Gate Characteristics of Thyristor: In order to trigger a thyristor, a positive gate to cathode voltage, Vg, and positive gate to cathode current Ig is applied. Both Vg and Ig are DC values. This will help to make the junction J2 in the thyristor forward biased.
Here we will discuss the forward gate characteristics of the thyristor that is the Vg – Ig curve along with the design of the firing circuit and its requirements.
Forward Gate Characteristics of Thyristor
Looking at the structure of a thyristor, the gate-cathode circuit of a thyristor is similar to the p-n junction diode. If Vg and Ig be the gate to cathode voltage and currents respectively then for a particular SCR or thyristor, the Vg– Ig characteristics give forward gate characteristics of thyristor that has spread between two respective curves.
The figure below shows the Vg– Ig characteristics (forward gate characteristics) curve of the thyristor and it is spread between curves 1 and 2. The spread is due to the difference in doping level between the gate-cathode p and n layers.
The scatter of the gate characteristics of the thyristor must be considered carefully while designing the gate trigger circuit.
The two curves in the forward gate characteristics of thyristors represent the lowest value of voltage to be applied to turn on the SCR (curve 1) and the highest value of voltage that can be safely applied to turn on the thyristor (curve 2).
Gate Characteristics of Thyristor Curves
In the curve, the maximum value of gate voltage Vgm and the maximum value of gate current Igm is specified.
Pgav is the rated or average gate power dissipation and to safely operate the thyristor and in order to prevent it from being damaged the mentioned limits should not be exceeded. If the limits are exceeded then it may permanently damage the gate-cathode junction.
The OX and OY on the curve represent the minimum required value of Vg and Ig for reliable turn-on of the thyristor.
The OA is the non-triggering gate voltage. This is specified by the manufacturer and any unwanted or noise signals in the gate signals must be less than OA.
The area ‘bcdefghb’ in the forward gate characteristics of the thyristor represents the area preferably for the driving of the gate of the thyristor.
Design of Firing Circuit
The figure below shows a gate triggering circuit connected across the gate of a thyristor.
From the circuit, applying Kirchhoff’s Voltage Law(KVL),
E = Vg + Ig VgRS
where E is the gate to source voltage
Vg and Ig are gates to cathode voltage and current respectively
RS is the gate to source resistance or internal resistance of the trigger source.
The short circuit current of the trigger circuit is given by ES/ Rs and it should be noted that the value of RS should be such that, the short circuit current is not harmful to the circuits and their components.
If the value of RS is significantly low, an external resistance of a suitable value can be connected across it.
The figure below shows a trigger circuit connected across the thyristor with resistance R1 connected across the gate and cathode. The R1 provides a path for the flow of leakage current.
Across junction J2, the thermally generated leakage current can be bypassed through R1 providing thermal stability.
The shunt resistance R1 also increases the holding and latching current levels. As some currents get bypassed through R1, the dv/dt capability of the thyristor increases.
The operating point is obtained by drawing a load line on the forward gate characteristics curve of the thyristor.
The load line AD is drawn by joining the gate-source voltage OD or ES and trigger circuit short circuit current OD or Igm where,
Igm = ES/R
The operating point (S) is the intersection of load line AD and curve 3.
OP and PS are the operating point gate current and gate voltage respectively.
The turn-on time can be minimized by keeping the operating point S close to the average gate power (Pgav). This will also ensure that there is the absence of unreliable turning-on of the thyristor.
The operating point S must be within the curve 1 and 2 and for reliable and fast turn-on it must be close to the Pgav.
In practice, the pulse signal is applied for triggering the thyristor. The turn-on time of the thyristor can be reduced by the application of a gate current of high magnitude. The width of the pulse must be greater than or equal to the turn-on time of the thyristor. Also, it must be ensured that the pulse width must be sufficient enough such that the anode current exceeds the latching current.
The gate power dissipation in pulse triggering must be less than the maximum gate power dissipation Pgm.
Frequency of Pulse Triggering
If Pgm is the maximum gate power dissipation, T be pulse width and T1 be the period.
If the frequency of the pulse, f= 1/ T1, then, Pgm. T. f ≥ Pgav.
The limiting case of the above expression is
In terms of duty cycle (). Duty cycle is the ratio of pulse width to periodicity.
So we have,
Finally for limiting cases,
High-Frequency Carrier Gating
Here, the thyristor is triggered by pulse triggering of the gate by the application of train pulses.
- Lower rating.
- Reduced Dimensions.
- Economical design.
Thyristor Protection Against Reverse Overvoltage
The trigger circuit may give voltage signals of high magnitudes that are dangerous for the thyristor. For protection, a diode called clamping diode is connected across the cathode and anode. The clamping diode ensures that the gate to cathode voltage does not exceed 1V.
A diode connected in series with the gate circuit will prevent the flow of negative gate-source current and limit this current value within the small reverse leakage current.