Non-intrinsic differential mode noise caused by ground current in an off-line power supply

Nonintrinsic differential-mode (DM) noise caused by ground current is common in switch-mode power supplies, but the phenomenon is first uncovered and reported in this paper. Using the zero-span mode of a spectrum analyzer allows one to investigate this mode of noise. It is found that this mode of noise is a voltage-generated noise, caused by the unbalance of the circuit. Understanding the phenomenon makes the EMI diagnosis and filter design less of a “black magic”