Eliminating Input Electrolytic Bulk Capacitors in Flyback-Based Universal Chargers with a Half-Bridge Series-Stacked Buffer

摘要

Flyback converters without power factor correction (PFC) function are common as the main power conversion stage for the emerging high power (65 W) universal ac (90-240 Vac) adapters using the power delivery capability of USB-C standard. In the conventional solution, large electrolytic capacitors are deployed after the ac line full-bridge rectifier to maintain a minimum dc-bus voltage for the flyback converters. The corresponding limited bus voltage ripple thus leads to large required capacitance. In this work, an active buffer with an additional half-bridge circuit is proposed to reduce the required capacitance and allow the use of high energy density ceramic capacitors to reduce the physical size of the needed input capacitor. The operation, control and sizing of passive components of the proposed active buffer are discussed, and a compact hardware prototype is designed and tested to validate the concept.