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-9
Thyristor Gate Characteristics...contd
Design Consideration

2. Design Considerations
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3. Contd...
• The design of the firing circuit can be carried
out with the help of the above figure.
• The first triggering ckt feeds power to the
gate-cathode ckt of the SCR and satisfies the
following equation
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s g g sE V I R 

4. Contd...
• Where,
• The value of Rs should be such that the
current Es/Rs does not cause any damage to
the source or to the SCR. Otherwise an
external resistance may be added in the circuit
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Es = Source voltage
Vg = G-K voltage
Ig = Gate current
Rs = Source internal resistance

5. Contd...
• A resistance R1 is also connected across G-K
terminals which provides an easy path to the flow
of leakage current between SCR terminals.
• If Ig(min) & Vg(min) are the min. gate current and
voltage to turn ON an SCR, then current through
R1 is Vg(min) / R1 & the ckt eqn. now becomes
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(min)
(min) (min)
1
g
s g g s
V
E V I R
R
 
   
 

6. Contd...
• Intersection of the load line of the triggering
source and the gate characteristics would give
the operating (triggering) point
• In order to minimize the turn ON time and
avoid unreliable turn ON, the triggering pt.
should within the safe gate drive area and as
close as possible to the power dissipation
curve (curve 3 in gate characteristics)
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7. Different Types of Triggering Signals
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8. 16-Oct-12 EE-321N, Lec-9 8
AC Signal
Short Duration Pulse
Long Duration Pulse
(as good as DC)
HF train/sequence
of pulses

9. 1. DC Signal
• A DC voltage of proper magnitude and polarity is
applied between the G & K of SCR such that G
becomes +ve w. r. t. K.
• When the applied voltage is sufficient to produce
the required gate current (IL), the device starts
conduction
• However, continuous application of DC signal
causes gate power dissipation & also there is no
isolation between the power & control circuit
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10. 2. AC Signal
• Most commonly used for SCRs used in AC
applications
• Provides proper isolation between the control &
power circuit
• Convenient control of firing angle is obtained by
changing the phase angle of the control signal
• However, gate drive is present only for half cycle
and presence of S/D transformer adds to the cost
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11. 3. Pulse Signal
• Most popular method of triggering the SCR
• Gate drive consists of a single pulse or a
sequence of HF pulses appearing periodically
• Requires pulse T/F for isolation
• The main advantage is that there is no need of
applying the signal continuously which
reduces the gate power dissipation drastically
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12. Contd...
• However, the average gate power dissipation
Pgav must be < the peak instantaneous gate
power dissipation Pgm
• If δ is the duty ratio/cycle, then
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gav gmP P 

13. Basic Requirements
Whatever be the method adopted for triggering,
the triggering signal has to satisfy the following
basic requirements:
 Signal should be of adequate amplitude &
sufficiently short rise time.
 It should be of adequate duration.
 It should occur at a time when the main
circuit conditions are favorable for
conduction
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14. Problem 1
An SCR has Vg-Ig characteristic given by the
equation
In a certain application, the gate voltage consists
of rectangular pulses of 12 V and of duration
50 µs with duty cycle 0.2. Determine:
(a) The value of external series resistance to
limit the power dissipation to 5 W.
(b) Average power dissipation
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1.5 8g gV I 

15. Problem 2
For an SCR, the G-K characteristic is given by a
straight line with gradient 16 V/amp passing
through the origin, the max turn ON time is 4 µs &
the min gate current to turn ON is 500 mA. If the
gate voltage is 15 V, determine:
(a) Resistance to be connected in series with the
SCR gate
(b) Gate power dissipation, given that pulse width
is equal to the turn ON time and the avg gate
power dissipation is 0.3 W.
(c) Max triggering frequency that will be possible
with pulse firing
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16. Solution on Board
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