Rectifier circuits
Now we come to the most popular
application of the diode: rectification. Simply defined,
rectification is the conversion of alternating current (AC) to direct
current (DC). This almost always involves the use of some device that
only allows one-way flow of electrons. As we have seen, this is exactly
what a semiconductor diode does. The simplest type of rectifier circuit
is the half-wave rectifier, so called because it only allows one
half of an AC waveform to pass through to the load:
For most power applications, half-wave
rectification is insufficient for the task. The harmonic content of the
rectifier's output waveform is very large and consequently difficult to
filter. Furthermore, AC power source only works to supply power to the
load once every half-cycle, meaning that much of its capacity is unused.
Half-wave rectification is, however, a very simple way to reduce power
to a resistive load. Some two-position lamp dimmer switches apply full
AC power to the lamp filament for "full" brightness and then half-wave
rectify it for a lesser light output:
In the "Dim" switch position, the
incandescent lamp receives approximately one-half the power it would
normally receive operating on full-wave AC. Because the half-wave
rectified power pulses far more rapidly than the filament has time to
heat up and cool down, the lamp does not blink. Instead, its filament
merely operates at a lesser temperature than normal, providing less
light output. This principle of "pulsing" power rapidly to a
slow-responding load device in order to control the electrical power
sent to it is very common in the world of industrial electronics. Since
the controlling device (the diode, in this case) is either fully
conducting or fully nonconducting at any given time, it dissipates
little heat energy while controlling load power, making this method of
power control very energy-efficient. This circuit is perhaps the crudest
possible method of pulsing power to a load, but it suffices as a
proof-of-concept application.
If we need to rectify AC power so as to
obtain the full use of both half-cycles of the sine wave, a
different rectifier circuit configuration must be used. Such a circuit
is called a full-wave rectifier. One type of full-wave rectifier,
called the center-tap design, uses a transformer with a
center-tapped secondary winding and two diodes, like this:
This circuit's operation is easily
understood one half-cycle at a time. Consider the first half-cycle, when
the source voltage polarity is positive (+) on top and negative (-) on
bottom. At this time, only the top diode is conducting; the bottom diode
is blocking current, and the load "sees" the first half of the sine
wave, positive on top and negative on bottom. Only the top half of the
transformer's secondary winding carries current during this half-cycle:
During the next half-cycle, the AC
polarity reverses. Now, the other diode and the other half of the
transformer's secondary winding carry current while the portions of the
circuit formerly carrying current during the last half-cycle sit idle.
The load still "sees" half of a sine wave, of the same polarity as
before: positive on top and negative on bottom:
One disadvantage of this full-wave
rectifier design is the necessity of a transformer with a center-tapped
secondary winding. If the circuit in question is one of high power, the
size and expense of a suitable transformer is significant. Consequently,
the center-tap rectifier design is seen only in low-power applications.
Another, more popular full-wave rectifier
design exists, and it is built around a four-diode bridge configuration.
For obvious reasons, this design is called a full-wave bridge:
Current directions in the full-wave
bridge rectifier circuit are as follows for each half-cycle of the AC
waveform:
Remembering the proper layout of diodes
in a full-wave bridge rectifier circuit can often be frustrating to the
new student of electronics. I've found that an alternative
representation of this circuit is easier both to remember and to
comprehend. It's the exact same circuit, except all diodes are drawn in
a horizontal attitude, all "pointing" the same direction:
One advantage of remembering this layout
for a bridge rectifier circuit is that it expands easily into a
polyphase version:
Each three-phase line connects between a
pair of diodes: one to route power to the positive (+) side of the load,
and the other to route power to the negative (-) side of the load.
Polyphase systems with more than three phases are easily accommodated
into a bridge rectifier scheme. Take for instance this six-phase bridge
rectifier circuit:
When polyphase AC is rectified, the
phase-shifted pulses overlap each other to produce a DC output that is
much "smoother" (has less AC content) than that produced by the
rectification of single-phase AC. This is a decided advantage in
high-power rectifier circuits, where the sheer physical size of
filtering components would be prohibitive but low-noise DC power must be
obtained. The following diagram shows the full-wave rectification of
three-phase AC:
In any case of rectification --
single-phase or polyphase -- the amount of AC voltage mixed with the
rectifier's DC output is called ripple voltage. In most cases,
since "pure" DC is the desired goal, ripple voltage is undesirable. If
the power levels are not too great, filtering networks may be employed
to reduce the amount of ripple in the output voltage.
Sometimes, the method of rectification is
referred to by counting the number of DC "pulses" output for every 360o
of electrical "rotation." A single-phase, half-wave rectifier circuit,
then, would be called a 1-pulse rectifier, because it produces a
single pulse during the time of one complete cycle (360o) of
the AC waveform. A single-phase, full-wave rectifier (regardless of
design, center-tap or bridge) would be called a 2-pulse
rectifier, because it outputs two pulses of DC during one AC cycle's
worth of time. A three-phase full-wave rectifier would be called a
6-pulse unit.
Modern electrical engineering convention
further describes the function of a rectifier circuit by using a
three-field notation of phases, ways, and number of
pulses. A single-phase, half-wave rectifier circuit is given the
somewhat cryptic designation of 1Ph1W1P (1 phase, 1 way, 1 pulse),
meaning that the AC supply voltage is single-phase, that current on each
phase of the AC supply lines moves in one direction (way) only, and that
there is a single pulse of DC produced for every 360o of
electrical rotation. A single-phase, full-wave, center-tap rectifier
circuit would be designated as 1Ph1W2P in this notational system: 1
phase, 1 way or direction of current in each winding half, and 2 pulses
or output voltage per cycle. A single-phase, full-wave, bridge rectifier
would be designated as 1Ph2W2P: the same as for the center-tap design,
except current can go both ways through the AC lines instead of
just one way. The three-phase bridge rectifier circuit shown earlier
would be called a 3Ph2W6P rectifier.
Is it possible to obtain more pulses than
twice the number of phases in a rectifier circuit? The answer to this
question is yes: especially in polyphase circuits. Through the creative
use of transformers, sets of full-wave rectifiers may be paralleled in
such a way that more than six pulses of DC are produced for three phases
of AC. A 30o phase shift is introduced from primary to
secondary of a three-phase transformer when the winding configurations
are not of the same type. In other words, a transformer connected either
Y-Δ or Δ-Y will exhibit this 30o phase shift, while a
transformer connected Y-Y or Δ-Δ will not. This phenomenon may be
exploited by having one transformer connected Y-Y feed a bridge
rectifier, and have another transformer connected Y-Δ feed a second
bridge rectifier, then parallel the DC outputs of both rectifiers. Since
the ripple voltage waveforms of the two rectifiers' outputs are
phase-shifted 30o from one another, their superposition
results in less ripple than either rectifier output considered
separately: 12 pulses per 360o instead of just six:
- REVIEW:
- Rectification is the conversion
of alternating current (AC) to direct current (DC).
- A half-wave rectifier is a
circuit that allows only one half-cycle of the AC voltage waveform to
be applied to the load, resulting in one non-alternating polarity
across it. The resulting DC delivered to the load "pulsates"
significantly.
- A full-wave rectifier is a
circuit that converts both half-cycles of the AC voltage waveform to
an unbroken series of voltage pulses of the same polarity. The
resulting DC delivered to the load doesn't "pulsate" as much.
- Polyphase alternating current, when
rectified, gives a much "smoother" DC waveform (less ripple
voltage) than rectified single-phase AC.
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