by Dr. Min Zhang, Chief EMC Consultant, Mach One Design Ltd
Contact details: www.mach1design.co.uk
Wide-Band-Gap devices such as SiC and GaN MOSFETs are slowly replacing Silicon based devices in high power density applications, such as an electric drive unit in an electric vehicle. One of the benefits of these devices is they can be switched on and off very fast. A sharp rise time of such devices can often be found between 1 ns to 10s of ns. The rapid change of voltage dV/dt could lead to severe EMI issues if proper design is not in place.
We learnt that the distance between the load and the power electronics switches can have a major impact on the switching waveform, hence on the EMI . So for these wide band gap devices, how long should the cable be to cause us concerns?
The Rise-Time Rule introduced in  can be used to evaluate this problem. It states that “if the length of the transmission line is less than one-third the distance a wave travels in one rise time, then terminating the line in its characteristics impedance is not necessary”. Assuming a switching device now has a 10 ns rise time, the rise-time distance is 3 m(assuming wave travels in the air in this case, as we use electrical drive unit as an example where often the connections between the power stage and the motor end-winding is through cables in free space, rather than tracks on the PCB). One-third of this distance is 1 m. For a well integrated motor drive system, the cable length is often kept well below 50 cm, therefore one shouldn’t worry too much of the possible reflections of the cable. However, this doesn’t necessarily mean that we should be comfortable of the system. As it can be seen from Figure 1, overshoot can easily occur depend on cable length and how fast the devices are switching.