We make use of various thyristor triggering methods to turn on the thyristor or to make the thyristor work in conduction mode of operation.
When (voltage across anode and cathode) VAK > 0 then SCR can be turned ON.
Following are some thyristor triggering methods.
Forward voltage Triggering:
This technique is already discussed in thyristor. Here we make VAK > VBO. But this may damage the SCR. So we do not usually use this method.
Here it is important to note that the magnitude of forwarding break overvoltage (VBO) and reverse breakdown voltage (VBR) are nearly the same but practically VBR is slightly greater than VBO.
During the design of the SCR circuits, we take VBO as the final voltage rating of the device.
It is the most simple, reliable, and efficient method. Here we apply a positive gate voltage between gate and cathode. When gate current gets established, charges are injected in the player, and the voltage at which forward break-over occurs gets reduced. The higher the gate current, the lower is the forward break overvoltage. The thyristor requires no gate current once the device gets turn ON in order to make the thyristor continue to operate in the ON state. We apply gate current in pulses instead of applying continuously. As the application of gate current in pulses will reduce the power losses in the gate circuit.
If the gate current is made zero before the anode current attains the value equal to the latching current, then the thyristor will again turn OFF.
Gate current is the order of 20 to 200 mA.
When VAK > 0, J-1 and J-3 are forward bias and J-2 is reverse biased. The reverse-biased J-2 acts as a capacitor due to charges across the junction J-2. If we suddenly apply a forward voltage, a charging current through junction capacitance Cj may turn ON the SCR.
Charging current ic is given by:
The Cj being a constant quantity, the expression of charging current reduces to: Therefore, if the rate of rising of forwarding voltage is high and greater than latching current ( IL), then charging current will increase and ic act as gate current to turn ON SCR.
Temperature or Light triggering:
If VAK > 0 but less than VBO, then J-1 and J-3 are forward biased and J-2 act as reverse biased. When photons fall on junction J2, they generate electrons and holes which neutralize the stored charges across the junction. So, depletion region width decreases, and finally, it starts conducting.
If VAK > 0 but less than VBO, then J-1 and J-3 are forward biased and J-2 act as reverse biased. With the increase in temperature, electrons and hole generation will increase thereby neutralizing the stored charges across the junction J-2. Consequently, the width of the depletion region decreases, and SCR gets turn ON.
What happens if we make the gate positive concerning the cathode during reverse blocking of thyristor?
We know that- Junction with lightly doped layers (at least on one side of the junction) requires large breakdown voltage and a junction with highly doped layers on both sides requires less breakdown voltage.
When a reverse voltage is applied to SCR, then J-1 and J-3 are reverse biased and J-2 is forward biased. J-3 has low breakdown voltage as it has heavily doped layers on both sides. J-1 has a large breakdown voltage as it has a lightly doped (N-) layer on its one side. So during reverse bias, most of the reverse voltage will appear across J-1.
Now if we apply positive in reverse bias SCR, breakdown of J-3 will take place and whole reverse voltage appear across J-1. This will cause an increase in reverse leakage current.
The flow of large leakage current along with high reverse voltage across J-1 results in high power loss across J-1. These losses heat the junction which may destroy SCR.
Notes on Thyristor Triggering Methods
- We make use of a carrier frequency gate driver to turn-on a thyristor in order to reduce the size of the pulse transformer.
- The main reason for connecting a pulse transformer at the output stage of the thyristor triggering circuit is to provide electrical isolation.
- We make use of high-frequency gate pulse modulation in triggering an SCR in order to minimize the saturation of the core of the pulse transformer.