Basic Analog Power Supply Design
62
January 2008
FIGURE 4. This is a nice bench power
supply that provides 0-30 volts at
one amp. It’s also current limited to
help eliminate unnecessary smoke
generation in your circuits. Meters to
monitor the voltage and current can
be added, if desired.
Ω is used for Rp, then the equation
becomes Vout = 1.25 (1 + 240/
Radj). Usually, this is accurate to
within a few percent.
A curious thing happens
when we replace Radj with a
power load instead of a resistor
(see Figure 3). The output voltage
varies with the load. We see that
this makes sense because the
power load variation is acting as a
variable resistor. The result is that
the current to the load remains
fixed but the voltage varies. In
other words, the voltage regulator
has changed into a current regulator. Instead of a constant voltage,
we have a constant current.
It is easily seen that the maximum current allowed depends
upon Rp. If Rp is made 1.25 Ω, then
up to a full amp of current can be
supplied. Note that this current is
limited by the 1.5 A maximum
output of the regulator and the
maximum output voltage obtainable
(which depends upon the input
voltage). This current regulation is
a very useful feature (of these
regulators) that is not often used.
Power Dissipation
Basically, the regulator acts like
a variable resistor in series with the
power supply. The amount of power
dissipated is simply the voltage
difference between the input and
output pins times the current
drawn. This creates some interesting
situations.
For example, suppose you
provide 35 V to the input of an
adjustable regulator and expect to
draw one amp at five volts and
also at 32 V. At five volts, there
will be a 30 V drop across the
regulator. With an amp of current,
the regulator will have to dissipate
30 W while providing five watts to
the load. It’s only 14% efficient. At
32 volts, there will be a three volt
drop which means that the regulator
must dissipate just three watts while
supplying 32 W to the load. This is
91% efficient.
It is clear that keeping the input
voltage as close to the output as
possible results in better efficiency
and less heat. (Note that with an input
voltage of 35 V and a current requirement of one amp the total power used
will always be 35 W. The more power
the load uses, the less the regulator
must dissipate.)
Often times, multiple voltages are
needed for a circuit. There is nothing
wrong with placing a five volt
regulator after a 10 V one. In this way,
the voltage drop for the five volt
regulator is shared between two
devices. However, if the two voltages
have significantly different current
requirements, this approach may not
be optimal. (You can use Ohm’s Law
to calculate the effective resistance
of the regulator given the desired
voltage and current. The power is then
calculated to be the resistance times
the current squared.)
Applying the Theory
We can now combine the information from the two articles to build a
practical power supply. We’ll start
with a basic supply and then discuss
how to change it to add features that
may be useful. Figure 4 provides a
practical circuit for an adjustable,
current limited and voltage regulated,
one amp power supply that goes from
zero volts to about 30 volts. (This
particular circuit has proven to be very
useful and adaptable in a number of
high-reliability designs for my clients,
usually with fixed current limit and
fixed output voltage.)
The raw DC circuit was described
in detail last month and will not be
discussed further except to note a
few minor changes. The power transformer is now specified as a 24 volt,
center-tapped, 1.0 to 1.5 amp device
instead of 25. 2 volts at two amps. The
reason for this change is three-fold.
The first is that some regulators
are limited to 35 V (most others
are rated to 40 V and there is a high
