Conducted EMI from SiC BJT boost converter and its dependence on the output voltage, current, and heatsink connection

In comparison to their Silicon (Si) counterparts, the Silicon Carbide (SiC) power transistors have lower on-state resistance and higher switching speed, power and temperature ratings. These advantages make it possible to build smaller, lighter and more efficient power converters. Unfortunately, all these benefits come at the price of higher conducted and radiated electromagnetic interference (EMI). This paper investigates the conducted disturbances from a 6 kW boost converter with SiC bipolar junction transistors (BJTs). The results show that the conducted emissions at the input of the converter are approximately proportional to the output voltage, but almost independent on the load current. The effect of the heatsink on the conducted EMI was studied as well. It was found that using separate heatsinks for the diode and the BJT did not affect the level of conducted emissions significantly, but the way of connecting the heatsink does. A floating heatsink is bad from an EMI point of view, and in many cases it may not be allowed for safety reasons. When the heatsink is grounded, alone or together with the negative terminal, the common-mode noise increases the EMI measured at the positive line and decreases the EMI on the negative line. However, this appears only in the lower frequency range. At higher frequencies, connecting the heatsink in any way is better than letting it float. Therefore, the best option is to connect the heatsink to the negative line of the boost converter, and if grounding is required, it may be grounded as well. This may not be possible in systems where the negative voltage bus is at non-zero potential.