UNIT – I

Power semiconductor devices and commutation

circuits

Introduction to power electronics:

Power Electronics is a field which combines Power (electric power), Electronics and Control systems.
Power engineering deals with the static and rotating power equipment for the generation, transmission
and distribution of electric power. Electronics deals with the study of solid state semiconductor power
devices and circuits for Power conversion to meet the desired control objectives (to control the output
voltage and output power). Power electronics may be defined as the subject of applications of solid state
power semiconductor devices (Thyristors) for the control and conversion of electric power. Power
electronics deals with the study and design of Thyristorised power controllers for variety of application
like Heat control, Light/Illumination control, Motor control - AC/DC motor drives used in industries,
High voltage power supplies, Vehicle propulsion systems, High voltage direct current (HVDC)
transmission.

Power Electronics refers to the process of controlling the flow of current and voltage and converting it to
a form that is suitable for user loads. The most desirable power electronic system is one whose efficiency
and reliability is 100%.

Take a look at the following block diagram. It shows the components of a Power Electronic system and
how they are interlinked.

Figure: 1.1. Block diagram of DC power supply

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A power electronic system converts electrical energy from one form to another and ensures the
following is achieved −

 Maximum efficiency

 Maximum reliability

 Maximum availability

 Minimum cost

 Least weight

 Small size

Applications of Power Electronics are classified into two types − Static Applications and Drive
Applications.

Static Applications
This utilizes moving and/or rotating mechanical parts such as welding, heating, cooling, and electro-
plating and DC power.

DC Power Supply

Figure: 1.2. Block diagram of DC power supply

Drive Applications
Drive applications have rotating parts such as motors. Examples include compressors, pumps, conveyer
belts and air conditioning systems.

Air Conditioning System

Power electronics is extensively used in air conditioners to control elements such as compressors. A
schematic diagram that shows how power electronics is used in air conditioners is shown below.

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 Figure: 1.3. Block diagram of Air Conditioning System

Power electronic applications

Commercial applications Heating Systems Ventilating, Air Conditioners, Central Refrigeration,

Lighting, Computers and Office equipments, Uninterruptible Power Supplies (UPS), Elevators, and
Emergency Lamps

Domestic applications Cooking Equipments, Lighting, Heating, Air Conditioners, Refrigerators &
Freezers, Personal Computers, Entertainment Equipments, UPS

Industrial applications Pumps, compressors, blowers and fans Machine tools, arc furnaces, induction
furnaces, lighting control circuits, industrial lasers, induction heating, welding equipments

Aerospace applications Space shuttle power supply systems, satellite power systems, aircraft power
systems.

Telecommunications Battery chargers, power supplies (DC and UPS), mobile cell phone battery
chargers

Transportation Traction control of electric vehicles, battery chargers for electric vehicles, electric
locomotives, street cars, trolley buses, automobile electronics including engine controls

Utility systems High voltage DC transmission (HVDC), static VAR compensation (SVC), Alternative
energy sources (wind, photovoltaic), fuel cells, energy storage systems, induced draft fans and boiler feed
water pumps

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Types of power electronic converters

1. Rectifiers (AC to DC converters): These converters convert constant ac voltage to variable dc

output voltage.
2. Choppers (DC to DC converters): Dc chopper converts fixed dc voltage to a controllable dc
output voltage.
3. Inverters (DC to AC converters): An inverter converts fixed dc voltage to a variable ac output
voltage.
4. AC voltage controllers: These converters converts fixed ac voltage to a variable ac output voltage
at same frequency.
5. Cycloconverters: These circuits convert input power at one frequency to output power at a
different frequency through one stage conversion.

Power semiconductor devices

i. Power Diodes.
ii. Power transistors (BJT's).
iii. Power MOSFETS.
iv. IGBT's.
v. Thyristors

Thyristors are a family of p-n-p-n structured power semiconductor switching devices

Power diodes

Power diodes are made of silicon p-n junction with two terminals, anode and cathode. P-N junction is
formed by alloying, diffusion and epitaxial growth. Modern techniques in diffusion and epitaxial
processes permit desired device characteristics. The diodes have the following advantages High
mechanical and thermal reliability High peak inverse voltage Low reverse current Low forward
voltage drop High efficiency Compactness.

Power transistors

Power transistors are devices that have controlled turn-on and turn-off characteristics. These devices
are used a switching devices and are operated in the saturation region resulting in low on-state voltage
drop. They are turned on when a current signal is given to base or control terminal. The transistor
remains on so long as the control signal is present. The switching speed of modern transistors is much
higher than that of thyristors and is used extensively in dc-dc and dc-ac converters. However their

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 voltage and current ratings are lower than those of thyristors and are therefore used in low to medium
power applications. Power transistors are classified as follows o Bipolar junction transistors(BJTs) o
Metal-oxide semiconductor filed-effect transistors(MOSFETs) o Static Induction transistors(SITs) o
Insulated-gate bipolar transistors(IGBTs)

Advantages of BJT’S

i. BJT’s have high switching frequencies since their turn-on and turn-off time are low.
ii. The turn-on losses of a BJT are small.
iii. BJT has controlled turn-on and turn-off characteristics since base drive control is possible.
iv. BJT does not require commutation circuits

Demerits of BJT

i. Drive circuit of BJT is complex.

ii. It has the problem of charge storage which sets a limit on switching frequencies.
iii. It cannot be used in parallel operation due to problems of negative temperature coefficient.

Thyristors – Silicon Controlled Rectifiers (SCR’s)

A silicon controlled rectifier or semiconductor-controlled rectifier is a four-layer solidstate current-

controlling device. The name "silicon controlled rectifier" is General Electric's trade name for a type of
thyristor.

SCRs are mainly used in electronic devices that require control of high voltage and power. This makes
them applicable in medium and high AC power operations such as motor control function.

An SCR conducts when a gate pulse is applied to it, just like a diode. It has four layers of
semiconductors that form two structures namely; NPNP or PNPN. In addition, it has three junctions
labeled as J1, J2 and J3 and three terminals(anode, cathode and a gate). An SCR is diagramatically
represented as shown below.

Figure: 1.4. Symbol of thyristor

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The anode connects to the P-type, cathode to the N-type and the gate to the P-type as shown below.

Figure: 1.5. Structure of thyristor

In an SCR, the intrinsic semiconductor is silicon to which the required dopants are infused. However,
doping a PNPN junction is dependent on the SCR application.

Modes of Operation in SCR

 OFF state (forward blocking mode) − Here the anode is assigned a positive voltage, the gate is
assigned a zero voltage (disconnected) and the cathode is assigned a negative voltage. As a
result, Junctions J1 and J3 are in forward bias while J2 is in reverse bias. J2 reaches its
breakdown avalanche value and starts to conduct. Below this value, the resistance of J1 is
significantly high and is thus said to be in the off state.

 ON state (conducting mode) − An SCR is brought to this state either by increasing the potential
difference between the anode and cathode above the avalanche voltage or by applying a positive
signal at the gate. Immediately the SCR starts to conduct, gate voltage is no longer needed to
maintain the ON state and is, therefore, switched off by −

o Decreasing the current flow through it to the lowest value called holding current

o Using a transistor placed across the junction.

 Reverse blocking − This compensates the drop in forward voltage. This is due to the fact that a
low doped region in P1 is needed. It is important to note that the voltage ratings of forward and
reverse blocking are equal.

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Characteristics of Thyristor
A thyristor is a four layer 3 junction p-n-p-n semiconductor device consisting of at least three p-n
junctions, functioning as an electrical switch for high power operations. It has three basic terminals,
namely the anode, cathode and the gate mounted on the semiconductor layers of the device. The symbolic
diagram and the basic circuit diagram for determining the characteristics of thyristor is shown in the
figure below,

V-I Characteristics of a Thyristor

Figure: 1.6. Circuit diagram for characteristics of SCR

From the circuit diagram above we can see the anode and cathode are connected to the supply voltage
through the load. Another secondary supply Es is applied between the gate and the cathode terminal which
supplies for the positive gate current when the switch S is closed. On giving the supply we get the
required V-I characteristics of a thyristor show in the figure below for anode to cathode voltage Vaand
anode current Ia as we can see from the circuit diagram. A detailed study of the characteristics reveal that
the thyristor has three basic modes of operation, namely the reverse blocking mode, forward blocking
(off-state) mode and forward conduction (on-state) mode. Which are discussed in great details below, to
understand the overall characteristics of a thyristor.

Reverse Blocking Mode of Thyristor

Initially for the reverse blocking mode of the thyristor, the cathode is made positive with respect to anode
by supplying voltage E and the gate to cathode supply voltage Es is detached initially by keeping switch S
open. For understanding this mode we should look into the fourth quadrant where the thyristor is reverse
biased.

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 Figure: 1.7. Reverse blocking mode of SCR

Here Junctions J1 and J3 are reverse biased whereas the junction J 2 is forward biased. The behavior of the
thyristor here is similar to that of two diodes are connected in series with reverse voltage applied across
them. As a result only a small leakage current of the order of a few μAmps flows. This is the reverse
blocking mode or the off-state, of the thyristor. If the reverse voltage is now increased, then at a particular
voltage, known as the critical 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, which
results in heating. This may lead to thyristor damage as the junction temperature may exceed its
permissible temperature rise. It should, therefore, be ensured that maximum working reverse voltage
across a thyristor does not exceed VBR. When reverse voltage applied across a thyristor is less than V BR,
the device offers very high impedance in the reverse direction. The SCR in the reverse blocking mode
may therefore be treated as open circuit.

Figure: 1.8. V- I characteristics of SCR

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Forward Blocking Mode Now considering the anode is positive with respect to the cathode, with gate
kept in open condition. The thyristor is now said to be forward biased as shown the figure below.

Figure: 1.9. Forward connection of SCR

As we can see the junctions J1 and J3 are now forward biased but junction J2 goes into reverse biased
condition. In this particular mode, a small current, called forward leakage current is allowed to flow
initially as shown in the diagram for characteristics of thyristor. Now, if we keep on increasing the
forward biased anode to cathode voltage.

In this particular mode, the thyristor conducts currents from anode to cathode with a very small voltage
drop across it. A thyristor is brought from forward blocking mode to forward conduction mode by turning
it on by exceeding the forward break over voltage or by applying a gate pulse between gate and cathode.
In this mode, thyristor is in on-state and behaves like a closed switch. Voltage drop across thyristor in the
on state is of the order of 1 to 2 V depending beyond a certain point, then the reverse biased junction J2
will have an avalanche breakdown at a voltage called forward break over voltage VB0 of the thyristor. But,
if we keep the forward voltage less than VBO, we can see from the characteristics of thyristor, that the
device offers high impedance. Thus even here the thyristor operates as an open switch during the forward
blocking mode.

Forward Conduction Mode

When the anode to cathode forward voltage is increased, with gate circuit open, the reverse junction J 2
will have an avalanche breakdown at forward break over voltage V BO leading to thyristor turn on. Once
the thyristor is turned on we can see from the diagram for characteristics of thyristor, that the point M at
once shifts toward N and then anywhere between N and K. Here NK represents the forward conduction
mode of the thyristor. In this mode of operation, the thyristor conducts maximum current with minimum

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voltage drop, this is known as the forward conduction forward conduction or the turn on mode of the
thyristor.

Two transistor analogy of SCR

Basic operating principle of SCR, can be easily understood by the two transistor model of SCR or
analogy of silicon controlled rectifier, as it is also a combination of P and N layers, shown in figure below

Figure: 1.10. Two transistor structure of SCR

This is a pnpn thyristor. If we bisect it through the dotted line then we will get two transistors i.e. one pnp
transistor with J1 and J2 junctions and another is with J2 and J3 junctions as shown in figure below.

Figure: 1. 11. Two transistor structure of SCR

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When the transistors are in off state, the relation between the collector current and emitter current is
shown below

Figure: 1. 12. Two transistors connection of SCR

Here, IC is collector current, IE is emitter current, ICBO is forward leakage current, α is common base

forward current gain and relationship between IC and IB is Where, IB is base current and β
is common emitter forward current gain. Let’s for transistor T1 this relation holds

And that for transistor T2

Now, by the analysis of two

transistors model we can get anode current,

From equation (i) and (ii), we get,

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If applied gate current is Ig then cathode current will be the summation of anode current and gate current

i.e.

By substituting this value of Ik in (iii) we get,

From this relation we can assure that with increasing the value of towards unity,

corresponding anode current will increase. Now the question is how increasing. Here is the
explanation using two transistor model of SCR. At the first stage when we apply a gate current Ig, it acts
as base current of T2 transistor i.e. IB2 = Ig and emitter current i.e. Ik = Ig of the T,2 transistor. Hence
establishment of the emitter current gives rise α2 as

Presence of base current will generate collector current as

This IC2 is nothing but base current IB1 of transistor T,1, which will cause the flow of collector current,

IC1 and IB1 lead to increase IC1 as

And hence, α1 increases. Now, new base current of T2 is

This will lead to increase emitter current

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and as a result α2 also increases and this further increases

As

α1 again increases. This continuous positive feedback effect increases towards unity and
anode current tends to flow at a very large value. The value current then can only be controlled by
external resistance of the circuit.

Turn on methods of SCR

The turning on Process of the SCR is known as Triggering. In other words, turning the SCR from
Forward-Blocking state to Forward-Conduction state is known as Triggering. The
various methods of SCR triggering are discussed here.

The various SCR triggering methods are

 Forward Voltage Triggering
 Thermal or Temperature Triggering
 Radiation or Light triggering
 dv/dt Triggering
 Gate Triggering

(a) Forward Voltage Triggering:-

 In this mode, an additional forward voltage is applied between anode and cathode.
 When the anode terminal is positive with respect to cathode (VAK), Junction J1 and J3 is forward
biased and junction J2 is reverse biased.
 No current flow due to depletion region in J2 is reverse biased (except leakage current).
 As VAK is further increased, at a voltage VBO (Forward Break Over Voltage) the junction J2
undergoes avalanche breakdown and so a current flows and the device tends to turn ON(even when
gate is open)

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(b) Thermal (or) Temperature Triggering:-
 The width of depletion layer of SCR decreases with increase in junction temperature.
 Therefore in SCR when VAR 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.
(c) 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 generated.
 When intensity of light becomes more than a normal value, the thyristor starts conducting.
 This type of SCRs are called as LASCR
(d) dv/dt Triggering:-
 When the device is forward biased, J1 and J3 are forward biased, J2 is reverse biased.
 Junction J2 behaves as a capacitor, due to the charges existing across the junction.
 If voltage across the device is V, the charge by Q and capacitance by C then,
ic =dQ/dt
Q=CV
ic =d(CV)/dt
=CdV/dt+VdC/dt
as dC/dt = 0
ic = CdV/dt
 Therefore when the rate of change of voltage across the device becomes large, the device may turn
ON, even if the voltage across the device is small.
(e) Gate Triggering:-
 This is most widely used SCR triggering method.
 Applying a positive voltage between gate and cathode can Turn ON a forward biased thyristor.
 When a positive voltage is applied at the gate terminal, charge carriers are injected in the inner P-
layer, thereby reducing the depletion layer thickness.
 As the applied voltage increases, the carrier injection increases, therefore the voltage at which
forward break-over occurs decreases.

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 Figure: 1. 13. V - I characteristics of SCR

 Three types of signals are used for gate triggering.

1. DC 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 disadvantage is that a continuous DC signal has to be applied. So gate power loss is high.

2. AC Gate Triggering:-

 Here AC source is used for gate signals.

 This scheme provides proper isolation between power and control circuit.
 Drawback of this scheme is that a separate transformer is required to step down ac supply.
 There are two methods of AC voltage triggering namely (i) R Triggering (ii) RC triggering

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(i) Resistance triggering:

The following circuit shows the resistance triggering.

Figure: 1. 14. Resistance triggering circuit of SCR

 In this method, the variable resistance R is used to control the gate current.
 Depending upon the value of R, when the magnitude of the gate current reaches the sufficient
value(latching current of the device) the SCR starts to conduct.
 The diode D is called as blocking diode. It prevents the gate cathode junction from getting damaged
in the negative half cycle.
 By considering that the gate circuit is purely resistive, the gate current is in phase with the applied
voltage.
 By using this method we can achieve maximum firing angle up to 90°.

(ii) RC Triggering

The following circuit shows the resistance-capacitance triggering.

Figure: 1. 15. Resistance Capacitance triggering circuit of SCR

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  By using this method 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 upon the voltage across the capacitor, when sufficient amount of gate current will flow in
the circuit, the SCR starts to conduct.
 In the negative half cycle, the capacitor C is charged up to the negative peak value through the
diode D2.
 Diode D1 is used to prevent the reverse break down of the gate cathode junction in the negative half
cycle.

3. Pulse Gate Triggering:-

 In this method the gate drive consists of a single pulse appearing periodically (or) a sequence of
high frequency pulses.
 This is known as carrier frequency gating.
 A pulse transformer is used for isolation.
 The main advantage is that there is no need of applying continuous signals, so the gate losses are
reduced.

Advantages of pulse train triggering:

 Low gate dissipation at higher gate current.

 Small gate isolating pulse transformer
 Low dissipation in reverse biased condition is possible. So simple trigger circuits are possible in
some cases
 When the first trigger pulse fails to trigger the SCR, the following pulses can succeed in latching
SCR. This important while
 Triggering inductive circuits and circuits having back emf's.

Turn off methods of SCR:

SCR can be turned ON by applying appropriate positive gate voltage between the gate and cathode
terminals, but it cannot be turned OFF through the gate terminal. The SCR can be brought back to the

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forward blocking state from the forward conduction state by reducing the anode or forward current below
the holding current level.

The turn OFF process of an SCR is called commutation. The term commutation means the transfer of
currents from one path to another. So the commutation circuit does this job by reducing the forward
current to zero so as to turn OFF the SCR or Thyristor.

To turn OFF the conducting SCR the below conditions must be satisfied.

 The anode or forward current of SCR must be reduced to zero or below the level of holding
current and then,
 A sufficient reverse voltage must be applied across the SCR to regain its forward blocking state.

When the SCR is turned OFF by reducing forward current to zero there exist excess charge carriers in
different layers. To regain the forward blocking state of an SCR, these excess carriers must be
recombined. Therefore, this recombination process is accelerated by applying a reverse voltage across the
SCR.

SCR Turn OFF Methods

The reverse voltage which causes to commutate the SCR is called commutation voltage. Depending on
the commutation voltage located, the commutation methods are classified into two major types.

Those are 1) Forced commutation and 2) Natural commutation. Let us discuss in brief about these
methods.

Forced Commutation

In case of DC circuits, there is no natural current zero to turn OFF the SCR. In such circuits, forward
current must be forced to zero with an external circuit to commutate the SCR hence named as forced
commutation.

This commutating circuit consists of components like inductors and capacitors called as commutating
components. These commutating components cause to apply a reverse voltage across the SCR that
immediately bring the current in the SCR to zero.

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Based on the manner in which the zero current achieved and arrangement of the commutating
components, forced commutation is classified into different types such as class A, B, C, D, and E. This
commutation is mainly used in chopper and inverter circuits.

Class A Commutation

This is also known as self commutation, or resonant commutation, or load commutation. In this
commutation, the source of commutation voltage is in the load. This load must be an under damped R-L-
C supplied with a DC supply so that natural zero is obtained.

The commutating components L and C are connected either parallel or series with the load resistance R as
shown below with waveforms of SCR current, voltage and capacitor voltage.

Figure: 1. 16. Class A Commutation circuit and waveforms

The value of load resistance and commutating components are so selected that they forms a under damped
resonant circuit to produce natural zero. When the thyristor or SCR is triggered, the forward currents
starts flowing through it and during this the capacitor is charged up to the value of E.

Once the capacitor is fully charged (more than the supply source voltage) the SCR becomes reverse
biased and hence the commutation of the device. The capacitor discharges through the load resistance to
make ready the circuit for the next cycle of operation. The time for switching OFF the SCR depends on
the resonant frequency which further depends on the L and C components.

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This method is simple and reliable. For high frequency operation which is in the range above 1000 Hz,
this type of commutation circuits is preferred due to the high values of L and C components.

Class B Commutation

This is also a self commutation circuit in which commutation of SCR is achieved automatically by L and
C components, once the SCR is turned ON. In this, the LC resonant circuit is connected across the SCR
but not in series with load as in case of class A commutation and hence the L and C components do not
carry the load current.

Figure: 1. 17. Class B Commutation circuit and waveforms

When the DC supply is applied to the circuit, the capacitor charges with an upper plate positive and lower
plate negative up to the supply voltage E. When the SCR is triggered, the current flows in two directions,
one is through E+ – SCR – R – E- and another one is the commutating current through L and C
components.

Once the SCR is turned ON, the capacitor is starts discharging through C+ – L – T – C-. When the
capacitor is fully discharged, it starts charging with a reverse polarity. Hence a reverse voltage applied
across the SCR which causes the commutating current IC to oppose load current IL.

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When the commutating current Ic is higher than the load current, the SCR will automatically turn OFF
and the capacitor charges with original polarity.

In the above process, the SCR is turned ON for some time and then automatically turned OFF for some
time. This is a continuous process and the desired frequency of ON/OFF depends on the values of L and
C. This type of commutation is mostly used in chopper circuits.

Class C Commutation

In this commutation method, the main SCR is to be commutated is connected in series with the load and
an additional or complementary SCR is connected in parallel with main SCR. This method is also called
as complementary commutation.

In this , SCR turns OFF with a reverse voltage of a charged capacitor. The figure below shows the
complementary commutation with appropriate waveforms.

Figure: 1. 18. Class C Commutation circuit and waveforms

Initially, both SCRs are in OFF state so the capacitor voltage is also zero. When the SCR1 or main SCR is
triggered, current starts flowing in two directions, one path is E+ – R1 – SCR1 – E- and another path is
the charging current E+ – R2- C+ – C- SCR1 – E- . Therefore, the capacitor starts charging up to the
value of E.

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When the SCR2 is triggered, SCR is turned ON and simultaneously a negative polarity is applied across
the SCR1. So this reverse voltage across the SCR1 immediately causes to turn OFF the SCR1. Now the
capacitor starts charging with a reverse polarity through the path of E+ – R1- C+ – C- SCR2 – E-. And
again, if the SCR 1 is triggered, discharging current of the capacitor turns OFF the SCR2.

This commutation is mainly used in single phase inverters with a centre tapped transformers. The Mc
Murray Bedford inverter is the best example of this commutation circuit. This is a very reliable method of
commutation and it is also useful even at frequencies below 1000Hz.

Class D Commutation

This is also called as auxiliary commutation because it uses an auxiliary SCR to switch the charged
capacitor. In this, the main SCR is commutated by the auxiliary SCR. The main SCR with load resistance
forms the power circuit while the diode D, inductor L and SCR2 forms the commutation circuit.

Figure: 1. 19. Class D Commutation circuit and waveforms

When the supply voltage E is applied, both SCRs are in OFF state and hence the capacitor voltage is zero.
In order to charge the capacitor, SCR2 must be triggered first. So the capacitor charges through the path
E+ – C+ – C- – SCR2- R- E-.

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When the capacitor is fully charged the SCR2 becomes turned OFF because no current flow through the
SCR2 when capacitor is charged fully. If the SCR1 is triggered, the current flows in two directions; one is
the load current path E+ – SCR1- R- E- and another one is commutation current path C+ – SCR1- L- D-
C.

As soon as the capacitor completely discharges, its polarities will be reversed but due to the presence of
diode the reverse discharge is not possible. When the SCR2 is triggered capacitor starts discharging
through C+ – SCR2- SCR1- C-. When this discharging current is more than the load current the SCR1
becomes turned OFF.

Again, the capacitor starts charging through the SCR2 to a supply voltage E and then the SCR2 is turned
OFF. Therefore, both SCRs are turned OFF and the above cyclic process is repeated. This commutation
method is mainly used in inverters and also used in the Jones chopper circuit.

Class E Commutation

This is also known as external pulse commutation. In this, an external pulse source is used to produce the
reverse voltage across the SCR. The circuit below shows the class E commutation circuit which uses a
pulse transformer to produce the commutating pulse and is designed with tight coupling between the
primary and secondary with a small air gap.

If the SCR need to be commutated, pulse duration equal to the turn OFF time of the SCR is applied.
When the SCR is triggered, load current flows through the pulse transformer. If the pulse is applied to the
primary of the pulse transformer, an emf or voltage is induced in the secondary of the pulse transformer.

This induced voltage is applied across the SCR as a reverse polarity and hence the SCR is turned OFF.
The capacitor offers a very low or zero impedance to the high frequency pulse.

Figure: 1. 20. Class E Commutation circuit and waveforms

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Natural Commutation

In natural commutation, the source of commutation voltage is the supply source itself. If the SCR is
connected to an AC supply, at every end of the positive half cycle the anode current goes through the
natural current zero and also immediately a reverse voltage is applied across the SCR. These are the
conditions to turn OFF the SCR.

This method of commutation is also called as source commutation, or line commutation, or class F
commutation. This commutation is possible with line commutated inverters, controlled rectifiers, cyclo
converters and AC voltage regulators because the supply is the AC source in all these converters.

Figure: 1. 21. Natural Commutation circuit and waveforms

Dynamic Turn OFF Switching Characteristics

The transition of an SCR from forward conduction state to forward blocking state is called as turn OFF or
commutation of SCR. As we know that once the SCR starts conducting, the gate has no control over it to
bring back to forward blocking or OFF state.

To turn OFF the SCR, the current must be reduced to a level below the holding current of SCR. We have
discussed various methods above to turn OFF the SCR in which SCR turn OFF is achieved by reducing

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the forward current to zero. But if we apply the forward voltage immediately after the current zero of
SCR, it starts conducting again even without gate triggering.

This is due to the presence of charge carriers in the four layers. Therefore, it is necessary to apply the
reverse voltage, over a finite time across the SCR to remove the charge carriers.

Hence the turn OFF time is defined as the time between the instant the anode current becomes zero and
the instant at which the SCR retains the forward blocking capability. The excess charge carriers from the
four layers must be removed to bring back the SCR to forward conduction mode.

This process takes place in two stages. In a first stage excess carriers from outer layers are removed and in
second stage excess carriers in the inner two layers are to be recombined. Hence, the total turn OFF time
tq is divided into two intervals; reverse recovery time trr and gate recovery time tgr.

tq = trr + tgr

The figure below shows the switching characteristics of SCR during turn ON and OFF. The time t 1 to t3 is
called as reverse recovery time; at the instant t1the anode current is zero and builds up in the reverse
direction which is called as reverse recovery current. This current removes the excess charge carriers from
outer layers during the time t1 to t3.

At instant t3, junctions J1 and J3 are able to block the reverse voltage but, the SCR is not yet able to block
the forward voltage due to the presence of excess charge carriers in junction J 2. These carriers can be
disappeared only by the way of recombination and this could be achieved by maintaining a reverse
voltage across the SCR.

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 Figure: 1. 22. Dynamic characteristics of SCR

Hence , during the time t3 to t4, the recombination of charges takes place and at the instant t 4, junction
J2 completely recovers. This time is called gate recovery time tgr.

 From the figure the turn OFF time is the time interval between the t 4 and t1. Generally, this time
varies from 10 to 100 microseconds. This turn OFF time tq is applicable to the individual SCR.
 The time required by the commutation circuit to apply the reverse voltage to commutate the SCR
is called the circuit turn OFF time (tc). For a safety margin or reliable commutation, this tc must
be greater than the tq otherwise commutation failure occurs.

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  The SCRs which have slow turn OFF time as in between 50 to 100 microseconds are called as
converter grade SCRs. These are used in phase controlled rectifiers, cyclo converters, AC voltage
regulators, etc.
 The SCRs which have fast turn OFF time as in between 3 to 50 microseconds are inverter grade
SCRs. These are costlier compared to converter grade and are used in choppers, force
commutated converters and inverters.

Resistance Firing Circuit

 The circuit below shows the resistance triggering of SCR where it is employed to drive the load
from the input AC supply. Resistance and diode combination circuit acts as a gate control circuitry
to switch the SCR in the desired condition.
 As the positive voltage applied, the SCR is forward biased and doesn’t conduct until its gate
current is more than minimum gate current of the SCR.
 When the gate current is applied by varying the resistance R2 such that the gate current should be
more than the minimum value of gate current, the SCR is turned ON. And hence the load current
starts flowing through the SCR.
 The SCR remains ON until the anode current is equal to the holding current of the SCR. And it
will switch OFF when the voltage applied is zero. So the load current is zero as the SCR acts as
open switch.
 The diode protects the gate drive circuit from reverse gate voltage during the negative half cycle
of the input. And Resistance R1 limits the current flowing through the gate terminal and its value
is such that the gate current should not exceed the maximum gate current.
 It is the simplest and economical type of triggering but limited for few applications due to its
disadvantages.
 In this, the triggering angle is limited to 90 degrees only. Because the applied voltage is maximum
at 90 degrees so the gate current has to reach minimum gate current value somewhere between
zero to 90 degrees.

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 Figure: 1. 23. R Firing circuit for SCR and corresponding waveforms

Resistance – Capacitacne (RC) Firing Circuit

 The limitation of resistance firing circuit can be overcome by the RC triggering circuit which
provides the firing angle control from 0 to 180 degrees. By changing the phase and amplitude of
the gate current, a large variation of firing angle is obtained using this circuit.
 Below figure shows the RC triggering circuit consisting of two diodes with an RC network
connected to turn the SCR.
 By varying the variable resistance, triggering or firing angle is controlled in a full positive half
cycle of the input signal.
 During the negative half cycle of the input signal, capacitor charges with lower plate positive
through diode D2 up to the maximum supply voltage Vmax. This voltage remains at -Vmax across
the capacitor till supply voltage attains zero crossing.
 During the positive half cycle of the input, the SCR becomes forward biased and the capacitor
starts charging through variable resistance to the triggering voltage value of the SCR.
 When the capacitor charging voltage is equal to the gate trigger voltage, SCR is turned ON and the
capacitor holds a small voltage. Therefore the capacitor voltage is helpful for triggering the SCR
even after 90 degrees of the input waveform.
 In this, diode D1 prevents the negative voltage between the gate and cathode during the negative
half cycle of the input through diode D2.

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 Figure: 1. 24. R Firing circuit for SCR

Figure: 1. 25. R Firing circuit waveforms of SCR

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UJT Firing Circuit

 It is the most common method of triggering the SCR because the prolonged pulses at the gate
using R and RC triggering methods cause more power dissipation at the gate so by using UJT (Uni
Junction Transistor) as triggering device the power loss is limited as it produce a train of pulses.
 The RC network is connected to the emitter terminal of the UJT which forms the timing circuit.
The capacitor is fixed while the resistance is variable and hence the charging rate of the capacitor
depends on the variable resistance means that the controlling of the RC time constant.
 When the voltage is applied, the capacitor starts charging through the variable resistance. By
varying the resistance value voltage across the capacitor get varied. Once the capacitor voltage is
equal to the peak value of the UJT, it starts conducting and hence produce a pulse output till the
voltage across the capacitor equal to the valley voltage Vv of the UJT. This process repeats and
produces a train of pulses at base terminal 1.
 The pulse output at the base terminal 1 is used to turn ON the SCR at predetermined time intervals

Figure: 1. 26. UJT Firing circuit for SCR and corresponding waveforms
Series and Parallel connections of SCRs

In many power control applications the required voltage and current ratings exceed the voltage and
current that can be provided by a single SCR. Under such situations the SCRs are required to be
connected in series or in parallel to meet the requirements. Sometimes even if the required rating is
available, multiple connections are employed for reasons of economy and easy availability of SCRs of
lower ratings. Like any other electrical equipment, characteristics/properties of two SCRs of same make
and ratings are never same and this leads to certain problems in the circuit. The mismatching of SCRs is
due to differences in

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 (i) turn-on time
(ii) turn-off time
(iii) Leakage current in forward direction
(iv) Leakage current in reverse direction and
(v) Recovery voltage.

Series Connection of an SCR

Figure: 1. 27. Series connection of SCRs

(i) Unequal distribution of voltage across SCRs

(ii) Difference in recovery characteristics.

Care must be taken to share the voltage equally. For steady-state conditions, voltage sharing is
achieved by using a resistance or a Zener diode in parallel with each SCR. For transient voltage
sharing a low non-inductive resistor and capacitor in series are placed across each SCR, as shown in
figure. Diodes D1 connected in parallel with resistor Rl, helps in dynamic stabilization. This circuit
reduces differences between blocking voltages of the two devices within permissible limits.
Additionally the R-C circuit can also serve the function of ‘snubber circuit‘. Values of R1 and C1 can
primarily be calculated for snubber circuit and a check can be made for equalization. If ΔQ is the
difference in recovery charge of two devices arising out of different recovery current for different
time and ΔV is the permissible difference in blocking voltage then

C1 = ΔQ/ ΔV

The value of resistance Rx should be sufficient to over damp the circuit. Since the capacitor C1 can
discharge through the SCR during turn-on, there can be excessive power dissipation, but the
switching current from C1 is limited by the resistor R1 This resistance also serves the purpose of

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damping out ‘ringing’ which is oscillation of C1 with the circuit inductance during commutation. All
the SCRs connected in series should be turned-on at the same time when signals are applied to their
gates simultaneously.

String efficiency =

This phenomenon increases the reliability of the string, but reduces the utilization of each SCR. Thus
string efficiency decreases. Reliability of string is measured by derating factor (DRF) which is given
by the expression

DRF = 1- string efficiency

Parallel Connection of an SCR

Figure: 1. 28. Parallel connection of SCRs

When the load current exceeds the SCR current rating, SCRs are connected in parallel to share the
load current. But when SCRs are operated in parallel, the current sharing between them may not be
proper. The device having lower dynamic resistance will tend to share more current. This will raise
the temperature of that particular device in comparison to other, thereby reducing further its dynamic
resistance and increasing current through it. This process is cumulative and continues till the device
gets punctured. Some other factors which directly or indirectly add to this problem are difference in
turn-on time, delay time, finger voltage and loop inductance.

Arrangement of SCRs in the cubicle also plays vital role. When the SCRs are connected in parallel, it
must be ensured that the latching current level of the all the SCRs is such that when gate pulse is
applied, all of them turn-on and remain on when the gate pulse is removed. Further the holding

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currents of the devices should not be so much different that at reduced load current one of the device
gets turned-off because of fall of current through it blow its holding current value. This is particularly
important because on increase in load current, the device which has stopped conducting cannot start
in the absence of gate pulse.

Another point to be considered is the on-state voltage across the device. For equal sharing of currents
by the devices voltage drop across the parallel paths must be equal. For operation of all the SCRs
connected in parallel at the same temperature, it becomes necessary to use a common heat sink for
their mounting, as illustrated in figure. Resistance compensation used for dc circuits is shown in
figure. In this circuit the resistors Rx and R2 are chosen so as to cause equal voltage drop in both
arms. Inductive compensation used for ac circuits is shown in figure The difference in characteristics
due to different turn-on time, delay time, finger voltage, latching current, holding current can be
minimized by using inductive compensation. Firing circuits giving high rate of rise can be used to
reduce mismatch of gate characteristics and delay time. Current sharing circuits must be designed so
as to distribute current equally at maximum temperature and maximum anode current. This is done to
ensure that the devices share current equally under worst operating conditions. Mechanical
arrangement of SCRs also plays an important role in reducing mismatching. Cylindrical construction
is perhaps the best from this point of view.

Derating:

Even with all the measures taken, it is preferable to derate the device for series/parallel operation.
Another reason for derating is poor cooling and heat dissipation as number of devices operates in the
same branch of the circuit. Normal derating factors are 10 to 15% for parallel connection of SCRs
depending upon the number of devices connected in parallel. Higher voltage safety factor is taken
when SCRs are connected in series.

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Numerical Problems:

1. The trigger circuit of a thyristor has a source voltage of 15V and the load line has a slope of -
120V per ampere. The minimum gate current to turn on the SCR is 25mA. Compute
i. Source resistance required in the gate circuit
ii. The trigger voltage and trigger current for an average gate power dissipation of 0.4
watts

Solution:

i. The slope of load line gives the required gate source resistance. From the load line,
series resistance required in the gate circuit is 120Ω
ii. Here VgIg = 0.4W
For the gate circuit Es = RsIg + Vg
15 = 120Ig +0.4/Ig

120Ig2 – 15 Ig + 0.4 = 0
Its solution gives Ig = 38.56mA or 86.44 mA

Vg = = 10.37V

Vg = = 4.627V

So choose the value for Ig which gives less voltage Ig = 86.44 mA and Vg = 4.627V from
minimum gate current of 25mA.

2. For an SCR the gate-cathode characteristic has a straight line slope of 130. For trigger source
voltage of 15V and allowable gate power dissipation of 0.5 watts, compute the gate source
resistance.
3. SCRs with a rating of 1000V and 200A are available to be used in a string to handle 6kV and 1kA.
Calculate the number of series and parallel units required in case de-rating factor is 0.1 and 0.2

35 | P a g e
 4. It is required to operate 250A SCR in parallel with 350A SCR with their respective on state
voltage drops of 1.6V and 1.2V. Calculate the value of resistance to be inserted in series with
each SCR so that the share the total load of 600A in proportion to their current ratings.
Snubber circuit

Due to overheating, over voltage, over current or excessive change in voltage or current switching devices
and circuit components may fail. From over current they can be protected by placing fuses at suitable
locations. Heat sinks and fans can be used to take the excess heat away from switching devices and other
components. Snubber circuits are needed to limit the rate of change in voltage or current (di/dt or dv/dt)
and over voltage during turn-on and turn-off. These are placed across the semiconductor devices for
protection as well as to improve the performance. Static dv/dt is a measure of the ability of a thyristor to
retain a blocking state under the influence of a voltage transient. These are also used across the relays and
switches to prevent arcing.

Necessity of Using the Snubber Circuit

These are placed across the various switching devices like transistors, thyristors, etc. Switching from ON
to OFF state results the impedance of the device suddenly changes to the high value. But this allows a
small current to flow through the switch. This induces a large voltage across the device. If this current
reduced at faster rate more is the induced voltage across the device and also if the switch is not capable of
withstanding this voltage the switch becomes burn out. So auxiliary path is needed to prevent this high
induced voltage

Similarly when the transition is from OFF to ON state, due to uneven distribution of the current through
the area of the switch overheating will takes place and eventually it will be burned. Here also snubber is
necessary to reduce the current at starting by making an alternate path.

Snubbers in switching mode provides one or more of the following functions

 Shape the load line of a bipolar switching transistor to keep it in its safe operating area.

 Reducing the voltages and currents during turn-ON and turn-OFF transient conditions.

 Removes energy from a switching transistor and dissipate the energy in a resistor to reduce junction
temperature.

 Limiting the rate of change of voltage and currents during the transients.

 Reduce ringing to limit the peak voltage on a switching transistor and lowering their frequency.

36 | P a g e
Design of RC Snubber Circuits:

There are many kinds of snubbers like RC, diode and solid state snubbers but the most commonly used
one is RC snubber circuit. This is applicable for both the rate of rise control and damping.

This circuit is a capacitor and series resistor connected across a switch. For designing the Snubber
circuits. The amount of energy is to dissipate in the snubber resistance is equal to the amount of energy is
stored in the capacitors. An RC Snubber placed across the switch can be used to reduce the peak voltage
at turn-off and to lamp the ring. An RC snubber circuit can be polarized or non-polarized. If you assume
the source has negligible impedance, the worst case peak current in the snubber circuit is

I = Vo/Rs and I = C.dv/dt

Figure: 1. 29. Forward-Polarized RC Snubber Circuit

For an appropriate forward-polarized RC snubber circuit a thyristor or a transistor is connected with an

anti-parallel diode. R will limit the forward dv/dt and R1 limits the discharge current of the capacitor
when transistor Q1 is turned on. These are used as overvoltage snubbers to clamp the voltage.

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 Figure: 1. 30. Reverse Polarized RC Snubber Circuit

Reverse polarized snubber circuit can be used to limit the reverse dv/dt. R1 will limit the discharge
current of the capacitor.

Figure: 1. 31. An un-polarized snubber circuit

An un-polarized snubber circuit is used when a pair of switching devices is used in anti-parallel. For
determining the resistor and capacitor values a simple design technique can be used. For this an optimum
design is needed. Hence a complex procedure will be used. These can be used to protect and thyristors.

Capacitors selection:

Snubber capacitors are subjected to high peak and RMS currents and high dv/dt. An example is turn-on
and turn-off current spikes in a typical RCD snubber capacitor. The pulse will have high peak and RMS
amplitudes. The snubber capacitor has to meet two requirements. First, the energy stored in the snubber
capacitor must be greater than the energy in the circuit’s inductance. Secondly, the time constant of
snubber circuits should me small compared to shortest on time expected, usually 10% of the on time. By
allowing the resistor to be effective in the ringing frequency this capacitor is used to minimize the

38 | P a g e
dissipation at switching frequency. The best design is selecting the impedance of the capacitor is same
that of resistor at the ringing frequency.

Resistors selection:

It is important that R in the RC snubber, have low self inductance. Inductance in R will increase the peak
voltage and it will tend to defeat the purpose of the snubber. Low inductance will also be desirable for R
in snubber but it is not critical since the effect of a small amount of inductance is to slightly increase the
reset time of C and it will reduce the peak current in switch at turn-on. The normal choice of R is usually
the carbon composition or metal film. The resistor power dissipation must be independent of the
resistance R because it dissipates the energy stored in the snubber capacitor in each transition of voltage
in the capacitor. If we select the resistor as that the characteristic impedance, the ringing is well damped.

When comparing the Quick design to optimum design, the required snubber resistor’s power capability
will be reduced. Usually the “Quick” design is completely adequate for final design. Going to the
“Optimum” approach is only if power efficiency and size constraints dictate the need for optimum
design.

Power Bipolar Junction Transistor (BJT)

Power BJT is used traditionally for many applications. However, IGBT (Insulated-Gate Bipolar
Transistor) and MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) have replaced it for
most of the applications but still they are used in some areas due to its lower saturation voltage over the
operating temperature range. IGBT and MOSFET have higher input capacitance as compared to BJT.
Thus, in case of IGBT and MOSFET, drive circuit must be capable to charge and discharge the internal
capacitances.

Figure: 1. 32. Symbol of transistor

39 | P a g e
The BJT is a three-layer and two-junction npn or pnp semiconductor device as given in Fig. 32. (a) and
(b).

Although BJTs have lower input capacitance as compared to MOSFET or IGBT, BJTs are
considerably slower in response due to low input impedance. BJTs use more silicon for the same drive
performance.

In the case of MOSFET studied earlier, power BJT is different in configuration as compared to simple
planar BJT. In planar BJT, collector and emitter is on the same side of the wafer while in power BJT it
is on the opposite edges as shown in Fig. 33. This is done to increase the power-handling capability of
BJT.

Figure: 1. 33. Structure of transistor

Power n-p-n transistors are widely used in high-voltage and high-current applications which will be
discussed later.

Input and output characteristics of planar BJT for common-emitter configuration are shown in Fig. 34.
These are current-voltage characteristics curves.

40 | P a g e
 Figure: 1. 34. Input and output characteristics of BJT

Metal-Oxide Semiconductor Field-Effect Transistor (Power)

MOSFET is a voltage-controlled majority carrier (or unipolar) three-terminal device. As compared to

the simple lateral channel MOSFET for low-power signals, power MOSFET has different structure. It
has a vertical channel structure where the source and the drain are on the opposite side of the silicon
wafer as shown in Figure. This opposite placement of the source and the drain increases the capability
of the power MOSFET to handle larger power.

N-channel enhancement type MOSFET is more common due to high mobility of electrons.

Figure: 1. 35. Symbol of MOSFET

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 Figure: 1. 36. Structure of MOSFET

Basic circuit diagram and output characteristics of an n-channel enhancement power MOSFET with load
connected are in Fig. 37 and Fig. 38 respectively.

Figure: 1. 37. Basic circuit diagram of n-channel enhancement power MOSFET

Drift region shown in Fig. 37 determines the voltage-blocking capability of the MOSFET.

When VGS = 0,

⇒ VDD makes it reverse biased and no current flows from drain to source.

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When VGS > 0,

⇒ Electrons form the current path as shown in Fig. 37. Thus, current from the drain to the source
flows. Now, if we will increase the gate-to-source voltage, drain current will also increase.

Figure: 1. 38. Output characteristics of an n-channel enhancement power MOSFET

For lower value of VDS, MOSFET works in a linear region where it has a constant resistance equal to
VDS / ID. For a fixed value of VGS and greater than threshold voltage VTH, MOSFET enters a saturation
region where the value of the drain current has a fixed value.

Besides the output characteristics curves, transfer characteristics of power MOSFET is also shown in
Fig. 39.

Figure: 1. 39. Transfer characteristics of an n-channel enhancement power MOSFET

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Insulated-Gate Bipolar Transistor (IGBT)

IGBT combines the physics of both BJT and power MOSFET to gain the advantages of both worlds. It
is controlled by the gate voltage. It has the high input impedance like a power MOSFET and has low
on-state power loss as in case of BJT. There is no even secondary breakdown and not have long
switching time as in case of BJT. It has better conduction characteristics as compared to MOSFET due
to bipolar nature. It has no body diode as in case of MOSFET but this can be seen as an advantage to
use external fast recovery diode for specific applications. They are replacing the MOSFET for most of
the high voltage applications with less conduction losses. Its physical cross-sectional structural
diagram and equivalent circuit diagram is presented in Fig. 40 to Fig. 41. It has three terminals called
collector, emitter and gate.

Figure: 1. 40. Cross -sectional structural diagram of IGBT

There is a p+ substrate which is not present in the MOSFET and responsible for the minority carrier
injection into the n-region. Gain of NPN terminal is reduced due to wide epitaxial base and n+ buffer
layer.

There are two structures of IGBTs based on doping of buffer layer:

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a) Punch-through IGBT: Heavily doped n buffer layer ➔ less switching time

b) Non-Punch-through IGBT: Lightly doped n buffer layer ➔ greater carrier lifetime ➔ increased
conductivity of drift region ➔ reduced on-state voltage drop

(Note: ➔ means implies)

Figure: 1. 41. Equivalent diagram of IGBT

Figure: 1. 42. Simplified Equivalent diagram of IGBT

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 Figure: 43. Equivalent diagram of IGBT

Based on this circuit diagram given in Fig. 43, forward characteristics and transfer characteristics are
obtained which are given in Fig. 44 and Fig. 45. Its switching characteristic is also shown in Fig. 45.

Figure: 1. 44. Forward characteristics of IGBT

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 Figure: 1.45. Transfer characteristics of IGBT

Figure: 1. 46. Dynamic characteristics of IGBT

(Note: Tdn : delay time ; Tr: rise time ; Tdf : delay time ; Tf1: initial fall time ; Tf2: final fall time)

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GTO (Gate Turn-off Thyristor)

GTO can be turned on with the positive gate current pulse and turned off with the negative gate current
pulse. Its capability to turn off is due to the diversion of PNP collector current by the gate and thus
breaking the regenerative feedback effect.

Actually the design of GTO is made in such a way that the pnp current gain of GTO is reduced. A
highly doped n spot in the anode p layer form a shorted emitter effect and ultimately decreases the
current gain of GTO for lower current regeneration and also the reverse voltage blocking capability.
This reduction in reverse blocking capability can be improved by diffusing gold but this reduces the
carrier lifetime. Moreover, it requires a special protection.

The symbol for GTO is shown in Fig.46.

Figure: 1. 47. Symbol of GTO

Overall switching speed of GTO is faster than thyristor (SCR) but voltage drop of GTO is larger. The
power range of GTO is better than BJT, IGBT or SCR.

The static voltage current characteristics of GTO are similar to SCR except that the latching current of
GTO is larger (about 2 A) as compared to SCR (around 100-500 mA).

The gate drive circuitry with switching characteristics is given in Fig. 48 and Fig. 49.

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 Figure: 1. 48. Gate Drive Circuit for GTO

Figure: 1. 49. Switching characteristics for GTO

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SCR Specifications and Ratings:
The main specifications of the SCR are its voltage rating and current rating. In this post, let us see various
ratings of thyristor.
Voltage Ratings
Peak Invese Voltage (VPIV)
The peak inverse voltage is defined as the maximum voltage which SCR can safely withstand in its OFF
state. The applied voltage should never be exceeded under any circumstances.
On State Voltage:
The voltage which appears across the SCR during its ON state is known as its ON state Voltage. The
maximum value of voltage which can appear across the SCR during its conducting state is called its
maximum on state voltage. Usually it will be 1V to 4V.
Finger Voltage:
The minimum voltage, which is required between the anode and cathode of an SCR to trigger it to
conduction mode, is called its finger voltage.

Rate of Rise of Voltage (dV/dt)

The rate at which the voltage across the device rises ( for forward condition) without triggering the
device, is known as its rate of rise of voltage.
Voltage Safety Factor:
The normal operating voltage of the SCR is kept well below its peak inverse voltage(VPIV) to avoid
puncture of SCR due to uncertain conditions. The operating voltage and peak inverse voltage are related
by voltage safety factor Vf
Vf = Peak inverse voltage / ( 2 x RMS value of input voltage)
Normaly Vf value lies between 2 and 2.5
Current Ratings:
The current carrying capacity of the device is known as its current rating.
It can be of two types.
1. Continuous
2. Intermittent.
Maximum average ON state current ( Imac):
This is the average value of maximum continuous sinusoidal ON state current with conduction angle
180deg, at frequency 40 to 60Hz, which should not be exceeded even with intensive cooling.

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Maximum rms ON-state current: (Imrc)
It is the rms value of the maximum continuous sinusoidal ON state current at the frequency 40 to 60 Hz
and conduction angle 180deg, which should not be exceeded even with intensive cooling.
Maximum surge - ON state Current (Imsc)
It is the maximum admissible peak value of a sinusoidal half cycle of tem milliseconds duration at a
frequency of 50Hz.
Latching Current (II)
It is the minimum current, which is required to latch the device from its OFF state to its ON state. In other
words, it is the minimum current required to trigger the device.
Holding Current (IH)
It is the minimum current required to hold the SCR conducting. In other words, It is the minimum current,
below which the device stops conducting and returns to its OFF state.
Gate Current:
The current which is applied to the gate of the device for control purposes is known as gate current.
Minimum Gate Current:
The minimum current required at the gate for triggering the device.
Maximum Gate Current:
The maximum current which can be applied to device safely. Current higher than this will damage the
gate terminal.
Gate Power Loss:
The mean power loss, which occurs due to flow of gate current between the gate and the main terminals.
Turn ON time:
The time taken by the device before getting latched from its OFF state to ON state. In other words, it is
the time for which the device waits before achieving its full conduction. Usually it will be 150 to 200μsec.
Turn OFF time:
After applying reverse voltage, the device takes a finite time to get switched OFF. This time is called as
turn-OFF time of the device. Usually it will be 200μsec.
Rate of rise of current (dI/dt)
The rate at which the current flowing in the device rises is known as its rate of rise (dI/dt) of current.

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Comparison between BJT and MOSFET:

Sl
BJT MOSFET
No

1 It is a Bipolar Device It is majority carrier Device

2 Current control Device Voltage control Device.

3 Output is controlled by controlling base current Output is controlled by controlling gate voltage

4 Negative temperature coefficient Positive temperature coefficient

5 So paralleling of BJT is difficult. So paralleling of this device is easy.

Dive circuit is complex. It should provide Dive circuit is simple. It should provide
6
constant current(Base current) constant voltage(gate voltage)

7 Losses are low. Losses are higher than BJTs.

8 So used in high power applications. Used in low power applications.

9 BJTs have high voltage and current ratings. They have less voltage and current ratings.

10 Switching frequency is lower than MOSFET. Switching frequency is high.

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