EE-321 N

Lecture-13

IC Based Triggering Circuit

Design Problem Based on UJT Trigger Circuit

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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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Solution

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1 1ln

1

1

1ln

1

T RCf

R

f C

h

h

p D

p BB D BB

V VV V V Vh

h

(a)

Solution...contd

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4

2

10

BB

RVh

1GT 1

1 2

BB

BB

V RV R

R R R

Solution...contd

(b)

(c)

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min max

BB pBB v

v p

R R R

V VV VR

I I

11

2 1

BB

BB

B BB B

RR

R

R R R

h

Solution...contd

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min min max

max max min

1ln

1

1ln

1

T R C f

T R C f

h

h

(d)

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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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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Pin Diagram

TCA 785 Based Trigger Circuit for SCR & TRIAC

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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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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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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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