power electronics

1. Power Electronics
(MCT-321)
Lecture # 12
Course Instructor:
Engr. Hassan Mujtaba
University of Engineering & Technology Lahore, Faisalabad Campus

2. Outline
• PWM inverters
• Principle of operation
• Performance parameters
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4. PWM Inverters
• An abbreviation of Pulse-Width-Modulated
Inverters.
• Inverter is a device which converts a DC input
supply voltage into a symmetrical AC voltage of
desired magnitude and frequency.
• It is also know as DC-AC converter.
• A variable output voltage can be obtained either by
changing input DC voltage or inverter gain.
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5. PWM Inverters
• Inverter gain is the ratio of output AC voltage to DC
input voltage.
• Square waves can be used for low and medium power
applications while high power applications require
sinusoidal wave-forms with minimum distortions.
• Inverters find applications in variable-speed ac motor
drives, induction heating, standby power supplies,
and uninterruptible power supplies.
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6. PWM Inverters
• Ideal and practical inverter have sinusoidal and non-
sinusoidal waveforms at output respectively.
• If the input dc is a voltage source, the inverter is called a
Voltage Source Inverter (VSI).
• Using high speed power semiconductor devices the
harmonic contents at the output can be reduced by PWM
techniques.
• Inputs are (battery, fuel cell, solar cell, other dc sources)
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7. Types of Inverters
• Inverters can be classified into two types:
1. Single-phase inverters
2. Three-phase inverters
• Both can be controlled by transistors like BJTs, MOSFETs, IGBTs etc. for turn-on
and turn-off.
• An inverter is called:
• VFI (voltage-fed-inverter) if input voltage remains constant.
• CFI (current-fed-inverter) if input current is maintained constant.
• A variable dc linked inverter if input voltage is controllable.(e.g Grid tied inverters)
• Resonant-pulse inverter if output voltage or current is forced to pass through zero by
creating LC resonant circuit.
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8. Principle of Operation
• It consist of two choppers which might be transistors.
• Both transistors are turn on alternatively for half cycles.
• The two transistors should not turn-on at the same time.
• These transistors can be replaced by any switching device.
• In case of inductive load, the energy is fed back to dc
source through diodes, both diodes will conduct in this
case are known as feedback diodes.
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9. Single-phase Inverter
• There are two types of single phase inverters:
• Half Bridge Inverter:
This type of inverter is the basic building block of a full bridge inverter. It
contains two switches and each of its capacitors has a voltage output equal
to Vdc/2. In addition, the switches complement each other, that is, if one is
switched ON the other one goes OFF.
• Full Bridge Inverter
This inverter circuit converts DC to AC. It achieves this by closing and opening
the switches in the right sequence. It has four different operating states which
are based on which switches are closed.
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10. Half-bridge Inverter
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11. Half-bridge Inverter
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12. Waveforms
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13. Performance Parameters
• Harmonic factor of nth harmonic, HFn
It is a measure of individual harmonic contribution. HFn versus
the control parameter gives the harmonic profile of the
inverter. Small HFn value is desired.
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14. Performance Parameters
• Total harmonic distortion, THD
It is a measure of closeness in shape between a waveform and
its fundamental component (sinusoidal waveform). THD = 0
means sinusoidal wave.
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15. Performance Parameters
• Distortion Factor, DF
DF indicates the amount of HD that remains in a particular
waveform after the harmonics of that waveform have been
subjected to the second order attenuation (i.e. divided by n2).
Thus it’s a measure of effectiveness in reducing unwanted
signals.
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16. Performance Parameters
• DF of nth harmonic component, DFn
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17. Performance Parameters
• Lowest-order harmonic LOH
LOH is that harmonic component whose frequency
is closest to the fundamental one. Its amplitude is
normally > 3% of the fundamental component.
High LOH is desired.
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21. Summary
• AC output voltage can be obtained by alternatively
connecting +ve and –ve terminals of DC supply
across the load through switching devices.
• Feedback diodes can be used to transfer the energy
stored in inductance back to DC supply.
• The parameters which define the performance and
measure the quality of inverter output voltage are:
HF, THD, DF, and LOH
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22. Coming Up
• Next Lecture:
• PWM inverters
• Single phase bridge inverter
• Three phase inverter
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