From: Douglas C. Smith ([email protected])
Date: Mon Mar 12 2001 - 11:23:06 PST
First: You are likely experiencing EFT, electrical fast transient,
bursts on the 48V. The likely level is up to 1000V peak on the 48 volt
bus. Try connecting an IEC 61000-4-4 generator set to 1000V through HV
decoupling caps to the 48 Volt bus. I will bet that will recreate the
problem. This is a common problem in 48V hot plug systems.
Second: normal scope measurements are completely unreliable in this kind
of environment. There are no commercial probes available that will work.
There are, however, many good ways to measure and fix the problem, but
are quite dependent on the system layout and beyond what I can write in
a short time here. Feel free to call me and I will see if I can give you
some suggestions. Some of these techniques are written up on my website,
http://www.dsmith.org , in a general fashion.
If you can get the system to pass with 1000V EFT on the 48 Volt bus, you
will likely have reliable operation with margin for the 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?
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