Part of Lecture series on EE321N, Power Electronics-I delivered by me during Fifth Semester of B.Tech. Electrical Engg., 2012 Z H College of Engg. & Technology, Aligarh Muslim University, Aligarh Please comment and feel free to ask anything related. Thanks!

1. EE-321 N
Lecture-13
IC Based Triggering Circuit

2. Design Problem Based on
UJT Trigger Circuit
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3. A UJT based triggering circuit has the following
specifications:
h = 0.65, Ip = 0.8 mA, Vp = 20 V, Iv = 4 mA,
Vv = 0.7 V, VD = 0.5 V, RBB = 5 kΩ, VGT = 0.4 V,
C = 0.05 µF, f = 2 kHz. Determine
(a) The values of R1, R2, R
(b) Check whether R is within permissible limits
(c) Values of RB1 & RB2
(d) Range of oscillator frequency
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4. Solution
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1 1
ln
1
1
1
ln
1
T RC
f
R
f C
h
h
 
   
 
  
 
    
p D
p BB D BB
V V
V V V Vh
h

    
(a)

5. Solution...contd
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4
2
10
BB
R
Vh
 
1
GT 1
1 2
BB
BB
V R
V R
R R R
  
 

6. Solution...contd
(b)
(c)
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min max
BB pBB v
v p
R R R
V VV V
R
I I
 

 
1
1
2 1
B
B
BB
B BB B
R
R
R
R R R
h   
  

7. Solution...contd
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min min max
max max min
1
ln
1
1
ln
1
T R C f
T R C f
h
h
 
   
 
 
   
 
(d)

8. Siemens Phase Control IC
TCA 785
• This phase control IC is intended to control
thyristors, triacs, and transistors. The trigger
pulses can be shifted within a phase angle
between 0° and 180°.
• Typical applications include converter
circuits, AC controllers and three-phase
current controllers.
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9. Contd...
Main Features:
• Reliable recognition of zero passage
• Large application scope
• May be used as zero point switch
• Three-phase operation possible (3 ICs)
• Output current 250 mA
• Large ramp current range
• Wide temperature range
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10. 31-Oct-12 EE-321N, Lec-13 10
Pin Diagram

11. TCA 785 Based Trigger Circuit
for SCR & TRIAC
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12. 31-Oct-12 12

13. Working
• DC power synchronized with AC mains is
supplied to the IC
• Ramp signal is generated internally in the IC
and compared with a control signal
• Trigger pulse is obtained when both the
signals are equal
• Pin 15 provides pulses in the +ve half cycle
whereas pin 14 provides pulses in –ve half
cycle (for TRIAC)
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15. Further Resources
1. Moreno et. al., “The Design of a Digital IC for
Thyristor Triggering”, Proceedings of Tenth
International Conference on VLSI Design,
1997.
2. Siemens TCA 785 datasheet (Available online)
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16. Home Assignment
Q3. Define Driver & Isolation circuits. Discuss
driver & isolation circuits for thyristors.
Emphasize on the use of pulse transformers
in such circuits.
Due Date 10. 11. 12, Saturday
Assignments will be submitted in the class
at the beginning of the lecture
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