Three level NPC dual active bridge capacitor voltage balancing switching modulation

A dual active bridge (DAB) converter is a widely used structure in the low voltage direct current (LVDC) distribution system. In the case of the conventional single-level DAB, the higher the input and output voltage, the higher the voltage stress that the power switch must withstand. To solve this problem, a multi-level structure can be applied to the DAB converter to have low voltage stress of the power switch. Among the multi-level structures, the neutral point clamped (NPC) type is the simplest and most effective in its structure. However, due to the use of two capacitors in series at the DC-link, a capacitor voltage imbalance occurs. If a capacitor voltage imbalance occurs, the voltage stress applied to the power switch can exceed the rated voltage. Conventional capacitor voltage balancing algorithm operates using PI controller. However, precise PI coefficient is required. Therefore, in this paper, a 3-level NPC based DAB converter is analyzed and a new capacitor voltage balancing algorithm without the PI controller is proposed. Finally, the above proposed algorithm is verified by the experimental results of a 1 kW prototype.