Modulation Method of a Full-Bridge Three-Level LLC Resonant Converter for Battery Charger of Electrical Vehicles

This paper proposes a modulation method for a full-bridge three-level LLC resonant converter. The target converter is known to enable fixed frequency operation, small resonant inductance ratio, and zero-current switching operation of rectifier diodes under wide output-voltage variation. Therefore, it is suitable for an application such as in battery chargers. A new parameter named “master duty,” which determines each voltage-level duty, is introduced to this modulation method. It simplifies the modulation method; therefore, a modification of the modulation is easy. It also makes possible seamless handling of two-level, three-level, and mixed modes. The modulation method is represented by the relationship between the master duty and each gate-pulse edge position. This representation can clearly depict a complex modulation such as a combination of pulse-width and phase-shift modulations. The voltage gain of the converter is analyzed using the first-harmonic approximation technique. The result of the analysis is verified by the circuit simulation. The voltage gain is also analyzed using lossy components, which is verified by the experimental result. A 385-V input 225- to 378-V/6.6-kW output prototype converter is fabricated for this verification. It shows 98.14% peak efficiency, and its power density is 1036 W/L.