Three-Leg Operation of Back-to-Back Converters

Bidirectional Integrated Motor Drive (IMD) systems require compact power converter designs that fit inside an electric machine housing. Therefore, high switching frequency is desirable, but it leads to increased heat generation, complicating the cooling strategy. Wide bandgap power semiconductors enable a reduction in switching losses, but this can be further improved by reducing the total number of switching transitions. This work introduces a Voltage Source Back-to-Back Converter (V-BBC) that is operated in a holistic manner to reduce the total switching losses. The proposed concept is based on the synergetic control of the two converters, requiring only three out of six legs to switch at any given switching cycle. An $\text{11}~\text{kW}$ hardware prototype is presented, and its design is optimized. It achieved a power density of $\rho =4.7 ~\mathrm{kW/dm^{3}}$ and measured efficiencies above 98.2% at $\text{100}~\text{kHz}$ and above 98.5% at $\text{50}~\text{kHz}$ switching frequency. Peak efficiencies were $\eta =98.68\%$ and $\eta =98.87\%$ for $\text{100}~\text{kHz}$ and $\text{50}~\text{kHz}$, respectively, for equal input and output frequencies and voltages. Losses were compared to conventional V-BBC operation using state-of-the-art discontinuous modulation, showing significant improvement across the entire operating range.