Common mode EMI reduction structure of EV/HEV inverters for high-speed switching

An inverter prototype, 2.9 L in size, for driving an 80-kW motor was fabricated, and its common mode current was reduced by up to 15 dB compared with that of a basic inverter. In this paper, the two main features of the electromagnetic interference (EMI) reduction structure are described. The first feature is that the P and N bus bars of the inverter prototype are designed to avoid a differential mode resonance in the FM radio frequency band. By the EMI modeling technology, the parasitic inductance and capacitance of the P and N bus bars were optimized and were embodied. The second feature is that the internal structure of the inverter prototype is designed to suppress the common mode current. By the modularized cooler technology, the water jacket was separated from the inverter housing. The P and N bus bars were separated from the housing by a great distance, making the parasitic capacitance between the bus bars and the housing became smaller. By the EMI modeling technology, the positive and negative capacitor electrodes are designed to have the same area, resulting in the same parasitic capacitance at a part of the positive and negative capacitor electrodes closest to the housing that suppresses the common mode current. Reducing the common mode current increases the switching speed, thus reducing the switching loss. This contributes to improving inverter efficiency.