Power Adapter with Line Voltage Control for USB Power Delivery

The demand for high-power-density high-efficiency power adapters is increasing due to the wide adopt of the USB Power Delivery as the charging protocol in consumer electronics. Flyback converters have conventionally prevailed for such applications. To further improve the power density of power adapters, an increase in switching frequency is required. However, this will increase the burden of the EMI filter, and the heat will be difficult to manage as the loss increases when size is reduced. Resonant converters have long been used for high frequency applications due to the easily-implemented soft switching and low current stresses. With the development of faster switching devices and new ferrites, the switching frequency of resonant converters is pushed as high as megahertz with acceptable thermal performance. In this paper, a series resonant converter is combined with the proposed line voltage control circuit to accommodate both the 120 VAC and the 220 VAC lines, as well as the wide output voltage range from 5 V to 20 V required by the USB Power Delivery. In the 60 W prototype, peak 94% efficiency is achieved at 1 MHz operation enabled by a budget MCU.