Download 1. The simple version

Power Supplies
Objective -- to supply a constant direct voltage VL in spite of variations in the
source voltage VS (e.g. from a transformer - rectifier - smoothing
arrangement) and the load current IL. (Less commonly they are required to
supply a constant direct current).
Perhaps the simplest idea
At least it is simple! The problems are that the value of R must be high to
produce a constant output voltage, but low to allow much output current to
flow. We can, however, use the circuit as the basis of a better circuit ...
The Series Regulator
The transistor operates as an emitter follower, so the effective output
resistance is low and variation in the output current will have little effect on IL.
VL will settle at a value which will make the two op-amp inputs of equal
voltage approximately, so VZ = R2VL/(R1 + R2) .. or VL = VZ(1 + R1/R2).
This circuit performs much better than the resistor and Zener arrangement
would on its own.
Such circuits are available in integrated form .. see RS, Farnell, etc.
catalogues.
A problem with it is that an output short-circuit will blow the series transistor it is usual to provide overcurrent protection. This can take two forms:
1. The simple version
R4
Q2
Q1
IL
R
R1
R3
VS
VL
+
-
R2
The operation is as before provided that the output current is not large
enough to drop more than about 0.7 volt across R4. If it is, the base current of
Q2 is diverted through the two diodes and further rise in I Lis prevented
(well, almost !).
The foldback circuit
IL
Q1
Rm
R
R1
R1
VS
VL
+
Q2
-
R2
R2
The circuit does not affect the performance of the regulator as long as there is
insufficient voltage dropped across Rm to switch transistor Q2 on. Once Q2
does switch on, the output current of the op-amp is diverted away from the
base of Q1, so Q1 is unable to supply all the "demanded" load current - and
the output current is limited to the short-circuit output current of the op-amp
(only typically 30 mA or so). This current is LESS than the full-load current -which is why the arrangement is called the foldback circuit.
The switched-mode circuit
L
VS
VL
C
Otherwise known as "diode-pump circuits", these supplies have the
advantage of much higher efficiency than the series regulator -- the switch is
either fully on (so there is no voltage drop across it) or fully off (so no current
flows in it) and, since power = VI, no power is dissipated in the switch. (In fact,
it will not be totally ideal and some power will be dissipated, but very little in
relation to that dissipated in the transistor of the series regulator). The switch
is opened and closed at high frequency (typically 50 kHz or above) and the
output voltage is equal to VS times the proportion of the time for which the
switch stays closed. The inductor current is "pumped up" whilst the switch is
closed; when it opens, the "flywheel diode" allows the load current to keep
flowing, though it will fall slightly in value until the switch closes again.
Feedback is used to control the "mark-space ratio" of the switch (the ratio of
ON time to OFF time) such as to open it more if VL rises and to close it more
if VL falls. Other circuits working on the same principle allow us to produce
voltages higher than Vs or of the opposite polarity to Vs (see Horowitz and
Hill, among other good books !)