From: Mendelsohn, Joseph P (Joseph) ([email protected])
Date: Wed Dec 06 2000 - 14:52:49 PST
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:[email protected]]
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:[email protected]>
To: 'Michael <mailto:[email protected]> Nudelman'
Cc: 'Dennis Schmitz' <mailto:[email protected]> ; si-list
<mailto:[email protected]>
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:[email protected]]
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:[email protected]>
[email protected]>
To: "'Dennis Schmitz'" < <mailto:[email protected]> [email protected]>
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:[email protected]]
> 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:[email protected]>
[email protected]>
> To: "si-list" < <mailto:[email protected]>
[email protected]>
> Cc: "'Dennis Schmitz'" < <mailto:[email protected]> [email protected]>
> 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:[email protected]]
> > 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:[email protected]]
> > 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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