Thyristor Characteristics, Two Transistor Model Of Thyristor & Thyrisror Turn On And Off.
Two Transistor Model Of
Thyristor & Thyrisror
Turn On And Off.
A thyristor is a four layer, semiconductor device of p-n-p-n structure
with three p-n junctions J1, J2 & J3 respectively.
It has three terminals, the anode, cathode and the gate.
Simplified Model Of SCR SCR Symbol
Forward blocking or off state condition.
Anode voltage is made +ve w.r.t. cathode, the junctions j1 & j3
are forward biased.
Junction j2 becomes reverse biased & only small leakage current
The SCR is then said to be in the forward blocking or off state.
Forward breakdown voltage Vbo.
If Vak is further increased j2 will breakdown due to avalanche
effect resulting in a large current through the device.
The corresponding voltage is called the forward breakdown
Now, the device is in forward conduction or ON state.
Forward blocking or off state
Forward breakdown voltage
Latching Current IL
After the SCR has switched on, there is a
minimum current required to sustain
This current is called the latching current
Usually IL is associated with turn ON of
Holding Current IH
SCR returns in its orignal off state if
anode current falls below low level called
holding current IH.
So, holding current IH is minimum anode
current to maintain thyristor in on state.
Usually IH is associated with turn off of
Effects of gate current on
If gate signal is applied, the
thyristor turn on before Vbo
So, forward voltage depends
upon magnitude of gate
Higher the gate current lower
the forward breakover voltage.
The typical gate current
magnitudes are of order of 20
o Ig(max) and Vg(max) are the
maximum gate current and
voltages that can flow through
the thyristor without damaging
o Vg (min) and Ig(min) are
minimum gate voltage and
current, below which thyristor
will not be turned-on.
o Hence to turn-on the thyristor
Ig(min) < Ig < Ig(max)
Vg (min) < Vg < Vg (max)
Thyristor Gate Characteristics
The operation of thyristor can also be explained in a
simple way by two transistor analogy.
One transister is pnp and second is npn.
The collector of one is attached with base of other &
TwoTransistor Model Of Thyristor
If in equation 2.9
SCR suddenly latches to the ON state from OFF state condition
This characteristic of device is called regenerative action.
The turning on Process of the SCR (turning the SCR from
Forward-Blocking state to Forward-Conduction state )is known
The various SCR triggering methods are
Forward Voltage Triggering
Thermal or Temperature Triggering
Radiation or Light triggering
Thyristor turn on Methods
Thermal Triggering (Temperature Triggering):
Depletion layer of SCR decreases with increase in junction temp.
In SCR when Vak is very near its breakdown voltage, the device is triggered
by increasing the junction temperature.
By increasing the junction temperature the reverse biased junction collapses
thus the device starts to conduct
Radiation Triggering (or) Light Triggering
For light triggered SCRs a special terminal niche is made inside the inner P
layer instead of gate terminal.
When light is allowed to strike this terminal, free charge carriers are
When intensity of light becomes more than a normal value, the thyristor starts
This type of SCRs are called as LASCR
Junction J2 behaves as a capacitor, due to the charges existing across
If voltage across the device is V, the charge by Q and capacitance by
ic = dQ/dt
Q = CV
ic = d(CV) / dt
ic= C. dV/dt + V. dC/dt
dC/dt = 0 (C = constant)
ic = C.dV/dt
The dV/dt across the device
becomes large & scr will turn on
This is most widely used SCR triggering method.
Three types .
1. DC Gate Triggering:-
2. AC Gate Triggering:-
I. Resistance triggering:
II. RC Triggering
3. Pulse Gate Triggering:-
A DC voltage of proper polarity is applied between gate and
cathode ( Gate terminal is positive with respect to Cathode).
When applied voltage is sufficient to produce the required gate
Current, the device starts conducting.
One drawback of this scheme is that both power and control circuits
are DC and there is no isolation between the two.
Another disadvantages is that a continuous DC signal has to be
applied. So gate power loss is high.
DC gate triggering:-
o Here AC source is used for gate signals.
o This scheme provides proper isolation between power and control
o Drawback of this scheme is that a separate transformer is required
to step down ac supply.
Two methods of AC voltage triggering namely
(i) R Triggering
(ii) RC triggering
AC Gate Triggering:-
o R is used to control the gate current.
o Depending on R, when the
gate current reaches the IL (latching)
the SCR starts to conduct.
o The diode D is called as blocking
diode. It prevents the gate cathode
junction from getting damaged in the
negative half cycle.
o By considering that the gate circuit
is purely resistive, the gate current is
in phase with the applied voltage.
o By using this method we can achieve
maximum firing angle up to 90.
Using this we can achieve firing
angle more than 90°.
In the positive half cycle, the
capacitor is charged through the
variable resistance R up to the peak
value of the applied voltage.
The variable resistor R controls the
charging time of the capacitor.
Depends on Vc, when sufficient
amount of gate current will flow in
the circuit, the SCR starts to
In the negative half cycle, the
capacitor C is charged up to the
negative peak value through the
o In this method the gate drive consists of a single pulse appearing
periodically (or) a sequence of high frequency pulses.
o This is known as carrier frequency gating.
1. Low gate dissipation at higher gate current.
2. Small gate isolating pulse transformer
3. Low dissipation in reverse biased condition is possible.So simple
trigger circuits are possible in some cases
4. When the first trigger pulse fails to trigger the SCR, the following
pulses can succeed in latching SCR.
Pulse Gate Triggering:-
The process of turning OFF SCR is defined as "Commutation"
There are two methods:
1. Natural Commutation
2. Forced Commutation
In AC circuit, the current always passes through zero for every half cycle.
As the current passes through natural zero, a reverse Voltage will
simultaneously appear across the device.
This will turn OFF the device immediately.
This process is called as natural commutation, since no external circuit is
required for this purpose.
Turning off methods of SCR
To turn OFF a thyristor, the forward anode current should
be brought to zero for sufficient time to allow the removal
of charged carriers.
In case of DC circuits the forward current should be
forced to zero by means of some external circuits.
This process is called as forced commutation.
The six distinct classes by which the SCR can be
turned off are:
1. Class A Self commutated by a resonating load.
2. Class B Self commutated by an L-C circuit.
3. Class C or L-C switched by another load carrying SCR.
4. Class D C or L-C switched by an auxiliary SCR Class.
5. E An external pulse source for commutation Class.
6. F AC line commutation.
Class of force commutation circuits
When the SCR is triggered, anode current
flows and charges up C.
The current through the SCR builds up
and completes a half cycle.
The inductor current will then attempt to
flow through the SCR in the reverse
direction and the SCR will be turned off.
The capacitor voltage is at its peak when
the SCR turns off and the capacitor
discharges into the resistance in an
Class A Commutation By Resonating Load
The Capacitor C charges up in the dot as positive before a
gate pulse is applied to the SCR.
The constant load current I(load) flows through R - L load.
This is ensured by the large reactance in series with the load
and the freewheeling diode clamping it.
A sinusoidal current flows through the resonant L- C circuit
to charge-up C with the dot as negative at the end of the half
cycle. This current will then reverse and flow through the
SCR in opposition to the load current for a small fraction of
the negative swing till the total current through the SCR
The SCR will turn off when the resonant–circuit (reverse)
current is just greater than the load current.
Class B Self Commutation By L-C Load
This configuration has two SCRs.
One of them may be the main SCR and the other
auxiliary. Both may be load current carrying main SCRs.
The configuration may have four SCRs with the load
across the capacitor, with the integral converter supplied
from a current source. Assume SCR2 is conducting.
C then charges up in the polarity shown.
When SCR1 is triggered, C is switched across SCR2 via
SCR1 and the discharge current of C opposes the flow
of load current in SCR2.
Class C Or LC Switched By Another
Load Carrying SCR
(Class C) can be converted to Class D if the
load current is carried by only one of the
SCR’s, the other acting as an auxiliary turn-off
The auxiliary SCR would have a resistor in its
anode lead of say ten times the load resistance.
Class D, L-C or C Switched By An SCR
The transformer is designed with sufficient iron and air gap
so as not to saturate. It is capable of carrying the load
current with a small voltage drop compared with the supply
When SCR1 is triggered, current flows through the load and
To turn SCR1 off a positive pulse is applied to the cathode
of the SCR from an external pulse generator via the pulse
transformer. The capacitor C is only charged to about 1 volt
and for the duration of the turn-off pulse it can be
considered to have zero impedance.
Thus the pulse from the transformer reverses the voltage
across the SCR, and it supplies the reverse recovery current
and holds the voltage negative for the required turn-off time.
Class E External Pulse Source For
If the supply is an alternating voltage, load current will flow during
the positive half cycle.
With a highly inductive load, the current may remain continuous for
some time till the energy trapped in the load inductance is dissipated.
During the negative half cycle, therefore, the SCR will turn off when
the load current becomes zero 'naturally'.
The negative polarity of the voltage appearing across the outgoing
SCR turns it off if the voltage persists for the rated turnoff period of
The duration of the half cycle must be definitely longer than the
turn-off time of the SCR.
If the 'fully-controlled' converter is used as an inverter with triggering
angles > 900, the converter triggering can be delayed till the 'margin
angle' which includes the overlap angle and the turn-off time of the
SCR - both dependent on the supply voltages.
Class F, AC Line Commutation