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Friday, 21 February 2014

Characteristics & Applications of Silicon Controlled Rectifiers, SCR (Thyristors)

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Thyristors are solid-state semiconductor devices. They have four layers of alternating P and N-type material.

Terminology of SCR

Breakover voltage

Breakover voltage is also called the forward-breakover voltage. It is the minimum forward voltage with the gate open so that the SCR starts conducting. In other words, the point with which the SCR is found to be turned ON. For example, if the breakover voltage of an SCR is 130V, then it can block a forward voltage until the supply voltage is less than 130 V  (< 130V).

Peak Reverse Voltage (PRV)

PRV is the maximum reverse voltage that can be applied to an SCR without conducting in the reverse direction.


Holding Current

It is the maximum anode current (with open gate) at which the SCR is turned off from on condition.
The SCR cannot be turned OFF by removing the Gate voltage. The only way to turn OFF or open the SCR is to reduce the supply voltage to almost zero. Here the internal transistor (refer to the below figure) comes out of saturation and opens the SCR.

Forward Current Rating (FCR)

FCR is the maximum anode current that an SCR is capable of passing without having destruction.
If an SCR has a forward current rating of 40 A, it means that the SCR can safely carry only 40 A. The current exceeds this value will result in SCR's destruction due to intensive heating at the junctions.

Importance of Peak Reverse Voltage (PRV)

For a practical application, when an SCR is used for rectification process, during the negative half cycle of given ac input supply, the reverse voltage is applied across the SCR. If Peak Reverse Voltage (PRV) is exceeded, there may be an avalanche breakdown. This will yields to the damage of the SCR (unless the external circuit limits the current).
The Commercial purpose SCRs has a rating of PRV upto 2.5kV.

V-I Characteristics of SCR

V-I characteristics of a SCR is obtained by plotting the Curve between the anode-cathode voltage(V) and anode current(I) of an SCR at constant Gate current.

SCR Characteristics
 Forward Characteristics of SCR (Forward Conduction)

o Anode is made +ve with respect to cathode.
o When supply voltage is increased from 0 V, then suddenly the SCR starts conducting. => (breakover voltage)
o Voltage drops at this point is shown by the dotted line.
o If proper gate current is made to flow through, then the SCR can close at smaller supply voltage.

Reverse Characteristics of SCR

o Anode is made -ve with respect to cathode.
o Initially the anode current retains a small value (viz. leakage current).
o Beyond a particular reverse voltage, the SCR starts conducting massive (avalanche) => (Reverse breakdown voltage).

Controlled Rectification Application

An application of SCRs is the controlled rectification.

o Two SCRs are connected across the center-tapped secondary. The above figure shows this.
o The gate signal for both SCRs is supplied from two circuits, which control the required gate supply.
o One SCR conducts in the positive half cycle and the second SCR conducts in the negative half cycle of the given input supply.
o Thus current through the load will be in the unidirectional form.
o The most important aspect of SCR's controlled rectification is the rectification process. Thus the output DC voltage can be controlled by using the Gate supply control circuits.
The graph shown in the above figure shows the shaded region to be rectified output. But the  non-shaded part denotes the absence of conduction. This will vary the output voltage.

Firing Angle

Firing Angle is the angle (in the input AC) at which the gate is triggered.
If the supply voltage v = VMAX sin θ and the firing angle is 'a' (alpha), then average voltage output can be obtained by the expression

Vav = VMAX / ∏ (1 + Cos α)

From the above application, it is clear that the SCR can also be used as a switch. The switch can be turned ON by firing the gate and turned OFF by reducing the anode current to less than the holding current.