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Construction of silicon control rectifier (SCR)

 SCR- silicon control rectifier

Construction

1. Anode and cathode region are highly drop where gate as moderately drop and drift layer are lightly drop.

2. SCR can be visualized as too BJT (PNP) and ( n p n) connected together

3. SCR is the minority carrier device and it has three p n junction .


Disadvantage of SCR

1. Switching is very least while T-off highest 

2. More switching loss

3. More conductivity modulation , least R-oN 

4. Minimum oN state drop and minimum conductivity loss 


# SCR contains three p n junction

J1 anode and drift - anode is highly drop , depletion low

J2 drift and gate - lightly drop , depletion low

J3 gate and cathode - highly drop , depletion low


Since there is light dropping on both side of J2 

So breakdown voltage is high for junction J2 



Reverse blocking


Let , 

          Anode is connected to negative terminal of supply which cathode is connected to positive terminal of supply 



1. J1 and J2 reverse biased while J2 forward biased 

2. Outer junction are reverse biased while iner junction is forward biased.

3. The entire reverse voltage is handled by J1 and  J3 while J2 doesn't handle much voltage

4.  Reverse breakdown voltage depends on breakdown voltage of J1 and J3 


5.  Depletion layer at J1 and J3 doesn't flow of majority carriers i.e hole from anode and electron from cathode.


Forward blocking


Anode is connected to positive terminal of battery and cathode to nagative terminal of battery


1. J1 , J3 forward biased and J2 is reverse biased

2. Outer junction are forward and inner junction is reverse biased 

3.  The entire forward voltage appear across junction J2

4.  Forward blocking capability depends on J2

5. Since breakdown voltage of J2 is highly drop due to light doping forward  breakdown voltage of SCR is generally more than reverse breakdown voltage .

Then it is asymmetric SCR .


Reverse in J2 allow a flow of minority carrier 

       Hence leakage current exist.


Forward conduction


1. When the forward voltage is greater than forward breakdown voltage junction j2 breakdown and a flow charge carrier flow .

2.  When such change carrier flow through device conductivity modulation take place in drift layer and R-oN rapidly reduces

     Voltage drop across device reduce and We see a negative resistance characteristics


Significant of gate current


1.  When the voltage is applied between gate and cathode , gate cathode junction  becomes forward biased.

2. Forward biased junction allows the flow of majority charge carrier, so electron entire in to gate region from cathode and holes from gate to cathode 

3. Electron gate region recombined with hole but gate is moderately dropped 

      So most of electron  are swept by E- field in to drift layer.

4. To compensate these electron battery between anode and cathode supplies holes to anode region.

5.   Some hole recombined with electron but restare swept by electric field in to gate.

6.  The hole from anode now acts as gate current and a regenerative process is setup .

7.   As gate current increase , more charges are available in device .

8.  Due to more charges present in device breakdown voltage is lowest.

   Breakdown voltage proposal to one bye charge density.


Forward breakdown voltage of devices reduces  as gate current increase.



Important question and answer


1.  A thyristor with the standard terminals of anode (A) , cathode (k) and gate (G) and the different junction J1 , J2 and J3 . when the thyristor is turned on and conducting- 


(A) J1 ,J2  are forward biased and J3 is reverse  biased

(B)  J1 and J3 are forward biased and J2 is reverse biased .

(C).  J2 and J3 is reverse biased and J1 is forward biased.

(D).  J1 ,  J2  and J3 are all forward biased.


Answer-(D)


2.  When cathode of thyristor model more positive then it s anode -

(A)  all the junction are forward biased 

(B). Outer junction are reverse biased and centre one is forward biased .

(C).  Outer junction are forward biased and centre one is reverse biased 

(D) all the junction are reverse biased


Answer - (B)


3.  The main advantage of IGBT over SCR in power electronics is -


(A) reduced weight

(B) self commutating capability

(C) very high reliability

(D) self cooling property


Answer - (B)


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