[SI-LIST] Re: 48V filtering
- From: "Abhishek, Dev" <dev.abhishek@xxxxxxxxx>
- To: <DavidS@xxxxxxxxxxxxxx>
- Date: Wed, 31 Dec 2003 16:02:09 +0530
I'd recommend the following:
1) Ultra low ESR 100 Volt caps at the input - a hot swap controller =
should precede the caps. Ensure that the capacitors provide sufficient =
capacity to handle the surge test as applicable for your requirements =
(FCC or CISPER)[x-caps]
2) Provide 100 V MOVs (300 Joules minimum is what the standards usually =
mandate - I have used as high as 1.5kJ), bidirectional
3) Common mode choke - two preferably, especially if you need to hit =
class B, Cisper
4) Ceramic capacitors, 100V, tune the value so that the filter provides =
30-40 db attenuation across the frequency spectrum in your system - =
each commmon mode input and output will require these
5) Ceramic, and electrolytic caps after the filter, close to the =
regulator
6) You might chose to add Y-caps - be sure to pick ultra-low leakage =
variety, at least 2.5kv, unpolarised. Resistive damping helps as well - =
use a couple of Melf's in series (10-30 MOhm range)
Optionally:
1) A crowbar circuit, actuated with a timer, to blow the fuse in the =
event of persistant over-voltage fault at the input
2) A discharge circuit, to discharge the electrolytics in the event that =
the card is pulled out of the back-plane.
3) Opto-isolated fault signalling, using a redundant card's power supply =
(if this has been provisioned in your system back-plane)
Placement and layout will play a critical role - pay attention to =
current paths while laying out the filter. Treat all signals as TNV =
(transmission network voltage). Provide 2kv isolation between any TNV =
signal and chassis ground and all small signal voltages. Relieve the =
planes below this section. Should that be unacceptable - use larger =
anti-pads on vias in this region to provide the necessary isolation. As =
a "cheat measure", use liberal conformal coating before the system is =
despatched for any regulatory testing!
Hope this helps.
Regards,
Dev
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of David Shapiro
Sent: Wednesday, December 31, 2003 1:04 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] 48V filtering
Dear All:
=20
I was requested to build telecom standard 48V entrance filter (coming =
from
plug to backplane).
Can anyone recommend what standards are involved and some reference =
design
if such exist?
=20
Thanks.
=20
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