A cost-effective circuit for three-level flying-capacitor buck converter combining the soft-start, flying capacitor pre-charging and snubber functions

The Three-Level Flying-Capacitor Buck (TL-FC-Buck) Converter is widely used in high voltage applications due to advantages such as the low voltage stresses on devices, double switching frequency on the inductor and the common ground between input and output. However, the flying capacitor needs to be pre-charged to half of the input voltage before the converter works. This paper proposes a novel and cost-effective auxiliary circuit, which integrates the pre-charging circuit with the inherent soft-start and snubber circuits. With the proposed approach, no additional components are required for pre-charging. The circuit thus becomes compact and cost-effective. The concept and analyses of the proposed circuit are presented in this paper, and the parameter design compromising the pre-charging time, voltage stresses of the devices during charging process and snubber losses are given. Finally, a prototype with 710V input, 230V output and 2.5kW rating power TL-FC-Buck converter is built to verify the design. Experimental results match the simulation quite well. With proper selection of parameters, the voltage stresses of devices can be acceptable, and the overall efficiency can still be high.