--- On Mon, 29 Jun 1998 16:02:43 -0400 "Schanker, Jack" <[email protected]> wrote:
> I wonder if anyone can help me with my confusion over the proper way to > evaluate conducted EMC on the DC input lines to DC-DC converters. Uhhh, "conducted EMC" is kinda fuzzy thinking. Very low conducted emissions will help to ensure Electromagnetic Compatibility with your environment! > > Our radios are available with a variety of slide-in modular supplies, > including 24 and 48 VDC. When DC power is ordered, it is up to the > customer to have a source, we don't provide a line operated DC supply. If your customer specifies that your radio work off 28 VDC power, then you are exempt from the commercial requirements associated with connection to the public power utilities. (I don't think I've ever come across a 28 VDC public power utility!) The customer may still want to impose a conducted emission limit on your radios, and may do this contractually with you. (The customer may want to protect other stuff on a common 28 VDC power buss, or even the source of the 28 VDC.) > When EMC labs have measured line conducted on the DC radios, they have > used either a "golden" or customer specified AC-DC converter (i.e., DC > power supply) and placed the LISN between the AC side of that supply and > the mains. Any conducted noise produced by the supply itself is > "subtracted out" supposedly leaving any noise put out by the radio and > its DC-DC converter. I wouldn't know how to "subtract out" the converter noise. I would be interested to see their technique and rationale. The emissions I would see on the power source side of an AC to DC converter would be the SUM of the converter's noise and (your radio's noise less losses and isolation due to the filtering/buffering of the converter). The only result that would count would be for the emissions to be completely under the emission limit, qualifying YOUR radio and someone elses converter, functioning as a system. Any different converter would be DIFFERENT, and all bets would be off. This actually makes the converter part of a deliverable "system" with your radio. > We are now looking at some DC-DC converters for their conducted noise > levels. The power supply manufacturer is measuring noise by using a > standard 50 uH LISN on the DC input line (as opposed to the AC supply > line). Is this generally considered an acceptable method of measurement? Fully acceptable, as per MIL-STD-461D/462D, where platforms often have a 28 VDC "public power utility". >;) The EUT creates a certain amount of RF power on its power input lines. That RF power applied across the impedance (varies between about 5 ohms at 10KHz and 47 ohms at greater than 1 MHz) of the LISN yields a voltage at the LISN sample port. The LISN is there to normalize the impedance (and to give you a convenient way to measure the voltage). Whether that impedance is similar to real life is another question! > On the one hand, you are looking at the noise where it first appears, > but on the other hand, regulatory concern is for noise getting back into > the mains. OK, so you are in a commercial environment. The way I see it, with your 28 VDC power input, you have no obligation to meet any conducted emission limits. The burden of compliance falls on the entity that connects you to the public mains. That's not you, and it's not the power converter manufacturer. It's the system integrator who decided to use your radio and the converter as a system. That integrator may try to get off the hook by contractually specifying some limits to you. But ultimately, the integrator has to prove the system performance, as a system. > Also, are the 50 uH chokes in commercial LISNs typically air > core so that they can carry DC without saturation ? It's not hard to design a 50uH air core inductor (perm=1) as a single layer solenoid. No need to complicate things with a core structure which may vary permeability as a function of current. > Our ultimate requirements are to meet FCC Part 15 and EN 55022. But YOU don't connect to the public mains directly, do you? Remember, this has all been directed toward conducted emission issues; you do need to comply with radiated requirements. > > What are the gorup's opinions ? > > Thanks, > > Jack > > Jacob Z. Schanker, P.E. > Director of Agency Compliance > California Microwave > 716 242 8454 (voice) > 716 242 8427 (fax) > [email protected] > ---------------------------------------> -------------------------- Ed Price [email protected] Electromagnetic Compatibility Lab Cubic Defense Systems San Diego, CA. USA 619-505-2780 List-Post: [email protected] Date: 06/29/98 Time: 14:56:55 --------------------------
