There is no such thing as a "noise free" switcher, only unaffected equipment.
The quietest switching power supplies tend to be medical grade. You don't
want RFI affecting life support equipment.
All switchers pull pulses of current from the rectified AC Line at frequencies
of a few kHz up to 1 Mhz. While this fundamental switching frequency and mostly
odd harmonics can be part of the problem, the real issue is the reverse
recovery time of the diodes and the storage time of the switching transistors.
These times can be from a few microseconds down to 100 nanoseconds or less. The
reciprocal of these recovery times is basis of the frequencies and the
harmonics they produce. Switch-mode power supply designers are always
pressured for higher and higher operating frequencies and faster recovery times
in order to shrink physical size and improve efficiency. Just think about to
latest generation of USB 3.0 power packs and how small they are for the amount
of power they produce.
The high frequency pulses produced by bipolar transistor designs can easily
have a fourier content in the 150 kHz to 30 MHz range. The MOSFET type
switchers can typically produce additional frequencies from 10 MHz on up to
more than 300 MHz. The stuff in the low end tends to be narrow band emissions
and the stuff on the high end tends to be broadband noise, for the most part.
Standard emissions testing these days set limits in a range from a few 10s of
kHz to 3 GHz and sometimes higher.
So the bottom line is to find a power pack with a noise profile that works in
your application. If it is of interest to the group, I may be able to pull up
some old plots from a spectrum analyzer, just to show what I mean by "noise
profile".
All the best, ~ Doug
Original Message
From: WD8ARZ
Sent: Wednesday, May 4, 2016 3:15 PM
To: rfi@contesting.com
Subject: Re: [RFI] DC Power supplies
AC switching type power supplies tend to be very rfi noisy. Transformer
type power supplies dont use the the dc to ac to dc switching that
generate that RFI .... and should be quieter power supplies. They are
heavier, with line isolation in most cases, and worth the extra expense
and weight. Another bad trait for those switching power supplies is that
there is generally not a true physical break the utility power feed
switch. Instead they just disable the output and leave the switching
circuits active and thus are RFI sources as long as they are plugged in.
73 from Bill - WD8ARZ
South Bend, Indiana
On 5/4/2016 2:51 PM, charlie@thegallos.com wrote:
> Hey Gang,
> We all know about buying old analog wall warts for power supplies to cut
> down on RFI, but sometimes, that is NOT going to happen
>
> snip snip
>
> I think it might be a real good idea if we could come up with a list of
> various "Line lump" (so they can be wired in) type power supplies at
> various power ratings that are KNOWN to be good, and sources for them
>
> I'd be more than happy to collect the list and put it up on my web site
>
> 73 de KG2V - Charlie - www.thegallos.com
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