The document discusses the triggering characteristics and gate drive circuits for thyristors, highlighting the range of gate voltage (vg) and gate current (ig) that can be utilized for effective triggering. It explains the preferred gate drive area and the significance of certain parameters like maximum gate power dissipation and the minimum gate voltage for successful triggering. Design considerations emphasize ensuring that the gate circuit operates within safe parameters to optimize performance and minimize switching losses.

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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