20 Jan
2010
20 Jan
'10
10:43 a.m.
If one looks at high-current regulators they have
multiple pass transistors
connected 'almost' in parallel, the catch is a small resistor is placed in the
emitter lead of each so the CE & BE circuits take on some linear
characteristics and distributes the current between the transistors. The
regulation sense point is then placed after all the resistors join together for
the output, so the resistors are inside the feedback loop and hence accounted
for in the regulation.
I've seen at least one PSU where one of those eqalisation resistors was
also used as the current sense resistor (the circuit assumed the load
current was shared approximatley equally between the pass transistors).
It looks neat on paper, but the problem comes if the pass transsitor
feeding the resistor also used for sensing opens. Then the circuit thinks
the PSU is delivering no current, and won't shut down even if the load is
shorted. Not what I would call a good design.
-tony