Power electronics digital notes b. Tech III year

(ii) RC Triggering 
The following circuit shows the resistance-capacitance triggering. 
Figure: 1. 15. Resistance Capacitance triggering circuit of SCR 

---

18 | 
P a g e

By using this method we can achieve firing angle more than 90°. 

In the positive half cycle, the capacitor is charged through the variable resistance R up to the peak 
value of the applied voltage. 

The variable resistor R controls the charging time of the capacitor. 

Depends upon the voltage across the capacitor, when sufficient amount of gate current will flow in 
the circuit, the SCR starts to conduct. 

In the negative half cycle, the capacitor C is charged up to the negative peak value through the 
diode D2. 

Diode D1 is used to prevent the reverse break down of the gate cathode junction in the negative half 
cycle. 
3. Pulse Gate Triggering:- 
 

In this method the gate drive consists of a single pulse appearing periodically (or) a sequence of 
high frequency pulses. 

This is known as carrier frequency gating. 

A pulse transformer is used for isolation.

The main advantage is that there is no need of applying continuous signals, so the gate losses are 
reduced. 
Advantages of pulse train triggering: 

Low gate dissipation at higher gate current. 

Small gate isolating pulse transformer 

Low dissipation in reverse biased condition is possible. So simple trigger circuits are possible in 
some cases 

When the first trigger pulse fails to trigger the SCR, the following pulses can succeed in latching 
SCR. This important while 

Triggering inductive circuits and circuits having back emf's. 
Turn off methods of SCR: 
SCR can be turned ON by applying appropriate positive gate voltage between the gate and cathode 
terminals, but it cannot be turned OFF through the gate terminal. The SCR can be brought back to the 

---

19 | 
P a g e
forward blocking state from the forward conduction state by reducing the anode or forward current below 
the holding current level. 
The turn OFF process of an SCR is called
 commutation
. The term commutation means the transfer of 
currents from one path to another. So the commutation circuit does this job by reducing the forward 
current to zero so as to turn OFF the SCR or Thyristor. 
To turn OFF the conducting SCR the below conditions must be satisfied. 

The anode or forward current of SCR must be reduced to zero or below the level of holding 
current and then, 

A sufficient reverse voltage must be applied across the SCR to regain its forward blocking state. 
When the SCR is turned OFF by reducing forward current to zero there exist excess charge carriers in 
different layers. To regain the forward blocking state of an SCR, these excess carriers must be 
recombined. Therefore, this recombination process is accelerated by applying a reverse voltage across the 
SCR.