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

From: Bob Lewandowski ([email protected])
Date: Wed Dec 06 2000 - 17:37:47 PST

I may have missed something in this earlier, but you said you have fuses on the
board. I didn't catch what the resonant frequency was either, so this is a real
hip shot.... Is it possible that the fuses and/or the connectors are the
inductance in your problem and the board planes are the capacitance. A
conventional cartridge type fuse with an on board holder is probably 10's of
nanohenrys (let's try 30nH), and depending on you plane spacings the capacitance
is probably in the 10's of nanofarads (let's try 10 mil plane spacing, 2 layers,
12" x 15", and FR-4, ~20 nF). That would yield a resonant frequency of about 6
or 7 MHz. X would be about 1 ohm and if Rs is less than 1 the Q is greater than
1.

If your frequency is significantly different, try measuring the capacitance of
the power plane structure, and then try to determine what inductance is
necessary to resonate at the observed frequency. Then look for something in the
system that is about that inductance. If you know the cap value, the frequency,
and the approx. Q from the damping rate you can start trying to add series R at
that frequency with some ferrites. Try replacing your fuse with a piece of wire
twice as long and see what happens to the resonant frequency, or add cap on the
board across the planes. Knowing one number in the formula gives you ideas
about the scale of the other number. Making value changes and observing
corresponding frequency changes verifies the theory.

---Bob Lewandowski
Vixel Corp.

Dennis Schmitz wrote:

> 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)" <[email protected]>
> To: "'Dennis Schmitz'" <[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)" <[email protected]com>
> > To: "si-list" <[email protected]>
> > Cc: "'Dennis Schmitz'" <[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> . 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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>

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