Jack:
Whoever told you this misunderstood the formula. A "fudge factor" is
applied to the formula for inner or outer layers. It is .048 for
outers, and .024 for inners, (which is indeed 50%), but the way it is
applied in the formula does not work that way. (this may not work for
all)
( ( max current ) )
( ( ------------ ) )
( ( fudge factor ) ) 1
( ------------------- ) ^ ----
( (temp rise)^.44 ) .725
Trace Width = -----------------------------------
copper thickness
And, yes, you need to additionally derate when there are a number of
heat sources (traces) in close proximity.
--
George Patrick
Tektronix, Inc.
Central Engineering, EDS Applications Support
P.O. Box 500, M/S 39-512
Beaverton, OR 97077-0001
* 503-627-5272 (voice) * 503-627-5587 (fax)
http://www.tektronix.com http://www.pcb-designer.com
"Off-Grid and Proud of it!"
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of Jack
Olson
Sent: Wednesday, May 09, 2007 08:30
To: [log in to unmask]
Subject: [DC] Current vs. Temperature (IPC vs. PCBTEMP)
I have a situation where I need to run several parallel traces,
all carrying 5amps simultaneously.
They are run side by side in batches of 3-4 (20 in all)
I remember hearing that the IPC chart tested external traces
and doubled the requirement for internal, so if I use a 40mil
trace for 5amps (2oz), an 80mil would suffice for internal.
I've since heard (from Jouppi?) that this internal number
is over-rated. Is that true?
If I use Doug Brook's PCBTEMP calculator (http://ultracad.com)
and type in my 2oz, 40mil, external trace I get a 16 degree rise,
which is fine. but when I switch to INTERNAL, with 80mil I get
a 29 degree rise. not acceptable.
SO MY QUESTION IS:
If the internal IPC chart was over-rated, why am I getting a
higher temp in the PCBTEMP calculator for internals?
I must have heard it wrong about the IPC charts?
Also, for any of you working with power out there, I'm concerned
that these calculators are thinking of a SINGLE trace around
a bunch of normal traces. How much different are my results
going to be if I am running LOTS of 5amp traces side by side?
I don't have access to ThermalMan or HavardThermal (or any
other tools except PCBTEMP)
hot and bothered,
Jack
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