Download Lecture-5: Thyristors - Dr. Imtiaz Hussain

Power Electronics
Lecture-5
Thyristors
Dr. Imtiaz Hussain
Assistant Professor
email: [email protected]
URL :http://imtiazhussainkalwar.weebly.com/
1
Introduction
• One of the most important type
semiconductor device.
of power
• Compared to transistors, thyristors have lower onstate 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.
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Introduction
• Thyristors
are
four-layer
semiconductor devices.
pnpn
power
• 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.
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Thyristors
• Bell Laboratories were the first to fabricate a siliconbased thyristor.
• Its first prototype was introduced by GE (USA) in 1957.
• Later on many other devices having characteristics
similar to of a thyristor were developed.
• These semiconductor devices are SCR, SCS, Triac, Diac,
PUT, GTO, e.t.c.
• This whole 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 .
K
A
G
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Thyristor/ SCR
• Thyristor can handle high currents and high voltages.
• Typical rating are 1.5kA & 10kV which responds 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 J1 and J3 are forward biased
and junction J2 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 offstate current.
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Thyristor/ SCR Operation
• If the anode voltage VAK is increased to a
sufficiently large value, the reverse biased
junction J2 would breakdown.
•
This is known as avalanche breakdown and the
corresponding voltage is called the forward
breakdown voltage VBO.
• Since the other two junctions J1 and J3 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 onstate.
• The voltage drop across the device in the onstate is due to the ohmic drop in the four layers
and is very small (in the region of 1 V).
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Thyristor/ SCR
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Thyristor Operating modes
Thyristors have three modes :
• Forward blocking mode:
Only leakage current flows,
so
thyristor
is
not
conducting.
• Forward conducting mode:
large forward current flows
through the thyristor.
• Reverse blocking mode:
When cathode voltage is
increased
to
reverse
breakdown
voltage
,
Avalanche
breakdown
occurs and large current
flows.
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Important characteristics
Latching Current IL
• This is the minimum anode current required to maintain
the thyristor in the on-state immediately after a thyristor
has been turned on and the gate signal has been removed.
• If a gate current greater than the threshold gate current is
applied until the anode current is greater than the latching
current IL then the thyristor will be turned on or triggered.
Holding Current IH
• This is the minimum anode current required to maintain
the thyristor in the on-state.
• To turn off a thyristor, the forward anode current must be
reduced below its holding current for a sufficient time for
mobile charge carriers to vacate the junction.
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Important characteristics
Reverse Current IR
• When the cathode voltage is positive with respect to the
anode, the junction J2 is forward biased but
junctions J1 and J3 are reverse biased. The thyristor is said
to be in the reverse blocking state and a reverse leakage
current known as reverse current IR will flow through the
device.
Forward Breakover Voltage VBO
• If the forward voltage VAK is increased beyond VBO , the
thyristor can be turned on. But such a turn-on could be
destructive. In practice the forward voltage is maintained
below VBO and the thyristor is turned on by applying a
positive gate signal between gate and cathode.
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Turn-on Characteristics
ton  td  tr
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13
Turn-off Characteristics
VAK
tC
tq
t
IA
Anode current
begins to
decrease
Commutation
di
dt
Recovery
t1 t2
Recombination
t3
t4
t5
t
tq=device
off time
tc=circuit
off time
tgr
trr
tq
tc
14
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The SCR can be turned on at its gate terminal.
With a dc
source, the
SCR stays
on after
it is gated.
Load
Anode
Cathode
Load
current
Gate
Gate pulse
occurs here
Time
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With an ac
source, the
SCR turns
off at the
zero-crossing.
Load
Anode
Cathode
Gate
Load
current
off
on
Gate pulse
occurs here
Time
Turns off here
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The gate can
be pulsed for
each positive
alternation.
Load
Anode
Cathode
Load
current
Gate
Time
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Load
Anode
Cathode
Gate
Load
current
The average
load current
can be
decreased
by gating
the SCR later.
Time
18
…. and later.
Load
Anode
Cathode
Load
current
Gate
Time
19
…. or, not
at all.
Load
Anode
Cathode
Load
current
Gate
No gate pulses: ILoad = 0
0
Time
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Thyristor turn-ON methods
•
Thyristor turning ON is also known as Triggering.
•
With anode is positive with respect to cathode, a thyristor
can be turned ON by any one of the following techniques :
–
–
–
–
–
Forward voltage triggering
Gate triggering
dv/dt triggering
Temperature triggering
Light triggering
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Forward Voltage Triggering
• When breakover voltage (VBO) across a thyristor is exceeded
than the rated maximum voltage of the device, thyristor
turns ON.
• At the breakover voltage the value of the thyristor anode
current is called the latching current (IL) .
• Breakover voltage triggering is not normally used as a
triggering method, and most circuit designs attempt to avoid
its occurrence.
• When a thyristor is triggered by exceeding VBO, the fall time
of the forward voltage is quite low (about 1/20th of the time
taken when the thyristor is gate-triggered).
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Gate Triggering
• Turning ON of thyristors by gate triggering is simple and
efficient method of firing the forward biased SCRs.
• In Gate Triggering, thyristor with forward breakover
voltage (VBO), higher than the normal working voltage is
chosen.
• Whenever thyristor’s turn-ON is required, a positive gate
voltage b/w gate and cathode is applied.
• Forward voltage at which device switches to on-state
depends upon the magnitude of gate current.
– Higher the gate current, lower is the forward
breakover voltage .
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Gate Triggering
• Turning ON of thyristors by gate triggering is simple and
efficient method of firing the forward biased SCRs.
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dv/dt triggering
• With forward voltage across anode & cathode of a thyristor, two
outer junctions (A & C) are forward biased but the inner junction
(J2) is reverse biased.
• The reversed biased junction J2 behaves like a capacitor because of
the space-charge present there.
• As p-n junction has capacitance, so larger the junction area the
larger the capacitance.
• If a voltage ramp is applied across the anode-to-cathode, a current
will flow in the device to charge the device capacitance according
to the relation:
• If the charging current becomes large enough, density of moving
current carriers in the device induces switch-on.
• This method of triggering is not desirable because high charging
current (Ic) may damage the thyristor.
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Temperature Triggering
• During forward blocking, most of the applied voltage
appears across reverse biased junction J2.
• This voltage across junction J2 associated with leakage
current may raise the temperature of this junction.
• With increase in temperature, leakage current through
junction J2 further increases.
• This cumulative process may turn on the SCR at some high
temperature.
• High temperature triggering may cause Thermal runaway
and is generally avoided.
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Light Triggering
• In this method light particles (photons) are made to
strike the reverse biased junction, which causes an
increase in the number of electron hole pairs and
triggering of the thyristor.
• For light-triggered SCRs, a slot (niche) is made in the
inner p-layer.
• When it is irradiated, free charge carriers are
generated just like when gate signal is applied b/w
gate and cathode.
• Pulse light of appropriate wavelength is guided by
optical fibers for irradiation.
• If the intensity of this light thrown on the recess
exceeds a certain value, forward-biased SCR is turned
on. Such a thyristor is known as light-activated SCR
(LASCR).
• Light-triggered thyristors is mostly used in highvoltage direct current (HVDC) transmission systems.
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Thyristor Gate Control Methods
• An easy method to switch ON a SCR into conduction is to apply
a proper positive signal to the gate.
• This signal should be applied when the thyristor is forward
biased and should be removed after the device has been
switched ON.
• Thyristor turn ON time should be in range of 1-4 micro
seconds, while turn-OFF time must be between 8-50 micro
seconds.
• Thyristor gate signal can be of three varieties.
– D.C Gate signal
– A.C Gate Signal
– Pulse
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Thyristor Gate Control Methods
D.C Gate signal: Application of a d.c gate signal causes the flow
of gate current which triggers the SCR.
– Disadvantage is that the gate signal has to be continuously
applied, resulting in power loss.
– Gate control circuit is also not isolated from the main
power circuit.
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Thyristor Gate Control Methods
A.C Gate Signal: In this method a phase - shifted a.c voltage derived from
the mains supplies the gate signal.
– Instant of firing can be controlled by phase angle control of the gate
signal.
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Thyristor Gate Control Methods
Pulse: Here the SCR is triggered by the application of a positive pulse of
correct magnitude.
– For Thyristors it is important to switched ON at proper instants in a
certain sequence.
– This can be done by train of the high frequency pulses at proper
instants through a logic circuit.
– A pulse transformer is used for circuit isolation.
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Thyristor Commutation
• Commutation: Process of turning off a conducting thyristor
• SCR cannot be turned OFF via the gate terminal.
• It will turn-off only after the anode current is negated
either naturally or using forced commutation techniques.
• Therefore, commutation can be classified as
– Natural commutation
– Forced commutation
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Line Commutation (Natural Commutation)
• Occurs only in AC circuits.
• Natural Commutation of thyristor takes place in
– AC Voltage Regulators
– Phase controlled rectifiers
– Cycloconverters
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Thyristor Turn-Off: Line-Commutated Thyristor Circuit
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Forced Commutation
• Applied to d.c circuits.
• If a thyristor is used in a DC circuit, when first turned on, it will stay
on until the current goes to zero. To turn off the thyristor it is
possible to use a Forced commutation circuit. The circuit creates a
reverse voltage over the thyristor (and a small reverse current) for
a short time, but long enough to turn off the thyristor.
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END OF LECTURE-5
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