> From:         steve@TLA.CO.NZ[SMTP:steve@TLA.CO.NZ]
> Sent:         Thursday, June 24, 2004 5:55 PM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      [EE:] Current sharing resistors

> Without going down the whole reverse engineering, desgn calculation
> path, does someone have a simple, rule of thumb value for emitter
> resistors to promote current sharing in paralleled bipolar transistors ?

. . .
> Thanks,

> Steve.

If two bipolar transistors had exactly the same base-emitter drop
while conducting the same collector currents, they could be connected
in parallel without doing anything. This will never happen and that is
why emitter resistors are needed. The rule of thumb is to equalize
the voltages between the common bases and the bottoms of the
paralleled emitter resistors while both transistors are conducting the
same current. Each of these voltages is made up of the Vbe part and
the I*R part. The Vbe part is intrinsic to the transistors; the I*R part
is controlled by choosing the resistor. The former drop is unpredictable,
so the I*R part is chosen to swamp it. That way, changes in the
base-emitter voltage become only a small part of the total voltage. In
practice, power losses limit the maximum resistor size, so aim
for a minimum I*R drop equal to Vbe, or I*R = 0.6 or R = 0.6 / I.
The more you increase R, the more equally the current will divide
and the less efficient the circuit will be.

John Power

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