rectifier half nd full

7/22/2019 rectifier half nd full

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Rectifier and regulator

In this lab you will construct and analyze a full wave rectifier and a shunt voltage

regulator. All component types in the example circuit are available in OrCAD Capture

libraries for simulation.

I. Introduction

1.1 The Full Wave Rectifier

he first building bloc! in the dc power supply is the full wave rectifier. he purpose of

the full wave rectifier "#$%& is to create a rectified ac output from a sinusoidal ac input signal. It

does this by using the nonlinear conductivity characteristics of diodes to direct the path of the

current.

#igure '. Common four(diode bridge configuration for the #$%

Diode Currents

Consider the current path in the diode bridge rectifier. In the positive half cycle of )in*

diodes D+ and D, will conduct. During the negative half cycle* diodes D- and D' will conduct.

As a result* the load will pass current in the same direction in each half cycle of the input.

Design Concerns %everse current does not exceed the brea!down value

ower dissipation limit / )dIdis not exceeded

Diode Voltages

#orward 0ias


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o If we consider a simple* piece(wise linear model for the diode I) curve* the diode

forward current is zero until )bias1/ )threshold* where )thresholdis 2.3 ) to 2.4 ).

he current increases abruptly as )bias increases further. Due to this turn(on or

threshold voltage associated with the diode in forward bias* we should expect a 2.3 to

2.4 ) voltage drop across each forward biased diode in the rectifier bridge. In thecase of the full wave rectifier diode bridge* there are two forward biased diodes in

series with the load in each half cycle of the input signal.

o he maximum output voltage "across load& will be )in ( - )threshold* or 5 )in( '.+ ).

o 6ince some current does flow for voltage bias below ) thresholdand the current rise

around is )threshold is more gradual than the piece(wise model* the actual diode

performance will differ from the simple model.

%everse 0ias

o In reverse bias "and neglecting reverse voltage brea!down&* the current through the

diode is approximately the reverse saturation current* Io. he voltage across the load

during reverse bias will be )out/ Io%load.

o In specifying a diode for use in a circuit* you must ta!e care that the limits for

forward and reverse voltage and current are not exceeded.

1.2 Filtered Full Wave Rectifier

he filtered full wave rectifier is created from the #$% by adding a capacitor across the

output.

#igure -. #iltered full wave rectifier


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6ince - ( ' 5 9-* where is the period of the sine wave* then

Peak Current Levels

Diodes in the bridge conduct only in the time period from 2 to '. he diode current

must replace the charge lost by the capacitor during its discharge.

I / d:9d / C;d)9d

As the magnitude of the filter capacitor increases* the pea! current through the diodes

must increase to replace the charge in less time. herefore it is not always best to choose the

largest value of C' available. In a dc power supply* you can rely on the stages following the

##$% to significantly improve the voltage regulation.

1. The !hunt Regulator

A shunt regulator may be placed between the filtered full wave rectifier and the load

resistance "impedance&. Its purpose is to minimize the variation in the voltage across the load* as

either the input voltage or the output resistance changes.

#igure +. #iltered #$% and shunt regulator

his regulator is called a shunt because it provides an additional path for current to flow*so that some current can bypass the load. he shunt regulator consists of a zener diode and a

resistor. he zener diode has a nearly constant voltage drop when used in reverse bias. he

resistor is chosen to maintain the zener in its proper wor!ing region* where it can provide

regulation and not exceed a maximum power limit.


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A simple model for the zener diode is a dc supply "battery& with a value of )zo* where

)zo is the effective zener voltage* * )z is the rated brea!down voltage* and %z is

the effective resistance of the zener* given by the inverse of the slope of the I) curve in the

wor!ing region.

#igure


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the input voltage and load current* both of which may change with time. hese terms must be

minimized for 7uality regulation.

In"ut !ensitivit# and Load !ensitivit#

Assume the input to the shunt regulator is )dc >9( )ripple. #or )in / )in"max& / )dc >)ripple* additional current is available from the source. o !eep )o / I8 %8 constant* some of

that current must be shunted through the zener diode. As long as Iz ? Iz"max&* as defined by the

maximum power dissipation for the zener* the circuit will safely regulate. Choose % to prevent

the zener from exceeding its maximum current limit.

#or )in / )in"min& / )dc ( )ripple* current drops. o !eep )o / I 8;%8constant* the

current through the zener diode must be reduced. o maintain regulation* Iz must not be reduced

below the !nee current. Choose % to maintain sufficient current through the zener@

he shunt regulator has several maor problems which prevent its common use as the sole

pre(regulation stage in dc power supplies@

o

$hen the load is open circuit* all current is shunted through the zener diode. hisre7uires an expensive* high power device.

o he line and load regulations values are high "5 '2 B or more&.

o he energy efficiency is low.

#or an improved design* the shunt regulator is used in conunction with a series pass

element with gain* usually a transistor* between the unregulated supply and the load.

II. Pro$ect Design

2.1 !i%ulation

Part 1&

o simulate the full wave rectifier circuit as shown in #igure '* the following components

should be used@


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'. Input AC voltage ")in&@ )in is a 1' V"eak and (' )* sinusoidal +ave.se )6I with

the setting@ )O## / 2* )AE8 / '2 and #%F: / 32

-. #ull wave rectifier "#$%&@ he full wave rectifier is constructed in the form of bridge

rectifier using four diodes "D1,-''-&.

,. 8oad resistor@ -22 G and


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-2 mAK %load .-< ohmK max - $att&. Choose any two points in the linear region

and use C%6O% function to display Id and )d of the two points. rint out from the

screen and it should loo! li!e that in #igure 3 except in the first 7uadrant.

-. Calculate the effective zener resistance %z and effective zener voltage )zo from the

e7uation or %z / ")z- )z'&9"Iz- Iz'& and )zo / )z' Iz';%z.

,. Calculate the value of the series resistor % "%


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Mz and ''2 )rms& using a '


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Comparison between the simulation results and the measurement results

Any other 7uestions* comments

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rectifier half nd full