Ignoramus12834 wrote:
On Sat, 26 Nov 2005 17:36:47 -0800, xray wrote:

On Sun, 27 Nov 2005 00:57:12 GMT, Ignoramus12834
wrote:


It is a AC/DC welder. Look at FETs T21 and T22, you can see that they
can be used to supply either +, or -, or +/- to the line labeled
+. Whereas line marked - is the center tapped point of the secondary.

I agree, except they appear to be bipolars, not FETs.


Could be. I thought that bipolar transistors are also field effect
transistors, that's why I said so. I am an amateur.
Bipolar Junction Transistors come in NPN and PNP types, and do not use
the field effect. They use base current to modulate (and amplify)
collector current.

Field effect transistors use the electric field between the gate and
source to modulate the resistance in the channel. Gate voltage
essentially controls drain current through this method. Generally,
gate current is zero.

Insulated Gate Bipolar Transistors are a combination of the two.
They really are a bipolar junction transistor with a FET connected
between the collector and base. They give many of the advantages
of both types. With the FET, the voltage drop across the drain-source
increases linearly with current. (It is a resistance, after all.)
With the Bipolar and IGBT, the voltage drop is almost flat with
changes in current, usually around 1.5 V. Regular bipolar junction
power transistors require a base current about 1/10th of the collector
current. With 100 A transistors, that amounts to 10 amps the control
circuit needs to deliver to the base. Darlington transistors have
at least 10 x higher gain, but the voltage drop will be double, a major
disadvantage. The IGBT solves that problem. The gate current required
is zero, once the gate capacitance has been charged.

One other pro/con thing about IGBTs is that, like other bipolars, the
forward voltage drop goes DOWN as they get hot. This effectively
prevents thermal runaway with single devices. But, it makes it a lot
harder to balance current in paralleled devices, as the hottest one
will have the lowest voltage drop, and thus get all the current.
(By comparison, the resistance of FETs goes UP with temperature,
making thermal runaway possible. But, it makes paralleling devices
a breeze, as they will self-equalize their current.)

Jon