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

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.

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.

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.

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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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

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

I2t 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.