6- Diode Applications

8/8/2019 6- Diode Applications

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DIODE APPLICATIONS

Half-wave Rectifiers, Full-wave

Rectifiers, DC power supplies(typical bock diagram), define

ripple, effect of capacitor filteringon ripple, basic zener diode

regulator.


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Diode Applications Clippers

Clampers Rectifiers


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Clippers Constant voltage difference across can

diode allows signals to be clipped

Vs

R

Vd

Vs Vd


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Clipper Variations Series, Parallel and Biasing

See Boylestad page 82


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CLAMPERS Clamps a signal to a different DC level

RC circuit where R is switched using diode

RC time constant given by: X!RC


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Diode On Operation Resistor Short

circuited

R=0

Xvery small

Capacitor charges

very quickly

Vdc

+ Vdc -

0


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Diode Off Operation Diode open circuited

X!RC

Twice voltage overresistor -

Vdc+

+ Vdc -

-2Vdc

lamperOutput


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Clamper OutputVdc

-Vdc

-2Vdc


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Clamper Variations See Boylestad page 86


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Rectifiers- Half waveVin

Vout

Vdc = 0.318 (Vm-Vd) PIV = VmApproximately


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Operation Modes

+

Vm

-

-

Vm

+

+ve half-cycle

Diode acts as shortcircuit

-ve half-cycle

Diode acts as opencircuit


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Rectifiers- Full WaveVin Vin

Vdc = 0.636 (Vm-2Vd) PIV = VmApproximately


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Operation modes

+

Vm

-

-

Vm

+

+ve half cycle

-ve half cycle


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Rectifiers centre tapped

+Vm

-

+Vm

-

Vin Vout

Vout- +

2Vm Vm

Vdc = 0.636 (Vm-Vd) PIV = 2VmApproximately


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Operation modes

-Vm

+

+Vm

-


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Linear Power Supplies

DC power supplies (typical bockdiagram), define ripple, effect of

capacitor filtering on ripple, basiczener diode regulator.


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Basic operation

Tx-former

Rectifier Filter Regulator

Load

Input


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Block Functions Transformer

Steps down voltage to useable value

Rectifier Turns AC into oscillating DC

Filter Gives more constant DC by reducing HF content

IC Regulator Gives more constant DC from DC with ripple

Independent of voltage variations


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Rectifiers Half-wave

Vdc=0.318 (Vm-Vd)

Full-wave

Vdc = 0.636 (Vm-2Vd)


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Power Supply Filters DC voltage required

High frequencies must be filtered out

Low pass filter

Capacitor impedance decreases withfrequency

HF shunted to ground

CjX

C [

1!


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C-Filter

Transformer Rectifier Filter


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Ripple and DC voltages

C

IrmsV

CR

VrmsV

dcr

L

dcr

4.2)(

4.2)(

!

!

C

Idcdc

17.4!

DC Voltage

Peak rectified voltage

Ripple voltage

NOTE:Idc measured in MILLI AmpsC measured in MICRORL measured in KILO ohms


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RC-Filter

Transformer Rectifier Filter


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RC-Filter:

Waveform characteristicsViewed as an simple RC-filter joined to

a simple C-filter

Calculate Vdc and Vr for simple C-filterfirst

Use voltage divider equations to

determine DC and AC voltages overload.


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RC-filter characteristics

(contd)

Vdc

Vrms

Calculated usingSimple capacitorFilter alone.

Entire system can be replacedBy these two sources

Actual voltage is the super-position of both


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Ripple Factor Ratio of AC ripple to

average DC value of

signal Smaller values mean

less ripple!

dc

r r sr )(!

Vr(p-p)Vdc


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Designing Filters Given:

Load current

Input voltage

Ripple factor

Use equations for Vr and Vdc

Divide equations to get ripple factor

Solve for C


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Voltage Regulation DC output voltage changes with circuit

loading

Smaller Change = Better Regulation!

Voltage regulation measured using full loadand no load voltages

%100.. v

!

FL

FLNL

V

VVRV

DC

Voltages


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Zener Diode Regulator Recall:

Zener Diodes have very steepcharacteristics

Relatively constant voltage for changes incurrent

Place in parallel with load to maintainfixed voltage across load


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Simple

Zener Regulator

Vz held by zener

Load drawsConstant current

Zener shunts current fluctuations

Varying current supplied


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Question Design a simple capacitive filter for a linear

power supply to meet the following

specifications: Peak transformer output voltage (Vm) : 50V

DC load current, (Idc) : 20mA

Ripple factor , (r) : 5%


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6- Diode Applications

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