Power Rectifier Lecture_Spring10

8/10/2019 Power Rectifier Lecture_Spring10

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Advanced Mechatronics, Georgia Tech

ME8843

ME 8843

Advanced Mechatronics

Instructor: Professor I. Charles Ume

Power Rectifiers


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ME8843 Outline

Motivation

Rectification Technologies

Types of Rectification

Rectification Circuits

Applications


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ME8843 Motivation

Early experiments with Direct

Current (DC) power relied on Leydenjars (rudimentary batteries) whichhad to be recharged via manuallabor (e.g. grad students)

Due to efficiency and safety reasons,Alternating Current (AC) is used forproviding electrical power

A means to convert AC to DC is

required - called RectificationAC Power

Transmission

Lines

Leyden Jar


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ME8843 Rectification Technologies

Electromechanical

Synchronous rectifier Used motor attached to metal contacts that switched direction ofcurrent flow in time with AC input voltage

Motor-generator set

An AC motor coupled to DC generator

Electrolytic

Two different material electrodes suspended in electrolyte

provide different resistance depending on current flow

Mercury arc rectifier

A sealed vessel with mercury in it provides DC power bytransmitting electricity through ionized mercury vapor

Capable of power on order of hundreds of kilowatts

Vacuum Tube

Capable of high voltages, but relatively low current


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ME8843 Mercury Vapor Rectifiers

From steel manufacturing plant in Germany


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ME8843 Rectification Based on Diode

Diodes provide compact, inexpensive means of

rectification

Can create rectifiers from multiple diodes or purchase

integrated module

Diode Rectifier ModulesDiodes


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ME8843 Types of Rectification

Full Wave Rectifier

Half Wave Rectifier

While output of the rectifiers is now DC (current only

flows in one direction), output oscillates


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ME8843Types of Rectification: Poly-phase

Industrial settings usually have 3-phase

power available for machines

Rectifying 3-phase power results in DC

voltage with less ripple

Three-phase full-wave bridge rectifier circuit

Input and output voltages for three-phase rectifier


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ME8843 Types of Rectification

Half Wave:

Negative components of sinewave are discarded

Full Wave:

Negative components are

inverted


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ME8843 Types of Rectification: Poly-phase

Input and output voltages for 3-phase rectifier


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ME8843 Rectification Circuit: Half-Wave

Rectification is most popular application of diode

It converts alternating current (AC) to direct current (DC).

It involves device that only allows one-way flow of

electrons, and this is exactly what semiconductor diode

does.

Simplest kind of rectifier circuit is half-waverectifier.

It only allows one half of AC waveform to pass through to

load.

Half-wave rectif ier circu it


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For most power applications, half-wave rectification is

insufficient for task.

Harmonic content of rectifier's output waveform is very large andconsequently difficult to filter.

AC power source only supplies power to load once every half-

cycle, meaning that much of its capacity is unused.

Half-wave rectification is, however, very simple way to reduce

power to resistive load.

Two-position lamp dimmer switches apply full AC power

to lamp filament for full brightness and then half-wave

rectify it for a lesser light output.

Half-wave rect i f ier appl icat ion: Two level lamp dimmer.


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In Dim switch position, incandescent lamp receives approximatelyone-half power it would normally receive operating on full-wave AC.

Because half-wave rectified power pulses far more rapidly than filamenthas time to heat up and cool down, lamp does not blink.

Instead, its filament merely operates at lesser temperature than normal,providing less light output.

This principle of pulsing power rapidly to slow-responding load

device to control electrical power sent to it is common in world ofindustrial electronics.

Since controlling device (diode, in this case) is either fullyconducting or fully non-conducting at any given time, it dissipateslittle heat energy while controlling load power, making this method of

power control very energy-efficient.

This circuit is perhaps crudest possible method of pulsing power to aload, but it suffices as a proof-of-concept application.


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ME8843 Rectifier Circuit: Full-Wave

If we need to rectify AC power to obtain full use of both

half-cycles of sine wave, different rectifier circuit

configuration must be used.

Such circuit is called full-waverectifier.

One kind of full-wave rectifier, called center-tapdesign, usestransformer with center-tapped secondary winding and two

diodes

Full-wave rectif ier, center-tapped d esign .


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ME8843 Rectifier circuit

This circuit's operation is easily understood one half-

cycle at time.

Consider first half-cycle, when source voltage polarity is

positive (+) on top and negative (-) on bottom. Only top diode is conducting; bottom diode is blocking current,

and load sees first half of sine wave.

Only top half of transformer's secondary winding carries current

during this half-cycle.

Ful l -wave center-tap rect i f ier : Top hal f of secon dary win ding cond ucts

dur ing posi t ive hal f -cycle o f input , del iver ing pos i t ive hal f -cycle to load.


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During next half-cycle, AC polarity reverses. Now, other

diode and other half of transformer's secondary winding

carry current while portions of circuit formerly carrying

current during last half-cycle sit idle. The load still sees half of sine wave, of same polarity

as before.

Full-wave center-tap recti f ier: During negative inpu t half-cyc le, bo ttom h alf

of secon dary wind ing c ond ucts, del iver ing a pos i t ive half -cycle to th e load.


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ME8843 Rectifier Circuit: Full-Wave

One disadvantage of this full-wave rectifier design is

necessity of transformer with center-tapped secondary

winding.

If circuit in question is one of high power, size and

expense of suitable transformer is significant.

Consequently, center-tap rectifier design is only seen in

low-power applications.


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ME8843 Rectifier circuit: Dual Polarity Full-Wave

The full-wave center-tapped rectifier polarity at load maybe reversed by changing direction of diodes.

Furthermore, reversed diodes can be paralleled withexisting positive-output rectifier.

The result is dual-polarity full-wave center-tappedrectifier.

Note that connectivity of diodes themselves is sameconfiguration as bridge.

Dual polari ty fu l l-wave center tap rect i f ier


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ME8843 Rectifier circuit: Full-Wave Bridge

Another, more popular full-wave rectifier design exists,

and it is built around four-diode bridge configuration.

For obvious reasons, this design is called full-wave

bridge.

Full-wave br idge r ect i f ier.


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ME8843 Rectifier circuit: Full-Wave Bridge Current directions for full-wave bridge rectifier circuit for

positive and negative half-cycles of AC source waveformare shown below and next page respectively.

Note that regardless of polarity of input, current flows insame direction through load.

That is, negative half-cycle of source is positive half-cycle at load.

Ful l -wave bridg e rect i f ier : Electron f low for po si t ive half-cycles


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Current flow is through two diodes in series for bothpolarities. Thus, two diode drops of source voltage are lost (0.72 = 1.4 V

for Si) in diodes.

This is disadvantage compared with full-wave center-tapdesign. This disadvantage is only problem in very low voltage power

supplies

Full-wave bridg e rect i f ier: Electro n flow fo r negative half=cy cles.


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Remembering proper layout of diodes in full-wave bridge

rectifier circuit can often be frustrating some times.

An alternative representation of this circuit is easier both

to remember and to comprehend.

It is exact same circuit, except all diodes are drawn in

horizontal attitude, all pointing same direction.

One advantage of remembering this layout for bridge

rectifier circuit is that it expands easily into poly-phase

version shown in next slide.

Alternat ive layout sty le for Ful l-wave b r idge rect i f ier.


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ME8843 Rectifier circuit: Polyphase-Three Phase

Each three-phase line connects between pair of diodes: One to route power to positive (+) side of load, and other to

route power to negative (-) side of load.

Poly-phase systems with more than three phases are

easily accommodated into bridge rectifier scheme. Takefor instance the six-phase bridge rectifier circuit in nextslide

Three-phase ful l-wave b ridg e rect i f ier circ uit .


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ME8843 Rectifier circuit: Polyphase-Six Phase

Six-phase ful l -wave br idg e rect i f ier circuit .


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ME8843 Rectifier circuit: Polyphase

When poly-phase AC is rectified, phase-shifted pulses

overlap each other to produce DC output that is muchsmoother

Has less AC content than that produced by rectification of single-

phase AC.

This is decided advantage in high-power rectifier circuits, where

sheer physical size of filtering components would be prohibitivebut low-noise DC power must be obtained.

The Figure in next slide shows full-wave rectification of three-

phase AC.


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ME8843 Rectifier circuit: Poly-phase

Three-phase AC and 3-phase ful l-wave rect i f ier o utp ut.


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In any case of rectification -- single-phaseor polyphase -- amount of AC voltage

mixed with rectifier's DC output is called

ripple voltage.

In most cases, since pure DC is desired

goal, ripple voltage is undesirable.

If power levels are not too great, filtering

networks may be employed to reduce

amount of ripple in output voltage.


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ME8843

Output ripple will always be present in circuits shown above

Amplitude of ripple can be reduced by adding smoothing capacitor

Capacitor and load (shown here as resistor) from low pass filter withtime constant T= RC

Time constant should be much longer than one ripple

For given ripple amplitude capacitor size (in microfarads) given by

f: line frequency

Iload: Load Current

Vrip: Amplitude of ripple voltage

NOTE: Voltage rating of the capacitor must be > 1.4*Voutand large

capacitors should have bleeder resistors for safety!

Output Ripple

C IloadfVrip

10 6 (Halfwave) or C Iload2fVrip

10 6 (Fullwave)


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Sometimes, method of rectification is referred to bycounting number of DC pulses output for every 360oofelectrical rotation.

A single-phase, half-wave rectifier circuit, then, would becalled 1-pulserectifier, because it produces single pulseduring time of one complete cycle (360o) of AC

waveform. A single-phase, full-wave rectifier (regardless of design,

center-tap or bridge) would be called 2-pulserectifier,because it outputs two pulses of DC during one ACcycle's worth of time.

A 3-phase full-wave rectifier would be called 6-pulseunit.


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ME8843 Rectifier Circuit: Output Voltage

Full wave rectification will produce voltage roughly equal

to

In practice, there will be small voltage drop across

diodes that will reduce this voltage

For accurate supplies, regulation is necessary

Vo 2Vi,RMS


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REVIEW:

Rectificationis conversion of alternating current (AC) to

direct current (DC). A half-waverectifier is circuit that allows only one half-

cycle of AC voltage waveform to be applied to load,resulting in one non-alternating polarity across it. The resulting DC delivered to load pulsates significantly.

A full-waverectifier is circuit that converts both half-cycles of AC voltage waveform to unbroken series ofvoltage pulses of same polarity. The resulting DC delivered to load doesn't pulsate as much.

Poly-phase alternating current, when rectified, givesmuch smoother DC waveform (less ripplevoltage) thanrectified single-phase AC.


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ME8843 Applications

DC Power supplies

Used to provide DC power to drive loads Radios

Used to rectify received radio signals as part of AM

demodulation

Signal to be transmitted is multiplied by a carrier wave

Diode in receiver rectifies signal

Carrier Wave

Audio SignalRectified Radio

Wave

Diode

Modulated

Signal Radio Transmission


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ME8843 Applications

Light Dimmer

Sends unrectified or half wave

AC power through light bulb

Automobile Alternators

The output of 3-phase AC

generator is rectified by diode

bridge

More reliable than DC

generator

6 Rectifier Diodes


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Advanced Mechatronics Georgia Tech

ME8843 References

http://en.wikipedia.org/wiki/Rectifier

http://en.wikipedia.org/wiki/Diode_bridge

http://www.allaboutcircuits.com/vol_3/chpt_3/4.html

http://my.integritynet.com.au/purdic/power1.html

http://electronics.howstuffworks.com/radio.htm
http://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://www.allaboutcircuits.com/vol_3/chpt_3/4.htmlhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://www.allaboutcircuits.com/vol_3/chpt_3/4.htmlhttp://www.allaboutcircuits.com/vol_3/chpt_3/4.htmlhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htmhttp://electronics.howstuffworks.com/radio.htm

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Power Rectifier Lecture_Spring10