Loss Reduction at Partial Loads of Multi-Level DAB Converters Using Adjusted Switching Patterns

A three-phase ( $3\boldsymbol{\phi }$ ) dual-active-bridge (DAB) dc-dc converter built with a medium frequency transformer with $\Delta$ Y-connected windings has low efficiency and low transformer utilization in partial loads. This makes the $3\boldsymbol{\phi }$ - $\Delta$ Y-DAB less attractive for applications where the performance at partial loads is crucial. This article presents control strategies utilizing extra degrees of freedom provided by multi-level converters to minimize the RMS current and improve the partial load operation of the $3\boldsymbol{\phi }$ - $\Delta$ Y-DAB converter. The duration of the zero-level voltage of the Y-side is adjusted with the help of multi-level converters, and the $\Delta$ -side dc-link voltage is controlled externally. It is shown that the RMS current can be reduced by 27% at no-load by increasing the duration of the zero-level voltage of the Y-side. In addition, the turn-on and turn-off currents of the switches of the Y-side can be decreased drastically for power levels below 42% of the nominal power. Considerable performance improvements can be accomplished by combining $\Delta$ -side dc-link reduction with the Y-side duty cycle variation. It is shown that the RMS current at no-load can be reduced by 100% to zero. This is also validated by measurements where more than 90% reduction in the RMS current is observed at no-load. Moreover, a dramatic decrease in the turn-on and turn-off currents of the switches of both bridges is achieved. Measurements highlighted a factor of nine reduction in no-load turn-off currents of the primary bridge. Moreover, a loss split calculation shows that the semiconductor losses can be decreased by 94% and 81% at 5% and 10% of nominal power, respectively.