2. • Structure & Symbol
• Turn ON techniques and V-I characteristics
• Switching characteristics & Protection circuits
• Commutation circuits
• Classification of Thyristors
Contents
2
3. Structure & Symbol
A thyristor is the most
important type of power
semiconductor devices.
Thyristors are extensively used
in power electronic circuits.
They are operated as bistable
switches from nonconducting
state to conducting state.
A thyristor is a four-layer semiconductor device of pnpn-structure with three pn junctions.
It has three terminals: anode, cathode, and gate. Figure above shows the sectional view of three
pn-junctions and the symbol of thyristor.
Thyristors are manufactured by diffusion.
3
4. Turn ON Techniques
Thermals: If temperature increases, The number of electron hole pair increases, The leakage
currents increases. α1 and α2 are increased. Due to the regenerative action, ( α1 + α2) may tend to
unity and the thyristor may be turned on. This type of turn-on is normally avoided.
Light: If light is allowed to strike the junction of the thyristor, the electron-hole pair increases; and
the thyristor will be turned on due to leakage current. The light-activated thyristors are turned on
by allowing light to strike the silicon wafers.
High Voltage: If VAK>VBO, sufficient leakage current will flow to initiate regenerative turn-on. This
type of turn-on may be destructive and should be avoided.
High dv/dt: If the rate of rise of cathode voltage is high, the charging current of the capacitive
junctions may be sufficient enough to turn on the thyristor. A high value of charging current may
damage the thyristor; and the device must be protected against high dv/dt.
Gate Current (IG>0): If a thyristor is forward biased, the injection of gate current by applying
positive voltage between the gate and cathode terminals would turn on the thyristor. As the gate
current increases, the forward blocking voltage decreases.
4
9. V-I characteristics curve
Two Transistor Model
The regenerative or latching action due to positive feedback can be demonstrated by using a two-
transistor mode of thyristor. A thyristor can be considered as two complementary transistors, one
pnp-transistor, Q1 and other npn-transistor, Q2 , as shown in Fig. below.
Figure: Two-transistor model of thyristor.
9
16. Switching characteristics
di/dt Limitations
A thyristor requires a minimum time to spread the current conduction uniformly
throughout the junctions. If the rate of rise of anode current is very fast compared
to the spreading velocity of a turn-on process, localized "hot-spot" heating will
occur due to high current density and the device may fall, as a result of excessive
temperature.
16
18. Switching characteristics
dv/dt limitations
18
If the switch S1 in Fig. below is closed at t = 0, a step voltage will be applied across
thyristor T1 and dv/dt may be high enough to turn on the device, The dv/dt can be
limited by connecting capacitor Cs.
33. Thyristor Commutation Circuits
33
Complementary Commutation
A complementary commutation is
used to transfer current between two
loads and such an arrangement is
shown in Fig. below. The firing of one
thyristor commutates the other one.
36. 36
Phase-Control Thyristor (or SCRs)
Control Characteristic: Turn-on with a pulse signal (Current for turn on); Turnoff
with natural commutation (No turn off control).
Switching frequency: Low 60 Hz i.e it is suited for low speed switching
applications.
Turn-off time: 50 to 100 μs.
On-state voltage drop: Varies typically from about 1.15 V for 600 V to 2.5 V
f for 4000 V devices; and for a 5500 A 1200 V thyristor it is typically 1.25 V.
Advantages: Simple turn-on; Latching device; Turn-on gain is very high; Low
cost; high voltage; and high current device.
Disadvantages: Low-switching speed; Most suited for line commutated
applications between 50 and 60 Hz; cannot be turned-off with gate control.
This is most suited for low-speed switching applications and is also known
as converter thyristor.
Classification of Thyristors
37. 37
Fast Switching Thyristors
Control Characteristic: Turn-on with a pulse signal (Current for
turn on); Turn-off with natural & forced commutation.
Switching frequency: Medium 5 kHz, these are used in high speed
switching application with forced commutation such as in inverter
and chopper circuit.
Turn-off time: 5 to 50 μs (fast turn off time).
On-state voltage drop: Low. For 2200A 1800 V thyristor is typically
1.7 V. The on-state forward voltage drop varies approximately as an
inverse function of the turn-of time.
dv/dt capability: high dv/dt o typically 1000 V/μs.
di/dt capability: high di/dt o typically 1000 A/μs.
Advantages: Same as the phase-controlled SCRs, except the turn-off
is faster. The fast turn-off and high di/dt are very important to
reduce the size and weight of commutating and/or reactive circuit
components.
Disadvantages: Similar to those of phase-controlled SCRs.
This type of thyristor
is also known as an
inverter thyristor.
Fast thyristors can be
made by diffusing heavy
metal ions such
as gold or platinum which
act as charge combination
centers into the silicon.
Classification of Thyristors
38. 38
Gate-Turn off Thyristors (GTOs)
Control Characteristic: Turn-on with a positive pulse signal; Turnoff
with a negative pulse signal (current for both turn-on and turn-off
control).
Switching frequency: Medium 5 kHz.
On-state voltage drop: Low but it has higher on-state voltage than
that of SCRs. 3.4 V for 550 A 1200 V GTO.
Advantages: Similar to the fast switching thyristors, except it will be
turned-off with a negative gate signal.
Disadvantages: Turn-off gain is low between 5 and 8 and it requires
a large gate current to turn-off a large on-state current; there is a
long tail current during turn-off; although a latching device, it
requires a minimum gate current to sustain on-state current.
Advantages of GTOs over SCRs:
1. Elimination commutating components in forced commutation, resulting in
reduction in cost, weight, and volume,
2. Reduction in acoustic and electromagnetic noise due to the elimination of
commutation chokes,
3. Faster turn-off permitting high switching frequencies, and
4. Improved efficiency of converters.
Classification of Thyristors
39. 39
Gate-Turn off Thyristors (GTOs)
V-I Characteristics of Gate Turn-Off Thyristor
Classification of Thyristors
40. 40
Gate-Turn off Thyristors (GTOs)
Why does an SCR fail to turn off with negative pulse?
There are two significant differences between a GTO
and a SCR. First the gate and cathode structures are
highly interdigitated, with various types of geometric
forms being used to lay out the gates and cathodes,
including complicated involute structures. The basic
goal is to maximize the periphery of the cathode and
minimize the distance form the gate to the center of
cathode region.
A second major difference is noted in the anode region
of GTO. At regular intervals, n+ regions penetrate the p-
type anode (p1 layer) to make contact with the n-
region that forms the n1 base layer. The n+ regions are
overlaid with the same metallization that contacts the
p-type anode resulting in a so-called anode short. The
anode - short structure is used to speed up the turn-off
of GTO. Due to these anode shorts, the reverse blocking
capacity of the GTO is reduced to the reverse
breakdown voltage of junction j3 and hence speeds up
the turn OFF mechanism.
Classification of Thyristors
41. 41
Bidirectional Triode Thyristors or Triode AC Semiconductor Switches (TRIACs)
A triac is equivalent to a pair of antiparallel connected SCRs. It has one gate for turning-
on in both directions. It can conduct in both directions and is normally used in
ac phase control (e.g. ac voltage controller)
Control Characteristic: Turn-on applying gate a pulse signal for current flow in both
directions; Turn-off with natural commutation (Current for turn-on; No turn-off
control).
Switching frequency: Low 60 Hz.
On-state voltage drop: Low.
Advantages: Same as the phase-controlled SCRs, except the current can flow in both
directions.
Disadvantages: Similar to those of phase-controlled SCRs; except for low-power
applications.
Classification of Thyristors
43. 43
Triode AC Semiconductor Switches (TRIACs)
Triac can conduct current irrespective of the voltage polarity of terminals MT1 and
MT2 with respect to each other and that of gate and terminal MT2. Consequently four
different possibilities of operation of triac exists.
Classification of Thyristors
45. 45
Triode AC Semiconductor Switches (TRIACs)
Advantages of Triacs over Antiparallel SCRs
A triac is equivalent to a pair of antiparallel connected SCRs.
1. Triacs can be triggered with positive or negative polarity voltages.
2. A triac needs a single heat sink of slightly larger size, whereas antiparallel thyristor pair needs two
heat sinks smaller sizes, but due to the clearance total space required is more for thyristors.
3. A Triac needs a single fuse for protection, which also simplifies construction.
Disadvantages of Triacs over Antiparallel SCRs
1. Triacs have low dv/dt rating compared to SCRs.
2. SCRs are available in larger rating compared to Triacs.
3. Since a Triac can be triggered in either direction, a trigger circuit with Triac needs careful
consideration.
4. Reliability of Triacs is less than that of SCRs.
5. Triac gets less time to turn off than a pair of SCRs would have in full waveform control circuit. Each
SCR would have an entire half cycle to achieve turn off if necessary. On the other hand, a traic must
turn off during the brief moment when the line passes through zero. If the load is inductive (as in a
motor), turn off can be difficult in a triac control circuit.
Classification of Thyristors
50. 50
Static Induction Thryistors
The characteristics of an SITH are similar to those of a MOSFET.
An SITH is normally turned on by applying a positive gate
voltage like normal thyristors and is turned off by application
of negative voltage to its gate. SITH has low on-state resistance
or voltage drop and it can be made with higher voltage and
current ratings (voltage rating can go up to 2500 V and the
current rating is limited to 500 A).
SITH has fast switching speeds and high dv/dt and di/dt
capabilities. The switching time is on the order of 1 to 6
microseconds.
This device is extremely process sensitive, and small
perturbations in the manufacturing process would produce
major changes in the device characteristics.
Classification of Thyristors
