An improved PDM control method for a high frequency quasi-resonat converter

This paper proposes a Pulse Density Modulation (PDM) method with improved transient for high frequency converters such as Class E DC/DC or quasi-resonant boost converters. In contrast to frequency or duty cycle control, the conventional PDM control method provides high efficiency at any load when modulation frequency is low. However, efficiency drops significantly for higher modulation frequencies due to the transient power losses in each power pulse. The proposed method decreases the power losses at the beginning of each pulse and reduces the voltage stress of the switch. Therefore, a higher modulation frequency and smaller filter size become feasible. Startup behavior of a quasi-resonant boost converter is analyzed in this paper. Soft switching conditions and peak voltage stress are studied as a function of the first switching cycle timing. In order to minimize the voltage overshoot and switching loss of the converter, the first switching period of each power pulse is chosen based on this analysis. A 1000 W, 3 MHz quasi-resonant boost converter with Class DE rectifier is implemented to verify the analysis and simulation results. A maximum efficiency of 96.7 percent is obtained. Further, the efficiency dropped less than three percent at one tenth of the load. About a 2% improvement in the light load efficiency is observed as compared to the conventional PDM method.