EMI, EMC, and Shielding Advanced EMC Topics Informational

How do I design a power line filter for an RF system to suppress conducted emissions below 30 MHz?

Q: How do I handle the filter for a switching power supply?

A switching power supply (SMPS) generates strong noise at the switching frequency (50-500 kHz) and its harmonics. The power line filter must provide: > 30-50 dB attenuation at the fundamental switching frequency, and > 20-40 dB at harmonics up to 30 MHz. Place the filter as close to the power entry point as possible to minimize the length of unfiltered wiring inside the enclosure. Also filter the DC output of the SMPS with LC networks to prevent conducted noise from reaching sensitive RF circuits.

A power line filter for an RF system suppresses conducted emissions below 30 MHz by inserting a low-pass filter network between the power supply and the equipment to prevent high-frequency noise generated by switching regulators, digital clocks, and RF circuits from conducted back onto the power lines where it would violate EMC regulatory limits (FCC Part 15, CISPR 32, MIL-STD-461). The filter design involves: identifying the noise characteristics (common-mode noise flows equally on both power conductors relative to ground; differential-mode noise flows between the power conductors; most switching power supply noise is common-mode at low frequencies and differential-mode at higher frequencies), selecting the filter topology (an LC pi-filter or T-filter provides the best attenuation per component: a typical configuration uses a common-mode choke inductor, typically 1-10 mH with high impedance at the noise frequencies, X-capacitors across the line for differential-mode filtering, typically 0.1-1 uF rated for line voltage, and Y-capacitors from each line to ground for common-mode filtering, typically 1-10 nF limited by leakage current safety requirements), designing for the required attenuation (the insertion loss must exceed the difference between the measured emission level and the regulatory limit at each frequency: typically 30-60 dB of attenuation is needed at the worst-case frequency), and considering safety requirements (Y-capacitors are limited in value by safety standards to restrict leakage current to < 0.5-3.5 mA depending on the equipment class; X-capacitors must be self-healing and rated for the line voltage).

Category: EMI, EMC, and Shielding

Updated: April 2026

Product Tie-In: Shielding, Gaskets, Absorbers, Filters

Power Line EMI Filter Design

Power line filters are mandatory in virtually every electronic product to meet conducted emission limits. The filter must suppress noise across the 150 kHz to 30 MHz conducted emission band while passing the 50/60 Hz power frequency with minimal voltage drop and power loss.

Common-Mode vs. Differential-Mode

Common-mode (CM): Noise current flows in the same direction on both power conductors and returns through the ground. Suppressed by: CM chokes (a bifilar-wound inductor that presents high impedance to CM signals and low impedance to DM/power signals) and Y-capacitors (bypassing CM noise to ground)
Differential-mode (DM): Noise current flows on one conductor and returns on the other. Suppressed by: series inductors on each line and X-capacitors across the line
Typical filter circuit: From line input: X-cap -> CM choke -> X-cap -> Y-caps to ground. This provides both CM and DM filtering with 40-80 dB of attenuation at 1-30 MHz

Power Line Filter Parameters

CM choke impedance: Z = j × 2pi × f × L_CM (typically 1-10 mH)
DM inductor: small leakage inductance of CM choke (50-200 uH)
X-capacitor cutoff: f_c = 1/(2pi sqrt(L_DM × C_X))
Y-capacitor leakage: I_leak = V_line × 2pi × f_line × C_Y
Insertion loss: IL = 20 log(V_without / V_with) [dB]

Common Questions

Frequently Asked Questions

What common-mode choke inductance do I need?

The CM choke inductance is determined by the required attenuation at the lowest noise frequency. For 40 dB attenuation at 500 kHz with 10 nF Y-capacitors: L_CM = 1/(4pi^2 × (500e3)^2 × 10e-9 × 10^(-40/20)) = approximately 1 mH. For higher attenuation or lower frequency: increase L_CM. Practical CM chokes range from 0.5 mH (SMD) to 30 mH (through-hole). Choose a choke rated for the full line current with adequate core saturation margin.

What are the safety requirements for Y-capacitors?

Y-capacitors connect from line to ground (safety ground). If a Y-cap fails short, it creates a shock hazard. Therefore: Y-caps must be safety-rated (Y1 class for reinforced insulation, Y2 class for basic insulation). Maximum capacitance is limited by leakage current standards: for Class I equipment (grounded): leakage < 3.5 mA (approx. C_Y < 23 nF for 240V/50Hz per line). For Class II equipment (double insulated): leakage < 0.25 mA (approx. C_Y < 1.7 nF). For medical equipment: even lower limits apply.

How do I handle the filter for a switching power supply?

A switching power supply (SMPS) generates strong noise at the switching frequency (50-500 kHz) and its harmonics. The power line filter must provide: > 30-50 dB attenuation at the fundamental switching frequency, and > 20-40 dB at harmonics up to 30 MHz. Place the filter as close to the power entry point as possible to minimize the length of unfiltered wiring inside the enclosure. Also filter the DC output of the SMPS with LC networks to prevent conducted noise from reaching sensitive RF circuits.

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