11

EL 124 EL 124 Electronic Devices and CircuitsElectronic Devices and Circuits

Diode ApplicationsDiode ApplicationsDiode ApplicationsDiode Applications

A. Professor A. Professor SajidSajid Ansari , Ansari , P.EngP.Eng. PMP.. PMP.

Department of Electrical EngineeringDepartment of Electrical EngineeringUsman Institute of TechnologyUsman Institute of Technology

An electrical device that converts Alternating Current (AC), which periodically An electrical device that converts Alternating Current (AC), which periodically reverses direction, to Direct Current (DC), which flows in only one direction. The reverses direction, to Direct Current (DC), which flows in only one direction. The process is known as Rectification.process is known as Rectification.

RectifierRectifier

Figure 2Figure 2--1 Block diagram of a rectifier and a dc power supply with a load. 1 Block diagram of a rectifier and a dc power supply with a load.

22

•• In half wave rectification of a In half wave rectification of a singlesingle--phase supply, either the phase supply, either the positive or negative half of the positive or negative half of the AC wave is passed, while the AC wave is passed, while the other half is blocked.other half is blocked.

HalfHalf--Wave Rectifier (HWR)Wave Rectifier (HWR)

•• Because only one half of the Because only one half of the input waveform reaches the input waveform reaches the output, mean voltage is lower. output, mean voltage is lower.

•• HalfHalf--wave rectification requires wave rectification requires a single diode in a singlea single diode in a single--phase phase supply, or three in a threesupply, or three in a three--phase supply. phase supply.

•• Rectifiers yield a unidirectional Rectifiers yield a unidirectional but pulsating direct current; but pulsating direct current; halfhalf--wave rectifiers produce far wave rectifiers produce far more ripple than fullmore ripple than full--wave wave rectifiers, and much more rectifiers, and much more filtering is needed to eliminate filtering is needed to eliminate harmonics of the AC frequency harmonics of the AC frequency from the output.from the output.

Figure 2Figure 2--2 Half2 Half--wave rectifier operation. The diode wave rectifier operation. The diode is considered to be ideal. is considered to be ideal.

Average Value of HWRAverage Value of HWR

Figure 2Figure 2––3 Average value of the half3 Average value of the half--wave rectified signal. wave rectified signal.

d θ0d θθsinV1

dtVT1V

2π

T

0DC

⎥⎦⎤

⎢⎣⎡ +=

=

∫∫

∫π

[ ]

πV

θcos2 πV

d θ0d θθsinV2 π

P

π

0

P

0 p

=

−=

⎥⎦⎢⎣+= ∫∫ π

33

RMS Value of HWRRMS Value of HWR

d θ0d θθsinVπ2

1

dtVT1V

2π

0

2p

T

0

2rms

2

2

⎥⎦⎤

⎢⎣⎡ +=

=

∫∫

∫π

π

( )

4V

θsinθπ4

V

d θ2θcos121

π2V

d θθπ2

V

P

π

0

P

P π

0P

2

2

0

22

2

221

sin

=

⎥⎦⎤

⎢⎣⎡ −=

⎟⎠⎞

⎜⎝⎛ −== ∫∫

π

V O

V D C

V r m s

t 0 t 1 t 2 t 3

V p

O

t

Effectt of Barrier PotentialEffectt of Barrier Potential

Figure 2Figure 2--5 The effect of the barrier potential on the half5 The effect of the barrier potential on the half--wave rectified output wave rectified output voltage is to reduce the peak value of the input by about 0.7 V. voltage is to reduce the peak value of the input by about 0.7 V.

44

VVpp

VVii(t)(t)

VV

Effectt of Barrier PotentialEffectt of Barrier Potential

00

--VVPP

ππ 22ππ tt

VVDD

VVoo(t)(t)

VVpp -- VVDD

00 ππ 22ππ

oo( )( )

tt

The PIV occurs at the peak of each halfThe PIV occurs at the peak of each half--cycle of the cycle of the input voltage when the diode is reverseinput voltage when the diode is reverse--biased. The biased. The diode must be cable of withstanding this voltage. In diode must be cable of withstanding this voltage. In this circuit the PIV occurs at the peak of eachthis circuit the PIV occurs at the peak of each

Peak Inverse Voltage (PIV)Peak Inverse Voltage (PIV)

this circuit, the PIV occurs at the peak of each this circuit, the PIV occurs at the peak of each negative halfnegative half--cycle. cycle.

55

HWR with TransformerHWR with Transformer--coupled coupled input voltage input voltage

NumericalNumerical

66

FullFull--Wave Rectifiers (FWR)Wave Rectifiers (FWR)

Figure 2Figure 2––1111 FullFull--wave rectification. wave rectification.

Average ValueAverage Value

dtV1VT

DC = ∫

[ ]θcosπ

V

dθθsinVπ1

dtVT

V

π

0

P

π

0 p

0DC

−=

= ∫

∫

πV2π

P

0

=

77

RMS ValueRMS Value

dtV1VT 22 = ∫

( )dθ2θcos121

πV

dθθsinVπ1

dtVT

V

P

π

0

2p

0rms

2

0

2

2

⎟⎠⎞

⎜⎝⎛ −=

=

∫

∫

∫

π

2V

θsinθπ2

V

P

π

0

P

2

2

221

=

⎥⎦⎤

⎢⎣⎡ −=

NumericalNumerical

88

CenterCenter--tapped FWRtapped FWR

Figure 2Figure 2––1313 A centerA center--tapped fulltapped full--wave rectifier. wave rectifier.

Note that the Note that the current current

Basic operation of Basic operation of CenterCenter--tapped tapped FWRFWR

through the through the load resistor is load resistor is in the same in the same direction direction during the during the entire input entire input cycle so thecycle so thecycle, so the cycle, so the output voltage output voltage always has the always has the same polarity. same polarity.

99

CenterCenter--tapped FWR with TR =1:1tapped FWR with TR =1:1

Figure 2Figure 2--15 Center15 Center--tapped fulltapped full--wave rectifier with a transformer wave rectifier with a transformer turns ratio of 1. turns ratio of 1. VpVp ((pripri) is the peak value of the primary voltage. ) is the peak value of the primary voltage.

CenterCenter--tapped FWR with TR =1:2tapped FWR with TR =1:2

Figure 2Figure 2--16 Center16 Center--tapped fulltapped full--wave rectifier with a transformer wave rectifier with a transformer turns ratio of 2. turns ratio of 2.

1010

Diode Inverse Voltage (PIV)Diode Inverse Voltage (PIV)

Figure 2Figure 2--17 Diode reverse voltage (D2 shown reverse17 Diode reverse voltage (D2 shown reverse--biased and biased and D1 shown forwardD1 shown forward--biased). biased).

Diode Inverse Voltage (PIV)Diode Inverse Voltage (PIV)

1111

FullFull--Wave Bridge RectifierWave Bridge Rectifier

Figure 2Figure 2––20 Operation of a bridge rectifier. 20 Operation of a bridge rectifier.

Bridge Operation Bridge Operation

Figure 2Figure 2––21 Bridge operation during a positive half21 Bridge operation during a positive half--cycle of cycle of the primary and secondary voltages. the primary and secondary voltages.

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VVpp

VVii(t)(t)

tt00

--VVPP

ππ 22ππ tt

VVoo(t)(t)

VVpp -- VVDD

00 ππ 22ππtt

Peak Inverse Voltages (PIV)Peak Inverse Voltages (PIV)

Figure 2Figure 2––22 Peak inverse voltages across diodes D3 and D4 in a 22 Peak inverse voltages across diodes D3 and D4 in a bridge rectifier during the positive halfbridge rectifier during the positive half--cycle of the secondary voltage. cycle of the secondary voltage.

1313

NumericalNumerical

Power Supply FiltersPower Supply Filters

Figure 2Figure 2––24 Power supply filtering. 24 Power supply filtering.

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Capacitor FilterCapacitor Filter

Figure 2Figure 2––2525 Operation of a halfOperation of a half--wave rectifier with a capacitorwave rectifier with a capacitor--input filter. The current indicates charging or discharging of the capainput filter. The current indicates charging or discharging of the capacitor. citor.

•• The variation in the capacitor voltage due to The variation in the capacitor voltage due to the charging and discharging is called the the charging and discharging is called the ripple voltageripple voltage

Ripple VoltageRipple Voltage

ripple voltage.ripple voltage.•• The smaller the ripple voltage, the better the The smaller the ripple voltage, the better the

filtering action.filtering action.•• For a given input frequency, the output For a given input frequency, the output

frequency of a full wave rectifier is frequency of a full wave rectifier is twicetwice that that of a half wave rectifierof a half wave rectifierof a half wave rectifier.of a half wave rectifier.

•• As a result, a full wave rectifier is easier to As a result, a full wave rectifier is easier to filter because of the shorter time between filter because of the shorter time between peaks.peaks.

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Ripple VoltageRipple Voltage

Ripple Voltage (HWR Vs. FWR)Ripple Voltage (HWR Vs. FWR)

1616

Ripple FactorRipple Factor

VVpp((rectrect) ) : Unfiltered peak rectified voltage : Unfiltered peak rectified voltage

VVrr(pp)(pp) : Peak to peak ripple voltage: Peak to peak ripple voltage

VVDCDC : Average value of ripple voltage: Average value of ripple voltage

CfRVV

rLDC

pprhwr 32

1)( ==

rfwr =Vr( pp )

VDC

=1

4 3 fR LC

VrVr and VDC determine the ripple factor. and VDC determine the ripple factor.

Ripple FactorRipple Factor

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Effects of REffects of RLL and Cand CC=C=1000µF1000µF

C=C=470µF470µFR=R=1500Ω1500Ω R=R=1000Ω1000Ω

C=C=100µF100µF R=R=500Ω500Ω

(a) (a) RRLL fixedfixed (b) (b) CC fixedfixed

Charging and Discharging of a Capacitor is based on its Natural Response Charging and Discharging of a Capacitor is based on its Natural Response (Resonant Response) and is depending on Time Constant ((Resonant Response) and is depending on Time Constant (ττ) of the circuit ) of the circuit

ττ = R= RLL CCGenerally, after 5Generally, after 5ττ the capacitor will reach to its extreme value (Maximum the capacitor will reach to its extreme value (Maximum or Minimum).or Minimum).

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Surge Current in the Capacitor Input FilterSurge Current in the Capacitor Input Filter•• When the power is first applied to a power supply, the When the power is first applied to a power supply, the

filter capacitor is uncharged.filter capacitor is uncharged.•• At the instant the switch is closed, voltage is connected to At the instant the switch is closed, voltage is connected to

th tifi d th h d itth tifi d th h d itthe rectifier and the uncharged capacitor appears as a the rectifier and the uncharged capacitor appears as a short.short.

•• An initial surge of current is produced through the An initial surge of current is produced through the forwardforward--biased diodes.biased diodes.

•• It is possible that the surge current could destroy the It is possible that the surge current could destroy the diodes, for this reason a surgediodes, for this reason a surge--limiting resistor limiting resistor RRsurgesurge, is , is sometimes connected. sometimes connected.

•• The value of this resistor must be small to avoid a The value of this resistor must be small to avoid a significant voltage drop across it.significant voltage drop across it.

•• The diode must have a forward current rating that can The diode must have a forward current rating that can handle the momentary surge of current.handle the momentary surge of current.

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Quiz #2Quiz #2Ch t #2Ch t #2Chapter #2Chapter #2

Tuesday 16Tuesday 16thth AprilApril

Please also see assignments on Please also see assignments on Faculty Share FolderFaculty Share Folder

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IC REGULATORSIC REGULATORS•• The most effective filter is a combination of a capacitorThe most effective filter is a combination of a capacitor--input input

filter with an Integrated Circuit (IC) regulator. filter with an Integrated Circuit (IC) regulator. •• IC Regulator takes output of rectifier and maintains a IC Regulator takes output of rectifier and maintains a

constant output voltage / current despite changes in the input.constant output voltage / current despite changes in the input.

2020

78xx Series 78xx Series FixedFixed Voltage Regulator ICVoltage Regulator IC

-- 33--terminal deviceterminal device-- Provide fixed positive DC voltageProvide fixed positive DC voltage-- Last 2 digits designate output DC voltage (e.g. 78Last 2 digits designate output DC voltage (e.g. 780505 -->> 5VDC output)5VDC output)Last 2 digits designate output DC voltage (e.g. 78Last 2 digits designate output DC voltage (e.g. 7805 05 5VDC output)5VDC output)-- Can produce output current up to in excess of 1A when used with Can produce output current up to in excess of 1A when used with

adequate heat sinkadequate heat sink-- The input voltage must be at least 2V above the output voltageThe input voltage must be at least 2V above the output voltage-- Thermal overload protection is availableThermal overload protection is available

LM3XX LM3XX AdjustableAdjustable Linear Voltage RegulatorLinear Voltage Regulator

•• Provide adjustable DC voltageProvide adjustable DC voltage•• In LM317, VIn LM317, Vout out can be varied from 1.2V to 37Vcan be varied from 1.2V to 37V•• LM317 can provide 1 5 A current to the loadLM317 can provide 1 5 A current to the load•• LM317 can provide 1.5 A current to the loadLM317 can provide 1.5 A current to the load

•• The counterpart of LM317 is LM 337 hence VThe counterpart of LM317 is LM 337 hence Voutout from from --1.2V to 1.2V to --37 V37 V

2121

Percent RegulationPercent RegulationThe regulation expressed as a percentage is a figure of merit The regulation expressed as a percentage is a figure of merit used to specify the performance of a voltage regulator. used to specify the performance of a voltage regulator. 11-- Line Regulation: Line Regulation: It is typically defined as a ratio of a It is typically defined as a ratio of a

change in output voltage for a corresponding change in change in output voltage for a corresponding change in the input voltage expressed as a percentage.the input voltage expressed as a percentage.

Line Regulation = (∆VLine Regulation = (∆Voutout / ∆V/ ∆Vinin) x100 ) x100 [%][%]22-- Load Regulation: Load Regulation: Load regulation specifies how much Load regulation specifies how much

change occurs in the output voltage over a certain range change occurs in the output voltage over a certain range of load current values, usually from minimum current (no of load current values, usually from minimum current (no load, NL) to maximum current (full load, FL). It is load, NL) to maximum current (full load, FL). It is normally expressed as a percentage.normally expressed as a percentage.Load Regulation =Load Regulation =

VVFLFL

(V(VNLNL –– VVFLFL )) x100x100

Example 2Example 2--9 D.I.Y.9 D.I.Y.

Clippers and ClampersClippers and Clampers•• Diode circuits, called Diode circuits, called ClippersClippers or or Limiters Limiters are sometimes are sometimes

used to clip off portions of the signal voltages above or below used to clip off portions of the signal voltages above or below certain levels.certain levels.

•• Another type of diode circuit, called a Another type of diode circuit, called a ClamperClamper, is used to , is used to restore a dc level to an electrical signal.restore a dc level to an electrical signal.

Diode Clipper / LimiterDiode Clipper / Limiter•• Diode Clipper or Limiter is an electronic circuit that limits orDiode Clipper or Limiter is an electronic circuit that limits orDiode Clipper or Limiter is an electronic circuit that limits or Diode Clipper or Limiter is an electronic circuit that limits or

clips off the positive or negative part of the input signal (See clips off the positive or negative part of the input signal (See Figure).Figure).

•• The negative or positive signal limiting function is based on if The negative or positive signal limiting function is based on if the diode is wired in a forward or reverse biased mode.the diode is wired in a forward or reverse biased mode.

2222

Diode Clippers / LimitersDiode Clippers / Limiters

Determining VDetermining VOUTOUT

• Whenever the input is below 0.7 V, the diode is reversed biased and appears as an open.Th t t lt V l k lik th ti• The output voltage VOUT looks like the negative part of the input, but magnitude is determined by the voltage divider formed by R1 and RL, as follows:

VV ==RRLL x Vx VININ

• If R1 is small compared to RL, then VOUT=VIN

VVOUTOUT ==RR11+R+RLL

x Vx VININ

Example 2Example 2--10 D.I.Y.10 D.I.Y.

2323

Biased LimitersBiased Limiters

A Positive Limiter A Positive Limiter with Positive Biaswith Positive Bias

A Positive Limiter with A Positive Limiter with Variable Positive BiasVariable Positive Bias

A Negative Limiter with A Negative Limiter with Negative BiasNegative Bias

Example 2Example 2--1111

Example 2Example 2--12 D.I.Y.12 D.I.Y.

2424

Diode Clampers / DC RestorersDiode Clampers / DC Restorers• Diode Clamper adds a dc level to an ac signal, also known as DC Restorers.• The net effect of the clamping action is the capacitor retains a charge

approximately equal to the peak value of the input less the diode drop.

Positive ClamperPositive Clamper Negative ClamperNegative Clamper

Example 2Example 2--13 D.I.Y.13 D.I.Y.

Voltage MultipliersVoltage Multipliers•• AA voltagevoltage multipliermultiplier isis anan electricalelectrical circuitcircuit thatthat

convertsconverts ACAC electricalelectrical powerpower fromfrom aa lowerlowerppvoltagevoltage toto aa higherhigher DCDC voltagevoltage byby meansmeans ofofcapacitorscapacitors andand diodesdiodes combinedcombined intointo aa networknetwork..

•• VoltageVoltage MultipliersMultipliers useuse clampingclamping actionaction totoincreaseincrease peakpeak rectifiedrectified voltagesvoltages withoutwithout necessitynecessityofof increasingincreasing thethe transformer’stransformer’s voltagevoltage ratingrating..

•• MultiplicationMultiplication factorsfactors ofof 22,, 33 andand 44 areare commoncommon..•• ApplicationsApplications:: UsedUsed inin HighHigh--VoltageVoltage LowLow--

CurrentCurrent applications,applications, suchsuch asas TVTV ReceiversReceivers..

2525

How Voltage Doubler Works?How Voltage Doubler Works?

Response Curve Response Curve (for Voltage Doubler)(for Voltage Doubler)

2626

Voltage TriplerVoltage Tripler

Response Curve Response Curve (for Voltage Tripler)(for Voltage Tripler)

2727

Voltage QuadruplerVoltage Quadrupler

Response Curve Response Curve (for Voltage Quadrupler)(for Voltage Quadrupler)

2828

Sajid Ansari , P.Eng. PMP.A. Professor (UIT).A. Professor (UIT).