static characteristics of a thyristor

Important Points About The V-I Characteristics of SCR

Forward Characteristics
When anode is positive w.r.t. cathode, the curve between V and I is called the forward characteristics.

In fig.1, OABC is the forward characteristics of SCR at IG=0.

If the supply voltage is increased from zero, a point reached (point A) when the SCR starts conducting.

Under this condition,the voltage across SCR suddenly drops as shown by dotted curve AB and most of supply voltage appears across the load resistance RL .

If proper gate current is made to flow, SCR can close at much smaller supply voltage.

Reverse Characteristics
When anode is negative w.r.t. cathode, the curve between V and I is known as reverse characteristics.

The reverse voltage does come across SCR when it is operated with a.c. supply.

If the reverse voltage is gradually increased, at first the anode current remains small (i.e. leakage current) and at some reverse voltage, avalanche breakdown occurs and the SCR starts conducting heavily in the reverse direction as shown by the curve DE.

This maximum reverse voltage at which SCR starts conducting heavily is known as reverse breakdown voltage.

SCR in Normal Operation
In order to operate the SCR in normal operation, the following points are kept in view:

The supply voltage is generally much less than breakover voltage.
The SCR is turned on by passing appropriate amount of gate current ( a few mA) and not by breakover voltage.
When SCR is operated from a.c.  supply, the peak reverse voltage which comes during negative half-cycle should not exceed the reverse breakdown voltage.
When SCR is to be turned OFF from the ON state, anode current should be reduced to holding current.
If gate current is increased above the required value, the SCR will close at much reduced supply voltage.
Important Terms In The V-I Characteristics of SCR
The following terms are much used in the study of SCR :

Breakover voltage
Peak reverse voltage
Holding current
Forward current rating
Circuit fusing rating
1.  Breakover Voltage
It is the minimum forward voltage, gate being open, at which SCR starts conducting heavily i.e. turned on.

Thus, if the breakover voltage of an SCR is 200 V, it means that it can block a forward voltage  (i.e. SCR remains open) as long as the supply voltage is less than 200 V. If the supply voltage is more than this value, then SCR will be turned on.

In practice, the SCR is operated with supply voltage less than breakover voltage and it is then turned on by means of a small voltage applied to the gate.

Commercially available SCRs have breakover voltages from about 50 V to 500 V.

2.  Peak Reverse Voltage (PRV)
It is the maximum reverse voltage  (cathode positive w.r.t. anode) that can be applied to an SCR without conducting in the reverse direction.

PRV is an important consideration while connecting an SCR in an a.c. circuit. During the negative half of a.c. supply, reverse voltage is applied across SCR. If PRV is exceeded, there may be avalanche breakdown and the SCR will be damaged if the external ciruit does not limit the current.

Commercially available SCRS have PRV ratings upto 2.5 kV.

3.  Holding Current
It is the maximum anode current, gate being open, at which SCR is turned OFF from ON condition.

When SCR is in the conducting state, it can not be turned OFF even if gate voltage is removed.

The only way to turn off or open the SCR is to reduce the supply voltage to almost zero at which point the internal transistor comes out of saturation and opens the SCR.

The anode current under this condition is very small (a few mA) and is called holding current.

Thus, if an SCR has a holding current of 5mA, it means that if anode current is made less than 5 mA, then SCR will be turned off.

4.  Forward Current Rating
It is the maximum anode current that an SCR is capable of passing without destruction.

Every SCR has a safe value of forward current which it can conduct. If the value of current exceeds this value, the SCR

SCR Thyristor summary

Silicon Controlled Rectifiers known commonly as Thyristors are three-junction PNPN semiconductor devices which can be regarded as two inter-connected transistors that can be used in the switching of heavy electrical loads. They can be latched-“ON” by a single pulse of positive current applied to their Gate terminal and will remain “ON” indefinitely until the Anode to Cathode current falls below their minimum latching level.

Static Characteristics of a Thyristor SCR

✔Thyristors are semiconductor devices that can operate only in the switching mode.
✔Thyristor are current operated devices, a small Gate current controls a larger Anode current.
✔Conducts current only when forward biased and triggering current applied to the Gate.
✔The thyristor acts like a rectifying diode once it is triggered “ON”.
✔Anode current must be greater than holding current to maintain conduction.
✔Blocks current flow when reverse biased, no matter if Gate current is applied.
✔Once triggered “ON”, will be latched “ON” conducting even when a gate current is no longer applied providing Anode current is above latching current.