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

Journal article, 2023

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 paper 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.