SMPS CM emissions
Consider a spherical SMPS ;<))
For the analysis the AC rectifier is not relevant.
Basically a SMPS power supply is
a switch connected between phases with
a coil before it and a rectifier C
combination behind it. All other
topologies are variations on the theme.
Like this
PLUS ---- COIL ---- DIODE------
| |
SWITCH CAP
| |
MINUS -------------------------
The current in the coil rises linear with the time
and descends linear with time on the operation
frequency of the switch.
This current must be provided by the source, of which
the high frequency parts should be filtered out by a
cap in the supply (may be part of the SMPS).
This is a differential current in plus and minus wire.
Enough DM filtering gets rid of it.
Regarding the enclosure around the SMPS, there are caps
between (hot) wiring and enclosure ground (I am not saying minus or smps
ground)
If conductor A carries +Vdiff and conductor B -Vdiff then,
as these capacitors are different (almost by design),
both conductors will have an different Vcommonmode to encl. ground.
Voila a source of CM current.
Another source is the difference in ground potential between
DC and AC side, often combat by a capacitor between primary
and secondary ground and this one creates those horrible leakage
currents Pete Perkins was referring to. Decent ground screens in
the insulation transformers can help.
Basically it is the physical asymmetries in the smps that create
Common mode currents.
Gert Gremmen
Van: [email protected] [mailto:[email protected]] Namens Pete Perkins
Verzonden: donderdag 11 december 2008 5:37
Aan: PSNet
Onderwerp: RE: Common-mode emissions from SMPS
Fellow travelers,
I'm pleased to see the explanations put forward on this discussion
thread.
It all fits with my own issue of measuring Touch Current leaking to
ground which has grown with the advent of SMPS.
If you want to see a collexion of TC waveforms that I've collected
>from an assortment of equipment go to Art Michael's www.safetylink.com
and
search on my name (Perkins) and you will find the waveform paper.
My ongoing interest has been in the LF portion of this current (up
to 1 MHz) rather than the HF EMC effects.
But the data shows that the current to earth has grown away from
sinusoidal waveforms since SMPS were introduced in equipment. As some
have
noticed, the crappiness factor has increased with time, especially with
the
addition of PFC.
We electric shock folks understand that the body reacts to peak
currents rather than the RMS currents normally considered in safety
standards. Getting rid of those old RMS meters and moving to a peak
reading
meter or scope gets to the correct TC value to be considered for
protection
>from electric shock. A large portion of the world still lives in the
Dark
Ages in this regard.
I remember dealing with the triplen harmonics in the 80's when they
were not well understood or attributed to SMPS in 3 phase systems. The
resulting transformer fires from these harmonics which circulated in
delta
primaries clouded the analysis (not my problem seemed to be a first
analysis
result). Accepting responsibility allowed the problem to be addressed
in a
rational way.
As with all of life, unintended consequences cloud perfectly clear
decisions that were made in good faith.
So all our lives keep getting more difficult as the technology is
improved.
Well, us technical folks see trouble as opportunity.
Keep the faith.
Enjoy the waveforms. :>)
Br, Pete
Peter E Perkins, PE
Principal Product Safety Engineer
[email protected]
-
This message is from the IEEE Product Safety Engineering Society
emc-pstc discussion list. To post a message to the list, send your
e-mail to <[email protected]>
All emc-pstc postings are archived and searchable on the web at:
http://www.ieeecommunities.org/emc-pstc
Graphics (in well-used formats), large files, etc. can be posted to that
URL.
Website: http://www.ieee-pses.org/
Instructions: http://listserv.ieee.org/request/user-guide.html
List rules: http://www.ieee-pses.org/listrules.html
For help, send mail to the list administrators:
Scott Douglas <[email protected]>
Mike Cantwell <[email protected]>
For policy questions, send mail to:
Jim Bacher: <[email protected]>
David Heald: <[email protected]>
-
This message is from the IEEE Product Safety Engineering Society emc-pstc
discussion list. To post a message to the list, send your e-mail to
<[email protected]>
All emc-pstc postings are archived and searchable on the web at:
http://www.ieeecommunities.org/emc-pstc
Graphics (in well-used formats), large files, etc. can be posted to that URL.
Website: http://www.ieee-pses.org/
Instructions: http://listserv.ieee.org/request/user-guide.html
List rules: http://www.ieee-pses.org/listrules.html
For help, send mail to the list administrators:
Scott Douglas <[email protected]>
Mike Cantwell <[email protected]>
For policy questions, send mail to:
Jim Bacher: <[email protected]>
David Heald: <[email protected]>
