The document discusses thyristors (also called SCRs). It describes thyristors as 4-layer 3-junction semiconductor devices that can be turned on by applying a gate current. Once on, the gate loses control and it remains on until the anode current drops below the holding current level. The document summarizes the construction, working principles, static and switching characteristics of thyristors including forward and reverse operation, latching/holding currents, turn on/off times. It also discusses different firing circuits used to trigger thyristors like R, RC, and UJT triggering.

41. 3. THYRISTOR
Thyristor is a 4 layer 3 junction semiconductor device. It has three terminals Gate (G), anode
(A) and cathode (K). Its working is similar to that of a diode except that we can control the
working of thyristor. The device conducts only in forward biased condition and block current
in reverse biased condition. The device turns ON only when a gate current is applied to it.
When the device is ON, the gate loses control over the device.
3.1 Construction
The figure below shows the construction of device. It has 4 layers, p-n-p-n. The bottom n-
layer forms the cathode and top p-layer forms the anode. The gate terminal is drawn from the
inner p-layer. Three junctions are formed between these 4 layers namely J1, J2, J3.
The threaded stud is used for mounting the device to a frame and to mount heat sink to
dissipate excess heat generated during the conduction. A metallic terminal is fixed to the
inner p-layer to form the gate terminal.
The thyristor is also called as SCR. SCR stands for Silicon Controlled Rectifier.
Silicon  Material used for making thyristor.
Controlled  The working can be controlled by gate current.
Rectifier  Since it works in forward biased condition, it act as a rectifier,
conducting only during positive half cycles

42. 3.2 Working
When a positive voltage is applied to anode, the junctions J1 and J3 becomes forward biased
and J2 remains reverse biased. The reverse biased junction blocks the conduction and at this
condition, a small leakage current flows through the device. When a positive gate current is
applied to the gate terminal, the width of the depletion region decreases and the junction J2
breaks down. When the junction J2 breaks down, the device starts conduction from anode to
cathode. SCR is a current controlled device as the gate current determined the conduction of
SCR.
3.3 Static I-V Characteristics of Thyristor
I-V characteristic is the plot between anode current Ia (Y-axis) and anode voltage Va (X-axis)
for a given gate current. The thyristor has 3 operating regions or operating modes.
 Reverse blocking mode.
 Forward blocking mode.
 Forward conduction mode.
The I-V chara is plotted for all the three modes for gate current = 0.

43.  Reverse Blocking Mode
In this mode, the thyristor is reverse biased with anode negative and cathode positive. In this
condition, J1 and J3 are reverse biased and J2 is forward biased. The device will not conduct
due to reverse biased junctions, but a small leakage current will flow through the device. This
current is called reverse leakage current. As the reverse biased voltage is increased, the
magnitude of reverse leakage current will remain constant till the reverse voltage reaches
VBR. At VBR, the junctions J1 and J3 breaks down and a large reverse current flows through
the device which is shown by a steep rise of Ia in the graph. VBR is the reverse breakdown
voltage.
 Forward Blocking Mode
In this mode, the thyristor is forward biased with anode positive and cathode negative. In this
condition, J1 and J3 are forward biased and J2 is reverse biased. The device will not conduct
due to reverse biased junction J2, but a small leakage current will flow through the device.
This current is called forward leakage current. As the forward biased voltage is increased, the
magnitude of forward leakage current will remain constant till the forward voltage reaches
VBO. At VBO, the junction J2 breaks down and a large current flows through the device which
is shown by a steep rise of Ia in the graph. VBO is the forward break over voltage. The region
in the graph from 0 to VBO is called the forward blocking region.
 Forward Condcution Mode
The region in the graph beyond VBO is the forward conduction region. At VBO, the junction J2
breaks down and a large forward current flows through the device. IBO is the current through
the device at VBO. When the device is turned ON the graph shift from point M to point N. The
current at point N is called Latching Current. The current at the lowest point on the line NK is
called Holding Current.
Latching Current : It is the minimum value of anode current which it must attain during
turn on process to maintain conduction when gate signal is removed.
Holding Current : It is the minimum value of anode current below which it must fall for
turning off thyristor.

44. 3.4 Switching Characteristics Of Thyristor
A thyristor is turned on by giving a positive gate current to the gate terminal. During turn ON
and turn OFF, the voltage across thyristor and current through it subjected to lots of
variations. This variation in current and voltage with respect to time is given by the switching
characteristics of thyristor. The characteristics during turn ON and turn OFF is described
below. The figure shows the switching characteristics of thyristor during turn ON and OFF.
 Turn On Characteristics
The thyristor is turned ON by giving a positive gate current. The turn ON time of thyristor is
the time required for the thyristor to change from forward blocking state to forward
conduction state. The turn ON time is divided into 3 intervals:
o Delay time, td
o Rise time, tr
o Spread time, tp
Delay Time (td) :
o It is the time measured from the instant at which gate current reached 0.9 Ig to the
instant at which the anode current reaches 0.1Ia.
o It may also be defined as the time required for the anode voltage to fall from Va to
0.9Va.
o In terms of anode current it can be defined as the time required for the anode current
to rise from forward leakage value to 0.1Ia.
o During this time, the anode current flows through a narrow path through the thyristor.
o The delay time can be decreased by increasing the gate current Ig and anode voltage
Va.
Rise Time (tr):
o It can be defined as the time required for the anode current to rise from 0.1Ia to 0.9Ia.
o Or it can be defined as the time required for the anode voltage to fall from 0.9Va to
0.1Va.
o During this time the magnitude of anode current increases with a high rate.
o The path through the thyristor through which the current flows now begins to spread
across the entire cross section of thyristor.

45. o The rate of spreading is less than the rate of change of anode current. This results in
high current flow through the narrow path.
o Due to short duration of rise time, the anode current will not spread over the entire
cross section.
o From the figure, we can see that the value of anode current and anode voltage is more
during the rise time, so it leads to high power dissipation compared to delay time and
spread time.
Spread Time (tp):
o It can be defined as the time required for the anode current to rise from 0.9Ia to final
value Ia.
o Or it can be defined as the time required for the anode voltage to fall from 0.1Va to
ON state voltage drop (1V to 1.5V).
o During this time the anode current is spread all over the entire cross section of
thyristor.
When the thyristor is fully turned ON, the gate signal can be removed. The total turn ON time
is the sum of delay time, rise time and spread time.
ton = td + tr + tp

47.  Turn Off Characteristics
Once the thyristor is turned ON, the gate losses control over the device. The thyristor can be
turned OFF by bringing anode current to a value below holding current. If a forward voltage
is applied to the thyristor when anode current is below holding current, the device may turn
ON by itself without any gate signal due to trapped charge carriers in the 4 layers of the
device. So these trapped charges must also be removed during the turn OFF process. The
process of turning OFF thyristor is also called commutation. The turn OFF (tq) time is the
time between the instant at which anode current becomes zero to the instant at which the
device regains its forward blocking capability. The turn OFF time is divided into 2 intervals:
o Reverse recovery time, trr
o Gate recovery time, tgr
Reverse Recovery Time (trr) :
o The anode current is decreased and becomes zero at t1. After t1, the anode current
flows in the reverse direction with the same di/dt slope.
o This reversal of anode current is due to trapped charges inside the 4 layers.
o This reverse current removes the charge carriers from the junctions J1 and J3.
o At t2, almost 60% of the charges are removed from the junctions and current begins to
decay after t2.
o This decay of reverse current builds up a reverse voltage across the thyristor.
o The reverse recovery period ends when the reverse current attains near zero value at
t3.
o The period between t1 and t3 is called reverse recovery time and it involves removal
of charges from J1 and J3
Gate Recovery Time (tgr) :
o During the reverse recovery time the charges from J1 and J3 are removed. But the
junction J2 has charges around it.
o These charges can be removed only by recombination.
o This is done by maintaining a reverse voltage across the thyristor. The magnitude of
this reverse voltage is not important.
o At t4, all the charges from junction J2 are removed and the thyristor is turned OFF.
o The period between t3 and t4 is called gate recovery time.

48. The turn OFF time is the sum of reverse recovery time and gate recovery time.
tq = trr + tgr
In practice there will be many thyristors in a circuit. The total turn OFF time of all these
thyristor is called circuit turn off time, tc.
3.5 Firing Circuits For Thyristors
Turning ON of thyristor is also called firing or triggering. The firing circuits of SCR are of 3
types.
o Resistance firing or R firing
o Resistance capacitance firing or RC firing
o UJT triggering
 R Firing
The circuit show the circuit diagram for R firing of SCR. The circuit consist of a ac voltage
source Vs (Vs=Vm Sin ωt), fixed resistances R and R1, variable resistance R2, diode D and
thyristor T. The current through the resistance branch can be varied by varying R2. Due to
diode D, the current through this branch will flow only during positive half cycle. The gate of
SCR is connected between D and R. So the gate voltage applied (Vg) will be equal to the
voltage drop across the resistor R. Whenever the drop across R becomes equal to the gate
threshold voltage Vgt, the thyristor starts conducting or in other words the thyristor is fired
when IR drop is equal to VGT. When the R2 is varied the current through this branch varies so
do the voltage drop across R. The variation in firing angle with respect to different values of
IR drop is shown in the waveform.

49. o Assume that the current I is less, the IR drop (Vg) will be less than VGT. In this
condition, the thyristor will not turn ON as in figure a.
o When the value of R2 is adjusted so that the Vg is equal VGT, the waveform of Vg
coincides with Vgt at angle 90O
.
o With further increase in current, the IR drop increases and the point of intersection of
Vgt and Vg moves toward 0.
o So we can say that in R firing, the firing angle can be adjusted only from 0 to 90O
.
o The current I and Vg will be maximum when R2 is 0. There is a maximum limit for
gate current and gate voltage. This maximum values of gate current (Igm) and gate
voltage (Vgm) is given by :
Igm = Vm/R1 or R1 = Vm/Igm
Vgm = Vm.R / (R1+R) or R = Vgm.R / (Vm-Vgm)
 RC Firing
The RC firing circuit is again classified into RC half wave triggering circuit and RC fullwave
triggering circuit.

50. 1. RC Hallfwave Triggering Circuit
The circuit shows the circuit diagram for RC halfwave firing of SCR. The circuit consist of a
ac voltage source Vs (Vs=Vm Sin ωt), variable resistance R, diodes D1 and D2, capacitor C
and thyristor T. The gate of SCR is connected between C and R through D1. So the gate
voltage applied (Vg) will be equal to the voltage across the capacitor C (Vc). The capacitor
charges to –Vm during the negative half cycle through diode D2. During the next positive
half cycle, the capacitor charges from –Vm to +Vm through R. Whenever the Vc becomes
equal to the gate threshold voltage Vgt, the thyristor starts conducting or in other words the
thyristor is fired when Vc is equal to Vgt. This is possible only during the charging from –
Vm to +Vm. When the R is varied the RC time constant varies. This will result in variation in
charging time of capacitor.
o When the value of R is adjusted such that charging time is more, the capacitor charges
to +Vm in a slow pace and coincides with Vgt at angle grater than 90O
.

51. o With further decrease charging time, capacitor charges in a fast pace and the point of
intersection of Vgt and Vc moves toward 0.
o So we can say that in RC firing, the firing angle can be adjusted from 0 to 180O
.
o The selection of R and C is as follows.
RC ≥ 1.3T/2
Vs ≥ RIgt + Vc (Igt = Gate current) (Vc = Vd + Vgt)
Vs ≥ RIgt + Vd + Vgt
i.e R ≤ (Vs – Vgt – Vd) / Igt
2. RC Fullwave Triggering Circuit
The circuit shows the circuit diagram for RC fullwave firing of SCR. The circuit consist of a
ac voltage source Vs (Vs=Vm Sin ωt), variable resistance R, diode rectifier, capacitor C and
thyristor T. The gate of SCR is connected between C and R through D1. So the gate voltage
applied (Vg) will be equal to the voltage across the capacitor C (Vc). Let the output of diode
voltage be Vd. The capacitor charges to Vd during each pulse output of diode rectifier
through R. Whenever the Vc becomes equal to the gate threshold voltage Vgt, the thyristor
starts conducting or in other words the thyristor is fired when Vc is equal to Vgt. So during
each pulse output of diode rectifier Vd, the capacitor charges and thyristor is triggered during
each half cycle of input AC.

52. o When the value of R is adjusted such that charging time is more, the capacitor charges
to Vd in a slow pace and coincides with Vgt at angle greater than 90O
.
o With further decrease charging time, capacitor charges in a fast pace and the point of
intersection of Vgt and Vc moves toward 0.
o Here thyristor is fired in each half cycle and the output is a full wave rectified DC.
o The selection of R and C is as follows.
RC ≥ 50T/2
i.e R ≤ (Vs – Vgt) / Igt
 UJT Triggering
The UJT triggering circuit is again classified into UJT oscillator triggering, Synchronized
UJT Triggering (Ramp triggering) and Ramp and Pedestral triggering.
1. UJT Oscillator Triggering
The circuit of UJT triggering is shown below. The circuit consist of resistor R, capacitor C,
UJT and biasing resistance R1 and R2. The capacitor charges through R with time constant
RC. The UJT turns ON when the capacitor voltage Vc reaches peak point voltage Vp of UJT.
At Vp, the E-B1 junction breaks down and UJT turns ON conducting from E to B1. The
capacitor discharges through E-B1-R1 resulting in a pulse voltage across R1 as shown in
figure.. This pulse voltage is fed to thyristor gate for turning ON.

53. UJT Oscillator triggering : Circuit and Waveform
2. Synchronized UJT Triggering (Ramp triggering)
The circuit of synchronized UJT triggering is shown below. The circuit consist of diode
rectifier, variable resistor R, potential divider R1, capacitor C, UJT and biasing resistance R2,
zener diode Z and pulse transformer. The resitance R1 reduces the voltage output of diode
rectifier. The zener diode acts as voltage regulator and clips the rectified voltage to a safe
value. The capacitor charges through R with time constant RC. The UJT turns ON when the
capacitor voltage Vc reaches peak point voltage Vp of UJT. At Vp, the E-B1 junction breaks
down and UJT turns ON conducting from E to B1. The capacitor discharges through E-B1-
and pulse transformer winding resulting in a pulse voltage in the winding. This voltage is
transformed to secondary winding by transformer action. The advantage of this circuit is that
more than one SCR can be fired from a single circuit.

54. 3. Ramp - Pedestral Triggering
The circuit of Ramp - Pedestral Triggering is shown below. The circuit consist of diode
rectifier, variable resistor R2, fixed resistor R, potential divider R1, capacitor C, UJT and
biasing resistance R3, zener diode Z and pulse transformer. The resitance R1 reduces the
voltage output of diode rectifier. The zener diode acts as voltage regulator and clips the
rectified voltage to a safe value. The capacitor initially charges through R2 to a pedestral
voltage Vpd. When Vc becomes Vpd, the capacitor now charges to Vz through R. The UJT
turns ON when the capacitor voltage Vc reaches peak point voltage Vp of UJT. At Vp, the E-
B1 junction breaks down and UJT turns ON conducting from E to B1. The capacitor
discharges through E-B1-and pulse transformer winding resulting in a pulse voltage in the
winding. This voltage is transformed to secondary winding by transformer action. The
advantage of this circuit is that more than one SCR can be fired from a single circuit. The
function of pedestral circuit (R2 and diode D) is to decrease the firing angle by a greater
extend and to reduce charging time of SCR.

55. 3.6 TRIAC firing using DIAC
The circuit shows the circuit diagram for TRIAC firing using DIAC. The circuit consist of a
ac voltage source Vs (Vs=Vm Sin ωt), variable resistance R, fixed resistance R1, capacitor C
and TRIAC T and DIAC D. The gate of TRIAC is connected between C and R through D. So
the gate voltage applied (Vg) will be equal to the voltage across the capacitor C (Vc). During
positive half cycle, the capacitor charges to +Vs. Whenever the Vc becomes equal to the gate
threshold voltage Vgt, the TRIAC starts conducting. During negative half cycle, the capacitor
charges to -Vs. Whenever the magnitude of Vc becomes equal to the gate threshold voltage
Vgt, the TRIAC starts conducting. So during each half cycle, the capacitor charges and
TRIAC is triggered during each half cycle of input AC.

56. 3.7 Thyristor Ratings
Some useful specifications of a thyristor related to its steady state characteristics as found in a
typical “manufacturer’s data sheet” will be discussed in this section.
 Voltage ratings
Peak Working Forward OFF state voltage (VDWM): It specifics the maximum forward (i.e,
anode positive with respect to the cathode) blocking state voltage that a thyristor can
withstand during working. It is useful for calculating the maximum RMS voltage of the ac
network in which the thyristor can be used. A margin for 10% increase in the ac network
voltage should be considered during calculation.
Peak repetitive off state forward voltage (VDRM): It refers to the peak forward transient
voltage that a thyristor can block repeatedly in the OFF state. This rating is specified at a
maximum allowable junction temperature with gate circuit open or with a specified biasing
resistance between gate and cathode. This type of repetitive transient voltage may appear
across a thyristor due to “commutation” of other thyristors or diodes in a converter circuit.
Peak non-repetitive off state forward voltage (VDSM): It refers to the allowable peak value of
the forward transient voltage that does not repeat. This type of over voltage may be caused
due to switching operation (i.e, circuit breaker opening or closing or lightning surge) in a
supply network. Its value is about 130% of VDRM. However, VDSM is less than the forward
break over voltage VBRF.

57. Peak working reverse voltage (VDWM): It is the maximum reverse voltage (i.e, anode
negative with respect to cathode) that a thyristor can with stand continuously. Normally, it is
equal to the peak negative value of the ac supply voltage.
Peak repetitive reverse voltage (VRRM): It specifies the peak reverse transient voltage that
may occur repeatedly during reverse bias condition of the thyristor at the maximum junction
temperature.
Peak non-repetitive reverse voltage (VRSM): It represents the peak value of the reverse
transient voltage that does not repeat. Its value is about 130% of VRRM. However, VRSM is
less than reverse break down voltage VBRR.
 Current Ratings
Maximum RMS current (IRMS): Heating of the resistive elements of a thyristor such as
metallic joints, leads and interfaces depends on the forward RMS current Irms. RMS current
rating is used as an upper limit for dc as well as pulsed current waveforms. This limit should
not be exceeded on a continuous basis.
Maximum average current (IAV): It is the maximum allowable average value of the forward
current such that
i. Peak junction temperature is not exceeded
ii. RMS current limit is not exceeded
Maximum Surge current (ISM): It specifies the maximum allowable non repetitive current
the device can withstand. The device is assumed to be operating under rated blocking voltage,
forward current and junction temperation before the surge current occurs. Following the
surge the device should be disconnected from the circuit and allowed to cool down. Surge

58. currents are assumed to be sine waves of power frequency with a minimum duration of ½
cycles. Manufacturers provide at least three different surge current ratings for different
durations.
For example
ISM=3000 A for 0.5 cycle
ISM=2100 A for 3 cycles
ISM=1800 A for 5 cycles
I2
t Rating: This rating in terms of A
2
S is a measure of the energy the device can absorb for a
short time (less than one half cycle of power frequency). This rating is used in the choice of
the protective fuse connected in series with the device.
Latching Current (IL): It is the minimum value of anode current which it must attain during
turn on process to maintain conduction when gate signal is removed.
Holding Current (IH): It is the minimum value of anode current below which it must fall for
turning off thyristor.
di/dt Rating: This rating specifies the maximum allowable rate of rise of anode current
during turn ON process. If the rate of rise of anode current is high, it may cause local hotspots
and may damage the thyristor. Local hot spots are formed because the rate of spreading of
anode current over the entire cross section is very less compared to rate of rise of anode
current.