[SI-LIST] Re: Pulse current handling capability of PCB

Traces heat by I^2*R loss. Traces cool by several means. Trace 
temperature is determined by the where the equilibrium is between the 
heating and cooling effects.

20us is so short there is effectively no cooling. In fact, there is 
negligible cooling for the first few seconds after a step-function 
pulse is applied. Studies suggest that trace equilibrium occurs (95% 
to 99%) after about 4 to 5 minutes. It gets half way to equilibrium 
in about 30 seconds. So you are looking at a fusing effect, not a 
trace current/heating effect.

UltraCAD has an article specifically on fusing effects on PCB traces 
( Fusing Current: When Traces Melt Without a Trace!) on it web site. 
Go to http://www.ultracad.com/article_outline.htm and look under the 
Trace Current/Temperature/Power/Resistance:  heading.

UltraCAD also has a calculator that specifically addresses fusing 
effects (among other things). You can find it at 
http://www.ultracad.com/calc.htm under the heading UCADPCB.exe. (This 
calculator will also take skin effect into consideration, if anyone cares!)

The calculator suggests that a 20us time frame is so short that trace 
width is not an issue (it is actually beyond the range of the 
calculator!) The calculator would suggest that for a 100us pulse and 
.5 Oz trace, the fusing width is about 8 mils. That means if you want 
the trace to FAIL (so as to trigger a controlled system shutdown 
after a catastrophic failure) you would size the trace somewhat 
narrower than this. If you want it to survive this pulse, you would 
size it somewhat larger, say 20 to 30 mils.

Doug Brooks




At 04:51 AM 9/15/2009, Deepak V.L wrote:

>Dear Experts,
>Could someone please guide me how to select a trace width for carrying a
>100A surge current for 20us over a PCB trace ?
>
>
>Thanks in advance,
>Deepak
>
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