Basic Analog Power Supply Design
FIGURE 5. The bridge rectifier
approach (top) provides full use of
the transformer power and with a
full-wave rectification. Additionally,
by changing the ground reference
(bottom), a dual voltage power
supply can be obtained.
output waveform displayed above.
(The filter capacitor is not shown
because by adding it, the waveform
changes to something like a DC
voltage.) It is useful to examine these
three basic circuits to identify the
strengths and weaknesses of them.
Figure 3 shows the basic half-wave
rectifier. The only redeeming characteristic of this is that it is very simple, using
only a single rectifier. The bad feature is
that it uses only half of the power cycle
making the theoretical efficiency of the
circuit less than 50% just to start. Often,
half-wave rectifier power supplies are
only 30% efficient. Since transformers
are expensive items, this inefficiency is
very costly. Secondly, the wave shape is
very difficult to filter. Half the time there
is no power at all coming from the transformer. Smoothing the output requires
very high values of capacitance.
It is rarely used for an analog
An interesting and important
thing happens when a filter capacitor is added to a half-wave rectifier circuit. The no-load voltage differential doubles. This is because
the capacitor stores energy from
the first half (positive part) of the
cycle. When the second half occurs, the
capacitor is holding the positive peak
voltage and the negative peak voltage is
applied to the other terminal causing a
full peak-to-peak voltage to be seen by
the capacitor and through that, the
diode. Thus, for a “ 25. 2 volt” transformer
above, the actual peak voltage seen by
these components can be over 80 volts!
Figure 4 (top circuit) is an example of a typical full-wave, center-tap
rectifier circuit. This is used today even
though, in most cases, it probably
shouldn’t be. It provides a nice output
that is fully rectified. This makes filtering relatively easy. It uses only two
rectifiers, so it’s pretty inexpensive.
However, it is no more efficient than
the half-wave circuit presented above.
This can be seen by re-drawing the
circuit with two transformers (Figure 4
A Note on Efficiency
Let me expain the difference between a full-wave rectifier design and a bridge,
especially from the point of view of efficiency.
Remember that I said half the power is wasted with a full-wave rectifier design.
It is self-evident if you consider that a full-wave design is really made up of two
half-wave designs. Combining two circuits that are each about 50% efficient does not
in any way increase the total efficiency of the circuit. It’s still about 50% overall.
The loss of efficiency comes from the amplitude of the resultant rectification. A
half-wave circuit clearly provides half the amplitude that is available from the transformer. So does the full-wave version, but it’s not as obvious. It only uses half of the
total available voltage of the transformer.
The output of the full-wave rectifier is 1.414 FIGURE A
times the voltage between the center tap
and the end. The output of a bridge is the
full end-to-end voltage of the transformer
times 1.414. This is shown in the attached
figure. For the 25. 2 volt transformer, the
full-wave rectifier develops about 19 volts
(unloaded). The bridge rectifier develops
about 38 volts (unloaded). So, the full-wave
rectifier does indeed waste about half the
available power from the transformer.
bottom). When this is done, it
becomes clear that the full-wave is really just two half-wave circuits connected
together. Half of each transformer
power cycle is not used. Thus, the
maximum theoretical efficiency is 50%
with real efficiencies around 30%.
The PIV of the circuit is one half of
the half-wave circuit because the input
voltage to the diodes is half of the
transformer output. The center tap
provides half the voltage to the two
ends of the transformer windings. So,
for the 25. 2 volt transformer example,
the PIV is 35. 6 volts plus the no-load
increase which is about 10% more.
Figure 5 presents the bridge rectifier circuit which should generally be
the first choice. The output is fully rectified so filtering is fairly easy. But most
importantly, it uses both halves of the
power cycle. This is the most efficient
design and gets the most out of the
expensive transformer. Adding two
diodes is much less expensive than
doubling the transformer power rating
(measured in “Volt-Amps” or VA).
The only drawback to this design
is that the power must pass through
two diodes with a resulting voltage
drop of 1.4 volts instead of 0.7 volts
for the other designs. Generally, this is
only a concern for low voltage power
supplies where the additional 0.7 volts
represents a substantial fraction of the
output. (In such instances, a switching
power supply is usually used rather
than either of the above circuits.)
Since there are two diodes being
used for each half-cycle, only half of
the transformer voltage is seen by
each. This makes the PIV equal to the
peak input voltage or 1.414 times the
transformer voltage, which is the same
as the full-wave circuit above.
A very nice feature of the bridge
rectifier is that the ground reference
can be changed to create a positive
and negative output voltage. This is
shown in the bottom of Figure 5.
Nowadays, nearly all filtering for an
analog power supply comes from a filter
capacitor. It is possible to use an inductor in series with the output, but at 60
Hz, these inductors must be quite large