lec6- diode applications rev

ECE01- Electronic Devices and Circuits Engr. Ehxell J. Landicho

DIODE APPLICATIONS

Because of their ability to conduct current in one

direction and block current in the other direction, diodes are

used in circuits called rectifiers that convert ac voltage into dc

voltage. Rectifiers are found in all dc power supplies that

operate from an ac voltage source. A power supply is an

essential part of each electronic system from the rectifier, the

half-wave rectifier.

The dc power supply converts the standard 220 V, 60 Hz ac

available at wall outlets into a constant dc voltage. It is one of

the most common electronic circuits that you will find. The dc

voltage produced by a power supply is used to power all

types of electronic circuits.

Basic block diagrams for a rectifier and complete

power supply are shown in the figure. The rectifier can be

either a half-wave rectifier or a full-wave rectifier. The

rectifier converts the ac input voltage to a pulsating dc

voltage, which is half-wave rectified as shown in the figure. A

block diagram for a complete power supply is shown. The

filter eliminates the fluctuations in the rectified voltage and

produces a relatively smooth dc voltage. The regulator is a

circuit that maintains a constant de voltage for variations in

the input line voltage or in the load. Regulators vary from a

single device to more complex integrated circuits. The load is

a circuit or device for which the power supply is producing

the dc voltage and load current.

RECTIFIER CIRCUITS

1. Half-Wave Rectifier

During the positive alternation of the 60 Hz input voltage, the

output voltage looks like the positive half of the input voltage.

The current path is through ground back to the source.

During the negative alternation of the input voltage, the

current i=0. so the output voltage is also 0.

60 Hz half-wave output voltage for three input cycles

Figure 2 illustrates the process called half-wave rectification.

A diode is connected to an ac source and to a load resistor. RV

forming a half-wave rectifier. Keep in mind that all ground

symbols represent the same point electrically. Let's examine

what happens during one cycle of the input voltage using the

ideal model for the diode. When the sinusoidal input voltage

(Vin ) goes positive, the diode is forward-biased and conducts

current through the load resistor, as shown in part (a). The

current produces an output voltage across the load RV which

has the same shape as the positive half-cycle of the input

voltage.

When the input voltage goes negative during the second half

of its cycle, the diode is reverse-biased. There is no current,

so the voltage across the load resistor is 0 V. as shown in

Figure 2(b). The net result is that only the positive half-cycles

of the ac input voltage appear across the load. Since the

output does not change polarity, it is a pulsating dc voltage

with a frequency of 60 Hz, as shown in part (c).

Average Value of the Half-Wave Output Voltage

The average value of the half-wave rectified output voltage is

the value you would measure on a de voltmeter.

Mathematically, it is determined by finding the area under

the curve over a full cycle, as illustrated in the figure below,

and then dividing by 2π, the number of radians in a full cycle.

The result of this is expressed in Equation, where VP is the

peak value of the voltage. This equation shows that VAVG is

approximately 31.8% of VP for a half-wave rectified voltage.


Example:What is the average value of the half-wave rectified

voltage in the figure.

Effect of the Barrier Potential on the Half-Wave Rectifier

Output

In the previous discussion, the diode was considered ideal.

When the practical diode model is used with the barrier

potential of 0.7 V taken into account, this is what happens.

During the positive half-cycle, the input voltage must

overcome the barrier potential before the diode becomes

forward-biased. This results in a half-wave output with a peak

value that is 0.7 V less than the peak value of the input, as

shown in the figure below. The expression for the peak

output voltage is

It is usually acceptable to use the ideal diode model, which

neglects the effect of the barrier potential, when the peak

value of the applied voltage is much greater than the barrier

potential (at least 10 V. as a rule of thumb). However, we will

use the practical model of a diode, taking the 0.7 V barrier

potential into account unless stated otherwise.

Peak Inverse Voltage (PIV)

The peak inverse voltage (PIV) equals the peak value of the

input voltage, and the diode must be capable of withstanding

this amount of repetitive reverse voltage. For the diode in the

figure below, the maximum value of reverse voltage,

designated as PlV occurs at the peak of each negative

alternation of the input voltage when the diode is reverse-

biased.

Half-Wave Rectifier with Transformer-Coupled Input Voltage

A transformer is often used to couple the ac input

voltage from the source to the rectifier, as shown in the

figure below. Transformer coupling provides two advantages.

First, it allows the source voltage to be stepped up or stepped

down as needed. Second, the ac source is electrically isolated

from the rectifier, thus preventing a shock hazard in the

secondary circuit.

From your study of basic ac circuits recall that the

secondary voltage of a transformer equals the turns ratio n

times the primary voltage, as expressed in Equation below.

We will define the turns ratio as the ratio of secondary turns

NSEC, to the primary turns,

Npri :

If n > I, the secondary voltage is greater than the primary

voltage. If n < 1, the secondary voltage is less than the

primary voltage. If n = I, then Vsec = Vpri.

The peak secondary voltage, VP(sec), in a transformer-coupled

half-wave rectifier is the same as Vp(in) in Equation 2-2_

Therefore, equation written in terms of Vp(sec) is

Therefore,


Example:

Determine the peak value of the output voltage for the figure

below if the turns ratio is 0.5.

FULL-WAVE RECTIFICATION

A full-wave rectifier allows unidirectional (one-way)

current through the load during the entire 360 0 of the input

cycle, whereas a half-wave rectifier allow¬ current through

the load only during one-half of the cycle. The result of full-

wave rectification is an output voltage with a frequency twice

the input frequency that pulsates every half-cycle of the

input, as shown in Figure.

The number of positive alternations that make up the fuJI-

wave rectified voltage is twice that of the half-wave voltage

for the same time interval. The average value, which is the

value measured on a de voltmeter, for a full-wave rectified

sinusoidal voltage is twice that of the half-wave, as shown in

the following formula:

Example:

Find the average value of the full-wave rectified voltage in

figure ¬below:

Types of Full-Wave Rectifiers

1. Bridge-Type Rectifier

Four diodes are connected in a bridge configuration

VDC = 0.636Vm

More widely used than center-tapped rectifier, using

four diodes (D1 to D4).

On the positive half cycle of the input sine wave, D1

and D2 are forward biased acting as closed switches

appearing in series with the load.

On the negative half cycle, D1 and D2 are reverse

biased and D3 and D4 are forward biased so current

flows through the load in the same direction.

2. Center-Tapped Full-wave Rectifier

Requires

Two diodes

Center-tapped transformer

VDC = 0.636Vm


Uses only two diodes but a center-tap transformer is

required.

During the positive half cycle, the upper diode is

forward-biased or closed while the lower diode is

reverse-biased or open

During the negative half cycle, the lower diode is

forward-biased and the upper diode is reverse-

biased.

BRIDGE Versus. CENTER-TAPPED RECTIFIER

A center-tap rectifier saves on the diodes used

compared to a diode bridge (2 vs. 4). The former also

has less diode drops in each half-cycle (0.7V vs.

1.4V).

However, center-tap transformers are more

expensive than the transformers used in bridge and

are harder to find.

Examples:

1. a) Show the voltage wavefonns across each half of the

secondary winding and across R L when a 100 V peak sine

wave is applied to the primary winding in the figure.

b) What minimum PIV rating must the diodes have?

2. Determine the peak output voltage for the bridge rectifier

in the figure below. Assuming the practical model, what PIV

rating is required for the diodes? The transformer is specified

to have a 12 V rms secondary voltage for the standard 110 V

across the primary.

3. Determine the output waveform for the network of the

figure below and calculate the output dc level and the

required PIV of each diode.

DIODE CLIPPERS

I. Series Clippers

The diode in a series clipper “clips” any voltage that

does not forward bias it:

A reverse-biasing polarity

A forward-biasing polarity less than 0.7 V (for a

silicon diode)

Adding a DC source in series with the clipping diode

changes the effective forward bias of the diode.


II. Parallel Clippers

The diode in a parallel clipper circuit “clips” any

voltage that forward bias it.

DC biasing can be added in series with the diode to

change the clipping level.

Summary of Clipper Circuits

DIODE CLAMPERS

A diode and capacitor can be combined to “clamp”

an AC signal to a specific DC level.

The input signal can be any type of waveform such

as sine, square, and triangle waves.

The DC source lets you adjust the DC camping level.

Summary of Clamper Circuits

ZENER DIODES

The Zener is a diode operated in reverse bias at the

Zener Voltage (Vz).

• When Vi VZ – The Zener is on – Voltage across the Zener is VZ – Zener current: IZ = IR – IRL


– The Zener Power: PZ = VZIZ – When Vi < VZ – The Zener is off – The Zener acts as an open circuit

Zener Resistor Values

If R is too large, the Zener diode cannot conduct

because the available amount of current is less than the

minimum current rating, IZK. The minimum current is given by:

The maximum value of resistance is:

If R is too small, the Zener current exceeds the

maximum current rating, IZM . The maximum current for the

circuit is given by:

The minimum value of resistance is:

VOLTAGE-MULTIPLIER CIRCUITS

Voltage multiplier circuits use a combination of

diodes and capacitors to step up the output voltage of

rectifier circuits.

Voltage Doubler

Voltage Tripler

Voltage Quadrupler

Voltage Doubler

This half-wave voltage doubler’s output can be calculated by:

Vout = VC2 = 2Vm

where Vm = peak secondary voltage of the transformer

• Positive Half-Cycle

o D1 conducts

o D2 is switched off

o Capacitor C1 charges to Vm

• Negative Half-Cycle

o D1 is switched off

o D2 conducts

o Capacitor C2 charges to Vm

Voltage Tripler and Quadrupler

Practical Applications

Rectifier Circuits

Conversions of AC to DC for DC operated

circuits

Battery Charging Circuits

Simple Diode Circuits

Protective Circuits against overcurrent

Polarity Reversal

Currents caused by an inductive kick in a

relay circuit

Zener Circuits

Overvoltage Protection

Setting Reference Voltages

ZKRL I II min

min

max L

ZL

I

VR

min

max

L

Z

L

LL

R

V

R

V I

Zi

ZL

VV

RVR

min


POWER SUPPLY A power supply is a device that supplies electric

power to an electrical load. The term is most commonly

applied to electric power converters that convert one form of

electrical energy to another, though it may also refer to

devices that convert another form of energy (mechanical,

chemical, solar) to electrical energy. A regulated power

supply is one that controls the output voltage or current to a

specific value; the controlled value is held nearly constant

despite variations in either load current or the voltage

supplied by the power supply's energy source.

POWER SUPPLY TYPES

• Battery

• AC Power supply

• DC Power Supply

• Linear Regulated Power supply

• AC/DC supply

• Switched-mode power supply

• Programmable power supply

• High-voltage power supply

DC SUPPLY Components of a DC Supply

A. Transformer

A transformer is a static electrical device that

transfers energy by inductive coupling between its winding

circuits. A varying current in the primary winding creates a

varying magnetic flux in the transformer's core and thus a

varying magnetic flux through the secondary winding. This

varying magnetic flux induces a varying electromotive force

(emf) or voltage in the secondary winding.

B. Rectifier

A rectifier is an electrical device that converts

alternating current (AC), which periodically reverses

direction, to direct current (DC), which flows in only one

direction. The process is known as rectification. Rectification

may serve in roles other than to generate direct current for

use as a source of power.

C. Filter

Electronic filters are electronic circuits which

perform signal processing functions, specifically to

remove unwanted frequency components from the

signal, to enhance wanted ones, or both.

A capacitor or capacitor circuit that is used to reduce

the variation of the output voltage from a rectifier.

The output of a filter is a RIPPLED DC.

Two most widely used filters are CAPACITOR-INPUT

filter and RC filter.

Ripple

Ripple is a small variation riding on the DC output

prior to small capacitor discharges between the

positive and negative pulses.

The ripple appears to be sawtooth-shaped and is

considered AC.

A small amount of ripple can be tolerated in some

circuits, and the lower the better.

D. Regulator

An electronic circuit or device that maintains an

essentially constant output voltage for a range of

input voltage or load values.

Regulators are classified as either DISCRETE or

INTEGRATED CIRCUIT (IC). Both discrete and IC

regulators are further classified as either LINEAR or

SWITCHING types.

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