An electrical circuit that converts alternating current into a direct current is called a rectifier. Rectifiers incorporate semiconductor devices like diodes, thyristors, IGBTS, etc. for the conversion of alternating current into a direct current. Depending upon the type of switching device used, rectifiers are classified into two types uncontrolled and controlled rectifiers.

Rectifier circuits employing only diodes are called uncontrolled rectifiers. Unlike a thyristor, a diode doesn’t have any gate terminal and it starts conducting when its anode-to-cathode voltage becomes positive. Thus we cannot control the conduction period for a diode, the conduction and commutation of the diode will depend upon the circuit conditions.

Thus the average dc output voltage of an uncontrolled rectifier cannot be adjustable which means the output voltage is fixed and cannot be varied. That is why rectifier circuits incorporating only diodes are called Uncontrolled Rectifiers.

However, for most of the applications, there is a requirement for a controllable dc power supply. A controllable or adjustable dc output can be obtained by using a controlled rectifier. Let us see the circuit construction and working of controlled rectifiers.

What is a Controlled Rectifier?

A controlled rectifier uses controllable semiconductor devices like thyristors, IGBTs, etc. We know that an uncontrolled rectifier gives a fixed dc output voltage, from a controlled rectifier we can get a controllable or adjustable dc output voltage.

Generally, thyristors are used in controlled rectifiers for which we can control the conduction period of the device even in forward bias conduction. A thyristor is turned ON by applying a voltage pulse to the gate terminal in its forward-biased condition and it is turned OFF when its anode current falls below the holding current value.

Thus by varying the time at which the thyristor is turned ON we can get a variable dc output voltage. Also, the thyristor is reverse-biased for each ac input cycle due to voltage reversal which reduces the anode current below the holding current value. Thus the thyristor is naturally commutated in a controlled rectifier which in turn eliminates the need for an extra commutation circuit.

Types of Controlled Rectifiers:

Controlled rectifiers are classified into two types single-phase controlled rectifiers and three-phase controlled rectifiers. The single-phase and three-phase rectifiers are further classified as,

• Single-phase Controlled Rectifiers
  • Half-wave controlled rectifiers
  • Full-wave controlled rectifiers
    • Half-controlled rectifiers
    • Fully-controlled rectifiers
• Three-phase Controlled Rectifiers
  • Half-wave controlled rectifiers
  • Full-wave controlled rectifiers
    • Half-controlled rectifiers
    • Fully-controlled rectifiers

Half-wave Controlled Rectifier:

A half-wave controlled rectifier converts only either the positive or negative half-cycle of ac input supply into dc. The circuit diagram of a half-wave controlled rectifier is similar to a half-wave uncontrolled rectifier except the diode is replaced by a thyristor as shown below.

In the above circuit, the thyristor will be forward-biased during the positive half-cycle but it will be in forward blocking state and blocks the flow of current through it. As soon as the gate signal is applied, the thyristor switches from OFF-state (forward blocking state) to ON-state (forward conduction state), and thus load gets connected across the supply.

During the negative half-cycle, the thyristor will be reverse biased and no current flows through the load. Therefore, only a half-cycle of ac input reaches the load. By varying the time at which the gate pulse is applied to the thyristor we can control the average dc output voltage.

Full-wave Controlled Rectifier:

A full-wave controlled rectifier converts the full cycle (both positive and negative half-cycle) of ac supply into dc. There are two types of full-wave rectifiers, center-tapped configuration, and bridge configuration. The center-tapped configuration uses two thyristors whereas the bridge configuration uses four thyristors.

In a center-tapped controlled rectifier, two thyristors are connected at the secondary side of a center-tapped transformer as shown below. During the positive half-cycle, thyristor T₁ will be forward-biased and allows current once it is triggered. During the negative half-cycle, thyristor T₂ will be forward-biased and allows current once it is triggered.

In bridge-type controlled rectifiers, four thyristors are connected in a closed-loop manner as shown below. During the positive half cycle, thyristors T₁ and T₂ will be forward-biased and allows current once it is triggered. During the negative half cycle, thyristors T₃ and T₄ will be reverse-biased and allows current once it is triggered.

Thus both center-tapped and bridge rectifiers convert positive and negative half-cycles of ac input into pulsating dc. By controlling the triggering pulse delay of thyristors we can control the average dc output voltage.

Half-Controlled Rectifier:

A rectifier circuit employing both diodes and thyristors is called a half-controlled rectifier. Due to the use of diodes in combination with thyristors, we don’t have complete control over the output voltage. Hence the circuit is known as a half-controlled rectifier. The below shows the circuit of a half-controlled rectifier.

By controlling the switching angle of the thyristors we can obtain a variable dc output voltage. A half-controlled rectifier is also known as a semi-converter.

Fully Controlled Rectifier:

A rectifier circuit employing only thyristors is called a fully-controlled rectifier. Since all the diodes are replaced by thyristors we have full control over the output voltage and thus rectifier circuit is said to be known as a Fully-controlled rectifier. The below shows the circuit of a fully-controlled rectifier.

In a fully-controlled rectifier, the power flow can be in both directions i.e., from ac supply to load and load to ac supply. When power flows from ac supply to the load the circuit works in rectifying mode and when power flows from the load to the ac supply the circuit works in inverting mode.

Advantages of Controlled Rectifiers:

• A variable output voltage can be obtained from a controlled rectifier.
• Since thyristors are naturally commutated there is no need for a commutation circuit.
• A smooth control over the output voltage.
• Thyristors can handle large voltages, currents, and power.

Disadvantages of Controlled Rectifiers:

• Since the cost of thyristors is more than diodes, controlled rectifiers are costlier than uncontrolled rectifiers.
• Controlled rectifiers are a little bit more complex than uncontrolled rectifiers.