By: Engr. Irfan Ahmed Halepoto

1
Instrumentation Power Electronic Systems
LECTURE11
THYRISTOR FUNDAMENTALS
By Engr. Irfan Ahmed Halepoto
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Limitation of power semiconductor devices

• Majority carrier devices, like Schottky diode,
  MOSFET exhibit very fast switching responses,
  controlled essentially by the charging of the
  device capacitances.
• However, forward voltage drops of these devices
  increases quickly with increasing breakdown
  voltage.
• Minority carrier devices, like BJT, IGBT can
  exhibit high breakdown voltages with relatively
  low forward voltage drop.
• But they can have longer switching times due to
  stored minority charges.
• Energy is lost during switching transitions, due
  to a variety of mechanisms.
• The resulting average power loss, or switching
  loss, is equal to this energy loss multiplied by
  the switching frequency.
• So need of a mechanism to have a compensation
  between these issues.

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THYRISTOR

• Thyristor, 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, that can handle high currents and high
  voltages, with better switching speed and
  improved breakdown voltage .
• Name thyristor, is derived by a combination of
  the capital letters from THYRatron and
  transISTOR.
• Thyristor has characteristics similar to a
  thyratron tube which is a type of gas filled
  tube used as a high energy electrical switch and
  controlled rectifier.
• But from the construction view point, a thyristor
  (pnpn device) belongs to transistor (pnp or npn
  device) family.
• This means that thyristor is a solid state device
  like a transistor and has characteristics similar
  to that of a thyratron tube.

4
THYRISTORS

• Thyristor (famous as Silicon Control
  Rectifier-SCR) 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 a three terminal (Anode, Cathode and
  Gate), three junctions and four layers
  solid-state semiconductor device, with silicon
  doped alternate material with P-N-P-N structure.
• Thyristor act as bistable switches.
• 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 Schematic Representation
6
Two-Transistor Model of Thyristors
7
Two-Transistor Model of Thyristors

• Two-transistor model is obtained by bisecting the
  two middle layers in two separate halves.
• Junctions J1J2 J2-J3 constitute pnp npn
  transistors separately.
• In transistors off-state, IC is related to IE as
  IC aIE ICBO
• where a is the common-base current gain and ICB0
  is collector-base leakage current of transistor.
• For transistor Q1, IC1 a1 Ia   ICBO1
  ...(01)
• Similarly, for transistor Q2, the collector
  current IC2 is given by
• IC2 a2 Ik   ICBO2 .. ( 02)
• Sum of two collector currents given by Eqs. (01)
  (02) is equal to the external circuit current
  Ia entering at anode terminal A. There fore  
• Ia IC1 IC2
• Ia a1 Ia ICBO1 a2 Ik   ICBO2 ...
  (03)
• When gate current is applied, then Ik Ia Ig .
  
• Substituting this value of Ik in Eq. (03) gives
• Ia a1 Ia ICBO1 a2 (Ia Ig )   ICBO2
• Or Ia a2 Ig ICBO1 ICBO2 /1-( a1 a2)

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Thyristor Internal constructional view
9
Thyristor- Operation Principle

• Thyristor has three p-n junctions (J1, J2, J3
  from the anode).
• When anode is at a positive potential (VAK) w.r.t
  cathode with no voltage applied at the gate,
  junctions J1 J3 are forward biased, while
  junction J2 is reverse biased.
• As J2 is reverse biased, no conduction takes
  place, so thyristor is in forward blocking state
  (OFF state).
• Now if VAK (forward voltage) is increased w.r.t
  cathode, forward leakage current will flow
  through the device.
• When this forward voltage reaches a value of
  breakdown voltage (VBO) of the thyristor, forward
  leakage current will reach saturation and reverse
  biased junction (J2) will have avalanche
  breakdown and thyristor starts conducting (ON
  state), known as forward conducting state .
• If Cathode is made more positive w.r.t anode,
  Junction J1 J3 will be reverse biased and
  junction J2 will be forward biased.
• A small reverse leakage current flows, this state
  is known as reverse blocking state.
• As cathode is made more and more positive, stage
  is reached when both junctions A C will be
  breakdown, this voltage is referd as reverse
  breakdown voltage (OFF state), and device is in
  reverse blocking state

10
Characteristics of Thyristors
11
Thyristor Operating modes

• Thyristors have three modes
• Forward blocking mode Anode is positive w.r.t
  cathode, but the anode voltage is less than the
  break over voltage (VBO) .
• only leakage current flows, so thyristor is not
  conducting .
• Forward conducting mode When anode voltage
  becomes greater than VBO, thyristor switches from
  forward blocking to forward conduction state, a
  large forward current flows.
• If the IGIG1, thyristor can be turned ON even
  when anode voltage is less than VBO.
• The current must be more than the latching
  current (IL).
• If the current reduced less than the holding
  current (IH), thyristor switches back to forward
  blocking state.
• Reverse blocking mode When cathode is more
  positive than anode , small reverse leakage
  current flows.
• However if cathode voltage is increased to
  reverse breakdown voltage , Avalanche breakdown
  occurs and large current flows.

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Thyristor turn-ON methods

• Thyristor turning ON is also known as Triggering.
  
• With anode 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).
• However, a thyristor switches faster with VBO
  turn-ON than with gate turn-ON, so permitted
  di/dt for breakover voltage turn-on is lower.

14
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.
• This means that thyristor will remain in forward
  blocking state with normal working voltage across
  anode and cathode with gate open.
• Whenever thyristors turn-ON is required, a
  positive gate voltage b/w gate and cathode is
  applied.
• With gate current established, charges are
  injected into the inner p layer and voltage at
  which forward breakover occurs is reduced.
• 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 .
• When positive gate current is applied, gate P
  layer is flooded with electrons from cathode, as
  cathode N layer is heavily doped as compared to
  gate P layer.
• As the thyristor is forward biased, some of these
  electrons reach junction J2.
• As a result, width of depletion layer around
  junction J2 is reduced.
• This causes junction J2 to breakdown at an
  applied voltage lower than forward breakover
  voltage VB0.
• If magnitude of gate current is increased, more
  electrons will reach junction J2, thus thyristor
  will get turned ON at a much lower forward
  applied voltage.

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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.

17
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
  high-voltage 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.
• 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.
• 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.
• Here, the gate looses are very low because the
  drive is discontinuous.

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Thyristor Commutation

• Commutation Process of turning off a conducting
  thyristor
• Current Commutation
• Voltage Commutation
• A thyristor can be turned ON by applying a
  positive voltage of about a volt or a current of
  a few tens of milliamps at the gate-cathode
  terminals.
• But 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.
• These methods of turn-off do not refer to those
  cases where the anode current is gradually
  reduced below Holding Current level manually or
  through a slow process.
• Once the SCR is turned ON, it remains ON even
  after removal of the gate signal, as long as a
  minimum current, the Holding Current (IH), is
  maintained in the main or rectifier circuit.

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Thyristor Turn-off Mechanism

• In all practical cases, a negative current flows
  through the device.
• This current returns to zero only after the
  reverse recovery time (trr) , when the SCR is
  said to have regained its reverse blocking
  capability.
• The device can block a forward voltage only after
  a further tfr, the forward recovery time has
  elapsed.
• Consequently, the SCR must continue to be
  reverse-biased for a minimum of tfr trr tq,
  the rated turn-off time of the device.
• The external circuit must therefore reverse bias
  the SCR for a time toff gt tq.
• Subsequently, the reapplied forward biasing
  voltage must rise at a dv/dt lt dv/dt (reapplied)
  rated. This dv/dt is less than the static
  counterpart.

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Thyristor Commutation Classification

• 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.
• A simple circuit consist of a precharged
  capacitor and a switch (e.g. another thyristor)
  parallel to the thyristor. When the switch is
  closed, the current is supplied by the capacitor
  for a short while. This cause a reversed voltage
  over the thyristor, and the thyristor is turned
  off.
• Commutation is achieved by reverse biasing
  thyristor or reducing the thysristor current
  below the holding current value.
• Commutating elements such as inductor, capacitors
  are used for commutation purpose.
• Force commutation is applied to choppers and
  inverters.
• Force Commutation methods
• Class A- Resonant Load
• Class B- Self commutation
• Class C- Auxiliary commutation
• Class D- Complimentary commutation
• Class E- External pulse commutation

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Thyristor Turn-Off Forced- Commutated Thyristor
Circuit
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THYRISTOR SWITCHING CHARACTERISTICS
Thyristor Turn-ON time for a resistive Load
Thyristor Turn -OFF time for a resistive Load
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THYRISTOR turn-ON turn-OFF Characteristics
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Thyristor protection circuits

• Reliable operation of a thyristor demands that
  its specified ratings are not exceeded.
• In practice, a thyristor may be subjected to
  overvoltages or overcurrents. During SCR turn-on,
  di/dt may be prohibitively large.
• There may be false triggering of SCR by high
  value of dv/dt.
• A spurious signal across gate-cathode terminals
  may lead to unwanted turn-on.
• A thyristor must be protected against all such
  abnormal conditions for satisfactory and reliable
  operation of SCR circuit and the equipment.
• SCRs are very delicate devices, their protection
  against abnormal operating conditions is,
  therefore, essential.
• The object of this section is to discuss various
  techniques adopted for the protection of SCRs.
• di/dt protection.
• dv/dtprotection.

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di/dt protection

• When a thyristor is forward biased and is turned
  on by a gate pulse, conduction of anode current
  begins in the immediate neighbourhood of the
  gate-cathode junction.
• Thereafter, the current spreads across the whole
  area of junction.
• The thyristor design permits the spread of
  conduction to the whole junction area as rapidly
  as possible.
• However, if the rate of rise of anode current,
  i.e. di/dt, is large as compared to the spread
  velocity of carriers, local hot spots will be
  formed near the gate connection on account of
  high current density.
• This localized heating may destroy the thyristor.
  Therefore, the rate of rise of anode current at
  the time of turn-on must be kept below the
  specified limiting value.
• The value of di/dt can be maintained below
  acceptable limit by using a small inductor,
  called di/dt inductor, in series with the anode
  circuit. Typical di/dt limit values of SCRs are
  20-500 A/µ sec.
• Local spot heating can also be avoided by
  ensuring that the conduction spreads to the whole
  area as rapidly as possible.
• This can be achieved by applying a gate current
  nearer to (but never greater than) the maximum
  specified gate current.

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di/dt Protection

• A thyristor requires a minimum time to spread the
  current conduction uniformly throughout the
  junctions
• Otherwise, a localized hot-spot heating may
  occur due to high current density.

32
dv/dt protection

• With forward voltage across the anode cathode
  of a thyristor, the 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.
• Let the capacitance of this junction be Cj. For
  any capacitor, i C dv/dt.
• In case it is assumed that entire forward voltage
  va appears across reverse biased junction J2 then
  charging current across the junction is given by
• i dQ/dt d(Cj Va )/dt
• iCj (d Va /dt) Va(d Cj /dt)
• i Cj dva /dt
• This charging or displacement current across
  junction J2 is collector currents of Q2 and Q1
  Currents IC2, IC1 will induce emitter current in
  Q2, Q1.
• In case rate of rise of anode voltage is large,
  the emitter currents will be large and as a
  result, a1 a2 will approach unity leading to
  eventual switching action of the thyristor.
• If the rate of rise of forward voltage dVa/dt is
  high, the charging current i will be more. This
  charging current plays the role of gate current
  and turns on the SCR even when gate signal is
  zero.
• Such phenomena of turning-on a thyristor, called
  dv/dt turn-on must be avoided as it leads to
  false operation of the thyristor circuit.
• For controllable operation of the thyristor, the
  rate of rise of forward anode to cathode voltage
  dVa/dt must be kept below the specified rated
  limit.
• Typical values of dv/dt are 20 500 V/µsec.
  False turn-on of a thyristor by large dv/dt can
  be prevented by using a snubber circuit in
  parallel with the device.

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Snubber circuit

• A snubber circuit consists of a series
  combination of resistance Rs and capacitance Cs
  in parallel with the thyristor as shown in Fig.
• Strictly speaking, a capacitor Cs in parallel
  with the device is sufficient to prevent unwanted
  dv/dt triggering of the SCR.
• When switch S is closed, a sudden voltage appears
  across the circuit. Capacitor Cs behaves like a
  short circuit, therefore voltage across SCR is
  zero.
• With the passage of time, voltage across Cs
  builds up at a slow rate such that dv/dt across
  Cs and therefore across SCR is less than the
  specified maximum dv/dt rating of the device.
• Here the question arises that if Cs is enough to
  prevent accidental turn-on of the device by
  dv/dt, what is the need of putting Rs in series
  with Cs ? The answer to this is as under.

34
snubber circuit (continue)

• Before SCR is fired by gate pulse, Cs charges to
  full voltage Vs. When the SCR is turned on,
  capacitor discharges through the SCR and sends a
  current equal to Vs / (resistance of local path
  formed by Cs and SCR).
• As this resistance is quite low, the turn-on
  di/dt will tend to be excessive and as a result,
  SCR may be destroyed. In order to limit the
  magnitude of discharge current, a resistance Rs
  is inserted in series with Cs as shown in Fig.
• Now when SCR is turned on, initial discharge
  current Vs/Rs is relatively small and turn-on
  di/dt is reduced.
• In actual practice Rs, Cs and the load circuit
  parameters should be such that dv/dt across Cs
  during its charging is less than the specified
  dv/dt rating of the SCR and discharge current at
  the turn-on of SCR is within reasonable limits.
• Normally, Rs Cs and load circuit parameters form
  an underdamped circuit so that dv/dt is limited
  to acceptable values.

35
Thyristor Family Members

• SCR Silicon Controlled Rectifier
• DIAC Diode on Alternating Current
• TRIAC Triode for Alternating Current
• SCS Silicon Control Switch
• SUS Silicon Unilateral Switch
• SBS Silicon Bidirectional Switch
• SIS Silicon Induction Switch
• LASCS Light Activated Silicon Control Switch
• LASCR Light Activated Silicon Control Rectifier
• SITh Static Induction Thyristor
• RCT Reverse Conducting Thyristor
• GTO Gate Turn-Off thyristor
• MCT MOSFET Controlled Thyristor
• ETOs Emitter Turn ON thyristor

36
Silicon-Controlled Rectifier (SCR)

• SCR is a synonym of thyristor
• SCR is a four-layer pnpn device.
• Has 3 terminals anode, cathode, and gate.
• In off state, it has a very high resistance.
• In on state, there is a small on (forward)
  resistance.
• Applications motor controls, time-delay
  circuits, heater controls, phase controls, etc.

37
Turning the SCR ON Method and its Characteristics

• The SCR can be turned on by exceeding the forward
  breakover voltage or by gate current.
• Notice that the gate current controls the amount
  of forward breakover voltage required for turning
  it on.
• VBR(F) decreases as IG is increased.

• The positive pulse of current at the gate turns
  on Q2 providing a path for IB1.
• Q1 then turns on providing more base current for
  Q2 even after the trigger is removed.
• Thus, the device stays on (latches).

38
Turning SCR Off

• The SCR will conduct as long as forward current
  exceeds IH.
• There are two ways to drop the SCR out of
  conduction
• Anode Current Interruption
• Forced Commutation.

39
Turning SCR Off Anode Current Interruption

• Anode current can be interrupted by breaking the
  anode current path , providing a path around the
  SCR, or dropping the anode voltage to the point
  that IA lt IH.

40
Turning The SCR Off Force Commutation

• Force commutation uses an external circuit to
  momentarily force current in the opposite
  direction to forward conduction.
• SCRs are commonly used in ac circuits, which
  forces the SCR out of conduction when the ac
  reverses.

41
SCR Characteristics Ratings

• Forward- breakover voltage, VBR(F) voltage at
  which SCR enters forward-conduction (ON) region.
• Holding current, IH value of anode current for
  SCR to remain in on region.
• Gate trigger current, IGT value of gate current
  to switch SCR on.
• Average forward current, IF (avg) maximum
  continuous anode current (dc) that the SCR can
  withstand.
• Reverse-breakdown voltage, VBR(R) maximum
  reverse voltage before SCR breaks into avalanche.

42
SCR Applications - dc motor control

• SCRs are used in a variety of power control
  applications.
• One of the most common applications is to use it
  in ac circuits to control a dc motor or appliance
  because the SCR can both rectify and control.
• The SCR is triggered on the positive cycle and
  turns off on the negative cycle.
• A circuit like this is useful for speed control
  for fans or power tools and other related
  applications.

43
SCR Applications- crowbar circuits

• Another application for SCRs is in crowbar
  circuits (which get their name from the idea of
  putting a crowbar across a voltage source and
  shorting it out!)
• The purpose of a crowbar circuit is to shut down
  a power supply in case of over-voltage.
• Once triggered, the SCR latches on.
• The SCR can handle a large current, which causes
  the fuse (or circuit breaker) to open.

44
DIAC (diode for alternating current)

• The DIAC is a five-layer device trigger diode
  that conducts current only after its breakdown
  voltage has been exceeded momentarily.
• When this occurs, the resistance of the diode
  abruptly decreases, leading to a sharp decrease
  in the voltage drop across the diode and,
  usually, a sharp increase in current flow through
  the diode.
• The diode remains "in conduction" until the
  current flow through it drops below a value
  characteristic for the device, called the holding
  current.
• Below this value, the diode switches back to its
  high-resistance (non-conducting) state.
• This behavior is bidirectional, meaning typically
  the same for both directions of current flow .
• their terminals are not labeled as anode and
  cathode but as A1 and A2 or MT1 ("Main Terminal")
  and MT2.
• Most DIACs have a breakdown voltage around 30 V.
• DIACs have no gate electrode, unlike some other
  thyristors they are commonly used to trigger,
  such as TRIACs.
• diac is normally used in ac circuits
• The drawback of the diac is that it cannot be
  triggered at just any point in the ac power
  cycle it triggers at its preset breakover
  voltage only. If we could add a gate to the diac,
  we could have variable control of the trigger
  point, and therefore a greater degree of control
  over just how much power will be applied to the
  line-powered device.

45
DIAC (diode for alternating current)
46
TRIAC (Triode for Alternating Current)

• Triac is five layer device that is able to pass
  current bidirectionally and is therefore behaves
  as an a.c. power control device.
• In triac , the main connections are simply named
  main terminal 1 (MT1) and main terminal 2 (MT2).
• The gate designation still applies, and is still
  used as it was with the SCR.
• The useful feature of the triac is that it not
  only carries current in either direction, but the
  gate trigger pulse can be either polarity
  regardless of the polarity of the main applied
  voltage.
• The gate can inject either free electrons or
  holes into the body of the triac to trigger
  conduction either way.
• So triac is referred to as a "four-quadrant"
  device.
• Triac is used in an ac environment, so it will
  always turn off when the applied voltage reaches
  zero at the end of the current half-cycle.
• If we apply a turn-on pulse at some controllable
  point after the start of each half cycle, we can
  directly control what percentage of that
  half-cycle gets applied to the load, which is
  typically connected in series with MT2.
• This makes the triac an ideal candidate for light
  dimmer controls and motor speed controls. This is
  a common application for triacs.

47
Triac operation

• The triac can be considered as two thyristors
  connected in antiparallel as shown in Fig .
• The single gate terminal is common to both
  thyristors.
• The main terminals MT1 and MT2 are connected to
  both p and n regions of the device and the
  current path through the layers of the device
  depends upon the polarity of the applied voltage
  between the main terminals.
• The device polarity is usually described with
  reference to MT1, where the term MT2 denotes
  that terminal MT2 is positive with respect to
  terminal MT1.

48
The Gate Turn-Off Thyristor (GTO)
49
GTOs Schematic representation
50
GTO Turn-on and Turn-off Pulses
51
Thyristor Summary

• A thyristor is a latching device and it can be
  turned on with a small gate pulse, typically
  100µs .
• Thyristors are generally off by line commutation
  due to the natural behavior of the input ac line
  supply.
• During the turn-off process, thyristors must be
  subjected to a reverse voltage for a certain
  minimum time known the turn-off.

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Summary Thyristors

• The thyristor family
• double injection yields lowest forward voltage
  drop in high voltage devices.
• More difficult to parallel than MOSFETs and IGBTs
• The SCR
• highest voltage and current ratings, low cost,
  passive turn-off transition
• The GTO
• intermediate ratings (less than SCR, somewhat
  more than IGBT). Slower than IGBT.
• Slower than MCT.
• Difficult to drive.
• The MCT
• So far, ratings lower than IGBT.
• Slower than IGBT.
• Easy to drive.
• Still emerging devices?

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Thyristor (SCR)
v-i characteristics

• If the forward breakover voltage (Vbo) is
  exceeded, the SCR self-triggers into the
  conducting state.
• The presence of gate current will reduce Vbo.
• Normal conditions for thyristors to turn on
• the device is in forward blocking state (i.e Vak
  is positive)
• a positive gate current (Ig) is applied at the
  gate
• Once conducting, the anode current is latched.
  Vak collapses to normal forward volt-drop,
  typically 1.5-3V.
• In reverse -biased mode, the SCR behaves like a
  diode.

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Thyristor Conduction
vo _

• Thyristor cannot be turned off by applying
  negative gate current. It can only be turned off
  if Ia goes negative (reverse)
• This happens when negative portion of the of
  sine-wave occurs (natural commutation).
• Another method of turning off is known as forced
  commutation,
• The anode current is diverted to another
  circuitry.

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Types of thyristors

• Phase controlled
• rectifying line frequency voltage and current for
  ac and dc motor drives
• large voltage (up to 7kV) and current (up to 4kA)
  capability
• low on-state voltage drop (1.5 to 3V)
• Inverter grade
• used in inverter and chopper
• Quite fast. Can be turned-on using
  force-commutation method.
• Light activated
• Similar to phase controlled, but triggered by
  pulse of light.
• Normally very high power ratings
• TRIAC
• Dual polarity thyristors