The FET switch capacitance is part of the capacitances
I was mentioning. In general, for a DM current to be converted
into CM current, asymmetries are required. Here it's capacitance
to ground, and I took a small part of the circuit as example.
Never mention green wires in EMC analyses. They are inductors.
The Y-caps (I read: filter caps)are there to short circuit all CM
current
to the chassis, and provide the proper return path to the parasitic
caps.
The question was, where CM current came from, not why there is
still CM current after filtering is applied.
It's not the Y-caps that we need to balance, but the circuit's
Parasitic C's, being the PCB traces, the FET surface, the coil
and any other conductive part connected into the circuit.
The Y caps are used on the boundary of the circuit, not internally.
You are right in that a 2 stage filtering is required, by using
a CM coil and Y-caps, to get decent results.
Beware however, that the voltages at the smps side of
the CM coil need to be free of DM signals. They are passed
trough the CM coil (by design) without attenuation.
Normally a well designed parallel cap (rectifier cap)will do that for
you,
but very often a substantial amount of interference voltage
in differential mode will remain there, and be passed through.
As this DM voltage is very low impedance, the Y caps
won't do much. In some cases a extra normal coil in the
supply part is required.
| CM-coil | single |
Y-cap | ended L |
| | |
---------chassis DM-CAP RECT CAP
| | |
Y-cap | |
| CM-coil | |
I left the rectifier out!!!
Gert Gremmen
Van: [email protected] [mailto:[email protected]] Namens Ken Javor
Verzonden: zondag 14 december 2008 1:12
Aan: Untitled
Onderwerp: Re: Common-mode emissions from SMPS
I didn't understand that part of Mr. Gremmen's post. Say there is no
line-to-ground capacitance at all between phase and neutral wires and
the
grounded equipment chassis. Or more accurately, that the reactance of
the
line-to-ground capacitance is a much higher impedance than 50 Ohms.
Given
the FET switch capacitance to ground, you are still going to drive
current
into the green wire and that current will be drawn from both phase and
neutral conductors.
The purpose of Y caps is of course to keep the cm current circulating
within
the equipment and not in the LISNs, but it isn't obvious to me that you
get
zero cm current if the two Y caps are perfectly balanced, and some net
cm
current if there is a disparity.
In fact, you typically need a cm choke to give you cm filter insertion
loss
at the low end, because leakage current requirements place an upper
bound on
Y cap values, which means they are ineffective at lower frequencies.
Ken Javor
Phone: (256) 650-5261
> From: John Woodgate <[email protected]>
> Date: Sat, 13 Dec 2008 16:49:07 +0000
> To: <[email protected]>
> Subject: Re: Common-mode emissions from SMPS
>
> In message <[email protected]>,
> dated Sat, 13 Dec 2008, Gert Gremmen <[email protected]> writes:
>
>> the two capacitances (+ wire and - wire) are inverse, you will need
to
>> know the difference between the tow caps and use that cor the CM
>> current. If both capacitances are equal, the CM current will be
>> essentially zero.
>
> So why not add a small capacitor so as to *make* the capacitances
equal?
> Adjust for minimum CM emission.
>
> Remember, you read it here first! (;-)
> --
> OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
> All these closing-downs have been caused by dozing clowns
> John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
>
> -
> ----------------------------------------------------------------
> 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]>
-
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]>
