Rectification uses diodes, thyristors, transistors, or converters to change an oscillating sinusoidal AC voltage source into a DC voltage source with a constant current. A single-phase or three-phase supply can be rectified into a constant DC level using half-wave, full-wave, uncontrolled, and fully controlled rectifiers, among other variations. One of the fundamental components of AC power conversion is a rectifier, which uses semiconductor diodes to achieve half-wave or full-wave rectification. Diodes are perfect for rectification because they permit alternating currents to pass through them in a forward direction while obstructing current flow in the opposite direction, resulting in a fixed DC voltage level.
What is Rectification?
Rectification is the process of using solid state
semiconductor devices to connect an AC power source to a linked DC load.
What is a single-phase rectifier?
A single-phase rectifier is a circuit that uses either
line-commutated or regulated ways to convert alternating current (AC) to direct
current (DC). The latter uses thyristors to provide for control over the power
conversion process.
Half-wave Rectification
The single-phase half-wave rectifier design eliminates the negative half of the AC supply waveform while passing the positive half. We can delete the positive half of the AC waveform and pass the negative halves by flipping the diode. Consequently, a sequence of either positive or negative pulses will be the output. As a result, the linked load, RL, receives neither voltage nor current for half of each cycle. To put it another way, because the half-wave rectifier only runs for half of the input cycle, the voltage across the load resistance, or RL, only has half waveforms, either positive or negative.
The equivalent DC value dropped across the load resistor, RL,
is only half of the sinusoidal waveform's value because of this pulsating DC.
Given that the sine function of the waveform has a maximum value of 1 (sin(90°)),
the average or mean DC value over half of a sinusoid is equal to 0.637 times
the maximum amplitude value. When the half-cycle is positive, AAVE
is equal to 0.637*AMAX. The average value of the waveform during
this negative half-cycle, however, will be zero, as indicated as the reverse-biased diode rectifies the negative half-cycles.
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| Half-Wave Rectifier and Rectified Output Waveform Image used courtesy of electronic-tutorial |
Full-wave Rectification
A full-wave rectifier produces a unidirectional output by
using both halves of the input sinusoidal waveform. It is essentially made up
of two half-wave rectifiers that are coupled in order to supply the load. In
order to produce a pulsing DC output, the single-phase full-wave rectifier uses
four diodes placed in a bridge arrangement.
Single-Phase Full-wave Bridge Rectifier
Because two of the four diodes in this bridge configuration are forward biased and the other two are reverse biased at any one time, full-wave rectification is possible. As a result, the conduction channel for the half-wave rectifier has two diodes rather than just one. Consequently, the two forward voltage drops of the serially linked diodes will cause a difference in voltage amplitude between VIN and VOUT. As before, we will assume perfect diodes for mathematical simplicity.
What is the operation of a single-phase full-wave rectifier? Diodes D1 and D4 are forward biased, and diodes D2 and D3 are reverse biased during the positive half cycle of VIN. Current then travels via D1–A–RL–B–D4 and returns to the supply during the input waveform's positive half cycle. Diodes D3 and D2 are forward biased, and diodes D4 and D1 are reverse biased during the negative half cycle of VIN. Current travels through D3, A, RL, B, and D2 and returns to the supply during the input waveform's negative half cycle.
Regardless of the input waveform's polarity, the positive and negative half-cycles in both situations result in positive output peaks, and as a result, the load current, i, always flows in the same direction via the load, RL between points or nodes A and B. At load, the source's negative half-cycle turns into a positive half-cycle.
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| Single-Phase Full-Wave Rectifier Image used courtesy of electronic-tutorial |
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| Rectified Waveform of Single-Phase Full-Wave Bridge Rectifier Image used courtesy of electronic-tutorial |
Full-wave Half-controlled Bridge Rectifier
Half-controlled Bridge Rectifier
Two thyristors and two diodes are used in the half-controlled rectifier design to regulate the average DC load voltage. According to what we learned in our thyristor tutorial, a thyristor will only conduct (or be in the "ON" state) when its gate (G) terminal receives a firing pulse and its anode (A) is more positive than its cathode (K). If not, it stays dormant. Additionally, we discovered that a thyristor is only "OFF" if its gate signal is cut off and the anode current drops below the thyristor's holding current, IH, due to reverse biasing caused by the AC supply voltage.
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| Half-Controlled Bridge Rectifier Image used courtesy of electronic-tutorial |
Fully-controlled Bridge Rectifier
The more popular term for single-phase fully-controlled bridge rectifiers is AC-to-DC converters. The speed control of DC machines frequently uses fully controlled bridge converters, which can be readily achieved by substituting thyristors for all four bridge rectifier diodes, as illustrated.
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| Fully-Controlled Bridge Rectifier Image used courtesy of electronic-tutorial |
Two thyristors per half-cycle are used to regulate the average DC load voltage in the fully controlled rectifier design. During the positive half-cycle, thyristors SCR1 and SCR4 fire together as a pair, and during the negative half-cycle, thyristors SCR3 and SCR4 likewise fire together as a pair. After SCR1 and SCR4, that is 180 degrees.
The four thyristors are then continuously switched as alternative pairs during continuous conduction mode of operation in order to maintain the average or equivalent DC output voltage. By altering the thyristor's firing delay angle (α), the output voltage can be completely regulated, much like with the half-controlled rectifier.
Conclusion
Single-phase rectifiers can be completely controlled full-wave bridge rectifiers with four thyristors or uncontrolled single-diode half-wave rectifiers to convert AC voltage to DC voltage. Because it only needs one diode, the half-wave rectifier has the advantages of simplicity and affordability.
But because only half of the input signal is used, resulting in a low average output voltage, it is not very efficient. Because it uses both half-cycles of the input sine wave to produce a greater average or equivalent DC output voltage, the full-wave rectifier is more efficient than the half-wave rectifier. The full-wave bridge circuit's requirement for four diodes is a drawback.
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