Hiya Jack-
What do I do?
The way I try to think about it is... if one trace is going to be running
15 degrees above ambient, then the trace right next to it has got a "new"
ambient somewhere between the original ambient and original+15. So yeah,
it's a factor. But I've got no scientific method to offer. Size 'em all
for a 5 degree rise and then start whittling if you have to....
In my experience, if it's a shortish span with some mass on both ends you
can cheat the charts quite a bit. If you can fatten or plane them
somewhere along the way (copper heatsink), it helps for temp rise, but
maybe not a good option for other reasons...
Always on vs. sometimes on is, of course, another thing to consider.
We've had some interesting arguments here through the years about just
what and when is 'ambient'. Wish you'd have been there; I'm sure you could
have made it even more interesting.
Carry on,
-Chris
Jack Olson <[log in to unmask]>
Sent by: DesignerCouncil <[log in to unmask]>
05/09/2007 11:30 AM
Please respond to
"(Designers Council Forum)" <[log in to unmask]>; Please respond to
Jack Olson <[log in to unmask]>
To
[log in to unmask]
cc
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
---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free service by IPC using LISTSERV
1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
To temporarily stop/(restart) delivery of DesignerCouncil send: SET
DesignerCouncil NOMAIL/(MAIL)
Search previous postings at: www.ipc.org > On-Line Resources & Databases >
E-mail Archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16
for additional information, or contact Keach Sasamori at [log in to unmask] or
847-615-7100 ext.2815
---------------------------------------------------------------------------------
"This e-mail message is intended only for the use of the intended
recipient(s).
The information contained therein may be confidential or privileged, and
its disclosure or reproduction is strictly prohibited.
If you are not the intended recipient, please return it immediately to its
sender at the above address and destroy it."
---------------------------------------------------------------------------------
DesignerCouncil Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF DesignerCouncil.
To temporarily stop/(restart) delivery of DesignerCouncil send: SET DesignerCouncil NOMAIL/(MAIL)
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
---------------------------------------------------------------------------------
|
