Q: How to reduce EMI emissions from PCB switching power supply circuits?
Answer
EMI from switching supplies comes from: (1) Switching transients at the MOSFET/diode node (2) discontinuous input current (3) ground noise. Countermeasures: slow down the switching edge with an RC snubber (10-100 Ohm + 100 pF-10 nF) at the switching node. Add input EMI filter (pi-filter with X2/Y2 capacitors) before the converter. Use spread-spectrum clocking if the converter IC supports it – reduces peak emissions by 5-10 dB. Shield the inductor with a copper Faraday cage on switching supplies >500 mW. Route the switching node on an internal layer with guard rings. Separate noisy sections from sensitive sections. Use EN55022/CISPR22 pre-compliance testing with a near-field probe during design.
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