Isolated Flying Capacitor Multilevel Converters

Abstract

This paper presents the topology known as the isolated flying capacitor multilevel flyback converter (FCMFC). The topology uses isolated flying capacitors (FC) for high-gain, DC-DC power conversion while maintaining primary-secondary isolation and improving the efficiency and capability of a standard flyback transformer circuit. Three converters have been developed and tested, one flyback as a control, a 3 level FCMFC, and a 4 level FCMFC to prove the gain and efficiency benefits of this new topology. The FCMFC prototypes achieve higher gain and efficiencies relative to a flyback design utilizing the same off-the-shelf transformer. The stress of the input and output switches is reduced because of the fractioning of the output voltage across multiple output stages, allowing future iterations to utilize lower-rated components. The operating parameters are 5V to 40V boosting for a 10W load at 250kHz. Utilizing the same semiconductor components, the flyback converter control prototype achieves 78% efficiency while the proposed FCMFC converters reach 85% and 82% efficiency, even with the added isolated power, gate drive, and bootstrap circuitry. The FCMFCs have a multiplying effect on the gain of the traditional flyback which also increases near the 90% duty cycle flyback threshold. Additional testing was done at light load and at 100kHz, 250kHz, 452kHz, and 500kHz.