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. Unit-2
Thyristor Triggering Circuits

2. EE-321 N
Lecture-8
Thyristor Gate Characteristics

3. Typical Switching Arrangement for
SCR
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IG

4. Introduction
• A thyristor can be triggered by the application of
+VG and hence a +IG supplied from a gate drive
circuit as shown
• However, the choice of (VG , IG) is not restricted to
a particular value, rather it varies over a wide
range
• It depends on the gate characteristics of the
device as shown next
• Any operating point (VG,IG) within the
safe/preferred gate drive area can be selected
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5. Typical Gate Characteristics of SCR
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t
u
v’t’
v
u’

6. Explanation
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• Above characteristics show the variation of VG
as a function of IG
• Some authors refer to this as characteristics
that of a diode b’coz G-K junction is a pn
junction
• The preferred gate drive area is mtt’uu’vv’m
which is bounded by the curves 1 to 5

7. Contd...
• Curves 1 & 2 correspond to the max possible
spread of the characteristics of the thyristor of
the same type. This is due to inadvertent
difference in doping of p and n layers
• Curve 3 represents a constant value of the
maximum permissible gate power dissipation
PGM
• Lines 4 & 5 outline the max limits for gate
voltage VGM and max gate current IGM
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8. Contd...
• VGD is the minimum gate voltage below which
a thyristor cannot be triggered at any
temperature (normally up to 100 °C)
• If the firing ckt generates a +ve gate signal
before the desired instant of triggering, it
should be ensured that it is < VGD. Similarly, all
noise signals should be < VGD
• Rectangle mnop is the area of unsuccessful
triggering and is specified by manufacturers
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9. Contd...
• For lower temperature, VGT and IGT increase
and therefore the size of the rectangle mnop
increases
• It should also be ensured that the gate circuit
load line should pass through the safe drive
area
• Most preferred zone is close to curve 3 which
in turn ↑s di/dt capability, minimizes turn ON
time and ↓s switching losses
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10. Design Considerations
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Next Turn

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