Re: [SI-LIST] : Backplane hot-insert rings like a bell.

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From: Dennis Schmitz (den@positron.net)
Date: Wed Dec 06 2000 - 16:12:53 PST


Joseph,

I'm sorry if I'm not being clear. I'm quite confident that it's the
backplane design. A new backplane stack removed the power planes from being
adjacent to the signals, but we can still measure the effect, although it
was greatly reduced (by a factor of four). We also added ferrites to the
circuit packs before the power was routed to the board planes and that
helped immensely. I'd like to attack all the sources, though, and it seems
that we can do better by adding capacitance to the backplane.

Dennis

----- Original Message -----
From: "Mendelsohn, Joseph P (Joseph)" <jmendelsohn@lucent.com>
To: "'Dennis Schmitz'" <den@positron.net>; "si-list"
<si-list@silab.eng.sun.com>
Sent: Wednesday, December 06, 2000 5:52 PM
Subject: RE: [SI-LIST] : Backplane hot-insert rings like a bell.

> Dennis,
> Your latest issue may not be related to the Backplane design and not
the
> circuit pack design. I don't want to state the obvious but you should
check
> this. I would suspect a grounding problem in the connector field. There
> are apparently signals passing through the pin field of the backplane
> connector. I would check to ensure that there is a ground plane left
> through the pin filed. Also, be very suspicious of signals that run
> adjacent to the -48V and -48V planes. These are supposed to be isolated
> from Chassis ground(Tellecordia spec).[please lets not have a discussion
on
> the requirement].
>
>
> Joseph
>
> -----Original Message-----
> From: Dennis Schmitz [mailto:den@positron.net]
> Sent: Wednesday, December 06, 2000 3:52 PM
> To: si-list
> Subject: Re: [SI-LIST] : Capacitor dielectric type
>
>
>
> The problem was that it coupled to the signal ground plane, which wouldn't
> ordinarily be a problem except for that single-ended signals used for
static
> stuff like board insert detection were picking it up as well, causing
false
> triggers.
>
> I'm convinced we have to detune the resonance to a lower frequency so that
> the ping is broader and of a lower amplitude. Pre-charging the planes of
the
> boards being inserted (to eliminate the excitation event) also seems
> prudent, but I wouldn't want to rely on that for system integrity.
>
> I was hoping for some solid research references to bear this out.
>
>
> ----- Original Message -----
> From: Mark Gill <mailto:gillmf@nortelnetworks.com>
> To: 'Michael <mailto:mnudelman@tellium.com> Nudelman'
> Cc: 'Dennis Schmitz' <mailto:den@positron.net> ; si-list
> <mailto:si-list@silab.eng.sun.com>
> Sent: Wednesday, December 06, 2000 2:10 PM
> Subject: RE: [SI-LIST] : Capacitor dielectric type
>
>
> Hey Mike -
>
> Decoupling the backplane is not required if the boards are correctly
> filtered/decoupled. None of the systems I have worked on require decaps
on
> the backplane (reliability problem), and none of them have ever exhibited
> this type of problem. But we do have good filtering/decoupling on the -48
V
> input to each card. Smart powering (which Joseph mentions) is a slicker
> approach, and very reasonable when considering switching power systems.
But
> depending on the actual parasitics (inductances, resistances, and
> capacitances), you certainly can get high frequency ringing upon
insertion -
> try simulating with SPICE to get a feel. The toughest parasitic should be
> the inductance. There are some ferrites which offer good resistive losses
> at higher frequencies. I would still call this an inrush phenomenon.
>
> -----Original Message-----
> From: Dennis Schmitz [mailto:den@positron.net]
> Sent: Wednesday, December 06, 2000 4:07 PM
> To: si-list
> Subject: [SI-LIST] : Backplane hot-insert rings like a bell.
>
>
> The disturbance is a damped sine transient with an amplitude of 4-8 volts,
> depending on which board is inserted and where it's measured.
>
> The backplane is 48 volts. Boards use an inrush limiter which causes
smaller
> events several microseconds later, but the primary event is when an
> uncharged board is plugged in. I isolated the effect by using a bare board
> with a connector and fuses. If I discharge the board before I insert it
(by
> shorting the power leads), it causes an event when it's inserted -- if I
> merely pull it out and re-insert it, there is no event (no, I wasn't
> surprised).
>
> There are no bypass caps on the backplane or any other component other
than
> connectors. The power plane is roughly 15x12 inches, although the
backplane
> is much larger.
>
> The effect is worst when there is only one board in the system (again, I'm
> not surprised).
>
> ----- Original Message -----
> From: "Mendelsohn, Joseph P (Joseph)" < <mailto:jmendelsohn@lucent.com>
> jmendelsohn@lucent.com>
> To: "'Dennis Schmitz'" < <mailto:den@positron.net> den@positron.net>
> Sent: Wednesday, December 06, 2000 3:01 PM
> Subject: RE: [SI-LIST] : Re: Backplane Board insertion problem
>
>
> > I assume that since you can connect the power plane of the board
directly
> > that you do not use any regulators in the design. That being the case,
> you
> > are probably charging the planes in the circuit board directly. This
> > combined with the inductance of the pins and whatever filtering is on
the
> > other packs can cause a resonance. Its not a power plane resonance, but
a
> > simple damped LC resonance. If that is the case, your problem is not
new.
> > Here are some questions?
> > 1) Define Disturbance.
> > 2) Are there any bypass caps on the backplane?( Qty and Value/type)
> > 3) How large is the power plane in the backplane vs. the power plane on
> the
> > board.
> > 4) How many circuit cards are plugged into the backplane when the
problem
> > occurs.
> > 5) What is the voltage?
> >
> > Joseph
> >
> > -----Original Message-----
> > From: Dennis Schmitz [mailto:den@positron.net]
> > Sent: Wednesday, December 06, 2000 11:31 AM
> > To: Mendelsohn, Joseph P (Joseph); si-list
> > Subject: [SI-LIST] : Re: Backplane Board insertion problem
> >
> >
> > We removed all components from the power section of the board, except
for
> > the fuses. We suspect that the capacitance of the power planes on the
> board
> > itself pings the backplane causing the resonant response.
> >
> > ----- Original Message -----
> > From: "Mendelsohn, Joseph P (Joseph)" < <mailto:jmendelsohn@lucent.com>
> jmendelsohn@lucent.com>
> > To: "si-list" < <mailto:si-list@silab.eng.sun.com>
> si-list@silab.eng.sun.com>
> > Cc: "'Dennis Schmitz'" < <mailto:den@positron.net> den@positron.net>
> > Sent: Wednesday, December 06, 2000 11:14 AM
> > Subject: Backplane Board insertion problem
> >
> >
> > > I would tend to agree with Mark. You probably do not have enough
> > > capacitance in the backplane, or you do not have an inrush(soft-start)
> > > circuit. There are many ways to avoid this problem. Check out the
> > > following brief in EDN
> > > <http://www.ednmag.com/ednmag/reg/1997/121897/26di_07.htm>
> http://www.ednmag.com/ednmag/reg/1997/121897/26di_07.htm
> > > < <http://www.ednmag.com/ednmag/reg/1997/121897/26di_07.htm>
> http://www.ednmag.com/ednmag/reg/1997/121897/26di_07.htm> . This type
> of
> > > circuit or, a simple MOSFET with a set RC time constant can help.
> > >
> > > Joseph
> > >
> > > -----Original Message-----
> > > From: Mark Gill [mailto:gillmf@nortelnetworks.com]
> > > Sent: Wednesday, December 06, 2000 9:22 AM
> > > To: 'Dennis Schmitz'; si-list
> > > Subject: RE: [SI-LIST] : Capacitor dielectric type
> > >
> > >
> > > Dennis -
> > >
> > > Your board insertion problem sounds more like an in-rush current
problem
> > > than power plane resonance. Do you have further details which would
> > confirm
> > > resonance over inrush?
> > >
> > > Mark
> > >
> > > -----Original Message-----
> > > From: Dennis Schmitz [mailto:den@positron.net]
> > > Sent: Tuesday, December 05, 2000 5:38 PM
> > > To: si-list
> > > Subject: Re: [SI-LIST] : Capacitor dielectric type
> > >
> > >
> > > Can some kind person please point me to some references on power-plane
> > > resonance? I am researching for a new backplane design of an existing
> > system
> > > where we are having some problems with board insertion causing
> > disturbances
> > > on the power distribution planes.
> > >
> > > Thanks in advance.
> > >
> > >
> >
> >
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>
>
>

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