1. Dr. Unnikrishnan P.C. Professor, EEE
EE305 Power Electronics
Dr. Unnikrishnan P.C.
Professor, EEE

2. Main features of firing circuits
In Thyristor Converters:
Power Circuit: High Voltage, >100 V
Gate Circuit : Low Voltage, 12 to 30 V
Isolation required between thyristor and gate pulse generating circuit- either by pulse transformers or opto-couplers

3. How to trigger the thyristor ?
R Triggering
RC triggering
UJT triggering
Pulse triggering using digital control

4. Half wave rectifier with R triggering
R1-limits gate current
R2-variable resistor to vary the firing angle
Rg-gate stabilizing resistor limits gate voltage
D –prevents gate current during negative half cycle of the ac supply voltage
We can achieve maximum firing angle up to 90°.

5. Design of R1
If R2 = 0 , gate current flows through RL, R1 , D and gate to cathode

6. Design of Rg Drop across Rg should not go beyond Vgm
Rg is high when R2=0

7. By varying R2 vGK can be varied
Design Steps
When OFF
By varying R2 vGK can be varied

8. Gate Threshold voltage
With different values of R2
Vgk
VGp>VGT
VGp=VGT
VGT
VGp<VGT
90°
<90° ωt
0 ° <α<= 90°
Triggering gate voltage, VGT: This is the voltage to apply across gate and cathode to reach the IGT current and then to trigger the device.

9. Half wave rectifier with RC Triggering
In the positive half cycle, the capacitor is charged through R1 and R2 up to the peak value of the applied voltage.
The variable resistor R2 controls the charging time of the capacitor. Higher R2 means higher time constant, more time required to get charged to gate trigger voltage, Large firing angle. We can achieve firing angle more than 90°.
A varying voltage applied across the gate and the cathode.
When this voltage reaches minimum gate trigger voltage ,thyristor is fired and turns ON.

10. Half wave rectifier with RC Triggering
A varying voltage applied across the gate and the cathode.
When this voltage reaches minimum gate trigger voltage ,thyristor is fired and turns ON.
In the negative half cycle, capacitor C is charged up to the negative peak value through resistors with Vab negative.
Diode D1 is used to prevent the reverse break down of the gate cathode junction in the negative half cycle.

11. Uni Junction Transistor (UJT)
Peak-Point Emitter Current. Ip. It is the minimum emitter current required to trigger the device (UJT).
Valley Point Voltage VV Emitter voltage at the valley point.

12. Uni Junction Transistor (UJT)

13. UJT Triggering UJT as Relaxation Oscillator
Can produce sharp pulses at constant frequency.
When Vs is applied, C charges through R, at a time constant RC

14. UJT as Relaxation Oscillator
When Vc reaches peak point voltage Vp UJT conducts and C discharges through R1
Produces pulses across R1.
This pulse is applied across the gate of the thyristor
When Vc less than the Valley voltage Vv, UJT turns OFF

15. Line Synchronised Triggering circuit
When the input is AC supply, some method is to be adopted to synchronize its oscillations with the frequency of AC supply.
For this, supply voltage of relaxation oscillator should be taken from AC supply through a diode rectifier and a zener diode.
Zener diode clips the voltage at VBB

16. Line Synchronized UJT Triggering Circuit
courtesy Motorola Semiconductor Products Inc.

17. Pulse triggering using digital control

18. Generation of trigger pulse using microcontroller

19. Optocouplers
Consists of IR LED & photo transistor
Provides electrical isolation b/w control ckt. & power ckt.
When the IR LED is turned ON, photo transistor also turns ON