Turn-On Methods Of A Thyristor
A thyristor operated can be non conducting state to conducting state in many ways explained as follows
- Forward Voltage Triggering
- Temperature Triggering (Thermal Triggering)
- Light Triggering (Radiation Triggering)
- dv/dt triggering
- Gate triggering
- D.C. Gate triggering
- A.C. Gate triggering
- Pulse Gate triggering
1. Forward Voltage Triggering
The anode – to – cathode forward voltage is increased and gate circuit kept open, reverse biased junction J2 will have an avalanche breakdown at a voltage like as forward break over voltage vBO. the break over voltage the SCR change into off state to on state (high voltage with low leakage current) . the thyristor voltage drop across during forward ON state.
2. Temperature Triggering (Thermal Triggering)
The Temperature Triggering like any other semiconductor , the width of the depletion layer of a thyristor decreased when on increasing the junction temperature, the breakdown voltage between the anode and cathode is very near, the thyristor can be triggered by increasing junction temperature. By changing the temperature value to a operation point (with in specified – limits) , a circumstances comes when the reverse biased junction get collapses create the device conduct. In this trigger is called “Temperature Triggering”.
3. Light Triggering (Radiation Triggering)
In this triggering method by radiation imparted the energy. Thyristor is attacked by a energy molecules such as photons (or) neutrons. With the help of this imparted external energy, electron and hole pairs are created the device , and same time the number of charge carrier increased. That lead to instant current flow to current in the device and triggering device. By occurring radiation the device must have high value of rate of change of voltage (dv/dt). this type triggering can achieved one of the example is LASCR (LIGHT ACTIVATED SILICON CONTROLLED RECTIFIER)
4.DV/DT Triggering
We know that the Junction J1 and J2 forward bias at forward voltage across the anode and cathode of the thyristor. Where as junction J3 become reverse bias. This reverse biased junction J2 the junction capacitor due to charging existing current will flow tending to turn on the device. if the voltage impressed across the device is denoted by V, the charge by Q and capacitance by Cj , then
ic =dQ/dt = d(Cj*v)/dt =(Cj dV/dt)+ (V* dCj / dt)
the rate of change of junction capacitance is also negligible because the junction capacitance is constant. The offering to charging current by the later term is negligible the above equation can change as
ic = Cj dV/dt
Since the device is larger across the rate of charge of voltage, the device may turn-on while though the voltage appearing across the device is less.
5. Gate Triggering
In this method used commonly triggering SCRs. In practically (laboratory) almost all the SCR devices are triggered by this process. the gate terminal the positive signal by applying at of the device, it can be activate much before the stated break over voltage. The conduction period of the SCR can be controlled by varying the gate signal within specified values of the minimum and maximum gate currents.
For a gate pulse applied between the gate and the cathode of the devices. Three types of pulse signal can be used for this purpose. The triggering signal dc or pulse signal or ac signals.
DC Gate triggering
DC gate triggering applied dc voltage of proper magnitude and polarity is between the gate and the cathode of the device in such a way that the gate becomes positive with respect to the cathode. When the applied voltage is enough to produce the required pulsating(gate) current , the thyristor turn on . One drawback of this scheme is that both the power and control circuits are dc and there is no isolation between the two.
this process is that a continuous dc signal has to be applied, at the gate causing more gate power loss disadvantage of the triggering.
AC Gate triggering
AC Gate triggering is mainly used for the gate signal in all application of thyristor control adopted for ac applications. This strategy provides the proper isolation between the power and the control circuits, the triggering angle control is attained very conveniently by changing the phase angle of the control signal.
But, the gate drive is maintained for one-half cycle after the device is turned ON, the voltage is applied between the gate and cathode during the negative half cycle . the drawback of this triggering is that a separate transformer is required to step down ac supply. Which adds to the cost.
Pulse Gate triggering
This triggering popularly used in device. The gate device consists of a single pulse appearing periodically or a sequence of high frequency pulses. This is called as carrier frequency gating. For isolation purpose the pulse transformer . The most important advantage is there is no need of applying continuous signals and since, the gate losses are very much reduced . its is also provided electrical isolation between the main device supply and its gating signal.