I recently had power supplies tested for conducted immunity per IEC1000-4-6, 'Conducted Immunity to RF Fields'. This test calls for injecting an RF signal in the range of 0.15-80MHz on the AC input cord to see how the system responds.
The power supplies had resistive loads attached, and were approximately 24" above the metal floor. There were no other leads or additional grounding wires attached to the unit. The technician then clamped a current transformer around the _entire_ AC power cord (line, neutral, and ground), and performed the test. The power supply passed without problem. Although I've never seen this test before, it appears that there would be no EMC stress on the power supply at all. The current transformer would simply be trying to induce a current into an open circuit. When I questioned the lab manager, he said it was a common-mode test. I asked him to identify the path the induced current was flowing in, but he couldn't. Furthermore, the power supply was fed from a simple EMI filter with a low RF impedance. Should a controlled impedance source (LISN) have been used? Was this test performed correctly? -- Patrick Lawler [email protected]
