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Power dissipation and series sensing resistor voltage drop are both reduced by direct coupling of emitter-base circuits for regulating semiconductor elements.
English (United States)
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Foldback Current Limiting in DC Power Supplies
Power dissipation and series sensing resistor voltage drop are both reduced
by direct coupling of emitter-base circuits for regulating semiconductor elements.
Fig. 1 illustrates an ideal current regulation pattern obtained through the
circuit of Fig. 2. The normal current to output 10 remains at a relatively constant
level, as shown at 11, until a knee current Ik is reached. If the output is shorted
to a voltage below Vo, the current is correspondingly reduced. If the output is
shorted to ground, the current is reduced in the limiting case to Isc. The control
circuit 16 in conventional series regulators directly drives the base of series
regulating transistor 18 which is coupled between output 10 and power source 20. Also in conventional circuits, the base 21 of current limiting device 22 is
normally coupled through the junction of resistors 24 and 25 which are connected
to the emitter of 18 rather than to the base 26. With such conventional circuits,
the voltage across the series regulating resistor 15 must become quite high in
order to forward bias 22 when the circuit is in current limiting mode. This
requires resistor 15 to be relatively large. For an output current of several
amperes, power dissipated in 15 can become appreciable. Further, such
conventional circuits cannot maintain adequate separation between Isc and Ik.
The slope of the foldback portion of the operating curve is directly proportional to