Ping Terry Given. PCB trace widths on motor drive.

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I know you do this shit terry!

I have just done the 'correct' calculations for the trace widths on a
motor drive i am designing. I have gone for worst case of 15A of
supply to the bridge @ 400Vdc. Realistically its probably only going
to be around 10A. So using 1oz copper, ambient 25degC and 10degC
derating i am coming up with a trace width of ~560mils with a
clearance of 103mils. Does this sound right? The connecter I am using
is 0.2" spacing so there is no way i can squeeze in a 560mil track
with 103mil spacing?


So if I am correct, how the fuck do you guys get a 560mil trace into a
FET? I am using one of those IRAMS beast from International Rectifier,
and the spacing between the input and output pins is only ~0.025" I
assume a certain degree of taper is permitted on the trace, but where
do you find this kind of info?


Further more, IIRC there is some standards (IPC?) regarding clearances
for mains rated gear, and IIRC does this not vary for different
classes of equipment?

As you can guess, power electronics is relatively new to me.

BTW: on my proto, a 160mil track of 1oz copper worked a treat, but the
biggest motor I have is about .4kW so i am probably only drawing
around 2A. Not even close to 10A!! I'm going to have to find me a
bigger motor.

Andy

Re: Ping Terry Given. PCB trace widths on motor drive.


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Yep, that sounds about right for the traditional calculations based on
the original IPC "warm and fuzzy" graphs. These "correct" calculations
are based on old and incomplete empirical data, so take'em with a grain
of salt.
In relality you can often get away with less than this for track width,
but it depends on your application and what you can tolerate in track
losses.
You can get your tracks tin plated to decrease the resistance, that
helps a *lot*.
If however you are using solder mask over bare copper then you have to
be more careful. Specifying 2oz copper is a wise move and common
practice for power stuff.

Personally I'd use more than 103mil for the spacing, just to be on the
safe side.

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You can't really find that kind of info, it's just "do the best you
can".
Using a thicker copper, tin plating, and using parallel traces on both
sides of the PCB (even multi layers too) are the tricks used, and keep
your necking as short as possible.

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IPC-2221 has general clearances listed, but not for equipment classes.
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always use a lot more than that if you can.

Hope that helps.

Dave :)


Re: Ping Terry Given. PCB trace widths on motor drive.



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Cheers for that dave. I just need someone to double check my maths. I
am using a fairly old copy (circa 5 years ago) copy of IPC standards,
and half the books are missing :)

I guess the next challeng is how to maximise clearance when there is
no room.

BTW: Roughly, what sort of percentage cost increase would one expect
these days by moving from 1oz to 2oz copper?

Re: Ping Terry Given. PCB trace widths on motor drive.



The Real Andy wrote:
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Yep, your calcs work out perfectly based on the IPC data.

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In this case you can use a routed slot between the tracks, that helps a
lot.
Although if the routing slot is small you might pay a premium for it
from the manufacturer. They complain they break lots of bits this way,
like it's *our* fault ;-)

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Not much, around 5-10% is a ballpark figure. Does vary a lot between
manufacturers of course. If you want high volume it pays to shop
around.

Dave :)


Re: Ping Terry Given. PCB trace widths on motor drive.


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Hi Guys,

I cant post to ABSE as my ISP doesnt support it :(

but I do have a PDF of the relevant graphs. email me, and I'll send it back.

0.56" is slightly off the curve, but comes in at around 12A for a 10K
dT. 0.2" will have a 20K dT at a bit over 10A.

as far as necking down to pins etc - think in terms of heatsinking. a
very short & skinny trace can carry a substantial current as long as
both ends are well heat-sunk, ie connected to a ruddy great lump of Cu.


0.103" is a bit small for creepage and clearance. the amount required is
a function of what it achieves, and the standards you are required to
meet. Most standards dont actually care about "working" insulation, only
safety stuff - IOW they dont care if the gear blows up. This also
explains how one can use T0-220 parts with legs very, very close together.

I'd go for >= 3.2mm if I could. think condensation etc.

Cheers
Terry

Re: Ping Terry Given. PCB trace widths on motor drive.



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Cheers for that. My email is a dot pearson at tpg dot com dot au

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