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April 2001

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Subject:
From:
Brian Ellis <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Thu, 26 Apr 2001 18:53:37 +0300
Content-Type:
text/plain
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text/plain (98 lines)
By coincidence, someone asked me a similar question just a couple of
days ago. I copy below my reply with his original e-mail below it:

Subject:
            Re: Can u please help me?
       Date:
            Mon, 23 Apr 2001 18:51:53 +0300
      From:
            Brian Ellis <[log in to unmask]>
        To:
            <snipped>
 References:
            1




<snip>

Unless I'm mistaken with my mental arithmetic, your mean current will be
about 60 mA. In view of the high thermal mass of the substrate,
equilibrium would be reached as if this were DC, in terms of the thermal
effect. It would therefore be theoretically OK to go down to a track
width of about 20 um for a temperature rise of c. 50°C (if you could
find someone to make it!!!).

This leaves four other factors: the dynamic IR drop. You would need to
make sure that you did not lose too many volts for your app, if the
conductors are long. It may help you to design your PCBs with the Vutrax
CAD system ( http://www.vutrax.co.uk ) which has a facility for
calculating the resistance of any net or track. The next one is the L/C
effect. If your pulses have a very sharp leading and/or trailing edge,
the L and C of the track may resonate, giving a ringing effect (undamped
high-pass filter effect), or it may cause the pulse edges to be rounded
off (damped low-pass filter effect. This will depend on many parameters.
The next problem may be cross-talk. With an obviously high dI/dt, there
may be considerable crosstalk to neighbouring tracks which may cause
undesirable effects. Lastly, there is EMC. If your tracks radiate, you
may need to take special measures to reduce the susceptibility (Vutrax
will also give an indication of sensitive EM radiation tracks, without
actually giving figures.). You may wish to consider tracks with a fixed
characteristic impedance to avoid these effects, if it is possible.

Brian
<snip> wrote:
>
> Dear Sir/Madame,
>                         I was wondering if you would be kind enough to give me a rough idea
of
> what the track width should be for the following specs:
>         100 microsecond pulses that are pulsed at 200Hz at 3Amps!!!
>
> THe mean current is obviously very low but I was a bit concerned about the
> "high" current. Would a 10 thou track width be sufficient?
>
> I need to pack as many tracks as possible in a confined area and thus need
> to find the minimum track width possible. Thanking you !!

I hope this helps

Brian
>

"May William D (Dean) CNIN" wrote:
>
> IPC-D-275 has information giving temperature rise of conductors, external
> and internal, for different constant current values. Is there any
> information that exists for determining the same information for pulsed
> currents? I would surmise that the conductor widths and weights would
> decrease from the DC values for the same temperature rises?
>
> Wm. Dean May
> NSWC Crane
> Bldg. 38, Code 8086
> 300 Highway 361
> Crane, IN 47522
> 812-854-3073
>
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