Q: How to reduce conducted emissions from switching power supplies below CISPR 32 limits?
Answer
Conducted emissions (150 kHz-30 MHz) from SMPS: (1) Input filter: add EMI filter (L-C-L pi filter) before the converter. X-capacitor (1-10 uF film) + common-mode choke + Y-capacitor (2.2 nF). Filter must be in a shielded compartment. (2) Spread spectrum: enable spread spectrum clocking on the SMPS IC if available – reduces peak emissions by 5-10 dB. (3) MOSFET snubber: add RC snubber across the switching MOSFET (10-100 Ohm + 100 pF-10 nF) to reduce ringing. (4) Soft switching: use synchronous rectification with zero-voltage switching (ZVS) to reduce di/dt. (5) Switching frequency: avoid frequency overlap with AM radio band (530-1700 kHz). Choose fsw > 1.7 MHz or <530 kHz. (6) PCB layout: keep switching node (SW/MOSFET junction) area minimal with guard ring. (7) Input cable: add ferrite clamp on the input cable. Pre-scan with LISN (Line Impedance Stabilization Network) and spectrum analyzer during development.
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