2-1 The Basic DC Power Supply

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DIODE

2-1 The Basic DC Power Supply

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• Power supply is a group of circuits that convert the

standard ac voltage

(220 V,50 Hz) provided by the wall

outlet to

constant dc voltage

.

• The voltage produced is used to power all types of

electronic circuits including:

• Consumer electronics (eg: radio, television, DVD,

etc.)

• Computers

• Industrial controllers

• Most laboratory instrumentation systems and

equipment

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The basic function of a DC power supply is to convert an AC

voltage to a constant DC voltage (AC

DC)

Either half or full-wave, the rectifier converts the

ac input voltage to a pulsating dc voltage.

Eliminates the fluctuations and produces smooth dc voltage

Maintains a constant dc

Voltage for variations in

input line voltage or in

the load.

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Transformer

• A transformer is a device that changes ac electric power at one voltage level to ac electric power at another voltage level through the action of a magnetic field.

• Simple transformer consist of: 1. Primary winding (input winding)

2. Secondary winding (output winding) 3. Magnetic core

• If the secondary has more turns than the primary, the output voltage across the secondary will be higher and the current will be smaller. If the secondary has fewer turns than the primary, the output voltage across the secondary will be lower and the current will be higher.

The general arrangement of a transformer

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There are three types of transformers: step-up, step-down, and

isolation.

These components are described as follows:

1. The step-up transformer provides a secondary voltage that is

greater than the primary voltage. Ex: a step-up transformer may provides a 440 V_ac output with a 220 V_ac input.

2. The step-down transformer provides a secondary voltage that is

less than the primary voltage. Ex: a step-down transformer may provides a 220 V_ac output with a 110 V_ac input.

3. An isolation transformer provides an output voltage that is equal to the input voltage. This type of transformer is used to isolate the power supply electrically from the ac power line.

+

+

+

+

+

+

2-1 The Basic DC Power Supply(cont.)

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The turns ratio of a transformer is equal to the voltage ratio of the

component

where

N

= the number of turns in the secondary

N

= the number of turns in the primary

V

= the secondary voltage

V

= the primary voltage

I

= the secondary current

I

= the primary current

2-1 Half-Wave Rectifiers

Diode – ability to conduct current in one direction and

block current in other direction

used in circuit called

RECTIFIER (ac

dc)

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•A half wave rectifier(ideal) allows conduction for only 180° or half of a complete cycle.

•During first one cycle:

-Vin goes positive – diode FB – conduct current -Vin goes negative – diode RB – no current- 0V

•The output frequency is the same as the input (same shape).

The average value

V

or V

:

-Measured on dc voltmeter

2-2 Half-Wave Rectifiers (cont.)

•

Practical Diode – barrier potential of 0.7V (Si) taken into

account.

•

During +ve half-cycle V

must overcome V

for forward

bias.

•

The peak o/p voltage:

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2-1 Half-Wave Rectifiers (cont.)

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- Peak inverse voltage

(PIV) is equal to the

peak value of the input

voltage.

Its the maximum voltage

across the diode when it

is in reverse bias.

The diode must be

capable of

withstanding this

amount of voltage.

2-1 Half-Wave Rectifiers (cont.)

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Transformers are often used to couple ac input to the rectifier.

The turns ratio, n of the primary to secondary determines the output versus the input.

The advantages of transformer coupling:

1) allows the source voltage to be stepped up or down

2) the ac source is electrically isolated from the rectifier, thus prevents shock hazards in the secondary circuit.

2-1 Half-Wave Rectifiers (cont.)

(Half-Wave Rectifier with Transformer-Coupled Input Voltage)

If n>1, Vsec is greater than Vpri.

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2-2 Full-Wave Rectifiers (cont.)

(Introduction)

A full-wave rectifier allows current to flow during both the positive and

negative half cycles or the full 360º whereas half-wave rectifier allows only

during one-half of the cycle.

The no. of +ve alternations is twice the half wave for the same time interval

The output frequency is twice the input frequency.

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▪ It employs four diodes arranged such that current flows in the direction through the load during each half of the cycle.

▪When Vin +ve, D1 and D2 FB and conduct current. A voltage across R

L looks

like +ve half of the input cycle. During this time, D3 and D4 are RB.

▪When Vin –ve, D3 and D4 are FB and conduct current. D1 and D2 are RB.

Used 4 diode: 2 diode in forward 2 diode in reverse

2 diode always in series with load

resistor during +ve and –ve half cycle . Without diode drop (ideal diode):

With diode drop (practical diode):

2-2 Full-Wave Rectifiers (cont.)

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PIV = V

_p(out)

PIV = V

_p(out)

+ 0.7

Power Supply Filters

• To reduce the fluctuations in the output voltage of half / full-wave rectifier – produces constant-level dc voltage.

• It is necessary – electronic circuits require a constant source to provide power & biasing for proper operation.

• Filters are implemented with capacitors.

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Regulators

Voltage regulation in power supply done using integrated circuit voltage

regulators.

To prevent changes in the filtered dc voltage/ to fix output dc voltage due to variations in input voltage or load.

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•In most power supply – 50 Hz ac power line voltage is converted to constant dc voltage.

•50 Hz pulsating dc output must be filtered to reduce the large voltage variation.

•Small amount of fluctuation in the filter o/p voltage - ripple

ripple

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A capacitor-input filter is a capacitor connected in parallel with R_L and its charge and discharge times will such that it fills in the “gaps” between each peak. This reduces variations of voltage. This voltage variation is called ripple voltage.

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Ripple Voltage: the variation in capacitor voltage due to the charging and discharging times.

The advantage of a full-wave rectifier over a half-wave is quite clear. The capacitor can more effectively reduce the ripple when the time between peaks is shorter.

Easier to filter

-shorted time between peaks.

-smaller ripple.

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Ripple factor: indication of the effectiveness of the filter

V_r(pp) = peak to peak ripple voltage;

V_DC = V_AVG = average value of filter’s output voltage.

•Lower ripple factor better filter

[can be lowered by increasing the value of filter capacitor or increasing the load resistance]

Effects of R

and C

Dr. RS

C=1000µF C=470µF

C=100µF

R=1500Ω

R=1000Ω

R=500Ω

(a) R

L fixed (b) C fixed

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2-5 Diode Limiting Circuits

•

Diode limiters/clippers – that limits/clips the portion of signal voltage

above or below certain level.

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

voltage to a specific value.

•Diode limiters/clippers – circuits that limits/clips the portion of signal

voltage above or below certain level.

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

voltage to a specific value.

•2 basic clipper configuration:

Negative clipper

2-5 Diode Limiters

Positive clipper

•

Forward-biased diode when i/p is +ve cycle.

•

Reverse-biased diode when i/p is in –ve cycle.

•

o/p signal is limit/clip to +0.7V during +ve cycle of i/p signal.

2-5 Diode Limiters

Negative clipper

•

Reverse-biased diode act as open cct during +ve cycle.

•

Forward-biased diode act as short cct during –ve cycle.

•

o/p signal is limit/clip to -0.7V during –ve cycle of i/p signal.

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2-5 Diode Limiters

Biased Limiters :

•

Use dc biasing source, V

to set limit on the circuit output voltage.

• This allow the circuit to clip input waveform at values other than

diode forward voltage, 0.7V.

• In each circuit, bias voltage is in series with shunt diode. As a result,

the diode conducts and clips the i/p waveform when signal voltage

equals sum of V

and V

.

• 2 type of biased limiters:

Positive Biased Limiter

• Positive limiter

• The voltage at point A must equal V_BIAS+0.7V before diode become FB and

conduct.

• Once diode begin to conduct, voltage at point A is limited to V_BIAS+0.7V, so all i/p

Negative Biased Limiter

• Negative limiter

• Voltage at point A must go below –V_BIAS - 0.7V to forward-bias the diode and initiate

Combination of Positive Limiter & Negative

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

Half-wave rectifiers only utilize half of the cycle to produce a DC voltage. Transformer Coupling allows voltage manipulation through its windings 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 secondary winding output whereas the center-tapped full wave uses only half.

Filtering and Regulating the output of a rectifier helps keep the DC voltage smooth and accurate

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