Research On A Very High Frequency SEPIC DC-DC Converter

Modern power electronics applications expect the power converters to achieve extremely high power density, high efficiency and fast transient response. Increasing the switching frequency is an effective way to reduce the energy storage requirements of the passive components, such as inductors and capacitors, improve the transient response, shrink the size and weight and increase the power density of the converter. Therefore, the converter with the very high switching frequency(30 MHz ~ 300 MHz) has been an international hotspot of the low power converters. Under this background, this thesis carries out a research on the Very High Frequency(VHF) DC-DC power converters.This thesis does the research on the very high frequency SEPIC resonant converter. Firstly, the comparison between the topology of the VHF SEPIC resonant converter and the conventional SEPIC converter is done. The characteristic and the analysis and design methods are also introduced respectively. Secondly, the conventional design method for the VHF SEPIC resonant converter and its drawbacks are presented. In the conventional method, the design of the rectifier stage can not realize the decoupling of the amplitude and phase of the fundamental input voltage and current; the design of the inverter stage is a high-order resonant tank design, which leads to more efforts in the design procedure. In order to solve the problem above, a decoupling design method is proposed for the resonant SEPIC converter. The proposed design method re-divides the resonant SEPIC topology to modify the rectifier topology using the duality theory. In the proposed design method, the design of the rectifier can decouple the design of the power factor and output power effectively, to achieve high efficiency and power density; for the design of the inverter stage, the order of the resonant tank is lowered, which simplify the procedure. Thirdly, the implementation of this converter is described, which contains the resonant gate driver and the hysteresis control circuits. More importantly, based on the loss analysis, it is noted that the loss of the resonant inductors is the dominant among the loss distribution. In order to further reduce this dominant loss and minimize the size of the inductor, a multi-layer structure of the PCB embedded inductors with large effective area is proposed. This structure of the four-layer solenoid PCB embedded inductors reduces the ac resistance and the loss effectively, which improves the power density and the efficiency of the converter.This thesis presents the experimental verification of two resonant SEPIC converters. In order to verify the proposed design method, a 30 MHz 15 V input 28 V/14 W output resonant SEPIC converter was implemented with the discrete air core inductors. The experimental waveforms are consistent with the simulating results. To further improve the power density and efficiency, another 25 W output VHF SEPIC converter with the proposed PCB embedded inductor was also implemented. It is verified that the proposed embedded inductor can be effectively applied. The power density of the power stage is over 200 W/inch3 and the efficiency can reach 82% at the rated output.