Column: EMC
(WBG) materials like silicon-carbide (SiC) and gallium-nitride (GaN) offer better performance. However, their faster switching at higher voltages means these devices’ dV/dt is higher too, leading to more EMI.
Switching events Tere are two aspects of a switching event: frequency and speed. EMI associated with these switching events leads engineers to mainly focus on the switching frequency, oſten overlooking the impact of switching speed. However, this speed is equally important and it is also the main source of EMI. Similarly, a high switching frequency
isn’t the only cause of EMI problems. Consider this example: an electrostatic discharge (ESD) event does not have the MHz of switching frequency, but the
rise time can be as short as 10-100ps. One ESD event could potentially radiate its energy to a nearby system and cause problems. Using the SPICE simulation soſtware, the
simulated switching event is seen to have 60kHz switching frequency, with 600V DC voltage and a duty ratio of 50%. Rise time is set to 12ns for a 5V/ns switching speed. Te spectrum analysis of the switching event is shown in Figure 1. Ideally, engineers would prefer the -40dB/decade roll-off to occur at a lower frequency point, because the noise spectrum decreases a lot faster aſter this crossing point. However, the roll-off point only depends on the rise time of a switching event, and normally there’s no option of shiſting it. Tis is because the rise time of a switching event oſten can’t be increased as higher rise time leads to
greater switching losses and lower system efficiencies. To demonstrate the effect of sharp rise time in Figure 2, a faster rise time (10V/ns) is simulated for comparison. As it can be seen, every time the switching speed is doubled, it results in a 3-6dB noise
increase from 1/πtrise. Once the spectrum characteristics of a
switching event are understood, it is then easy to design a filter that can suppress the noise at the frequency range of interest. For instance, a motor drive causes conducted emission between 100kHz-10MHz, so the lower frequency range noise (< 1MHz) oſten needs differential-mode filtering. Between 1MHz and 10MHz, some form
of common-mode filtering is needed. A three-phase filter that has sufficient attenuation in this range would be a good choice. One example is a C-L-C (π) filter, shown in Figure 3.
Figure 3: REO CNW 103 three-phase filter gives good attenuation in the lower to mid frequency range
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