RE: [SI-LIST] : hot plug in

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From: Mendelsohn, Joseph P (Joseph) (jmendelsohn@lucent.com)
Date: Mon Mar 12 2001 - 08:36:03 PST


Sounds familiar. By any stretch of the imagination is the soft start
located after the LC filtering or before the LC filtering? If the LC filter
is before the soft-start, you are probably dealing with ringing on the 48V
caused by the inrush of the capacitors(pretty common). What tends to happen
is other packs will see disturbance on the 48V especially if there are no
bypass caps physically mounted on the backplane. When this is the case, the
inrush current must come from the other packs with LC filters. This can
cause the Undervoltage/Overvoltage to trip shutting down the SMPS module as
the L's kick the voltage during the discharge event.

Your comment "1" needs some clarification. Did you remove the pins from the
backplane during this test? Where did you apply power, before or after the
softstart/LC filter?

If this is the case, you need to either move your LC filter after the
softstart, or add an inrush current limiter before the LC filter. The less
expensive way is to use the softstart as the inrush current limiter.

Joseph

-----Original Message-----
From: Krishnakumar [mailto:kkps@rapid5.com]
Sent: Monday, March 12, 2001 11:13 AM
To: si-list@silab.eng.sun.com
Subject: [SI-LIST] : hot plug in

i am having this problem, can anybody help?

we have a new system with a new type of backplane. this backplane is a
virtual back plane - it is made up of strips - one strip for each card.
power distribution is through cables. we distribute -48V, -48V RETURN, and
Digital GND through cables. all cards get frame ground through the screws.

our cards have 48V to 5/3.3V DC to DC converters. we have soft start
circuitry on the cards, and have good (LC) filters on -48V. i believe we
have good decoupling on 5V and 3.3V. we have stacked pins for power pins.
cards get Frame GND first, then Digital GND, then -48V return and then -48V.

now this is the problem. i get data errors on one card when i plug in
another card.

i made the following observations:

1. the error seems to reduce if i apply power to the board using a banana
jack rather than plugging in the board.
2. adding inductors on the Digital Ground lead does not help. (we already
have inductors on -48V and -48V return)
3. scope readings show big ringing on almost all signals - N48V Return,
Digital GND, 3.3V, 5V. there is no ringing on N48V as such(surprise to me).
but i donot believe in the scope readings. scope shows ringing on Digital
GND with respect to Digital ground itself. Looks like it is measuring noise
on some big ground loop. (i am using a scope with the Ground pin removed
from the power cord.) the amplitude of ringing, for ex., on 3.3V, is 7V to
0V, dying down in about 100-500us, the ringing frequency is 50MHz. should my
first attempt be to kill this ringing?

can anybody suggest some thing to help me out?

thanks
bye

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