Lecture2 BE

8/14/2019 Lecture2 BE

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Basic Electronics

Lecture # 2

Diode as a Rectifier


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The Basic DC power Supply

The DC power supply converts thestandard AC Voltage into a constant DCVoltage.


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A diode is connected toan ac source whichprovides the inputvoltage, Vin and to loadresistor RL, forming a

half-wave rectifier. When positive cycle

comes then diode allowsit and works in forwardbias and when negative

cycle comes then diodedoes not allow it andworks in reverse bias.

The Half WaveRectifier


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Average value of the Half-Wave Output Voltage

The average value of the half wave rectified output voltage is thevalue you would measure on a dc voltmeter.

Mathematically, it is determined by the area under the curve over afull cycle, and then dividing by 2

, the number of radians in a fullcycle.

PAVG

VV Where VP is the peak value of the voltage.


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Effect of the Barrier Potential on the Half-Wave Rectifier Output

During the +ve half-cycle, the input voltage mustovercome the barrier potential before the diode becomesforward biased.

For silicon diode this results in a half-wave output with a

peak value that is 0.7V less than the peak value of input.VVV inPP 7.0)(


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Peak Inverse Voltage (PIV)

The maximum value of Reverse voltage, designated aspeak inverse voltage (PIV), occurs at the peak of eachnegative alternation of the input voltage when the diodeis reverse-biased.

The PIV equals the peak value of the input voltage, andthe diode must be capable of withstanding this amountof repetitive reverse voltage.

)(inPVPIV


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Full-Wave Rectifier

Full-Wave rectifier allows unidirectional (one-way) currentto the load during the entire 360o of the input cycle.

Full-Wave rectification is an output voltage with afrequency twice the input frequency that pulsates every

half-cycle of the input.

PAVG

VV

2


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Full-Wave Center-Taped Rectifier

The full-wave center-taped rectifier uses twodiodes connected to the secondary center-tapedtransformer.

Half of the total secondary voltage appears b/w

the centered tap end of the secondary windingwith opposite polarities.


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For the +ve half cycle of the input voltage. This condition forward-biases D1 and reverse

biases D2. Forve half cycle of the input voltage. This condition reverse-biases D1 and forward

biases D2. Because the output current during both the

positive and negative portions of the input cycle

is in the same through the load, the outputvoltage developed across the load resistor is afull-wave rectified DC voltage.

Full-Wave Center-Taped Rectifier


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Output Voltage of Center-Taped Rectifier

The output voltage of center-taped rectifier isalways one-half of the secondary voltage lessthe diode drop.

VV

Vout 7.02

sec


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Peak Inverse Voltage

Peak inverse voltage across either diode in thecenter-taped full wave rectifier is

VVPIV outp 7.02 )(


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The Full-Wave Bridge Rectifier The full wave rectifier uses four diodes, as shown in the

following figure.

When the input cycle ispositive, diodes D1 and D2are forward biased andconduct current. Duringthis time, diodes D3 and

D4 are reverse-biased.

When the input cycle isnegative, diodes D3 andD4 are forward biased and

conduct current. Duringthis time, diodes D1 andD2 are reverse-biased.


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Bridge Output Voltage

During the +ve half-cycle of the total secondary voltage,diodes D1 and D2 are forward-biased. The same is truewhen D3 and D4 during the negative half-cycle.

(sec))( PoutP VV

Two diodes are always in series with the load resistor

during the +ve andve half-cycles. If diode drops are takeninto account, the output voltage is

Neglecting the diode drops

VVV PoutP 4.1(sec))(


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Peak Inverse Voltage

PIV is ideally equal to the secondary voltage.

If the diode drops of the forward-biased diodes areincluded, the peak inverse voltage across each reverse-biased diode in terms of VP(out) is

)(outPVPIV

VVPIV outP 7.0)(


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Power Supply Filters

A power supply filter ideally eliminates thefluctuations in the output voltage of a half-wave or full-wave rectifier and produces a

constant-level DC Voltage.


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Half-Wave rectifier with Capacitor Filter

A half-wave rectifier with a

capacitor filter as shown inthe figure. During the +ve quarter cycle

of the input, the diode isforward-biased, allowing thecapacitor to charge to within0.7V of the input peak.

When the input begins todecrease below its peak, thecapacitor retains its chargeand the diode becomesreverse-biased.

During the remaining part of

the cycle, the capacitor candischarge only through theload resistance, which isnormally long compared tothe period of input.


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Ripple Voltage The capacitor quickly charges and slowly discharges through RL after

the positive peak.

The variation in the capacitor voltage due to charging anddischarging is called the ripple voltage.

The ripple factor ( r ) is an indication of the effectiveness of the filterand is defined as

DC

ppr

V

Vr

)(Where Vr(pp) is the peak-to-peak ripple voltage.

VDC is the dc (average) value of the filters output voltage.

The ripple factor can be lowered by increasing the value of the filter capacitor or

increasing the load resistance.


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Full-Wave rectifier with Capacitor Filter

For full wave rectified capacitor filter, approximations for the peak-to-peak ripple voltage, Vr(pp), and the DC value of the filter outputvoltage, VDC, are given in the following experession. Vp(rect) is thepeak rectified voltage.

)(

)()(

2

11

1

rectp

L

DC

rectp

L

ppr

VCfR

V

VCfR

V


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Power Supply Filters And Regulators

Being that the capacitorappears as a short duringthe initial charging, thecurrent through the diodescan momentarily be quite

high. To reduce risk ofdamaging the diodes, asurge current limitingresistor is placed in serieswith the filter and load.


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Power Supply Filters And RegulatorsRegulation is the last step in eliminating the remaining ripple and maintaining

the output voltage to a specific value. Typically this regulation is performed byan integrated circuit regulator. There are many different types used based onthe voltage and current requirements.


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Power Supply Filters And Regulators

How well the regulation is performed by a regulator is measured by its

regulation percentage. There are two types of regulation, line andload. Line and load regulation percentage is simply a ratio of changein voltage (line) or current (load) stated as a percentage.

Line Regulation = ( VOUT/ VIN)100%

Load Regulation = (VNL VFL)/VFL)100%


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Diode Limiters

Limiting circuits limit the positive or negative amount of an input voltage

to a specific value.

This positive limiter will limit the output to VBIAS + .7V


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Diode Limiters

The desired amount of limitation can be attained by a power supply orvoltage divider. The amount clipped can be adjusted with different levels ofVBIAS.

This positive limiter will limitthe output to VBIAS + .7V

The voltage divider provides theVBIAS . VBIAS =(R3/R2+R3)VSUPPLY


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Diode ClampersA diode clamper adds a DC level to an AC voltage. The capacitor charges to thepeak of the supply minus the diode drop. Once charged, the capacitor acts like a

battery in series with the input voltage. The AC voltage will ride along with theDC voltage. The polarity arrangement of the diode determines whether the DCvoltage is negative or positive.


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Voltage Multipliers

Clamping action can be used to increase peak rectified voltage. Once C1 and

C2 charges to the peak voltage they act like two batteries in series, effectivelydoubling the voltage output. The current capacity for voltage multipliers is low.


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Voltage MultipliersThe full-wave voltage doubler arrangement of diodes and capacitors

takes advantage of both positive and negative peaks to charge thecapacitors giving it more current capacity. Voltage triplers andquadruplers utilize three and four diode-capacitor arrangementsrespectively.


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Voltage MultipliersVoltage Tripler


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Voltage MultipliersVoltage Quadrupler


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The basic function of a power supply to give us a smooth ripple freeDC voltage from an AC voltage.

Half-wave rectifiers only utilize half of the cycle to produce a DCvoltage.

Transformer Coupling allows voltage manipulation through itswindings ratio.

Full-Wave rectifiers efficiently make use of the whole cycle.This makes it easier to filter.

The full-wave bridge rectifier allows use of the full secondarywinding output whereas the center-tapped full wave uses only half.

Summary


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Summary

Filtering and Regulating the output of a rectifier helps keep the DCvoltage smooth and accurate.

Limiters are used to set the output peak(s) to a given value.

Clampers are used to add a DC voltage to an AC voltage.

Voltage Multipliers allow a doubling, tripling, or quadrupling of rectifiedDC voltage for low current applications.

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Lecture2 BE