Basic Analog
Power Supply
Design
by Gerard Fonte
Part 1
There is the old saying:
“You can give a man a fish
and he will eat for a day or
you can teach a man to fish
and he will eat forever.”
There are many articles that
give the reader a specific
design for building a power
supply. And it must be made
clear that there is nothing
wrong with these cook-book
designs. They often have very
good performance. But they
don’t teach the readers how
to design a power supply by
themselves. This two-part
article will start from the
beginning and explain every
step necessary to build a
basic analog power supply.
The design will focus on the
ubiquitous “three-terminal
regulator” and include a
number of enhancements to
the basic design.
It is always important to remember
that the power supply — either for a
particular product or as a general
piece of test equipment — has the
potential to electrocute the user, start a
fire, or destroy the device it is powering.
Obviously, these are not good things. For
that reason, it is critical to approach this
design conservatively. Provide plenty of
margin for components. A well-designed
power supply is one that is never noticed.
Input Power Conversion
Figure 1 shows the fundamental
design for a typical analog power
supply. It consists of three main components: Input power conversion and
conditioning, rectification and filtering,
and regulation. The input power conversion is typically a power transformer
and is the only method considered here.
However, there are a couple of points
FIGURE 1. A basic analog power supply
consists of three parts. The first two are
discussed in this article and the last in
the next installment.
that are important to mention.
The first is that 117 VAC (Volts
Alternating Current) is really an RMS (Root
Mean Square) measurement. (Note that I
have seen ordinary household power
specified anywhere from 110 VAC to 125
VAC. I just measured mine and found it
to be precisely 120.0 VAC.) An RMS measurement of a sine wave is much lower than
the actual peak voltage and represents the
equivalent DC (Direct Current) voltage
needed to provide the same power. The
RMS conversion varies according to the
wave shape and for a sine wave the value
is 1.414. This means that the deviation
around zero volts is actually 169.7 volts
(for my 120 VAC power). The power goes
from -169.7 volts to +169.7 volts each
cycle. Therefore, the peak-to-peak voltage
is actually 339.4 volts!
This voltage becomes especially
important when adding bypass capacitors to the main power lines to suppress
noise from entering or leaving the
power supply (a common situation). If
you think the actual voltage is 120 volts,
you may use 150 volt capacitors. As you
72
December 2007
