A push-pull Class E converter with improved PDM control

In this paper, an improved Pulsed Density Modulation (PDM) method for high frequency converters such as push/pull Class E DC-DC or quasi resonant boost converters is proposed. Conventional PDM control provides high efficiency at any load when modulation frequency is low. However, the efficiency drops significantly at higher modulation frequencies due to hard-switching during transients. The proposed method not only reduces the transient power loss of the converter, but also decreases the voltage stress of the switches. Startup behavior of a class E push-pull converter is analyzed. Soft switching conditions and the switches' peak voltages are studied as a function of the first switching cycle. In each power pulse, the first cycle timing of the converter is calculated to minimize the voltage overshoot and switching loss of the converter. A 1 kW, 3 MHz push-pull Class-E inverter with Class-DE rectifier is implemented to verify the analysis and simulation results. A maximum efficiency of 93 percent is obtained. Further, the efficiency drops only 2 percent at one tenth of the load. In addition, about 1/5 reduction in the switches' peak voltage is obtained as compared to the conventional PDM method.