Design and Control of GaN based Three-Phase/ Single-Phase Combo Three-Level Flying Capacitor PFC for OBC Applications

As power demand for on-board chargers (OBC) is increasing to meet the fast-charging requirements, 800 V DC bus voltage is being adopted to reduce the copper losses as compared to 400 V DC bus voltage for PFC in OBCs. For higher voltage operation, multi-level converter topologies employ the benefit of reducing device stress and common-mode voltage. Hence a GaN based three-level flying capacitor converter is presented in this paper for 800 V DC bus voltage OBC. Since flying capacitor converter has the benefit of multiplying the inductor current frequency as compared to the switching frequency thus reducing filter size, it becomes attractive for high density OBC applications. The design of the converter, specifically the GaN based flying capacitor phase leg considering symmetrical gate loop and power loop design for all the devices is discussed in detail. It is critical to achieve symmetry in gate loop and power loops as a small asymmetry can cause unbalance in flying capacitor voltage owing to the small rise and fall times in GaN devices. Also, an active feedback method for balancing of flying capacitor voltage is presented which works on the principle of adjusting duty ratios to charge/ discharge the flying capacitor to achieve good balancing.