Hi,
today i was looking at the schematic for the hp e3631 psu, and one thing puzzled me. (http://circuitslab.case.edu/manuals/Agilent_E3631_Power_Supply_Service_Guide.pdf , page 130).
What is the benefit ( if any) of having the pass-transistor (pnp) on the low side, instead on the high side.
Thanks in advance.
BR Jonas
It usually comes down to the ability to have less voltage drop from the unregulated side to the regulated side. With an NPN pass device, the unregulated side has to be at least 1 Vbe above the regulated side (and usually a bit more) so that the regulation circuitry has enough headroom to adjust the base voltage above the emitter which is sitting on the regulated output rail. With a PNP pass device, the control voltage on the base is 1 Vbe below the emitter, which is sitting on the unregulated side, so there's no headroom issue at all. Plus, the PNP can be run close to saturation in the worst case, provided the regulator circuit can run enough current through the base to keep the regulation happy. Thus, with less Vce on the pass device, the regulator can will be more efficient and can tolerate less of a regulator dropout voltage (voltage across the pass device).
The downside is that the PNP pass device is essentially a high impedance output (current source), and the regulator needs to be able to respond cleanly to load changes, which places more difficult requirements on the regulator feedback loop. With an NPN, the output is taken from an emitter follower, which is a low impedance, making it less sensitive to load changes.