[RFI] Switch mode Power Supplies

[Dale Svetanoff]

Folks,

As most of you know, the topic of RFI issues with switching (or "switch mode") power supplies pops up here quite frequently.  I was delighted to see that the February issue of QST  addresses the subject with lab tests on 4 more such power sources.  (See pages 56 thru 59 for all of the details.)  Make certain that if you are interested in noise issues with switching power supplies, you read the entire article carefully.  It is also worthwhile to track back and see the prior power supply test articles the League has published, and which are referenced in the footnotes.

Keep in mind that all switch mode power supplies have both radiated and conducted emissions.  The name of the game is to buy those having the lowest levels of emissions in YOUR operating frequency bands of interest.  Thus, study the plots and tables well if you are primarily an HF operator, as most of the emissions occur in the HF bands.  (An exception was the TenTec supply, which was very noisy in the 160m band, considered to be MF.)  Those who operate with HF antennas inside their houses (especially in the attic) or with antennas close to the house and the house wiring is not shielded (no conduit or metal armor cladding), might very well end up with RFI issues from a power supply when trying to receive weaker signals in some of the HF bands.  While there are ways to reduce conducted emissions from the power supplies, the best alternatives might be to either stay with linear (heavy) power supplies or to move the antennas further away from the house wiring.

Related comment: This reflector also gets frequent posts regarding industrial switching power supplies for use in amateur radio applications, especially for 28 VDC output or very high current at 13.6 VDC.  If a power supply was intended for the industrial or commercial market, the most it may have to meet in the way of emissions requirements is set forth in FCC Part 15, Class A limits.  These limits are far more lax than the Class B limits for computing devices used in a home or office environment.  In short, such switching power supplies are almost guaranteed to be "screamers", expecially in the HF region.  Stay with linear technology supplies unless you are prepared to test and design filters or RFI suppressors for these special power units.  The cost of the "fixes" could easily add up to the purchase cost of the power supply alone, or even exceed it.   

My thanks to ARRL for another excellent article in helping hams make better buying decisions.

73, Dale
WA9ENA