chapter 2(dioderectifier uniten)

8/3/2019 Chapter 2(DiodeRectifier UNITEN)

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Power Electronics (EEED243)

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Chapter 2AC to DC CONVERSION (RECTIFIER)

Single-phase, half wave rectifierUncontrolled

R loadR-L loadR-C loadControlledFree wheeling diode

Single-phase, full wave rectifierR loadR-L load,Controlled R, R-L loadcontinuous and discontinuous current mode

Three-phase rectifier

uncontrolledcontrolled

Rectifiers

DEFINITION: Converting AC (from mains or other AC source) to DC power byusing power diodes or by controlling the firing angles of thyristors/controllableswitches.

Basic block diagram

Input can be single or multi-phase (e.g. 3-phase).

Output can be made fixed or variable

Applications: DC welder, DC motor drive, Battery charger,DC power supply,HVDC


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Introduction

The single-phase diode rectifiers are the simplest static converters of electrical power.Power diodes employed in the circuits are uncontrolled semiconductor power switches.

Diode as we have already seen, turns-on when a positive voltage is applied at its anode

with the respect to its cathode.In most applications, the rectifier circuits are supplied from the utility source. In the

analysis of diode rectifier circuits in this section we will assume that diodes are ideal.

Single-Phase, half-wave with R- load

RMS Voltage


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DC voltage is fixed at 0.318 or 31.8% of the peak value.

RMS voltage is reduced from 0.707 (normal sinusoidial RMS) to 0.5 or 50% of peakvalue.

Half wave is not practical because of the high distortion supply current. The supplycurrent contains DC component that may saturate the input transformer.

Half-Wave with R-L Load

This is a first order differential equation. Solution is in the form of:

where: if, in are known as forced and natural response, respectively. From thediagram, forced response is:


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R-L Load


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R-L waveform

RMS Current and Power Factor

The average (DC) current is

The RMS current is:

Power absorbed by the load is:


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Power Factor is computed from definition:

where P is the real power supplied by the source, which is equal to the power absorbed by

the load. S is the apparent power supplied by the source i.e.


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Half wave rectifier, R-C Load


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Freewheeling Diode (FWD)


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Operation of FWD

Note that both D1and D2cannot be turned on at the same time.

For a positive cycle voltage source,

D1 is on, D2 is off

The equivalent circuit is shown in Figure (b)The voltage across the R-L load is the same as the source voltage.

For a negative cycle voltage source,

D1 is off, D2 is on

The equivalent circuit is shown in Figure (c)

The voltage across the R-L load is zero.

However, the inductor contains energy from positive cycle. The

load

current still circulates through the R-L path.

But in contrast with the normal half wave rectifier, the

circuit inFigure (c) does not consist of supply voltage in its loop.

Hence the negative part of vo as shown in the normal half-wavedisappear

FWD- Continuous load current

The inclusion of FWD results in continuous load current as shownbelow

Note that the output voltage has no negative part.


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Full wave rectifier with R load

6. Bridge Circuit

Bridge Waveform


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For the bridge rectifier, the basic observations are as follows:

1. Diodes D1 and D2 conduct together, and D3 and D4 conduct together. Kirchoffsvoltage law around the loop containing the source, D1 and D3 shows that D1 and D3

cannot be on at the same time. Same principle applied to D2 and D4. The load current

can be positive or zero but never negative.2. The voltage across the load is +Vs when D1 and D2 are on. The voltage across the load

is Vs when D3 and D4 are on.

3. The maximum voltage across a reverse biased diode is the peak value of the source.

4. The current entering the bridge from the source is iD1- iD4, which is symmetric aboutzero. Therefore, the average source current is zero.

5. The rms source current is the same as the rms load current. The source current is the

same as the load current for half of the source period and is the negative of the loadcurrent for the other half. The squares of the load and source currents are the same, so

the rms currents are equal.

6. The fundamental frequency of the output voltage is 2, where is the frequency of

the ac input, since two periods of the output occur for every period of the input. TheFourier series of the output consists of a dc term and the even harmonics of the source

frequency.


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2) Center-tapped circuit

Center-tapped circuit

Center-tapped rectifier requires center-tap transformer. Bridge does not.

Center tap requires only two diodes, compared to four for bridge. Hence, per

half-cycle only one diode volt-drop is experienced.Conduction losses is half of

bridge.

However, the diodes ratings for center-tapped is twice than bridge.


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Full wave bridge, R-L load


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CONTROLLED RECTIFIER


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RMS voltage and current

Average voltage

Average current RMS current

The power absorbed by the load

[ ] ==

coscos2

)sin(2

1 VmtdtVmVo

=

dtiIo )(2

1

( ) ]2

1[ 2

dtiIRMS =


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Power Electronics (EEED243) 22

chapter 2(dioderectifier uniten)

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