Download Thyristors

Power Electronics(2150903)
Topic:
“Thyristors”
Guided By: Prof. Purva Mistry
Branch : Electrical Engineering
Div: B
Batch : B1
Presented By:
Aditya Mehra(130120109001)
Rijay Doshi(130120109007)
Jay Kared (130120109016)
PRESENTATION ON :
I-V characteristics
Turn on methods, Switching
characteristics
Two transistor model
Silicon Controlled Rectifier
• A Silicon Controlled Rectifier (or thyristor) is a four layer
solid state device that controls current flow
• The name “silicon controlled rectifier” is a trade name for the
type of thyristor commercialized at General Electric in 1957
• The name thyristor has been derived from words “thyratron
tube ” and ‘transistor’ because it has characteristics which is
combination of both.
• It is unidirectional
• It has a rating of 1200V / 1500A with switching frequencies
ranging from 1KHz to 20KHz.
Construction of different semiconductor devices(for extra
information)
Thyristor Volt-Ampere Characteristic
Modes of operation of SCR
• Reverse blocking mode
• Forward blocking mode
• Forward conduction mode
Reverse blocking mode:
 When the anode is at a positive potential VAK with
respect to the cathode with no voltage applied at the
gate, junctions J1 and J3 are forward biased, while
junction J2 is reverse biased. As J2 is reverse biased,
no conduction takes place.
Now if VAK is increased beyond the
breakdown voltage VBO of the thyristor,
avalanche breakdown of J2 takes place and the
thyristor starts conducting.
If a positive potential VG is applied at the gate
terminal with respect to the cathode, the
breakdown of the junction J2 occurs at a lower
value of VAK. By selecting an appropriate
value of VG, the thyristor can be switched into
the on state suddenly.
• Forward Blocking mode: When anode is given
positive voltage and cathode is given negative
voltage, scr is forward biased , junctions J1 and J2
are forward biased but J2 is reverse biased.
• Forward conduction mode: When scr is in
forward conduction mode and the anode voltage
is increased then reverse voltage breakdown for
junction J2 takes place , unlike diode the
breakdown occurs in forward region that is
reason for calling it forward break over voltage .
It can also be achieved by applying gate pulse in
forward conduction mode.
THYRISTOR TURN-ON
METHODS
• Forward voltage triggering : The SCR can
be turned on by increasing forward biasing
voltage until forward break over point but this
method is not employed because it can damage
SCR.
• Gate triggering:
• Simple , reliable and efficent.
• When gate pulse is applied , electrons are
injected into p layer of SCR and magnitude of
break over voltage is reduced.
Effects of Increasing Gate Current
• Since n layer is heavily doped e’s go from n to
p layer. This causes reduction in depletion
layer of Junction J2 and forward break over
voltage is reduced.
• Higher gate pulse , lower is break over voltage.
• dv/dt triggering:
• In forward conduction mode junction J2 is
reverse biased due to depletion layer.
• There are space charges around depletion
layer, so J2 behaves like capacitance.
dC j
dQ d C j .Va 
dVa
ic 

 Cj
 Va.
dt
dt
dt
dt
• Ic = current due to capacitance charging
• Va = Voltage due to capacitance
dC j
•Junction capacitance is constant,
0
dt
•Therefore, i  C dVa
c
j
dt
•The charging current of capacitance plays role
of gate when rate of change of voltage is high
and SCR is triggered , even when actual gate
current is zero.
• Thermal triggering: In forward blocking state
reverse leakage current is present , the
magnitude of reverse leakage current increases
with increase in temperature. This reverse
leakage current can trigger conduction in SCR
• Light triggering : LASCR(Light Activated
SCR)
• In this type of SCR , the triggering is done by
light instead of gate pulse. Light plays role of
gate current.
Two-Transistor Model
• When transistor is in off state,
• Ic = α Ie + IcBO
• For a PnP transistor,
• Ic1 = α1 Ia + IcBO1
• For a PnP transistor, Ic2 = α Ik + IcBO2
• Ia = Ic1+ Ic2
• Ia= α2 Ik + IcBO2+
•
α1 Ia + IcBO1
Ia= α2 (Ia+Ig) + IcBO2+ α1 Ia + IcBO1
 2  I CBO1  I CBO 2
Ia 
I  (1   2 )
Switching Characteristics of
Thyristor
•
•
•
•
Switching Characteristics during turn-on:
Total turn on time=
Delay time td +Rise time tr + Spread time tp
Delay time : The time measured from instant at
which gate current reaches 0.9Ig to the instant
at which anode current reaches 0.1 Ia.
• Rise time : The time taken by anode current to
tise from 0.1 Ia to 0.9Ia.
• Spread time : The time taken by anode current to rise
from 0.9Ia to Ia.
• During turn on time SCR should be considered
Charge controlled device.
• Greater the magnitude of gate pulse lesser, time it
takes to inject charge.
• The maximum power loss takes place during on time.
• The turn on time ranges from 1to 4 microseconds.
Switching characteristics during turn-off:
The process of SCR from conduction state to forward
blocking state is called commutation process or turnoff process.
• Turn off time = Reverse recovery time + Gate
recovery time.
• Reverse recovery time: Time taken by SCR to remove
holes from p layer and electrons from n layer.
• Gate recovery time :The electrons and holes of
depletion layer are trapped around junction J2.The
only way to terminate them is by recombination of
holes and electrons. The time taken by recombination
of holes and electrons is called Gate recovery time.
• The SCR is truly off only when depletion layer has
been eliminated.
References:
• POWER ELECTRONICS by P.S Bhimbra.
• POWER ELECTRONICS by Dr.K.B Khanchandani.