3904/3906 tranny specs

[Rene Schmitz]

At 23:31 05.01.99 +0100, you wrote:

>No, not really...
>But I've experienced another nasty thing about trannies as a switch.
>The transistor was used in a voltage-shifter circuit and I had to connect a
>square-generator (about 1Hz) to it.
>I started the generator and left it alone for about half an hour.
>When I came back the tranny was broken.
>When I compared load-line and transistor-curve (ofcourse after the usual
>cursing and wishing all trannies to hell :-))it seems to be that the
>Vbe-capacitor (specific to every tranny) was the cause.
>The transistor has switched to slow (the cap slowed it down) between on and
>off-state, became hot and broke down...
>
>Never got this problem before...
>
>Strange huh, any other explainations?

This problem might also lie in the reduced safe operation area of transistors,
that is you can't rely on the maximum power dissipation alone, and there is
a further restriction on the maximum current for a certain power. 

I doubt that you blew the tranny because of the miller cap, since even if
you draw substantial power for a few microseconds there is still almost a
second of no power dissipation.

One cure for such problems is to use small signal mosfets in such
applications, they come in TTL /CMOS compatible flavors (with thresholds of
1.5V or so),
and they reduce your driving circuitry since you can ommit the base
resistor, and these are free from second breakdown and reduced safe
operation area. 
(But watch out for problems arising out of the gate capacitance.)

Rene


             ,   : (uzs159 at uni-bonn.de)
|") [" |\ | ["   : (http://www.uni-bonn.de/~uzs159)
|"\ [_ | \| [_   : (http://members.xoom.com/Rene_Schmitz)