PE_Converters.ppt

Power Electronics
• Power electronics is the branch of electrical engineering that
deals with the processing of high voltages and currents to
deliver power that supports a variety of needs.
• All the areas, from space applications to household electronic
equipment, need a steady and dependable electric power with
the desired conditions or specifications.
• Power supply in one form is transformed into another form by
processing the energy using controlled mechanisms supplying
regulated and controlled power.
• Power electronics is the application of electronics to the
control and conversion of electric power.
• There are various Power semiconductor devices such as:
a) Power Diodes
b) Power transistors c) Power Mosfets d) IGBTs e) Thyristors.

Thyristors (Silicon Controlled Rectifiers (SCR))
• Thyristors are a family of p-n-p-n structured power
semiconductor switching devices.
A) Symbol of Thyristors:
B) Structure of Thyristors:
• The anode connects to the P-type, cathode to the N-type and
the gate to the P-type shown above.

Thyristors
• There are three Junctions as shown below.
• Very Imp: It is necessary that all three junctions should be
forward biased for thyristor to turn ON.

Thyristors
C) Working:
• Thyristors works in three modes:
1) Forward blocking mode:
• In this mode, +ve voltage is applied to Anode and –ve voltage to
cathode.
• Thus J1 is FB, J2 is RB and J3 is FB and hence thyristor will not
conduct (turned ON).

Thyristors
2) Forward Conduction mode:
• In this mode, the positive voltage is applied to Gate and hence
J2 will also be turned ON.
• Hence the thyristor will turn ON.

Thyristors
3) Reverse Blocking mode:
• In this mode, the -ve voltage is applied to Anode and +ve
voltage is applied to Cathode hence J1 is RB , J3 is RB and J2 is
FB.
• Hence the thyristor will not turn ON.

Types of power electronics converter circuits
Power Electronics Converter Circuits
DC to AC
Converter
AC to DC
Converter
DC to DC
Converter
AC to AC
Converter

AC to DC Converter (Rectifier)
• It is also called as Rectifier.
• There are basically two types of Rectifier.
A) Half wave Controlled Rectifier
B) Full Wave Controlled Rectifier

A) Half Wave Controlled Rectifier:
1) Circuit diagram:
Where,

• Half Wave Controlled Rectifier circuit consists of SCR (Silicon
Controlled Rectifier) / thyristor, an AC voltage source and load.
• The load may be purely resistive, Inductive or a combination of
resistance and inductance.
• For simplicity, we will consider a resistive load.
2) Working:
• To understand the working, we must note the following points
about SCR/ Thyristor given in box below.
1. SCR will be turned ON only if it is forward biased and
Gate signal is applied.
2. SCR will be turned OFF when current through it
reaches below holding current and reverse voltage is
applied.

• Let us assume that thyristor T is fired at a firing angle of α.
• This means when wt = α, gate signal will be applied and SCR
will start conducting. Refer the figure below.

• Thyristor T is forward biased for the positive half cycle of
supply voltage.
• The load output voltage is zero till SCR is fired. Once SCR is fired
at an angle of α, SCR starts conducting.
• But as soon as the supply voltage becomes zero at ωt = π, the
load current will become zero and after ωt = π, SCR is reversed
biased.
• Thus thyristor T will turn off at ωt = π and will remain in OFF
condition till it is fired again at ωt = (2π+α).
• Therefore, the load output voltage and current for one
complete cycle of input supply voltage may be written as
Vo = VmSinωt (for α≤ωt≤ π)
i_o = VmSinωt / R (for for α≤ωt≤ π)

3) Formulas:
i) Average value of load output voltage:
ii) RMS value of load output voltage:

4) Numericals:
A) A half-wave rectifier circuit employing an SCR is adjusted to
have a gate current of 1mA. The forward breakdown voltage of
SCR is 100 V for Ig = 1mA. If a sinusoidal voltage of 200 V peak is
applied, find i) firing angle , ii) conduction angle and iii) average
current.
Assume load resistance = 100 ohm.

Sol:
i)
ii)
iii)

B) An SCR half-wave rectifier has a forward breakdown voltage of
150 V when a gate current of 1 mA flows in the gate circuit. If a
sinusoidal voltage of 400 V peak is applied, find:
i) Firing angle , ii) average output voltage , iii) average current for
load resistance of 200 ohm, iv) power output

Sol:

C) An a.c. voltage v = 240 sin314 t is applied to an SCR half-wave
rectifier. If the SCR has a forward breakdown voltage of 180 V, find
the time during which SCR remains off.

Sol:
• The SCR will remain off till the voltage across it reaches 180 V.
This is shown in Figure below.
• Clearly, SCR will remain off for t second.

B) Full Wave Controlled Rectifier (Bridge Configuration):
1) Circuit diagram:
Fig. 1 (a)

2) Working:

 Mode 1 (α to π):
• Here the L is positive and N is negative.
• Hence T_1 and T_2 are Forward biased while T_3 and T_4 are
Reversed biased.
• Also at ωt = α, the T_1 and T_2 are fired hence T_1 and T_2
conducts.
• Therefore, the current flows in the following manner:
from point L ---> through T_1----> through load---> through
T_2----> to point N.
• At ωt = π, the supply voltage goes through zero and hence T_1
and T_2 are turned OFF.

 Mode 2 (π to π+ α ):
• At ωt = π, the supply becomes zero, after ωt = π the supply
voltage reverses polarities.
• Thus in this mode of operation, no SCR conducts.
 Mode 3 (π+ α to 2π):
• Now the L is –ve and N is +ve.
• Hence T_1 and T_2 are Reverse biased while T_3 and T_4 are
Forward biased.
• Also at ωt = π + α, the T_3 and T_4 are fired hence T_3 and T_4
conducts.
• Therefore, the current flows in the following manner:
from point N ---> through T_3----> through load---> through
T_4----> to point L.
• At ωt = 2π, the supply voltage goes through zero and hence T_3
and T_4 are turned OFF.

DC to AC Converter (Inverter)
• It converts fixed DC to variable AC.
• In variable AC we can vary the magnitude as well as frequency
of the AC signal.
• Video Explanation:

DC to DC Converter (Chopper)
• The DC-to-DC converters or Choppers convert one level of DC
voltage to another level.
• There basically two types of DC to DC converter, viz., Buck
Converter and Boost Converter.
• A Buck Converter outputs a lower voltage than the original
voltage, while a Boost Converter supplies a higher voltage.
• As chopper can increase or decrease the DC voltage level at the
output side, so similar to AC transformer, it is also known as DC
transformer.

AC to AC Converter
• AC to AC converters is used for converting the AC waveforms
with one particular frequency and magnitude to AC waveform
with another frequency at another magnitude.
• This conversion is mainly required in case of speed controlling
of machines, for low frequency and variable voltage magnitude
applications as well.
• The ac voltage controllers are classified into two types based on
the type of input ac supply applied to the circuit.
a) Single Phase AC-AC Converter
i) Single phase half wave ac voltage controller
ii) Single phase full wave ac voltage controller
b) Three Phase AC-AC Converter
i) Three phase half wave ac voltage controller
ii) Three phase full wave ac voltage controller

PE_Converters.ppt

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