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70 DESIGN007 MAGAZINE I MAY 2025 High-frequency EMC Noise in DC Circuits Figure 1: Power conversion stage of avionics unit. Article by Karen Burnham EMC United EMC isn't black magic, but it's easy to under- stand why it seems that way. When looking at a schematic like that in Figure 1, it looks like you're only dealing with DC signals all across the board. ere's a 28 VDC input that goes through an EMI filter, then gets converted to 12 VDC power. Except in extremely rare cir- cumstances involving equipment sensitive to magnetostatic fields, DC electricity will never be part of an EMC problem. Harmonics e biggest problem occurs when you use a switching operation to convert power from one form to another—something most mod- ern electronics do all the time. For power and thermal efficiency reasons, we want switching waveforms to get as close to a perfect square wave as possible—the faster the rise and fall time, the less loss and heating you have in the operation. e problem is that there's a trade- off: Faster rise and fall times necessarily gen- erate high-frequency electrical signals. en, if provisions aren't made for controlling the high-frequency noise signals, they can escape to cause signal integrity and EMC problems throughout the board and system. Let's illustrate this with a simple function generator. e beginning setting is a 5 VDC square wave switching at 20 MHz—some-