Loss Balance and Transient DC-Bias Suppression Strategies in Three-Level DAB Converters Modulated With Five DoFs

In the three-level (3 L) neutral-point-clamped (NPC) dual-active-bridge (DAB) converter, both sides of the converter can adapt to higher DC bus voltage, and more control degrees of freedom (DoFs) are provided compared to the conventional DAB converter. In the PWM generating technique with 5 DoFs, loss imbalance may develop in inner switches and outer switches because the leading or lagging arm is introduced, affecting system reliability. In addition, the transient DC-bias issue may also arise in the 3L-NPC-DAB converter modulated with 5 DoFs, leading to the transformer saturation. In this paper, two strategies are proposed to suppress the loss imbalance and the transient DC-bias in the current stress and magnetizing branches. It is also general and applicable for SPS, DPS, and TPS modulations. Furthermore, the loss balance strategy and DC-bias suppression strategy are applied in the PWM generation technique simultaneously without additional components. With the proposed solutions, the loss imbalances of inner switches and outer switches are suppressed and DC-bias in the integral of voltages on both sides of the 3L-NPC-DAB converter is abolished. The effectiveness of the proposed strategies is validated by a series of experiments conducted under varying situations.