I've specified a FET in a product because it worked well and 
the price was good.
I've now been informed that the quoted price was wrong and 
that the actual price is about double. This can be accepted 
if needs be, but a cheaper device would be good and should 
be available. Finding it is proving harder than I expected.

I think the Chinese suppliers are just trying to say that 
they are not interested as their 10k price is slightly 
above Digikeys somewhat lower volume price.

I think the best solution will probably be to use the part I 
originally specified and then look for better pricing.

Specification is moderately modest apart from one parameter.
Power supply is 3V to 4V with gate drive about 2V nominal. 
More can be had if utterly necessary by driver redesign.

N Channel.
25V +
1A+
Rdson 0.1 ohm genuine at 1A. *
Vgs on <= 2V at Id=1A and Rdson <= 0.1 ohm.
Price - low. Under $US0.20. Lower better.
Package - TO92 or SMD, not SO pkgs.
Volume - notionally 10,000 batches to start.
100,000+ or ++ pa.

Actual peak current is probably closer to 500 mA but the 1A 
spec gives some margin.

The catch is the Vgs. Zetex make parts that meet this spec 
but the price is generally 'rather too high'.

I had specified the superb Philips PHT8N06LT.
55V, 7.5A, 1.8W, 0.08r Rdson nom, SOT223.

Vgs(to) = 1.0 - 2.0 volts.
While it is 'sailing close to the wind' it is still within 
formal specs worst case if slightly more than 2V gate drive 
is available. The saving grace is that it is operated at far 
below it's max Id and at such low currents very low gate 
voltages are tolerable.

Alternatives:

The FDN337N mentioned here recently is a very nice part but 
its Vgs actual performance is somewhat inferior. It works 
but you can easily see the performance difference. $US0.12 
Digikey.

Lower Vds parts (eg 20V) have lower Vgs requirements. Alas, 
my maximum Vds is just over 20 volts so I can't risk a 20V 
part - even though it would probably survive.




        Russell







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