Mr. Lomax is quite the sleuth! Yes, it's the CPC5602. What would be much
harder to find is the heat-sink discussion, as it is within the datasheet
for a different part -- the CPC5610/5611.
(www.cpclare.com/home/PDFs.nsf/www/cpc5610_r11_31.pdf/$File/cpc5610_r11_31.p
df (whew!)) The rather brief note (on page 19) says:
"The CPC5602C is a ...[FET with] a typical on-resistance of 8 ohms, a
breakdown voltage exceeding 350V and is designed in a SOT-223 package. In
some current limiting conditions, the FET is required to dissipate almost 2
watts of power. The FET must have a sufficient heat sink to handle this
thermal load. If the heat sink is integrated into the PCB layout, a heat
sink may be designed with: 4 PCB layers, a heat sink area of 1.5 cm x 0.4 cm
and all 4 layers connected by 20 plated through holes. This provides a total
area of 5.4 sq. cm. This is sufficient for the expected heat dissipation of
the FET during operation under current limit."
I couldn't quite come up with 5.4 sq. cm from their measurements, unless you
count BOTH sides of EACH layer, and count the hole plating as two-sided
also! Anyway, my pads are about 0.8 sq cm (per layer), and their spec is
0.6 sq cm, so I feel pretty safe.
I had already set the connections to direct, not thermal relief. I'm
planning to go with Mr. Lomax's suggestion to not connect to the inner
planes, so I can avoid the whole split-plane issue. This will also make
maintenance of the design a lot easier (this is the first board spin of this
design, with another turn or two quite likely).
Thanks again!
Dwight Harm
Trax Softworks, Inc.
> -----Original Message-----
> From: Abd ul-Rahman Lomax [mailto:[EMAIL PROTECTED]]
> Sent: Sunday, May 20, 2001 5:07 PM
<snip>
> This got my curiosity up, so I tracked down the part which Mr. Harm is
> using. There is only one SOT-223 part made by CP Clare, if I am correct,
> the CPC5602 DAA. I read the app notes, which did not mention heat sinking
> of the part as far as I could find. There is a reference design, available
> as gerber. I downloaded the files and loaded them....
<snip>
> Mr. Harm, you should increase the heat sink areas if you can. Use copper
> pour on the top and bottom to do this, and set the vias for direct
connect.
> This, by lowering thermal resistance between the heat sink areas and the
> ambient air, will do much more to improve thermal performance than would
> any possible effect from inner plane copper. Allow the unbroken normal
> inner planes to be under the part because they may indeed help with heat
> dissipation; they will carry heat out from under the part to the rest of
> the board. Use 1 ounce copper on the inner planes if possible. It is the
> split plane that will not help, because it does not increase thermal
transfer area.
<snip>
> [EMAIL PROTECTED]
> Abdulrahman Lomax
> P.O. Box 690
> El Verano, CA 95433
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