[SI-LIST] Re: SMPS capacitor
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: gnuarm.2006@xxxxxxxxx
- Date: Mon, 17 Feb 2014 20:46:15 -0500
Rick
let me a bit more clear. I was speaking with respect to the posed question
indicating that the bypass capacitors were going to be placed remotely from
the current transient generating source.
If the huge current transient from an SMPS is allowed to pass through vias,
without being filtered by bypass capacitors, all bets are off. The
resulting cavity mode current injection can cause significant noise and EMI
problems. I have seen 12V input filtering capacitors placed on the
opposite side of the board from the input to power mosfets in a SMPS.
Terrible, horrible, no good, very bad things happened. (i.e. the full
brunt of the transient current was allowed to pass through vias.)
The takeaway is to filter the noise as close to the source as is possible.
Scott
On Mon, Feb 17, 2014 at 7:55 PM, Rick Collins <gnuarm.2006@xxxxxxxxx> wrote:
> I'm a little confused. How do you get the
> current from the IC to the ground plane if the
> plane is not on the component layer without using
> a via? I don't think I've *ever* seen a SMPS 4
> layer board with the ground and power planes
> other than in the inner layers. Are you
> suggesting that these boards can't pass emissions
> testing? Perhaps you could qualify "hosed" in a more technical sense?
>
> I have seen vias used in any number of reference
> designs by reputable SMPS manufacturers like TI and ADI.
>
> Rick
>
>
> At 05:40 PM 2/17/2014, Scott McMorrow wrote:
> >Ivor
> >If the ripple current passes through vias anywhere along the path, you are
> >hosed. ( I actually had a different word in mind.) Board radiated
> >emissions is generally due to either instantaneous currents crossing split
> >planes, or passing through vias. SMPS have hellacious instantaneous
> >currents.
> >
> >Same applies to the input and the output of the supply.
> >
> >regards,
> >
> >Scott
> >
> >
> >Scott McMorrow
> >His Exalted Eminence the Grand Poobah
> >Teraspeed Consulting Group LLC
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--
Scott McMorrow
Teraspeed Consulting Group LLC
16 Stormy Brook Rd
Falmouth, ME 04105
(401) 284-1827 Business
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
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