Thyristor

1. THYISTOR TURN-ON.
A thyristor is turned on by increasing the anode
current. This can be accomplished in one of the
following ways.
1. Temperature Triggering.
If the temperature of a thyristor is high, there will be an
increase in the number of electron hole pairs, which
would increase the leakage current. This increase in
currents would cause‘1’ and ‘2’ to increase. Due to
regenrative action,(1 +2 ) may tend to be unity and
the thyristor may be turned on. This type of turn-on
may cause thermal runaway and is normally avoided.
2. Light Triggering.
When light is thrown on the gate-cathode junction
through a light window, the electron-hole pairs will
increase ( free charge carriers electrons and holes are
generated ). If the intensity of this light exceeds a
certain value, the thyristor is turned on.Such a thyristor
is known as light activated SCR (LASCR).

2. 3. Forward Voltage Triggering.
When the forward anode to cathode is greater than the
forward breakdown voltage VBO.
Sufficient leakage current will flow to initiate
regenrative turn-on. This type of turn-on may be
destructive and should be avoided.
4. dv / dt Triggering.
With forward voltage across the anode and cathode of
a thyristor, the two junctions are forward biased but the
inner junction J2 is reverse biased.This junction has
the characteristics of a capacitive due to charges
existing across the junction.If the entire anode to
cathode forward voltage Va appears across junction J2
and the charge is denoted by ‘q’ than a charging
current ‘I’ given by equation
 Gate Triggering. The gate triggering is the most common
method of turning on the SCRs, because this method lends itself
accurately for turning on the SCRs at the desired instant of time

3. I = (dq / d t),  d (Cj , Va )/ d t  Cj dVa / d t + Va
dCj / d t
As Cj, the capacitive of junction ‘J2’ is almost constant,
the current is given by
i = Cj dVa / d t
If the rate of rise of forward voltage ‘dVa / d t’ is high,
the charging current plays the role of gate current and
turns on the thyristor even when gate signal is zero.
Such phenomena of turning on a thyristor, called ‘dVa
/ d t’ 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 / d t’ must be kept
below the specified rated limit. Typically ‘dV / d t’ are
20-500v / sec. False turn-on of a thyristor can be
prevented by using a snubber circuit in parallel with the
device.

4. GATE CURRENT.
If a thyristor is forward biased, the injection of gate
current by applying positive gate voltage between the
gate and cathode terminals would turn on the thyristor.
As the gate current is increased, the forward blocking
voltage is decreased.
The following points should be considered in designing
the gate control circuit:
1. The gate signal should be removed after the
thyristor turned on. A continuous gating signal would
increase the power loss in the gate junction.
2. While thyristor is reverse biased. There should be no
gate signal; otherwise, the thyristor may fail due to an
increased leakage current.
3. The width of gate pulse tG must be longer than the
time required for the anode current to rise to the
holding current value IH. In practice, the pulse width tG
is normally made more than the turn on time ton of the
thyristor

6. FIRING CIRCUITS FOR THYRISTORS
An SCR can be switched from off-state to on-state in
several ways; these are forward voltage triggering,
dv / dt triggering, temperature triggering, light
triggering and gate triggering. The gate triggering is
the most common common method of turning on the
SCRs, because this method lends itself accurately for
turning on the SCRs at the desired instant of time.
MAIN FEATURES OF FIRING CIRCUITS
The most common method for controlling the onset
of conduction in an SCR is by means of gate voltage
control. The gate control circuit is also called firing, or
triggering circuit. These gating circuits are usually low
power electronics circuits. A firing circuit should fulfil
the following two functions.

7. If power circuit has more than one SCR, the firing
circuit should produce gating pulses for each SCR at
the desired instant for proper operation of the power
circuit. These pulses must be periodic in nature and
the sequence of firing must correspond with the type of
thyristorised power controller. For example, in a single
phase converter using two SCRs, the triggering circuit
must produce one firing pulse in each half cycle ; in a
3-phase full converter using six SCRs, gating circuit
must produce one trigger pulse after every 60 degree
interval
The control signal generated by a firing circuit may not
be able to turn –on an SCR. It is therefore common to
feed the voltage pulses to a driver circuit and then to
gate-cathode circuit. A driver circuit consists of a pulse
amplifier and a pulse transformer

8. A firing circuit scheme, in general consists of the components shown
in above fig. . A regulated DC power supply is obtained from an
alternating voltage source. Pulse generator, supplied from both
AC and DC sources, gives out voltage pulses which are then fed
to pulse amplifier for their amplification. Shielded cables transmit
the amplified pulses to pulse transformers. The function of pulse
transformer is to isolate the low voltage gate-cathode circuit from
the high voltage anode-cathode circuit

9. Types of Thyristor Firing Circuits
1. Resistance Firing Circuit
2. RC Firing Circuit
3.UJT Firing Circuit
4.Pulse Transformer Firing Circuit
Resistance Firing Circuit
Resistance triggering circuit is the simplest and the
most economical method.This however, suffer from a
limited range of firing angle control (0 to 90 degree),
great dependence on temperature and differnce in
performance between individual SCRs
 R C FIRING CIRCUITS
The limited range of firing angle control by resistance
firing circuit can be overcome by RC firing circuit.
The firing angle control range from 0 degree to 180
degree

10. Types of Thyristor Firing Circuits
 UJT triggering circuits.
Resistance and RC triggering circuits give prolonged
pulses. As a result, power dissipation in the gate circuit
is large. This difficulty can be overcome by UJT
triggering circuits.

11. RESISTANCE FIRING CIRCUITS
Theory of operation
As shown in the circuit, R2 is the variable resistance, R
is the stabilizing resistance. In case R2 is zero, gate
current may flow from source, through load, R1, Diode
D, and gate to cathode. This current should not
exceed permissible gate current . This current can be
limit with the value of R1

12. OPERATION OF RESISTANCE FIRING CIRCUITS
 It is thus seen that function of R1 is to limit the gate
current to a safe value as R2 is varied.
 Resistance R should have such a value that maximum
voltage drop across it does not exceed maximum
possible gate voltage

13. R C FIRING CIRCUITS
The limited range of firing angle control by
resistance firing circuit can be overcome by
RC firing circuit.

14. Theory of operation of RC Firing
Circuit
Fig illustrates RC triggering circuit.
 By varying the value of R, firing angle can be
controlled from 0 to 180 degree.
 In the negative half cycle, C charges through D2 .
This capacitor voltage remains constant at –Vm until
supply voltage attains zero value.
 When capacitor charges to positive voltage equal to
gate trigger voltage Vgt, SCR is fired and after this,
capacitor holds to a small positive voltage.
 Diode D1 is used to prevent the breakdown of
cathode to gate junction through D2 during the
negative half cycle.

15. Unijunction Transistor (UJT).
It is a three terminal device . The device input, is
called the emitter, has a resistance which rapidly
decreases when the input voltage reaches a certain
level. This is termed a “negative resistance
characteristics’’.
three terminals called the Emitter (E), Base-one(B1)
and Base-two(B2). It is made up of an N-type base to
which P-type emitter is embedded. P-type emitter is
heavily doped and N-type base is lightly doped

16. UJT Equivalent Circuit & Characteristics Curve

17. UJT Firing Circuit
 The unijunction transistor is a highly efficient switch ;
its switching time is in the range of nanoseconds.
Since UJT exhibits negative resistance
characteristics,
 Fig. (a) shows a circuit diagram with UJT working in
the oscillator mode. The external resistances R1 R2
are small in comparison with the internal resistances
RB1, RB2 of UJT bases

18. Operation of UJT Firing Circuit
In Fig. (a), when source voltage VBB is applied, capacitor C
begins to charge through R exponentially towards VBB, During
this charging, emitter circuit of UJT is an open circuit. The
capacitor voltage vC, equal to emitter voltage vE, is given by
VC = VE = VBB( 1 – e-t/RC)
The time constant of the charge circuit is 1 = RC
When this emitter voltage vE (or vC) reaches the
peak-point voltage VP (=  VBB + VD), the unijunction
between E – B1 breaks down. As a result, UJT turns
on and capacitor C rapidly discharges through low
resistance R1 with a time constant t2 = R1C. Here t2 is
much smaller than t1. When the emitter voltage
decays to the valley-point voltage VV, UJT turns off

19. Pulse Transformer Firing Circuit
Sometimes pulse transformers are used in firing
circuits for thyristors and GTOs, for isolation between
the gate circuit and the load circuit. The main reason
for this is that the load may use a high voltage ac
supply, and the firing circuit may use a low voltage.
The transformer generally used arc either l:l two-
winding, or l'l:l three-winding types. These have
transformers have
a low winding resistance, and a low leakage
resistance. The pulse transformer provides electrical
isolation as it transfers a pulse from the primary 1o the
secondary coil. The secondary coil of the pulse
transformer is connected directly between the gate
and the cathode, or may have series resistor, or a
series diode to prevent reverse gate current.
.

20. There are various ways of connecting the pulse
transformer to trigger the thyristor. Figure shows the
basic pulse transformer coupling to drive a single
thyristor

21. A pulse at the output of the pulse generator is given
to the primary of the pulse transformer, this is
transmitted faithfully at its secondary terminal through
the resistor R to the gate of the thyristor. Figure 3.19
shows another way of using a pulse transformer to
drive an anti-parallel pair of thyristors.

22. Here a three-winding transformer provides complete
isolation and the pulse generator must supply enough
energy to trigger both thyristors. Note the black dots
on the primary and secondary windings. These dots
are used to indicate the polarity of the windings.
Transformer polarity is defined as the relative
direction of the induced voltages in the primary and
secondary windings with respect to the winding
terminals. The dot is used to indicate which windings
have the same instantaneous polarity

23. Pulse Transformers
Pulse transformers are used quite often in firing circuits
for ,SCRs and GTOs. This transformer has usually two
secondaries. The turns ratio from primary to the two
secondaries is 2:1:1 or 1:1:1. These transformers are
designed to have low winding resistance, low leakage
reactar~ce and Iow interwinding capacitance. The
advantages of using pulse transformers in triggering
semiconductor devices are:
(a) They provide isolation of low voltage firing circuit
from high voltage anode-cathode power circuit and
(a) The trigger pulse can be coupled to one or more
devices from the same trigger source by means of pulse
transformer.
A square pulse at the primary terminals of a pulse
transformer may be transmitted at its secondary
terminals faithfully as a square wave or it may be

24. transmitted as a derivative of the input waveform.
A general layout of the trigger circuit using a pulse
transformer is shown in Fig. 2 Here, R1 limits the current
in the primary circuit of pulse transformer. In practice,
trigger pulses are preferred due to the following reasons:

25. (a) This pulse waveform is suitable for injecting a large
charge in the gate circuit for reliable turn on.
(b) The duration of this pulse is small, and therefore, no
significant heating of the gate circuit is observed.
(c) The fact stated (b) as mentioned permits Va to be
raised to a suitable high value so that a hard drive of
SCR is obtained. A device with a hard drive can
withstand high di/dt at the anode circuit, which is
desirable.