A High Frequency Class Φ₂ DC-DC Converter With Synchronous Rectifier

Morden applications are posing higher and higher requirements on the density, system efficieny and transient response of the power converters. Increasing the switching frequency is an enffective way to achieve this goal. It helps reduce the size of enery-storage components, such as inductors and capacitors, to shrink the system volume and weight, and to enhance the dynamic responses. Therefore, high-switching-frequency（above 3MHz） converters have become an international research hotspot. Under this background, this thesis carried out a research on the high frequency DC-DC converter.This thesis proposes a high frequency Class Φ₂ DC-DC converter with synchronous rectifier, which is suitable for low-voltage output application. Key features of the converters are: 1) zerovoltage-swtiching（ZVS） of the main MOSFET with low voltage stress; 2) zero-voltage-swtiching（ZVS） and zero-current-swtiching（ZCS） of the SR MOSFET; 3) parasitics components of the converter are fully taken into consideration; 4) all inductors and capacitors take part in resonace to reduce the volume and the weight of the converter. Firstly, the topology derivation and operation principle of the inverter are analyzed based on the impendance model. Secondly, the design method of the converter is presented. Concerning the coupling of amplitude and phase of the fundamental voltage and current in the rectifier stage, the duty cycle and system efficiency are considered in the proposed design method. For the design of the inverter stage, the impendence of the resonant tank across the main MOSFET is utilized in the design process. Thirdly, two different resonent gate drives are proposed for the main switch and the SR switch. One is the multistage resonant gate drive. The other one is a self-oscillating resonant driver. At last, the hysteresis on-off control and it design procedure are proposed.A prototype of 10 MHz 12V input 5V/10 W output resonant converter was built in the lab to verify theoretical analysis. Experimental results validated the theory analysis. To demonstrate the advantages of the synchronaztion rectifier, another resonant coverter with diode rectifier is implemented. The converter with synchronous rectifier has the effiency of 79.5% at full load, while the effiency of the converter with diode rectifier is 77.4%. The future work is to optimize the gate drive of the SR and improve the effiency.