SCR stands for silicon controlled rectifier.it belongs to thyristor family .In the thyristor family most efficient and most using controlled device is SCR.SCR is gate controlled device,to operate in conducting mode we should trigger.some triggering methods of scr to turn on.
It is internally developed mechanism. When the SCR is in forward blocking state J2 behaves as capacitor due to charges existing across junction. It is clearly understood from the fig given below. When switch is open then V1=0 if it is closed then V1=100 and if we consider dt=1ms
Extra Point :
Various TURN-ON methods of Thyristor or SCR :-
When anode is positive with respect to cathode there are different methods to turn ON thyristor.
1. Forward voltage triggering
2. Gate triggering
3. dv/dt triggering
4.Temperature triggering
5. Light triggering.
1. Forward voltage triggering:
When positive of the supply is connected to anode and negative of the supply is connected to cathode then J1 and J3 are forward biased and J2 is reverse biased as described in above section. Now if the forward voltage that is supply voltage is increased then the temperature increases and electron holes pairs are created which leads to avalanche breakdown so this leads to breakage of barrier and large current flows in forward direction. So the thyristor comes into conduction in this way due to avalanche breakdown. The voltage at which breakage of junction takes place is called forward break over voltage(VBO).At VBO thyristor comes from off state to ON state. At off state it acts as open circuit so voltage is high and current is less at ON state it acts as short circuit so voltage is less and current is high. But generally this method is not preferred. The device can bear only the VBO voltage if the applied voltage exceeds VBO it damages because of increase in temperature and breakdown of barrier i.e; reverse biased junction and flow of large amount of current. So the final voltage rating of the thyristor is considered as VBO.
2. Gate triggering :
When anode is connected to positive terminal of supply and cathode to negative terminal of supply whose value is less than VBO and when supply is given to gate through Es then more electrons from outer n layer reaches inner P layer since current flows from inner p layer to outer n layer(convectional current direction is opposite to direction flow of electrons) and also outer n layer is heavily doped compared to inner p layer so as result inner p becomes more positive than inner n layer so potential barrier at J2 decreases as a result breakdown of the reverse biased junction takes place at a voltage less than breakdown voltage VBO. The voltage at which thyristor will turn on depends on the current flowing from gate to cathode. As flow of current from anode to cathode increases voltage at which thyristor turns on decreases. Once the thyristor comes into ON state and supply to gate is removed then also thyristor will be in ON state.
LATCHING CURRENT: Gate supply can be removed only after significant anode current is reached before this if we remove gate supply then thyristor will remain in OFF state only. So the minimum anode current above which the thyristor will remain in ON state even if gate signal is removed is called as latching current.
HOLDING CURRENT: Once the thyristor starts conducting gate will lose control so in order to turn OFF thyristor the anode current must be reduced below a low level value called holding current then only thyristor will turn OFF.
Generally latching current is more than holding current.
3. dv/dt triggering:
Cj=1uf
i=Cdv/dt
=1*10-6 *(100-0/10-3)
=0.1=100ma.
If this current is more than rated gate current of SCR then SCR starts conducting. While applying the forward voltage SCR will start conducting sometimes without giving trigger pulse. The junction capacitance is subjected to rate of change of voltage(dv/dt), it results in charging current(I=C*dv/dt). If this current is more then thyristor conducts(due to availability of free charges inJ2) since breakdown of J2 place due to increase in rate of change of voltage and so charging current flows. This is not applicable because we cannot have exact control on thyristor turn on time. So Ic must be 0 but it is practically not possible so we have to maintain low dv/dt value. If rate of change of voltage is high then SCR will turn on even when the voltage across device is small.4.Temperature triggering:
When SCR is in forward blocking mode then supply voltage is applied across it whose voltage is less than VBO some forward leakage current will occur so junction temperature increases if we are able to supply more temperature at this stage then break down takes place at voltage below VBO that is at supply voltage only, because increase in temperature leads to formation of electron hole pairs and SCR will start conducting.
5. Light triggering:
For light triggering of SCR a recess is made in inner P layer as shown in the figure. Then a light of appropriate wavelength is irradiated on it. So electrons are ejected from this layer due to photoelectric effect and thyristor will be in conduction due to availability of free electrons in reverse biased J2 . A LASCR will have light source or a gate supply or both. If both are present the Es voltage is applied to gate at a voltage less than the voltage required to turn on thyristor and light is irradiated on it so thyristor will turn ON at less gate voltage. Higher the voltage applied to gate and cathode lower the light intensity required.
In the above various TURN-ON methods of Thyristor or SCR we prefer to use gate triggering to turn on thyristor.
Extra Point :
The main difference between forward voltage triggering and dv/dt triggering is that in forward voltage triggering directly supply voltage is applied without any switch and applied voltage is gradually increased above VBO and conduction takes place due to avalanche breakdown but in dv/dt triggering switch is used so rate of change of voltage occurs and thyristor turns on due to charging current(if it is more than rated current of SCR).
