Light Load Efficiency Improvement for Three-Phase Inverter Employing Valley Switching Under Mixed TCM/DCM Operation

In this article, a mixed triangular conduction mode (TCM)/discontinuous conduction mode (DCM) operation employing valley switching (VS) is proposed to improve the light load efficiency for three-phase inverter. Under TCM operation, zero-voltage switching (ZVS) can be achieved easily at full load range, but the switching frequency increases rapidly at light load or the zero crossing of load current, which significantly increases the turn-
off
loss. Therefore, VS under DCM is adopted to limit the switching frequency around the zero crossing of load current. The optimum mode changing point for the overall efficiency is determined based on the loss analysis under TCM and DCM. Current ripple prediction is implemented based on fully digital control instead of current zero-crossing detection. No auxiliary circuit or high frequency sensor is needed. Owing to the high switching frequency and low inductance of the filter, the power density and the dynamic response can be significantly increased. A 3-kW experimental prototype interfacing a 400 V dc with three-phase 110 V ac is developed to validate the proposed control. The results demonstrate that the light load efficiency is improved by 0.4% to 4.2%.