4.Rectifiers

Diode Rectifiers

Block Diagram ofDC Power Supply

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Diodes in Rectifier Circuits

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Half-wave Rectifier

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V1 V2

N1 N2

The primary and secondary voltages of transformer are related as V2/V1=N2/N1V1= rms primary voltage

V2=rms secondary voltageN1= no of turns in primary coilN2=no of turns in secondary coil

Vi=Vm sinωt

•During +ve half cycle the diode is forward biased.•Therefore a current IL flows through load resistor RL. In forward bias ,the voltage drop across the diode is very small (0.7V for Si and 0.3V for Ge).•For practical purposes we assume that the diode is ideal so the voltage drop is zero

•During –ve half cycle the diode is reverse biased•Therefore the diode does not conduct and hence no current flows IL=0•And hence V0=0

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Full-wave Rectifier(Centre-tap)

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Full-wave Rectifier(Bridge)

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Diode Ratings1. Current Handling Capacity.2. Peak Inverse Voltage (PIV ): It is the

largest reverse voltage that is expected to appear across the diode. Usually, diodes are selected which have VZ at least 50 % greater than PIV.

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PIV Ratings• For half-wave rectifier : Vm

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Figures of Merit of a Rectifier

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1. Ripple Factor (1. Ripple Factor (rr) :) :

2. Rectification Efficiency (η) :

Performance of Half-wave Rectifier

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Total RMS Value :

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RMS Value of AC Components :

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Note that this is a general result. Now, for Half-wave Rectifier :

Performance of Full-wave Rectifier

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Its period is , and not 2.

How much is its period in terms of ωt ?

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Total RMS Value :

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Ripple Factor :

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Rectification Efficiency :

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Filter CircuitsThe output of a rectifier is not purely dc. It

also has some ac components, called ripples.The ripples are maximum in half-wave

rectifier, and are less in full-wave rectifierFilter circuit converts pulsating output of a

rectifier into a steady dc level.Filters simply smoothen out the ripples.

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There are two ways to do it :1. Ripples are bypassed around a the load by

using a shunt capacitor.2. Ripples can be limited to a low value by a

series inductor. A filter circuit is generally a

combination of capacitors and inductors.

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Filtering action depends upon the fact that:Capacitor allows ac only to pass.Inductor allow dc only to pass.

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Shunt Capacitor Filter

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Operation of Capacitor Filter

During the conduction period, the capacitor gets charged and stores energy.

During the non-conduction period, the capacitor discharges through the load resistance delivering energy to it.

Capacitor gets charged to the peak value quickly because charging time constant is almost zero.

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Discharging time constant is quite large, because it discharges through the load resistance.

As XC << RL, the ripples are bypassed through the capacitor and only dc component flows through the load resistance.

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• Obviously, a large-value capacitor should be preferred.

• Any disadvantage ?• Ans . : Yes. Diode may get damaged.• How does the ripple factor vary with increase

in the load current ?• Ans. : Increases.

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Review• Block Diagram of

DC Power Supply.• Half-wave Rectifier.• Full-wave Rectifier

(Centre-tap).• Full-wave Rectifier

(Bridge).• Diode Ratings.

1. Current Handling Capacity.2. PIV Ratings.

• Figures of Merit of a Rectifier :

1. Ripple Factor.2. Rectification Efficiency.

• Performance of Half-wave Rectifier.

Idc , Irms , I’rms , r , η• Performance of Full-wave

Rectifier.

Idc , Irms , I’rms , r , η• Filters.

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