced to the proposed RGD. The control stage can block the circulating current in the drive circuit, so that the resonant gate voltage can realize fast shutdown and buildup during ON-OFF operation, which ensures fast transient response and high close-loop efficiency.A 10 MHz prototype with 18 V input, 5 V/2 A output was built to verify the functionality of the isolated resonant converter and the proposed RGD. The experimental results agree well with the theoretical analysis and simulation results. Moreover, to verify the loss-saving advantages of the proposed RGD, two other prototypes rectified by the diodes and SR with conventional RGD were built respectively. Significant efficiency improvement owing to the proposed RGD is achieved in wide load range compared to the other two prototypes. At full load of 2 A, the proposed RGD improves the converter efficiency from 80.2% using the conventional RGD to 82%?an improvement of 1.8%?. Compared to the efficiency of 77.3% using the diode rectification, the proposed RGD achieves the efficiency improvement of 4.7% at full load.