- SCR Definition: An SCR (silicon-controlled rectifier) is defined as a type of thyristor used for controlling power in circuits.
- Gate Characteristics of SCR: The gate characteristics of SCR include the safe limits for gate voltage and current, which are crucial for proper operation.
- Maximum and Minimum Limits: Each SCR has specified maximum and minimum limits for gate voltage and current to ensure safe operation.
- Non-Triggering Voltage (Vng): Vng is the voltage below which noise and unwanted signals should remain to prevent accidental activation of the SCR.
- Load Line and Operating Point: The operating point, determined by the load line, should be close to the maximum average gate power dissipation (Pgav) to minimize turn-on time and avoid accidental activation.
Gate characteristic of thyristor or SCR gives us a brief idea to operate it within a safe region of applied gate voltage and current. So this is a very important characteristic regarding thyristor. At the time of manufacturing each SCR or thyristor is specified with the maximum gate voltage limit (Vg-max), gate current limit (Ig-max) and maximum average gate power dissipation limit (Pgav). These limits should not be exceeded to protect the SCR from damage and there is also a specified minimum voltage (Vg-min) and minimum current (Ig-min) for proper operation of a thyristor.
The SCR also has a gate non-triggering voltage (Vng), which is specified during manufacturing.
All noise and unwanted signals should be below this voltage to prevent accidental activation of the thyristor.
Curve 1 shows the minimum voltage needed to turn on the SCR, while curve 2 shows the maximum safe voltage. The safe operating area for the SCR is within the region marked bcdefghb.
Now, from the triggering circuit, we get,
Where,
Es = gate source voltage
Vg = gate cathode voltage
Ig = gate current
Rs = gate source resistance
A load line of gate source voltage is drawn as AD where OA = Es and OD = Es/Rs which is trigger circuit short circuit current. Now, let a VI characteristic of gate circuit is given by curve 3. The intersection point of load line (AD) and curve 3 is called as operating point S. It is evident that S must lie between S1 and S2 on the load line. For decreasing the turn ON time and to avoid unwanted turn ON of the device, operating point should be as close to Pgav as possible. Slop of AD = source resistance Rs. Minimum amount of Rs can be determined by drawing a tangent to the Pgav carve from the point A.
