piclist 2002\09\27\232613a >
Thread: 2-tran SMPS 5v reg now 90% efficicient!
face BY : Russell McMahon email (remove spam text)

> Transistors in TO92 tend to go bad when dissipating more than 0.5W or so
> (even that is a lot).

It is of course not ideal to run anything at it's maximum rated dissipation,
but note that these Zetex transistors are NOT TO92 - they just "look like"
TO92. They are Zetex's proprietary E-line package which they rate at 1.2w
unheatsunk dissipation. They seem to know what they are doing. They are a
thinner package than TO92 and I believe they do special things with the
leadframe to ensure minimum thermal resistance.

Looking at the specs of E-Line parts shows something interesting - the
Max_current x Vsat product is usually approximately 1.2 watts. ie they seem
to current rate their parts based on maximum allowable dissipation. They
also frequently publish a peak current rating which is several times the
continuous current rating. While the E-Line package is not intended to have
a heatsink added, this suggests that if it WAS heat-sunk then the current
ratings could probably be increased - probably at least to the peak current.
This of course would take the parts out of spec and anything could happen
but it would be interesting to see what results could be obtained. As
Rth-junction_case is usually lower than Rth-case_ambient for small parts
this suggests a well designed heatsink may increase allowable dissipation
several times.

Zetex produce parts virtually without equal from other sources. Typically
several amps with betas of 100+ and very low Vsats in TO92 sized packages.
The price is probably the only disincentive but even that's often acceptable
when the other factors are considered. For those who wonder wjere Zetex came
from, they used to be "Newmarket" in the very early days of the
semoconductor revolution. For those who haven't heard of Newmarket .....

       Russell McMahon

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