Enhanced Active Thermal Balancing Strategy for Three-Phase Dual-Active Bridge Converter Suppressing Transformer Flux Saturation

To improve the power efficiency of three-phase dual-active bridge (DAB3) under light-load conditions, triple phase-shift (TPS) modulations with variable duty cycles have been developed. However, with variable duty cycles, asymmetric phase currents due to the asymmetric phase voltages occur. These asymmetric currents and voltages result in different switch currents, leading to thermal imbalance between the top and bottom switching devices. To mitigate this thermal imbalance and enhance lifetime of DAB3 under TPS modulations, an active thermal balancing (ATB) strategy has been introduced. The existing ATB method periodically exchanges switch currents between the top and bottom switches by inverting gate signals over every switching cycle. Nevertheless, the inverted gate signals lead to a flux trajectory with significantly increased flux peak, potentially causing transformer flux saturation issues. To avoid flux saturation, this article proposes an enhanced ATB strategy that not only inverts gate signals but also reverses the applied sequence of phase voltages periodically. By alternately applying original phase-voltage in a positive sequence and inverted phase-voltage in a negative sequence, the flux trajectory is modified to achieve thermal balancing without flux saturation. Experimental tests validate the effectiveness of the proposed ATB strategy.