Peter L. Peres wrote:
>
> Imho only the FET qualifies (wow at what price) for a 3A supply, and I say
> that without looking at max. dissipation which you did not list.
> Transistors in TO92 tend to go bad when dissipating more than 0.5W or so
> (even that is a lot).

Some very good points Peter, and i'm one of the
crowd who *really believe* in using parts much
tougher than they need to be, remember the years
I spent in TV and VCR repair... :o)

BUT, in this case the 3A, 4A etc ratings are the
official safe AVERAGE current, 10A or 15A etc are
the acceptable peak currents.

The 3A switcher is ON for approx 40% of the time,
at 3A. The average Pon is 1.2A, and add a safe 20% for
switching slope. Never more than about 1.5A average.
Peak current is of course 3A.

Now these transistors are NOT the normal T092 things,
they are advertised as special low-sat high performance
devices suitable for switching supplies.

So I have no qualms about using a special switcher
marked as a 3A average 10A peak device in a 1.5A
average 3A peak application. :o)

And YES I was probably going to heatsink them too,
it doesn't take much size or expense to sink a TO92
sized pack.

>  According to your efficiency figures a 3A/5V supply
> will have 15W out and burn 1.5W on the circuit (assuming efficiency stays
> as good with the higher current, which I doubt). Even if only 1/3 goes to
> the TO92 it will try to melt.

Actually my 3A version is running very well at 18W in,
15W out, 3W wasted. 83%. This is with a clunky old
DARLINGTON PNP, which I have properly tested at 0.95v
Vce sat @ 3A. At 40% on time that is 1.14W, and changing
to the Zetex part at 0.27v 3A sat will drop that 1.14W
back to 0.324W, saving almost a watt.

There's not a lot I can do with the buck diode, the one
I have now is 0.36v @ 3A x 60% which is 0.648W and
pretty good really. Shopping for super diodes shows
very few that perform better than this.

After the 1.14W lost on the darlington and 0.648W on the
buck diode there is a total 1.2W lost mainly on the
inductor, which at 3A is about 0.133 ohms and about what
my inductor measures. I haven't had much luck finding
a 3A inductor less than 0.1 ohms yet either! :o)

> *>ZTX951, PNP, 60v, 4A av, 15A pk,
> *>0.3V Vce @ 4A, beta=100 (TO92 flat)
> *>$2.57 (100 1.32) (price in AUD)

That's $1.41 (100 = 0.73) US price.
:o)
-Roman

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