Basics of Rectifiers and Filters

8/11/2019 Basics of Rectifiers and Filters

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Chapter 1Common Diode Applications

Basic Power Supply Circuits


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Power Supply A group of circuits used to convert ac to dc.

Rectifier Converts ac to pulsating dc.

Filter

Reduces variations in the rectifier output. Voltage regulator Maintains a constant dc outputvoltage.


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TransformersTransformers are described in terms of the relationshipbetween secondary voltage (V S) and primary voltage (V P).

Step-up

VS is greater than V P.Step-down VS is less than V P.Isolation VS is approximatelyequal to V P.


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Transformer Voltage and Current

Ratios

S

P

P

S

P

S

I

I

V

V

N

N


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Half-wave RectifiersHalf-wave rectifier A diode placed in series betweena transformer (or ac line input) and its load.


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Positive Half-wave RectifiersThis circuit converts an

ac input to a series ofpositive pulses.


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Average Load Voltage and Current Average voltage (V ave ) The dc equivalent of avoltage waveform.

Average current (I ave ) The dc equivalent of acurrent waveform.

For the output from a half-wave rectifier:

pk V

V ave

pk I

I ave


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Negative Half-wave Rectifiers

This circuit converts anac input to a series ofnegative pulses.


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Peak Inverse Voltage (PIV)Peak inverse voltage (PIV) The maximum diode

reverse bias produced by a given circuit.

For the diode in a half-wave rectifier :

(pk)PIV S V


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Full-wave Rectifier


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Full-wave Rectifier OperationDiodes conduct duringalternate half cycles of

the input signal.

VL(pk) is approximatelyhalf the value of V S(pk) .

The circuit producestwo positive half-cyclesfor each input cycle.


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Average Load Voltage and Current Average voltage (V ave ) The dc equivalent of a voltagewaveform.

Average current (Iave

) The dc equivalent of a currentwaveform.

For the output from a full-wave rectifier :

pk 2V V ave

pk 2 I I ave


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Peak Inverse Voltage (PIV)Peak inverse voltage (PIV) The maximum diodereverse bias produced by a given circuit.

For the diode in a full-wave rectifier :

(pk)PIV S V

(pk)2PIV LV


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Negative Full-wave RectifiersThe negative full-wave rectifier converts anac input to a series of negative pulses.


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Full-Wave Bridge RectifiersThe most commonly used because:

It does not require the use of a center-tapped transformer.It can be coupled directly to the ac power line.It produces a higher dc output than a comparable full-wavecenter-tapped rectifier.


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Bridge Rectifier Operation

Conduction alternates between two diode pairs.


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Calculating load voltage andcurrent relationships

V7.0PIV

2

V4.1

S(pk)

aveave

)(pk ave

(pk))(pk

V

R

V I

V V

V V

L

L

S L


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Working with RectifiersRectifiers are high current circuits, so I FR B can have asignificant impact on diode forward voltage (V F ).

Cooling is often used to minimize the effects of highpower rectifier reverse current. Components may becooled using a fan and/or a heat sink.

High transformer tolerances can introduce significantdiscrepancies between calculated and measuredcurrent and voltage values.


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Integrated RectifiersIntegrated Rectifier A rectifier circuit that is etchedon a single piece of silicon (housed in a single case). IC rectifiers are:

Cheaper to produceEasier to troubleshoot

The diodes in an IC rectifier operate at the sametemperature, so they have equal values of forward

voltage and leakage current.IC rectifier cases are designed to be attached easilyto a heat sink.


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Filters A filter reduces the voltage and current variationsin the output from a rectifier.


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Ripple Voltage (Vr)

Ripple voltage (V r) The variation in the filter outputvoltage.


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The Basic Capacitive Filter

The capacitor:charges through the

rectifier diode.discharges throughthe load.

1

11

5

5

C RT

C RT

L D

DC


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Surge Current

The high initial current through a power supply.Surge current lasts only long enough for the

filter capacitor to charge.

BW

S

R R

V

I ) pk (

surge


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Limiting Surge Current

Surge current can be limited by:Inserting a current limiting resistor between the filter

capacitor and the rectifier.Using a low-value filter capacitor (which shortens theduration of the surge).Using an inductive filter.


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Filter output voltages

C

t I

V

V V V

Lr

r

2 pk dc


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Filtering and Half-waveRectifier PIV

) pk (2PIV S V


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LC Filters


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Voltage Regulators

Voltage regulator The final circuit in the dc powersupply.


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Zener Voltage Regulators


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Regulator Response toLoad Variations


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Zener Reduction of V r

r

S L Z

L Z r V

R R Z

R Z V

)||(

||(out)


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A Basic DC Power Supply

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Basics of Rectifiers and Filters