Suppose the regulator can source 1 amp by itself. This implies that your series-pass transistor will have to source the rest. This usually means that, at approximately 1 amp, you want the transistor to "turn on". This is usually done via a resistor at the base pin that drops 0.7V when the current reaches 1 amp. In this image, the transistor is "on" at approximately 0.7v/3 ohm = 230 mA or so. Note also that the voltage at the base is at a lower potential than the emitter because of the PNP transistor used.Say I were to want 2A to 3A output, does that mean Iq = 2A/3A?
How do I determine Ireg? Where would I get that?
OH! Missed that. I thought they were illustrating something else the way it was written.IQ1 is the transistor collector current. It is shown a littrle too low down on the schematic.
Alternatively, start with a lower input voltage such that the difference between the output voltage and input voltage is minimal; this will prevent the transistor from dissipating prohibitive amounts of powerHave you looked into switching power supply options?
For the current, you'll probably be looking at an external MOSFET switch, and the inductor won't be very cheap, but it would get your supplies stable with the current required without 100 Watts of heat to dissipate from the power supply alone.
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by Luke James