A Thyristor can be switched from a non conducting state to a conducting state in several methods:
Forward voltage triggering:
In this method when anode to cathode forward voltage is increased with gate circuit open, then the reverse bias junction J2 will have a avalanche breakdown at a voltage called forward break over voltage VBO. At this voltage thyristor or SCR changes from OFF state to ON state. The forward voltage drop across the SCR during ON state is of the order of 1 to 1.5V and increases slightly with increase in the load current
Thermal Triggering (Temperature Triggering):
Width of the depletion layer of the thyristor decreases on increasing the junction temperature. Thus in the SCR when the voltage applied is very near to the breakdown voltage, the device can be triggered by increasing its junction temperature. By applying the temperature to certain extent, a situation comes when the reverse biased junction collapse making the device to conduct. This method of triggering the thyristor by heating is known as the Thermal Triggering process.
Radiation Triggering (Light Triggering):
Thyristors are bombarded with energy particles such as neutrons and protons. Light energy is focused on the depletion region results in the formation of charge carriers. This lead to instantaneous flow of current with in the device and the triggering of the device.
In this method of triggering if the applied rate of change of voltage is large, then the device will turn on even though the voltage appearing across the device is small. We know that when SCR is applied with forward voltage across the anode and cathode, junctions j1 and j3 will be in forward bias and junction j2 will be in reverse bias. This reverse biased junction j2 will have the characteristics of the capacitor due to the charges exist across the junction. If the forward voltage is suddenly applied a charging current will flow tending to turn on the SCR. This magnitude of the charging current depends on the rate of change of applied voltage.
This is the most commonly used method for triggering the SCR or thyristor. For gate triggering a signal is applied across the gate and cathode of the device. By applying a positive signal at the gate terminal of the SCR it will be triggered much before the specified break over voltage. Three types of signals can be used for triggering the SCR. They are either dc signal,ac signal or pulse signal.
DC Gate triggering:
In this type of triggering a dc voltage of proper magnitude and polarity is applied between the gate and cathode such that gate becomes positive with respect to the cathode. When the applied voltage is sufficient to produce required gate current the device starts conducting
- Both the power and control circuits are dc and there is no isolation between the two
- continuous dc signal has to be applied to turn on the device from turning off. Hence power loss at the gate
AC Gate triggering:
ac source is most commonly used triggering source for thyristor for ac applications.
Have the advantages than dc source such as power isolation between the power and the control circuits and firing angle can be controlled by changing the phase angle of the control signal.
- Gate drive is maintained for one half cycle of the device is turned ON.
- Severe reverse voltage is applied across gate and cathode during negative half cycle.
The drawback of this scheme is that a separate transformer is required to step down the ac supply increasing the cost
Pulse Gate Triggering:
This is the most popular method for triggering the SCR. In this method gate drive consists of single pulse appearing periodically or sequence of high frequency pulses. This is known as carrier frequency gating. A pulse transformer is used for isolation. The main advantage of this method is there is no need to apply continuous gate signal and hence gate losses are very much reduced. Electrical isolation is also provided between the main device supply and its gating signals.