Thyristors - Thyristor A thyristor is one of the most...

20/03/2017
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Thyristors and their application
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• A thyristor is one of the most important types of
power semiconductor devices.
• Compared to transistors, thyristors have lower on-
state conduction losses and higher power handling
capability.
• However, they have worse switching performances
than transistors.
• Name ‘thyristor’, is derived by a combination of the
capital letters from THYRatron and transISTOR .
Thyristor
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• Thyristors are four-layer pnpn power
semiconductor devices.
• These devices switch between conducting and
nonconducting states in response to a control
signal.
• Thyristors are used in timing circuits, AC motor
speed control and switching circuits.
Thyristors …
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Thyristors …
• Bell Laboratories were the first to fabricate a silicon-
based thyristor.
• Its first prototype was introduced by GE (USA) in 1957.
• Later on many other devices having characteristics
similar to those of a thyristor were developed.
• The semiconductor devices are: SCR, SCS, Triac, Diac,
PUT, GTO, e.t.c.
• This family of semiconductor devices is given the
name thyristors.
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Thyristor/ SCR
• SCR is a three terminal, four layers solid state
semiconductor device, each layer consisting
of alternately N-type or P-type material, i.e;
P-N-P-N,
• It can handle high currents and high voltages,
with better switching speed and improved
breakdown voltage .
A K
G
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Thyristor/ SCR…
• Thyristor can handle high currents and high voltages.
• Typical ratings are 1.5kA & 10kV which corresponds to 15MW
power handling capacity.
• This power can be controlled by a gate current of about 1A
only.
• Thyristor acts as a bistable switch.
– It conducts when gate receives a current pulse, and
continue to conduct as long as forward biased (till device
voltage is not reversed).
– They stay ON once they are triggered, and will go OFF only
if current is too low or when triggered off.
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Thyristor/ SCR Operation
• When the anode voltage is made
positive with respect to the cathode,
junctions J
1
and J
3
are forward biased
and junction J
2
is reverse biased.
• The thyristor is said to be in
the forward blocking or off-state
condition .
• A small leakage current flows from
anode to cathode and is called the off-
state current .
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Thyristor/ SCR Operation…
• If the anode voltage V
AK
is increased to a
sufficiently large value, the reverse biased
junction J
2
would breakdown.
• This is known as avalanche breakdown and the
corresponding voltage is called the forward
breakdown voltage V
BO
.
• Since the other two junctions J
1
and J
3
are
already forward biased, there will be free
movement of carriers across all three junctions.
• This results in a large forward current and the
device is now said to be in a conducting or on-
state .