[R-390] OT: Pass Transistor Question

[2002tii]

Bob wrote:

>The equalizing resistor use is the result of broader mfr tolerance 
>in days of yore.  You pointed out a reference to a 1978 
>handbook.  Remember in that time when we built ridiculous bridge 
>rectifiers for HB Amplifiers?  Same philosophy!
>
>If you are using more modern components, don't need the 
>equalizing!  If using "OLD" saved parts, equalize!

I have to disagree with you there.  In the base conducting region 
(i.e., with the transistor's base-emitter voltage above its threshold 
voltage), a transistor's transconductance is very high -- the 
collector-emitter current changes a lot for a very small (millivolts) 
change in base-emitter voltage.  Because of the variability of the 
fabrication process, the threshold voltages of individual transistors 
exhibit enough variability (tens to several hundreds of millivolts) 
that it is virtually impossible to match them accurately enough that 
they will share current equally.  Picking two 2N3055s of the same 
production lot at random almost guarantees that the current between 
un-ballasted transistors operated in parallel will be different by at 
least a factor of 10 (delivering 5.5 amps total, one transistor will 
carry 5 amps and the other will carry only 0.5 amps).  More likely, 
the factor will be closer to 100.  Even matching them carefully by 
sorting through hundreds of transistors, you are unlikely to do 
better than 2:1 without ballasting.  Worse, the threshold voltage of 
a bipolar transistor decreases with temperature, so the transistor 
that starts off carrying more current ends up carrying an 
ever-increasing share of the total current because it runs 
increasingly warmer.

Running bipolar transistors in parallel without ballasting is an 
almost certain invitation to disaster.

With lateral MOSFETS, on the other hand, the output current declines 
with temperature so they tend to self-balance.  You cannot rely on 
self-balancing in the linear (non-saturated) active region with 
vertical MOSFETS, however, and most of the power MOSFETS on the 
market today use the vertical architecture.

Best regards,

Don