Every switching converter is a tiny radio transmitter you didn't ask for. PWM transistors chop current at 20 kHz to 2 MHz, and those sharp edges inject high-frequency noise back onto the AC mains and out through every cable. EMI filters are the passive networks that keep that noise contained so your device passes FCC Part 15 or CISPR 32 and doesn't scramble the neighbor's WiFi.

Conducted EMI travels two ways on a two-wire input:

• Differential mode (DM): noise current flows out on line and back on neutral — the same loop as your load current. Caused by ripple on the input capacitor.
• Common mode (CM): noise current flows out on both line and neutral together, returning through earth ground via parasitic capacitance. Caused by fast dV/dt edges on the switching node coupling to the chassis.

The classic filter has four parts:

• X-capacitors (line-to-neutral): shunt DM noise. Must be self-healing film caps rated to fail safe — a shorted X-cap across mains would be a fire.
• Y-capacitors (line-to-ground and neutral-to-ground): shunt CM noise to earth. Limited to ~4.7 nF in most applications because leakage current to ground is a shock hazard — UL caps the touch current at 0.5 mA.
• Common-mode choke: two windings on a single core wound so load current fluxes cancel (no saturation) but CM noise sees high inductance. This is the big toroid you see on every laptop brick PCB.
• Differential-mode inductor: often just the leakage inductance of the CM choke, sometimes a separate part for high power.

Rule of thumb for filter corner frequency: place it at least a decade below your switching frequency. For a 100 kHz converter, target a 10 kHz corner. With a typical 2.2 nF Y-cap and 10 mH CM choke, corner ≈ 1/(2π√LC) = 1/(2π × √(10e-3 × 2.2e-9)) ≈ 34 kHz — workable for a 500 kHz supply, marginal for 100 kHz.

Real-world example: a 65W USB-C laptop charger. Open one up and right after the AC inlet you'll find an X2-cap (typically 0.1–0.47 µF), a CM choke (often 5–20 mH on a ferrite toroid), and two small ceramic Y-caps tied to the secondary-side ground through the safety isolation. Without these, the charger would still work fine — but it would never pass conducted emissions testing, and FCC certification is a hard ship-blocker.

EMI filtering is usually the last thing designed and the first thing that delays your product launch. Layout matters as much as component values: a great filter ruined by a sloppy ground return is a common cause of last-minute compliance failures.