Re: 2N3055 failure (power supply)

In article <4596d350$0$13301$ecde5a14@xxxxxxxxxxxxxxxx>,
Paul E. Schoen <pstech@xxxxxxxxx> wrote:
[snip]
So, even without negative feedback current sensing, there is
adequate sharing at levels where it counts, and the largest
device does finally take its proper share of the load. Also, the
positive temperature coefficient of device resistance even above
saturation will provide even better sharing as devices heat up.

I had a serious rant here a few years ago about the
way in which manufacturers of VMOS devices perpetuated
the myth that "MOSFETs allow easy paralleling".

In fact nothing could be further from the truth.

For the VMOS, at fixed Vgs, dId/dT is positive. This
means that if any one MOSFET in a bank runs slightly
warmer it will take more current, getting hotter, so
taking more current..... and so on. Thermal runaway.

Look at Fig.3 of the IRF1405. At Vgs= 4.5V, Id is
4.2A at Tj=25C, rising to 28A at 175C. That's roughly
+0.15A/C. For the IRF1405 dI/dT does not approach
zero (and go negative) until Id is about 180A.

Hitachi realised this problem 20-odd years ago and
they produced power MOSFETs where dI/dT went negative
at only about 100mA. ISTR they were called lateral
or long channel MOSFETs, an entirely different process
to the VMOS and it's derivatives. The negative dI/dT
did allow easy paralleling and those Hitachi MOSFETs
were widely used in successful high power audio amps.

--
Tony Williams.
.