1. What is Power Electronics?

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 and Motor control -
AC/DC motor drives used in industries, High voltage power supplies, Vehicle propulsion
systems, High voltage direct current (HVDC) transmission.
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. Cycloconverter: These circuits convert input power at one frequency to output power
at a different frequency through one stage conversion.

Power semiconductor devices

a. Power Diodes.
b. Power transistors (BJT's).
c. Power MOSFETS.
d. IGBT's.
e. Thyristors
2. What is Turning On 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
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 Player, 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.

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 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.

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.

3.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.
4.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-LC 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.

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.
5.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.

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-.

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- DC.

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.
6.What is Dual Converter?

Dual converter, the name itself says two converters. It is really an electronic converter or circuit
which comprises of two converters. One will perform as rectifier and the other will perform as
inverter. Therefore, we can say that double processes will occur at a moment. Here, two full
converters are arranged in anti-parallel pattern and linked to the same dc load. These converters
can provide four quadrant operations. The basic block diagram is shown below

Modes of Operation of Dual Converter

There are two functional modes: Non-circulating current mode and circulating mode.

Non Circulating Current Mode  One converter will perform at a time. So there is no
circulating current between the converters.
 During the converter 1 operation, firing angle (α1) will be 0<α1< 90o ; Vdc and Idc are
positive.  During the converter 2 operation, firing angle (α2) will be 0<α2< 90o ; Vdc
and Idc are negative. Circulating Current Mode
 Two converters will be in the ON condition at the same time. So circulating current is
present.  The firing angles are adjusted such that firing angle of converter 1 (α1) + firing
angle of converter 2 (α2) = 180o .
 Converter 1 performs as a controlled rectifier when firing angle be 0<α1< 90o and
Converter 2 performs as an inverter when the firing angle be 90o <α2< 180o . In this
condition, Vdc and Idc are positive.
 Converter 1 performs as an inverter when firing angle be 90o <α1< 180o and
Converter 2 performs as a controlled rectifier when the firing angle be 0<α2< 90o In this
condition, Vdc and Idc are negative.
 The four quadrant operation is shown below

7.Single Phase Dual Converter

The source of this type of converter will be single-phase supply. Consider, the converter
is in no circulating mode of operation. The input is given to the converter 1 which
converts the AC to DC by the method of rectification. It is then given to the load after
filtering. Then, this DC is provided to the converter 2 as input. This converter performs
as inverter and converts this DC to AC. Thus, we get AC as output. The circuit diagram is
shown below
 8. Sketch and explain working of Single Phase fully controlled rectifier with
freewheeling diode ?
 This rectifier formed four SCRs. At ‘α’ T1 is triggered T1 & T2 are triggered and they
conduct from ‘α’ to ‘π ‘. The voltage across the inductor forward biases the
Freewheeling diode which conducts until the energy in the inductor falls to zero. At ‘π ‘+
‘α’ T3 and T4 are triggered. The current transfer from T1T2 to FD and From FD to T3T4
and so on. With FD, the advantages are 1. The average volt output increases 2. PF is
improved since FD prevents power flow DC side to AC. The Disadvantage is that
Regeneration is not possible due to FD.

9. Sketch and explain 6 Pulse converter?

10. Sketch and explain Single Phase Full Wave rectifier with R Load ?
11. Explain the working principle of Chopper?
 A chopper is a high speed ON/OFF switch. It connected source to load and disconnects
the load from the source at a fast speed. Figure below represents the simple circuit to
show its working principle.
 In this circuit, the switch SW is chopper. This switch can be made ON and OFF at a very
high speed. In this way, the load may be connected and disconnected from the supply
source Vs. When the switch is ON, the load voltage is equal to the source voltage Vs and
when the switch is OFF, load voltage becomes equal to ZERO. Thus, a chopped voltage
across the load is obtained . The output voltage i.e. voltage across the load is shown in
figure below.

12. Sketch and explain working of step up chopper ?

 Switch OFF period: When the chopper CH is switched OFF, the current through the L can
not die instantaneously rather it decays exponentially. Due to this behavior of L, it will
force the current through the diode D and load for the entire time period T OFF. This is
shown in figure below.
 Since, the current through the inductor L tends to decrease, the polarity of the emf
induced in inductor L is reversed as shown in above figure. As a result, the voltage
across the load becomes equal to the sum of source voltage and emf induced in
inductor. Thus, the output voltage exceeds the source voltage Vs. The load / output
voltage may be written as below. Vo = Vs + L(di/dt)
 Thus, the circuit works as a step-up chopper. It may be noted here that, the voltage
across the load increases because the inductor releases its stored energy to the load
during the OFF period.
 12. What is Cycloconverter ?
 A cycloconverter converts a constant voltage, constant frequency AC waveform to
another AC waveform of a lower frequency by synthesizing the output waveform from
segments of the AC supply without an intermediate DC link”. One particular property of
Cycloconverters is that it does not use a DC link in the conversion process thus making it
highly efficient. The conversion is done by using power electronic switches likes
Thyristors and switching them in a logical manner. Normally these Thyristors will be
separated into two half, the positive half and the negative half. Each half will be made to
conduct by turning them during each half cycle of the AC form thus enabling bi-
directional power flow.

13. Explain working of SMPS ?

 In this power source, a high voltage DC power is directly acquired from a DC power
source. Then, this high voltage DC power is switched usually in the range of 15KHz-5KHz.
And, then it is fed to a step down transformer unit of 50Hz. The o/p of this transformer
is fed to the rectifier, them this rectified o/p power is used as a source for loads, and the
oscillator ON time is controlled and a closed loop regulator is formed.
The switching-power supply o/p is regulated by using Pulse Width Modulation shown in
the above circuit, the switch is driven by the PWM oscillator, then indirectly the step
down transformer is controlled when the power fed to the transformer.

14.