power electronics(ii)

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Uncontrolled & controlled Rectifiers

Engr. Jahangeer badar soomro

CLASSIFICATION OF RECTIFIER CIRCUITS

1. Single Phase Half Wave Uncontrolled Rectifier

2. Single Phase Full Wave Uncontrolled Rectifier

3. Three Phase Full Wave Uncontrolled Rectifier

4. Single Phase Half Wave Controlled Rectifier

5. Single Phase Full Wave Controlled Rectifier

6. Three Phase Full Wave Controlled Rectifier.

Single Phase Half Wave Uncontrolled Rectifier

Single Phase Full Wave Uncontrolled Rectifier

using center tap

Single Phase Full Wave Uncontrolled Rectifier

using Bridge

Important Question

• Explain the operation of single phase half wave converter with RL load with the help of circuit diagrams and waveforms. Also discuss that why flywheel diode is connected in such type of circuits.

• https://www.youtube.com/watch?v=F57tKwX63M0

Also discuss that why flywheel diode is connected in such type of circuits

Single Phase Full Wave Controlled Rectifier Using Center tap/Two Pulse Converter/Mid point Converter

Single Phase Full Wave Controlled Rectifier Using Bridge

• You guys now know very well about this circuit that how it will operate.

• This is our today lab.

• Design this circuit in MATLAB. And show the satisfactory results.

Three Phase Full Wave Uncontrolled Rectifier

• https://www.youtube.com/watch?v=Ij9aCSC8QBY

• It is your assignment to cover three phase uncontrolled rectifier from this video.

• Very easily explained and informative.

• You must listen this video.

Three Phase Full Wave controlled Rectifier

• https://www.youtube.com/watch?v=Ij9aCSC8QBY

• It is your assignment to cover three phase controlled rectifier from this video.

• Very easily explained and informative.

• You must listen this video.

AC VOLTAGE CONTROLLER/REGULATOR.

ENGR.JAHANGEER BADAR

DC TO DC CONVERTEROR

CHOPPER

ENGR. JAHANGEER BADAR SOOMRO

Efficiency & Power Losses

• Efficiency is a good measure of the success of a givenconverter technology.

• With very small amount of power lost, the converterelements can be packaged with high density, leading to aconverter of small size and weight, and of low temperaturerise.

• How can we build a circuit that changes the voltage, yetdissipates negligible power?

Linear Regulators Vs Switch Mode Conversion Converter

• Linear Regulators are inefficient • Linear Regulators have a high drop out voltage. • They are limited to output voltage smaller than input

voltage .• Linear regulators can however provide a very high

quality output voltage but they are used for application of low power level as low drop-out-voltage voltage regulators.

• At higher power levels switching regulators are used.• Switched mode conversion offers you high efficiency as

discussed in class. (Due to switching phenomenon)

Linear Regulators Vs Switch Mode Conversion Converter

• A 7 minute video comparing beautifully linear regulator vs switch mode conversion converter.

• https://www.youtube.com/watch?v=giGRrODKJSE

Principle of Step Down Chopper

Design procedure for Buck Converter• Calculate D to obtain required output voltage.

• Select a particular switching frequency:

– –preferably >20KHz for negligible acoustic noise

• Higher fs results in smaller L, but higher device losses.

– Thus lowering efficiency and larger heat sink.

Design procedure for Buck Converter

• Inductor requirement

• C Calculation

• Possible switching devices: MOSFET, IGBT and BJT. Low powerMOSFET can reach MHz range.

𝑟 =1 − 𝐷

8𝐿𝐶𝑓2

𝐿 ≥ 𝐿𝑚𝑖𝑛 =1 − 𝐷

2𝑓𝑅

Example-2

• A buck converter has an input voltage of 50V and output of 25V.The switching frequency is 10KHz. The power output is 125W. (a)Determine the duty ratio, (b) value of L to ensure continuouscurrent, (c) value of capacitance to limit the output voltage ripplefactor to 0.5%.

Solution

(a) output voltage

𝑉𝑜 = 𝐷𝑉𝑠

(b) Value of L

𝐷 =𝑉𝑜𝑉𝑠

𝐷 =25

50= 0.5

𝐿 ≥ 𝐿𝑚𝑖𝑛 =1 − 𝐷

2𝑓𝑅

𝐿 ≥ 𝐿𝑚𝑖𝑛 =1 − 0.5

2 × 10𝐾𝑅

𝑃𝑜 =𝑉2

Example-2

• Resistance is calculated as

• L must at least be 10 times greater than Lmin.

𝑅 =𝑉2

𝑃𝑜=252

125= 5Ω

𝐿 ≥ 𝐿𝑚𝑖𝑛 =1 − 0.5

2 × 10𝐾× 5

𝐿 ≥ 𝐿𝑚𝑖𝑛 =2.5

20𝐾= 125𝜇𝐻

𝐿 = 1.25𝑚𝐻

Example-2

• Value of capacitance to limit the output voltage ripple factor to0.5% can be calculated using following equation.

(c) Value of C

𝑟 =1 − 𝐷

8𝐿𝐶𝑓2

0.005 =1 − 0.5

8 × 1.25𝑚 × 𝐶 × (10𝐾)2

𝐶 =1 − 0.5

8 × 1.25𝑚 × 0.005 × (10𝐾)2

𝐶 =0.5

5000= 100𝜇𝐹

Step-Up Converter (Boost Converter)

• Boost Converter Design

• Minimum inductor value

• Capacitor Value

𝐿𝑚𝑖𝑛 =𝐷(1 − 𝐷)2𝑅

𝑟 =𝐷

𝑅𝐶𝑓

http://www.learnabout-electronics.org/PSU/psu33.php

Buck-Boost ConverterPlease refer to following link for excellent material on buck-boost.

Concept Of Commutation

Engr. Jahangeer Badar Soomeo

Power Converters

Engr. Jahangeer Badar Soomro

One particular reason why you need Power Electronics is that you do not have DC transformers and AC batteries.

DC TO AC CONVERTEROR

IINVERTER

ENGR.JAHANGEER BADAR

WHAT IS INVERTER?

• Thyristor based inverters are used for high power applications.

• For low and medium power inverters fully controlled devices such as Power BJT and Power Mosfet, IGBT, GTO, SIT are used. In addition to having fully controlled these devices high switching frequencies.

• Therefore these devices may be employed very efficiently in inverters where the output voltage is controlled using PWM techniques.

TYPICAL APPLICATIONS

– Un-interruptible power supply (UPS)

TYPICAL APPLICATIONS

– HVDC (High Voltage Direct Current)

Types of Inverters

• There are three basic types of dc-ac converters depending ontheir AC output waveform:

– Square wave Inverters

– Modified sine wave Inverters

– Pure sine wave Inverters

Square Wave Inverters

– The square wave is the simplest and cheapest type, but nowadays itis practically not used commercially because of low power quality(THD≈45%).

Modified Sine wave Inverters

• The modified sine wave topologies provide rectangular pulses withsome dead spots between positive and negative half-cycles.

• They are suitable for most electronic loads, although their THD isalmost 24%.

• They are the most popular low-cost inverters on the consumermarket today,

Pure Sine Wave Inverters

– A true sine wave inverter produces output with the lowest totalharmonic distortion (normally below 3%).

– It is the most expensive type of AC source, which is used whenthere is a need for a sinusoidal output for certain devices, suchas medical equipment, laser printers, stereos, etc.

– This type is also used in grid-connected applications.

Simple square-wave inverter

• To illustrate the concept of AC waveform generation

AC Waveform Generation

AC Waveforms

Output voltage harmonics

• Harmonics may cause degradation of equipment(Equipment need to be “de-rated”).

• Total Harmonic Distortion (THD) is a measure todetermine the “quality” of a given waveform.

𝑇𝐻𝐷𝑣 = 𝑛=2∞ 𝑉𝑛,𝑅𝑀𝑆

𝑉1,𝑅𝑀𝑆

𝑇𝐻𝐷𝑖 = 𝑛=2∞ 𝐼𝑛,𝑅𝑀𝑆

𝐼1,𝑅𝑀𝑆

Fourier Series

• Study of harmonics requires understanding of wave shapes.

• Fourier Series is a tool to analyse wave shapes.

• Where,

𝑣 𝑡 = 𝑎𝑜 +

𝑛=1

𝑎𝑛 cos 𝑛𝜃 + 𝑏𝑛 sin 𝑛𝜃

𝑎𝑛 =1

𝑣 𝑡 cos 𝑛𝜃 𝑑𝜃

𝑏𝑛 =1

𝑣 𝑡 sin 𝑛𝜃 𝑑𝜃

𝑎𝑜 =1

𝑣 𝑡 𝑑𝜃

Filtering

• Low-pass filter is normally fitted at the inverter output toreduce the high frequency harmonics.

1-∅ Voltage source Inverters

• Single phase voltage source inverters are of two types.

– Single Phase Half Bridge voltage source inverters

– Single Phase full Bridge voltage source inverters

CycloConverter

Power Transistors & Power MOSFETS

INSULATED GATE BIPOLAR

TRANSISTOR (IGBT)IGBT is a voltage controlled device. It has high input

impedance like a MOSFET and low on-state conduction losses

like a BJT.

Figure below shows the basic silicon cross-section of an IGBT.

Its construction is same as power MOSFET except that n+ layer at

the drain in a power MOSFET is replaced by P+ substrate called

collector.

n Bufferlayer

Gate Gate

Emitter

Collector

http://www.learnabout-electronics.org/Downloads/Fig3116_new.swfConceptual understanding of JFET

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