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Lecture Note

4 Thyristor Family PSDs

Prepared by Dr. Oday A Ahmed Website: https://odayahmeduot.wordpress.com Email: [email protected]

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Lecture Note 4: Thyristor Family PSDs Instructure: Dr. Oday A Ahmed

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Contents of this Lecture:

SCR Operation and Characteristics

TRIAC Operation and Characteristics

DIAC Operation and Characteristics

GTO Operation and Characteristics

What is a Thyristor?

A thyristor is the most important type of power semiconductor devices. They are

extensively used in power electronic circuits. They are operated as bi-stable

switches from non-conducting to conducting state.

A thyristor is a four layer, semiconductor of p-n-p-n structure with three p-n

junctions. It has three terminals, the anode, cathode and the gate. The word

thyristor is coined from thyratron and transistor. It was invented in the year 1957

at Bell Labs. The Different types of Thyristors are

o SCR: silicon-controlled rectifiers

o GTO: Gate Turnoff Thyristor

o TRIAC: Triode on AC

SCR

Thyristor SCR is a general class of a four-layer PNPN semiconducting device, as

shown below:

SCRs have the highest power handling capability. They have a rating of

1200V / 1500A with switching frequencies ranging from 1 KHz to 20 KHz.


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Used as a latching switch that can be turned on by the control terminal but

cannot be turned off by the gate.

The structure of the Silicon Controlled Rectifier (SCR also called thyristor)

consists of variously doped P and N conducting layers with three external

connections named anode A, cathode K and gate G. It can be represented as two

series power diodes:

SCR Construction and Operation

The construction of SCR shows that the gate terminal is kept nearer the cathode.

The approximate thickness of each layer and doping densities are as indicated in

the Fig.4.2. In terms of their lateral dimensions, Thyristors are the largest

semiconductor devices made. A complete silicon wafer as large as ten centimeters

in diameter may be used to make a single high power thyristor.

Fig.4.2 Structure of a generic thyristor

The operation of SCR can best be understood by thinking of its internal PNPN

structure as a two-transistor arrangement as shown below. The upper PNP layers

A K

G


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act as a transistor, Q1, and the lower NPN layers act as a transistor, Q2. Note that

the two middle layers are "shared”.

When the gate current ΙG is zero, the device is in the off state. In this state,

the very high resistance between the anode and cathode can be

approximated by an open switch.

When ΙG is zero, the leakage current from Q1 is not enough to switch ON

Q2.

When a positive pulse of current (trigger) is applied to the gate, both

transistors turn on (the anode must be more positive than the cathode). This

action is shown in Figure below.

o ΙB2 turns on Q2, providing a path for ΙC2 = ΙB1 into the Q2 collector,

thus turning on Q1. Where, ΙC2 =hfe ΙB2, thus ΙC2> ΙB2

o The collector current ΙC1 of Q1 provides additional base current ΙB2

for Q2 (IB2 will increase results in increasing in IC2 which leads to

increases in IB1 and ΙC2) so that Q2 stays in conduction after the trigger

pulse is removed from the gate. This operation is called positive

feedback.

o By this regenerative action, Q2 sustains the saturated conduction of

Q1 by providing a path for ΙB1; in turn, Q1 sustains the saturated

conduction of Q2 by providing ΙB2. Thus, the device stays on

(latches) once it is triggered ON, on the condition the IA>IAlatching.

o In this state, the very low resistance between the anode and cathode

can be approximated by a closed switch.


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V-I characteristics of a thyristor

An elementary circuit diagram for obtaining static I-V characteristics of a

thyristor is shown in Fig.4.3. The anode and cathode are connected to main

source through the load. The gate and cathode are fed from a source Es which

provides positive gate current from gate to cathode. A typical V-I characteristics

of a thyristor is shown Fig.4.3b.

Fig.4.3

From SCR characteristic reveals that a thyristor has three basic modes of

operation:

1) Reverse blocking mode:

In this mode, J1 and J3 are reverse biased, and

junction J2 is forward biased (Why?). The SCR

doesn’t conduct due to this reverse bias. A very

small current flows from cathode to anode. This

current is called reverse leakage current of the

SCR. This mode is called reverse blocking mode.

If the reverse voltage is increased, then at a critical breakdown

level, called reverse breakdown voltage VBR. An avalanche

occurs at J1 and J3 and the reverse current increases rapidly. A

large current associated with VBR gives rise to more losses in the

SCR. This may lead to thyristor damage as the junction

temperature may exceed its permissible temperature rise. It

(a)

(b)


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should, therefore, be ensured that maximum working reverse voltage across a

thyristor does not exceed VBB.

2) Forward blocking Mode:

The SCR is said to be forward when anode

is made positive with respect to cathode.

Due to this forward bias, the junctions J1

and J3 are forwards biased and J2 is

revers biased. Hence the forward voltage

is to be hold by junction J2. A very small

current flow in this region. This current

called forward leakage current. If the

forward voltage>breakover voltage, the

SCR goes from forward blocking mode to

forward conduction mode.

3) Forward Conduction Mode:

When anode to cathode forward voltage is increased with gate circuit open,

reverse biased junction J2 will have an avalanche breakdown at a voltage called

forward breakover voltage VBO. After this breakdown, thyristor gets turned on.

This method is not recommended (Why?), a positive gate pulse between gate and

cathode can be used to turn it on at lower forward voltage, as shown in Figure

below:

Latching Current IL

This is the minimum anode current required to maintain the thyristor in the on-

state immediately after a thyristor has been turned on and the gate signal has been


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removed. If a gate current, greater than the threshold gate current is applied until

the anode current is greater than the latching current IL then the thyristor will be

turned on or triggered.

Holding Current IH

To turn off a thyristor, the forward anode current must be reduced below its

holding current for a sufficient time for mobile charge carriers to vacate the

junction. If the anode current is not maintained below IH for long enough, the

thyristor will not have returned to the fully blocking state by the time the anode-

to-cathode voltage rises again. It might then return to the conducting state without

an externally applied gate current.

Example 1: The SCR shown has the latching current of 20mA and is fired by the

pulse of width 50µs. Determine whether the SCR triggers or not.

Solution:

When the SCR T1 is turned on, a step of

voltage is applied to the RL load. Thus,

the current via RL can be obtained as:

𝑉𝑠 = 𝑖(𝑡)𝑅 − 𝐿𝑑𝑖(𝑡)

𝑑𝑡

By applying Laplace transform,

𝑉𝑠 = 𝐼𝑠𝑅 − 𝐿𝑠𝐼𝑠

Then,

𝐼𝑠 =𝑉𝑠

𝑅 − 𝐿𝑠=

𝑉𝑠𝑅⁄

(1 − 𝐿𝑅⁄ 𝑠)

By applying inverse Laplace transform, i(t) can be obtained as:


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𝑖(𝑡) =𝑉𝑠

𝑅(1 − 𝑒−𝑡

𝑅

𝐿 )

Here observe that the SCR will be latched if i(t) is greater than latching current

when gate triggering pulse is removed after 50µsec. Hence,

𝑖(𝑡) =100

20(1 − 𝑒−50×10−6×

200.5 ) = 10𝑚𝐴

Hence the SCE will not be triggered since:

𝑖(𝑡) = 10𝑚𝐴 < 𝐼𝐿 = 20𝑚𝐴

Example 2: A SCR is connected in series with a 0.5H inductor and 20Ω

resistance. A 100V DC voltage is applied to this circuit. If the latching current is

4mA, find the minimum width of the gate trigger pulse required to properly turn-

on the SCR.

Solution:

The equivalent circuit is shown aside:

𝐼𝐿 = 4𝑚𝐴

𝑖(𝑡) =𝑉𝑠

𝑅(1 − 𝑒−𝑡

𝑅

𝐿 )

When 𝑖(𝑡) is equal to latching current 𝐼𝐿, SCR must be turned ON. Hence, if

𝑖(𝑡) = 𝐼𝐿

𝐼𝐿 =𝑉𝑠

𝑅(1 − 𝑒−𝑡

𝑅

𝐿 )

4×10−3 =100

20(1 − 𝑒−𝑡

20

0.5 )

Solving above equation by taking the ln of two sides, the required width to trigger

the SCR is equal to:𝑡 = 20𝜇𝑠𝑒𝑐

Thyristor Turn ON

Thermal Turn on: If the temperature of the thyristor is high, there will be an

increase in charge carriers which would increase the leakage current. This would

cause an increase in α1 & α2 and the thyristor may turn on. This type of turn on

may cause thermal run away and is usually avoided.


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Light: If light be allowed to fall on the junctions of a

thyristor, charge carrier concentration would increase

which may turn on the SCR. SCRs are turned on by

allowing light to strike the silicon wafer. When the

intensity of light becomes more than a normal value, SCR

starts conducting. The wavelength of the light waves can

be guided by an optic fiber. This type of SCR is called

Activated Silicon Controlled Rectifier (LASCR) and are

built with both light and gate triggering arrangement.

High Voltage Triggering: This is triggering without application of gate voltage

with only application of a large voltage across the anode-cathode such that it is

greater than the forward breakdown voltage VBO . This type of turn on is

destructive and should be avoided.

Gate Triggering: Gate triggering is the method practically employed to turn-on

the thyristor.

Instead of applying a continuous (DC) gate drive, the

pulsed gate drive is used. The gate voltage and current

are applied in the form of high frequency pulses. The

frequency of these pulses is up to l0 kHz. Hence, the

width of the pulse can be up to 100 microseconds. The

pulsed gate drive is applied for following reasons

(advantages):

i) The thyristor has small turn-on time i.e. up to 5

microseconds. Hence, a pulse of gate drive is sufficient

to turn-on the thyristor.

ii) Once thyristor turns-on, there is no need of gate drive. Hence, gate drive in the

form of pulses is suitable.

iii) The DC gate voltage and current increases losses in the thyristor. Pulsed gate

drive has reduced losses.

iv) The pulsed gate drive can be easily passed

through isolation transformers to isolate

thyristor and trigger circuit.

Usually, a train of pulses is used rather than

single pulse. This is to insure the SCR turned-on. If the first pulse fails to turn on


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the SCR, then the second and successive pluses are available to turn on the SCR.

This is can be clarified as shown below:

Requirement of Gate Drive

The gate drive has to satisfy the following requirements:

i) The maximum gate power should not be exceeded by gate drive; otherwise

thyristor will be damaged.

ii) The gate voltage and current should be within the limits specified by gate

characteristics (as shown below) for successful turn-on.

Ig(max) is the maximum gate current that

can flow through the thyristor without

damaging it. Similarly, Vg(max) is the

maximum gate voltage to be applied.

Similarly, Vg(min) and Ig(min) are

minimum gate voltage and current,

below which thyristor will not be

turned-on. Hence to turn-on the

thyristor successfully the gate current

and voltage should be

Ig(min) < Ig < Ig(max)

Vg (min) < Vg < Vg(max)

The gate characteristic also shows the curve for constant gate power (Pg). Thus,

for reliable turn-on, the (Vg, Ig) point must lie in the shaded area. It turns-on

thyristor successfully. Note that any spurious voltage/current spikes at the gate

must be less than Vg(min) and Ig(min) to avoid false triggering of the thyristor.

Fig.7


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iii) The gate drive should be preferably pulsed.

In case of pulsed drive the following relation

must be satisfied:

(Maximum gate power (Pgmax) x pulse width (Tp)) x (Pulse frequency (f)) ≤

Allowable average gate power (Pav),

iv) The width of the pulse should be sufficient to turn-on the thyristor successfully

TP>>tON.

v) The gate drive should be isolated electrically from the thyristor. This avoids

any damage to the trigger circuit if in case thyristor is damaged.

vi) The gate drive should not exceed permissible negative gate to cathode voltage,

otherwise the thyristor is damaged.

vii) The gate drive circuit should not sink current out of the thyristor after turn-

on.

Example 3: A SCR has a linearized gate-cathode characteristic of slope 25 V/A.

A gate current of 200mA turns the thyristor on in 16µs. The gate source voltage

is 10V. The manufacturer’s average maximum power for the gate is 400mW.

Pulse firing is used. Calculate:

(a) the value of the gate series resistance;

(b) the gate power dissipation during turn-on;

(c) the frequency of the gate pulses.

Solution

a) The total gate resistance included the

internal gate resistance is

RG=VGC/IG=10/0.2=50Ω.

From gate-cathode characteristic of SCR,

the internal gate resistance rG is: 25Ω

Thus, the required gate series resistance is

50-25=25 Ω

b) PG=𝐼𝐺2𝑟𝐺=1W

c) 𝑓 =𝑃𝐺_𝑎𝑣𝑔

𝑃𝐺𝑡𝑜𝑛 =

0.4

1×16×10−6= 25𝑘𝐻𝑧


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Example 4: The range of spread of gate-cathode characteristics for a certain

thyristor can be linearized to between

15V/A and 10V/A. The

manufacturer's data gives the

maximum gate power dissipation as

5W. Sketch the characteristic up to

Vcc=15V and Ic = 1.5A, and insert the

PG(max av) line. With the gate firing

circuit as shown in Fig. aside, a 1:1

isolating transformer, Vp amplitude of

20V, and Rl = R2 = 20Ω, determine the possible range of VGC and IG.

Solution

The characteristic is sketched in Fig

aside

Load line AB can be inserted.

This gives an operating region between C and D, i.e. about 5-7V for VGC and

0.4-0.5A for IG.

dv/dt Triggering: Under transient

conditions, the capacitances of the p-n

junction will influence the characteristics

of a thyristor. If the thyristor is in the

blocking state, a rapidly rising voltage

applied across the device would cause a

high current to flow through the device

resulting in turn-on. If ij2 is the current

through the junction j2 and Cj2 is the

junction capacitance and Vj2 is the voltage

across j2, then


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From the above equation, we see that if dv/dt is large, ij2 will be large. A high

value of charging current may damage the thyristor and the device must be

protected against high dv/dt. The manufacturers specify the allowable dv/dt.

SCR Turn OFF Methods

To turn fully off the SCR, the anode current must be less than holding current that

can be achieved through two method:

a) By using a resistor in the way if the SCR

To turn off the SCR, R should be increase until anode current reach to the holding

current. Hence, the turn off and on can be clarified further as shown in the

hysteresis loop.

b) By using commutation circuits: commutation circuit is classified into

natural and forced commutation

Note: if DC applied to SCR, the SCR can turn off either reduce current as shown

in a or by using forced commutation

Natural Commutation: if the AC source is applied to

SCR, then this is called natural commutation

(commutation means the transfer from on period to the

off period).

R

Hysteresis Loop

R


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Forced Commutation: The process of turning OFF a thyristor or SCR by using

external circuits is known as Forced Commutation. This method

of commutation is used for D.C. Commutation.

Triac

SCR can be used to control lamps, motors, or heaters etc. However, one of the

problems of using a SCR for controlling such circuits is that like a diode, the

“SCR” is a unidirectional device, meaning that it passes current in one direction

only, from Anode to Cathode.

Circuits like shown below

can be used to obtain full-

wave power control in two-

directions but this increases

both the complexity and

number of components used

in the switching circuit.

a “Triode AC Switch”

or Triac for short which is also a member of the thyristor family that be used as

a solid-state power switching device but more importantly it is a “bidirectional”

device.

In other words, a Triac can be triggered into conduction by both positive and

negative voltages applied to its Anode and with both positive and negative trigger

pulses applied to its Gate terminal making it a two-quadrant switching Gate

controlled device.

A Triac behaves just like two conventional thyristors connected together in

inverse parallel (back-to-back) with respect to each other and because of this


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arrangement the two thyristors share a common Gate terminal all within a

single three-terminal package.

Mode_1 Mode_2

Mode_3 Mode_4


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Four modes in which a Triac can be operated are shown using the Triacs I-V

characteristics curves.

Ι + Mode = MT2 current positive (+ve), Gate current positive (+ve)

Ι – Mode = MT2 current positive (+ve), Gate current negative (-ve)

ΙΙΙ + Mode = MT2 current negative (-ve), Gate current positive (+ve)

ΙΙΙ – Mode = MT2 current negative (-ve), Gate current negative (-ve)

In Quadrant Ι, the Triac is usually triggered into conduction by a positive gate

current, labelled above as mode Ι+. But it can also be triggered by a negative gate

current, mode Ι–.

Similarly, in Quadrant ΙΙΙ, triggering with a negative gate current, –ΙG is also

common, mode ΙΙΙ– along with mode ΙΙΙ+. Modes Ι– and ΙΙΙ+ are, however, less

sensitive configurations requiring a greater gate current to cause triggering than

the more common Triac triggering modes of Ι+ and ΙΙΙ–.

Diac

The DIAC is a diode that conducts electrical current only after its

breakover voltage, VBO, has been reached momentarily. When breakdown

occurs, the diode enters a region of negative dynamic resistance, leading to a

decrease in the voltage drop across the diode and, usually, a sharp increase in

current through the diode. The diode remains in conduction until the current

through it drops below a value characteristic for the device, called the holding

current, IH. Below this value, the diode switches back to its high-resistance, non-

https://en.wikipedia.org/wiki/Diode

https://en.wikipedia.org/wiki/Electrical_current

https://en.wikipedia.org/wiki/Voltage

https://en.wikipedia.org/wiki/Negative_dynamic_resistance


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conducting state. This behavior is bidirectional, meaning typically the same for

both directions of current. Most DIACs have a three-layer structure with

breakover voltage of approximately 30 V.

Triac and Diac Applications

A common type of Triac switching circuit

uses phase control to vary the amount of

voltage, and therefore power applied to a

load, in this case a motor, for both the

positive and negative halves of the input

waveform. This type of AC motor speed

control gives a fully variable and linear control

because the voltage can be adjusted from zero to

the full applied voltage as shown.

This basic phase triggering circuit uses the Triac

in series with the motor across an AC sinusoidal

supply. The variable resistor, VR1 is used to

control the amount of phase shift on the gate of

the Triac which in turn controls the amount of voltage applied to the motor

by turning it ON at different times during the AC cycle.

The Triac’s triggering voltage is derived from the VR1 – C1 combination via

the Diac (The diac is a bidirectional semiconductor device that helps provide a

sharp trigger current pulse to fully turn-ON the triac).

At the start of each cycle, C1 charges up via the variable resistor, VR1. This

continues until the voltage across C1 is sufficient to trigger the diac into

conduction which in turn allows capacitor, C1 to discharge into the gate of the


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Triac turning it “ON”. Once the Triac is triggered into conduction and saturates,

it effectively shorts out the gate triggering phase control circuit connected in

parallel across it and the Triac takes control for the remainder of the half-cycle.

As we have seen above, the Triac turns-OFF automatically at the end of the half-

cycle and the VR1 – C1 triggering process starts again on the next half cycle.

GTO

A gate turn-off thyristor (GTO) is a thyristor which is

turned on or off by the gate. Like a SCR, GTO can be

triggered by into the conducting state by a pulse of

positive gate current. However, unlike the SCR, a pulse

of negative current at the gate terminal can cause its turn-

off. This feature lead to use it of more compact inverter

and chopper circuits since no commutation circuits are

required.

There are three significant differences between a GTO and a conventional

thyristor.

1- The gate and cathode structures are highly interdigitated, with various

types of geometric forms being used to layout the gates and cathodes. The

basic goal is to maximize the periphery of the cathode and minimize the

distance from the gate to the centre of a cathode region.

2- The cathode areas are usually formed by etching away the silicon

surrounding the cathodes so that they

appear as islands or mesas.

3- The n+ regions are overlaid with the same

metallization that contacts the p-type anode

resulting in a so-called anode short. The

anode-short structure is used to speed up

the turn-off of the GTO.

The i-v characteristic of a GTO, as shown in the

figure below, in the forward direction is identical

to that of a conventional thyristor. However, in the

reverse direction, the GTO has virtually no

blocking capability because of the anode-short

structure. The only junction that blocks in the reverse direction is junction J3, and

it has a rather low breakdown voltage (20-30 V typically) because of the large

doping densities on both sides of the junction.

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