eng 201- diode applications

7/24/2019 ENG 201- Diode Applications

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1

Diode Applications(ENG-201)Dr. Hamdy Abd Elhamid

[email protected]
mailto:[email protected]:[email protected]


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Outline

1- Rectifiers:- Half-Wave,

- Full Wave,

- Bridge .

2- Signal Clipping

- Positive Clipping- Negative Clipping

3- Clamper

- Positive Clamper

- Negative Clamper

4- Voltage regulators


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Block diagram of a rectifier and a dc power supply with a load.


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Half-wave rectifier operation. The diode is considered to be ideal.


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The effect of the barrier potential on the half-wave rectified outputvoltage is to reduce the peak value of the input by about 0.7 V.

Vp

0

-VP

2

Vi(t)

t

VD

Vp- VD

0 2

Vo(t)

t


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Average value of the half-wave rectified signal.

V

cos2V

d0dsinV2

1

dtVT

1V

P

0

P

2

0 p

T

0DC


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Root Mean Square (RMS) Value

4

V

sin4

V

d2cos12

1

2

Vd

2

V

d0dsinV21

dtVT

1V

P

0

P

P

0

P

2

0

2p

T

0

2

rms

2

2

0

2

2

2

2

2

22

1

sin

VDC

Vrms

t0

t1

t2

t3

Vp

VO

t


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The PIVoccurs at the peak of each half-cycle of the input voltage when the diode isreverse-biased. In this circuit, the PIV occurs at the peak of each negative half-cycle.


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Half-wave rectifier with transformer-coupled input voltage.


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Full-wave rectification.

A center-tapped full-wave rectifier. Full-Wave Bridge Rectifier
http://c/Documents%20and%20Settings/Dr%20Hamdy/Local%20Settings/Temp/Lecture%205%20Bridge%20rectifier.ppthttp://c/Documents%20and%20Settings/Dr%20Hamdy/Local%20Settings/Temp/Lecture%205%20Bridge%20rectifier.ppthttp://c/Documents%20and%20Settings/Dr%20Hamdy/Local%20Settings/Temp/Lecture%205%20Bridge%20rectifier.ppthttp://c/Documents%20and%20Settings/Dr%20Hamdy/Local%20Settings/Temp/Lecture%205%20Bridge%20rectifier.ppt


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Basic operation of a center-tapped full-wave rectifier. Note that the current through theload resistor is in the same direction during theentire input cycle, so the output voltagealways has the same polarity.

A center-tapped full-wave rectifier.


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Average Value

V2

cos

V

dsinV

1

dtVT

1V

P

0

P

0 p

T

0DC

2

V

sin2

V

d2cos12

1

V

dsinV

1

dtVT

1V

P

0

P

P

0

2

p

T

0

2

rms

2

2

0

2

2

2

22

1

RMS Value


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Center-tapped full-wave rectifier with a transformer turns ratio of 1. Vp(pri) is the peak value of the primaryvoltage.

Center-tapped full-wave rectifier with a transformer turns ratio of 2.


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Full-Wave Bridge Rectifier


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Full-Wave Bridge Rectifier


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16

Bridge operation during a positive half-cycle of the primary and secondary voltages.


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Vp

0

-VP

2

Vi(t)

t

Vp- VD

0 2

Vo(t)

t

17

DC Value =???


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18

Peak inverse voltages across diodes D3and D4in a bridge rectifier duringthe positive half-cycle of the secondary voltage.


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19

Power supply filtering.

Download datasheet


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Operation of a half-wave rectifier with a capacitor-input filter. The currentindicates charging or discharging of the capacitor.

Vr(pp)=


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21

Half-wave ripple voltage (green line).

Vr(pp)=


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22

Vr and VDCdetermine the ripple factor.

Vp(rect) : unfiltered peak rectified voltage

Vr(pp) : peak to peak ripple voltage

VDC : average value of ripple voltage

rhwrVr(pp)

VDC

1

2 3fRLC

rfwrVr(pp )

VDC

1

4 3fRLC


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Effects of RLand C

C=1000F C=470F

C=100F

R=1500

R=1000

R=500

(a) RLfixed (b) Cfixed


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Type of

Rectifier

PIV

Half Wave Peak value of the input secondary voltage, Vs(peak)

Full Wave :

Center-Tapped

2Vsecondary(peak)- V

Full Wave:Bridge

Vsecondary(peak)- V


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25

Diode Limiters (Clipper)

Clippers are networks that employ diodes to clip

away a of an input signalwithout distorting the

remaining part of the applied waveform.


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Diode Limiters (Clipper)

in

L

L

out

VRR

R

V

1


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27

Example

What would you expect to see displayed on an oscilloscope connected across RL in thelimiter shown in above figure.

VVk

kV

RR

RV in

L

L

out 09.9101.1

0.1

1

Assume approximate diode is used, Vd = 0.7 V


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Biased Limiters (Clippers)

A positive limiter

A negative limiter


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Modified Biased Limiters (Clippers)


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Summary

Limiters(Clippers)

In this examples

VD= 0

In analysis, VD

= 0

or VD= 0.7 V can

be used. Both are

right assumption.


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Diode Clampers

Positive clamper operation.

Negative clamper operation


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Diode Clamper (Square wave)

Diode Clamper

Diode ON stateDiode OF state Output

VVc= 0 ; Vc= V; Vo= 0.7 V but

ideal Vo= 0V

-V - Vc- Vo = 0; Vc = V

Vo= -2 V


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Summary of Clamper Circuits


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Voltage Multipliers

Voltage Doubler

Voltage Tripler

Voltage Quadrupler

Voltage multiplier circuits use a combination of diodes and capacitors

to step up the output voltage of rectifier circuits.


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Voltage Doubler

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Positive Half-Cycle

o D1conducts

o D2is switched off

o Capacitor C1charges to Vm

Negative Half-Cycleo D1is switched off

o D2conducts

o Capacitor C2charges to Vm

Vout = VC2= 2Vm


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Zener Diodes

The zener diode

silicon pn-junction device-designed for operate inthe reverse-biased region

Schematic diagram shown that this particular zener

circuit will work to maintain 10 V across the load

Zener diode symbol


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Zener Diodes

Breakdown voltageset by controlling the doping level during manufacture

When diode reached reverse breakdownvoltage remains constant- current changedrastically

If zener diode isFBoperates the same as a rectifier diode

A zener diodeis much like a normal diodebut if it is placed in the circuit in reverse

bias and operates in reverse breakdown.

Note that its forward characteristics are just like a normal diode.

1.8V200V


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Zener Diodes

The reverse voltage (VR) is increasedthe reverse current (IR) remains extremely

small up to the kneeof the curve

Reverse currentcalled the zener current, IZ

At the bottom of the knee- the zener breakdown voltage (VZ) remains constant

although it increase slightly as the zener current, IZincrease.

IZKmin. current required to maintain voltage regulation

IZMmax. amount of current the diode can handle without being damage/destroyed

IZTthe current level at which the VZrating of diode is measured (specified on adata sheet)

The zener diode maintains a constant voltage for value of reverse current rating

from IZKto IZM


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Zener Diodes(Zener Equivalent Circuit)

Since the actual voltage is not ideally vertical, the change in zener current

produces a small change in zener voltage

By ohms law:

Normaly -Zz is specified at IZT

Z

I

Z

V

Z

Z

Z

I

VZ

Zener impedance


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Zener Diodes Applications

Zener diode can be used as

1. Voltage regulator for providing stable reference

voltages

2. Simple limiters or clippers


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Zener Regulation with a Variable Load

The zener diode maintains a nearly constant voltage across RL

as long as the zener

current is greater than IZKand less than IZM

When the o/p terminal of the zener diode is open (RL=)-load current is zero and all of

the current is through the zener When a load resistor (R) is connected, current flow through zener & load RL, IL, IZ

The zener diode continues to regulate the voltage until IZ reaches its min value , IZK At this point, the load current is max. , the total current throughRremains

essentially constant.


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10/26/201542

Example .Fig. (x) shows the zener regulator. Calculate (i) current through the series

resistance (ii) minimum and maximum load currents and (iii) minimum and maximum

zener currents. Comment on the results.

Solution.


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10/26/201543

Example .A zener regulator has VZ = 15V. The input voltage may vary from 22 V to 40 V

and load current from 20 mA to 100 mA. To hold load voltage constant under all

conditions, what should be the value of series resistance ?

Solution. In order that zener regulator may hold output voltage constant under alloperating conditions, it must operate in the breakdown region. In other words,

there must be zener current for all input voltages and load currents. The worst

case occurs when the input voltage is minimum and load current is maximum

because then zener current drops to a minimum.


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Zener Limiting

Zener diode also can be used in ac applications to limit voltage swings to desired level

a) To limit the +ve peak of a signal voltage to the selected zener voltage- Duringve alternation, zener arts asFBdiode & limits theve voltage to -0.7V

b) Zener diode is turn around

-Theve peak is by zener action & +ve voltage is limited to +0.7V

c) Two back-to-back zeners limit both peaks to the zener voltage7V

-During the +ve alternation, D2 is functioning as the zener limiterD1 is functioning

as aFBdiode.-During theve alternation-the roles are reversed