Hi John,

 

 

Most small switchmode power supplys are nowadays equipped with two terminal mains plugs, eliminating cost for country-dependent and clumsy three terminal plugs with a protective earth pin. To reduce RF emissions, they usually contain a shunt capacitor between output and one (or sometimes both) of the mains lines. This is meant to minimize radiation of switching harmonics from the oputput DC lines due to capacitive leakage in the ferrite transformer. As there is no ground to shunt to, going to mains primary is the only option. The capacitor is usually several 100 pF to a couple of nF, a tradeoff between EMC (especially at low frequencies) and 50 Hz leakage currents to the output, and being a safety-critical component it needs to be rated for high surge voltages. Whether the leakage current is present or not may depend on the polarity of the plug - try reversing it!

 

The best solution is to externally ground the output or connected devices to protective earth, which will eliminate the 50 Hz leakage, and very likely improve LF interference a lot. 

 

I haven't seen 25 Hz periodics from one of my power supplies yet. But it may be related to a method of improving light-load conversion efficiency by intermittent switcher operation. 

Best 73,

Markus (DF6NM)