Three basic questions about linear regulators (DC power supply):
1 - The op amp controlling the pass transistor gets at the inputs, the reference voltage (user control) and a sample of the output voltage:
a) In a closed loop, it controls, in a continuous way, the pass transistor driving voltage. From all what I've read so far, a quite common topology.
b) As a comparator in an open loop , it controls, in an on/off way, the pass transistor driving voltage. Quite uncommon, at least for me. I run across such a topology in an article from ELEKTOR (Spanish version - No. 260 and/or 261, if i recall properly).
Question 1: The first seems more appropriate for a smooth more ripple free control of the output. Am I right? What are the advantages of each topology?
-------------------------------------------------------------------------------------------
2 - Question 2: is it imperative for the positive rail pass transistor to be an NPN type (and PNP for the negative rail)?
Before posting I made a quick search and I couldn't find examples of the opposite but I think I saw some somewhere.
If both types may be used, which should I prefer and why?
--------------------------------------------------------------------------------------------------------------------------------------------------
3 - Sensing the current may be done at four places:
a) between the rectifier and the pass transistor.
b) between the pass transistor and the load
c) between the "common" (output) and the "common" of the regulator circuit itself. ELEKTOR issue above, again.
d) in the ground "rail" which I've already discarded for a dual supply.
I've implemented in the past, a) and b).
Raw voltage in a) could demands high voltage components but voltage variations are much more limited, while b) gives chances to be in more comfortable voltages.
Intrinsecally by design, in c) the load is always riding some mV above the regulator's common due to the sensing resistor and affects the actual voltage feedback to the opamp in charge..
Question 3: which is best? Besides the drawbacks I've mentioned, is there anything else to account for?
----------------------------------------------------------------------------------------------------------------
Sorry for the long posting. I would like to reinvent my wheel at least once!
Gracias for enlightening replies.
Agustín Tomás
1 - The op amp controlling the pass transistor gets at the inputs, the reference voltage (user control) and a sample of the output voltage:
a) In a closed loop, it controls, in a continuous way, the pass transistor driving voltage. From all what I've read so far, a quite common topology.
b) As a comparator in an open loop , it controls, in an on/off way, the pass transistor driving voltage. Quite uncommon, at least for me. I run across such a topology in an article from ELEKTOR (Spanish version - No. 260 and/or 261, if i recall properly).
Question 1: The first seems more appropriate for a smooth more ripple free control of the output. Am I right? What are the advantages of each topology?
-------------------------------------------------------------------------------------------
2 - Question 2: is it imperative for the positive rail pass transistor to be an NPN type (and PNP for the negative rail)?
Before posting I made a quick search and I couldn't find examples of the opposite but I think I saw some somewhere.
If both types may be used, which should I prefer and why?
--------------------------------------------------------------------------------------------------------------------------------------------------
3 - Sensing the current may be done at four places:
a) between the rectifier and the pass transistor.
b) between the pass transistor and the load
c) between the "common" (output) and the "common" of the regulator circuit itself. ELEKTOR issue above, again.
d) in the ground "rail" which I've already discarded for a dual supply.
I've implemented in the past, a) and b).
Raw voltage in a) could demands high voltage components but voltage variations are much more limited, while b) gives chances to be in more comfortable voltages.
Intrinsecally by design, in c) the load is always riding some mV above the regulator's common due to the sensing resistor and affects the actual voltage feedback to the opamp in charge..
Question 3: which is best? Besides the drawbacks I've mentioned, is there anything else to account for?
----------------------------------------------------------------------------------------------------------------
Sorry for the long posting. I would like to reinvent my wheel at least once!
Gracias for enlightening replies.
Agustín Tomás