When you build an isolated offline supply — a flyback phone charger, a medical device, an appliance controller — you have a transformer separating mains-referenced primary from touch-safe secondary. But there's a dirty secret: high-frequency switching noise couples across that isolation barrier through the transformer's interwinding capacitance, typically 20-200 pF. That noise flows out through your DC output cable as common-mode current, radiates like an antenna, and blows your EMC pre-compliance scan into orbit.

The fix is a Y-capacitor: a small ceramic (typically 470 pF to 4.7 nF) bridging primary ground (rectified mains return) to secondary ground. It provides a low-impedance return path for common-mode noise before that current has to flow through the earth loop and your output cable.

Why not just any capacitor? Because this cap sits across mains isolation. If it shorts, the user gets full line voltage on the "safe" side. Y-caps are safety-rated — Y1 (double insulation, ~8 kV surge) or Y2 (basic insulation, ~5 kV) — and they're engineered to fail open, not short. Never substitute a generic ceramic.

Sizing rule of thumb: Y-cap value is bounded by leakage current limits, not by noise suppression. IEC 60950/62368 caps touch-current at 0.25 mA for typical Class II equipment. At 240 VAC, 50 Hz:

• I_leak = 2π·f·C·V = 2π·50·C·240
• Setting I_leak = 0.25 mA → C_max ≈ 3.3 nF
• Medical (IEC 60601) tightens this to 0.1 mA → C_max ≈ 1.3 nF

In practice, designers pick 1 nF for universal-input consumer gear and 470 pF for medical. Bigger is better for EMC but worse for leakage — and worse for the tingle a user feels touching an ungrounded laptop chassis (that's Y-cap leakage flowing through you to earth).

Concrete example: A 65 W USB-C GaN charger uses a flyback with ~50 pF transformer interwinding capacitance. Without a Y-cap, radiated emissions at 30-100 MHz exceed CISPR 32 Class B by 15 dB. Adding a 2.2 nF Y1 cap between the primary bulk cap negative and secondary GND drops emissions by 12-18 dB — usually enough to pass. Physical placement matters: keep the loop from Y-cap to transformer short, or the parasitic inductance defeats the whole point.

Watch for the ground-referenced pitfall: on 3-prong grounded supplies, Y-cap leakage flows to safety earth harmlessly. On 2-prong Class II supplies, it flows through whatever the user touches — hence the strict limits.