Pat, Probably you meant IEC 1000-4-6; anyway, the test setup sounds shaky because the one configuration that would RF current to flow seems to be missing. I'm assuming that your power supply, like the vast majority of all power supplies, will end up in an enclosure that will be connected to other devices that are essentially RF grounded - forming a complete loop for RF. (When I cite a spec hereafter I'm referring to ENV 50141:1993, I don't have IEC 1000-4-6, yet.)
In your case, it appears the lab tested an open circuit stub. I would expect very little RF current will flow through the power supply until you reached about 1/4th wavelength (of the power cable + power supply) and multiples of 1/4th wave +N*1/2 wave. At 80 MHz this is roughly one meter. Since your maximum cable length from EUT to RF coupler is spec'd by the test to be 30 cm, the power supply will see very little RF current flow - so I'd expect it to pass easily in the configuration you mentioned. (I also noted that the spec calls out 10 cm EUT's height above the reference plane, even if it is a table top device.) IMO the test should be repeated with the power supply's secondary common tied directly to the chamber floor to form a closed loop. (From the setup drawings in the spec, it appears that a closed loop is assumed - though not explicitly called out.) Perhaps both test cases are valid, but only experience will show which is truely worse-case; and then, there are always surprises looming. A line filter vs a LISN as the decoupler? The spec calls out an inductance of 280 microHenries, with reactance of greater-than or equal-to 260 Ohms up to 26 MHz, then 150 Ohms above 26 MHz. It's anybody's guess whether an off-the-shelf line filter meets this spec, and a 50 microHenry LISN doesn't seem likely either (being barely 50 Ohms when terminated). Has anyone else tried both test cases? (Dang, I need a chamber...) Regards, Eric Lifsey National Instruments [email protected] on 10/31/97 07:22:34 PM Please respond to [email protected] To: [email protected] cc: (bcc: Eric Lifsey/AUS/NIC) Subject: IEC1000-4-3 Cconducted immunity test setup 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]
