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RAJIV GANDHI COLLEGE OF ENGINEERING AND TECHNOLOGY IT-T32 ELECTRONIC DEVICES AND CIRCUIT

UNIT-I

Diode current equation V-I Characteristics of PN Junction diode-application –Half wave rectifier and full wave rectifiers with and without filters derivation of ripple factor rectification of efficiency and TUF-Zener diode and its application , clippers ,Clampers ,voltage multiplier.

PN JUNCTION DIODES

A p–n junction is a boundary or interface between two types of semiconductor

Material, p-type and n-type, inside a single crystal of semiconductor.

It is created by doping, for example by ion implantation, diffusion of dopants, or

Byepitaxy (growing a layer of crystal doped with one type of dopant on top of a layer of

Crystal doped with another type of dopant).

If two separate pieces of material were used, this would introduce a grain

boundary between the semiconductors that would severely inhibit its utility by

scattering the electrons and holes.

p–n junctions are elementary "building blocks" of most semiconductor electronic

devices such as diodes, transistors, solar cells, LEDs, and integrated circuits; they are the

active sites where the electronic action of the device takes place.

https://en.wikipedia.org/wiki/Semiconductors

https://en.wikipedia.org/wiki/Semiconductors

https://en.wikipedia.org/wiki/Grain_boundary

https://en.wikipedia.org/wiki/Grain_boundary

https://en.wikipedia.org/wiki/Scattering

https://en.wikipedia.org/wiki/Electron_hole

https://en.wikipedia.org/wiki/Electron_hole

https://en.wikipedia.org/wiki/Semiconductor_device

https://en.wikipedia.org/wiki/Semiconductor_device


Forward Biased Diode

With the externally applied voltage, a potential difference is altered between the P and N

regions. When positive terminal of the source is connected to the P side and the negative

terminal is connected to N side then the junction diode is said to be connected in forward

bias condition. Forward bias lowers the potential across the PN junction.

The majority charge carriers in N and P regions are attracted towards the PN junction and

the width of the depletion layer decreases with diffusion of the majority charge carriers.

The external biasing causes a departure from the state of equilibrium and a misalignment

of Fermi levels in the P and N regions, and also in the depletion layer. So an electric field

is induced in a direction converse to that of the incorporated field.

The presence of two different Fermi levels in the depletion layer represents a state of

quasi-equilibrium. The amount of charge Q stored in the diode is proportional to the

current I flowing in the diode.

With the increase in forward bias greater than the built in potential, at a particular value

the depletion region becomes very much thinner so that a large number of majority

charge carriers can cross the PN junction and conducts an electric current. The current

flowing up to built in potential is called as ZERO current or KNEE current.


Forward Biased Diode Characteristics

With the increase in applied external forward bias, the width of the depletion layer

becomes thin and forward current in a PN junction diode starts to increase abruptly after

the KNEE point of forward I-V characteristic curve. Firstly, a small amount of current

called as reverse saturation current exists due to the presence of the contact potential and

the related electric field. While the electrons and holes are freely crossing the junction

and causes diffusion current that flows in the opposite direction to the reverse saturation

current.

The net result of applying forward bias is to reduce the height of the potential barrier by

an amount of eV. The majority carrier current in the PN junction diode increases by an

exponential factor of eV/kT.

As result the total amount of current becomes I = Is * exp(eV/kT), where Is is constant.

The excess free majority charge carrier holes and electrons that enter the N and P regions

respectively, acts as a minority carriers and recombine with the local majority carriers in

N and P regions. This concentration consequently decreases with the distance from the

PN junction and this process is named as minority carrier injection.

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SUBJECT NAME: Electronic Devices and Circuits


The forward characteristic of a PN junction diode is non linear, i.e., not a straight line.

This type of forward characteristic shows that resistance is not constant during the

operation of the PN junction.

The slope of the forward characteristic of a PN junction diode will become very steep

quickly. This shows that resistance is very low in forward bias of the junction diode.

The value of forward current is directly proportional to the external power supply and

inversely proportional to the internal resistance of the junction diode.

Applying forward bias to the PN junction diode causes a low impedance path for the

junction diode, allows for conducting a large amount of current known as infinite current.

This large amount current starts to flow above the KNEE point in the forward

characteristic with the application of a small amount of external potential. The potential

difference across the junction or at the two N and P regions is maintained constant by the

action of depletion layer.

The maximum amount of current to be conducted is kept limited by the load resistor,

because when the diode conducts more current than the usual specifications of the diode,

the excess current results in the dissipation of heat and also leads to severe damage of the

device.


Reverse Biased Diode

When positive terminal of the source is connected to the N side and the negative terminal

is connected to P side, then the junction diode is said to be connected in reverse bias

condition. In this type of connection majority charge carriers are attracted away from the

depletion layer by their respective battery terminals connected to PN junction.

The Fermi level on N side is lower than the Fermi level on P side. Positive terminal

attracts the electrons away from the junction in N side and negative terminal attracts the

holes away from the junction in P side. As a result of it, the width of the potential barrier

increases that impedes the flow of majority carriers in N side and P side.

The width of the free space charge layer increases, thereby electric field at the PN

junction increases and the PN junction diode acts as a resistor. But the time of diode

acting as a resistor is very low.

There will be no recombination of majority carriers taken place at the PN junction; thus,

no conduction of electric current. The current that flows in a PN junction diode is the

small leakage current, due to minority carriers generated at the depletion layer or

minority carriers which drift across the PN junction. Finally, the result is that the growth

in the width of the depletion layer presents a high impedance path which acts as an

insulator.

In reverse bias condition, no current flows through the PN junction diode with increase in

the amount of applied external voltage. However, leakage current due to minority charge

carriers flows in the PN junction diode that can be measured in micro amperes.

As the reverse bias potential to the PN junction diode increases ultimately leads to PN

junction reverse voltage breakdown and the diode current is controlled by external

circuit. Reverse breakdown depends on the doping levels of the P and N regions. With

the increase in reverse bias further, PN junction diode become short circuited due to

overheat in the circuit and maximum circuit current flows in the PN junction diode.

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Reverse Biased Diode Characteristics:

V-I Characteristics of PN Junction Diode

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In the current–voltage characteristics of junction diode, from the first quadrant in the

figure current in the forward bias is incredibly low if the input voltage applied to the

diode is lower than the threshold voltage (Vr).

The threshold voltage is additionally referred to as cut-in voltage. Once the forward bias

input voltage surpasses the cut-in voltage (0.3 V for germanium diode, 0.6-0.7 V for

silicon diode), the current spectacularly increases, as a result the diode functions as short-

circuit.

The reverse bias characteristic curve of diode is shown in the fourth quadrant of the

figure above. The current in the reverse bias is low till breakdown is reached and

therefore the diode looks like as open circuit. When the reverse bias input voltage has

reached the breakdown voltage, reverse current increases spectacularly.

Diode current equations

The diode equation gives an expression for the current through a diode as a function of

voltage. The Ideal Diode Law, expressed as:

Where:

I = the net current flowing through the diode;

I0 = "dark saturation current", the diode leakage current density in the absence of light;

V = applied voltage across the terminals of the diode;

8 SUBJECT NAME: Electronic Devices and Circuits


q = absolute value of electron charge;

k = Boltzmann's constant; and

T = absolute temperature (K).

The "dark saturation current" (I0) is an extremely important parameter which

differentiates one diode from another. I0 is a measure of the recombination in a device. A

diode with a larger recombination will have a larger I0.

I0 increases as T increases; and

I0 decreases as material quality increases.

At 300K,kT/q = 25.85 mV, the "thermal voltage".

Non-Ideal Diodes

For actual diodes, the expression becomes:

Where:

n = ideality factor, a number between 1 and 2 which typically increases as the current

decreases.

The diode equation is plotted on the interactive graph below. Change the saturation

current and watch the changing of IV curve. Note that although you can simply vary the

temperature and ideality factor the resulting IV curves are misleading.

In the simulation it is implied that the input parameters are independent but they are not.

In real devices, the saturation current is strongly dependent on the device temperature.

Similarly, mechanisms that change the ideality factor also impact the saturation current.

Temperature effects are discussed in more detail on the Effect of Temperature page.

Diode switching time

In electronics, a step recovery diode (SRD) is a semiconductor junction diode having the

ability to generate extremely short pulses. It is also called snap-off diode or charge-

storage diode or memory varactor, and has a variety of uses in microwave electronics

as pulse generator or parametric amplifier.

When diodes switch from forward conduction to reverse cut-off, a reverse current flows

briefly as stored charge is removed. It is the abruptness with which this reverse current

ceases which characterizes the step recovery diode.

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http://www.pveducation.org/pvcdrom/appendicies/constants

http://www.pveducation.org/pvcdrom/appendicies/constants

https://en.wikipedia.org/wiki/Parametric_amplifier

https://en.wikipedia.org/wiki/Parametric_amplifier


Generally, the PN junction of a small Signal Diode is encapsulated in glass to protect the

PN junction, and usually have a red or black band at one end of their body to help

identify which end is the cathode terminal. The most widely used of all the glass

encapsulated signal diodes is the very common 1N4148 and its equivalent 1N914 signal

diode.

Small signal and switching diodes have much lower power and current ratings, around

150mA, 500mW maximum compared to rectifier diodes, but they can function better in

high frequency applications or in clipping and switching applications that deal with short-

duration pulse waveforms.

The characteristics of a signal point contact diode are different for both germanium and

silicon types and are given as:

Germanium Signal Diodes – These have a low reverse resistance value giving

a lower forward volt drop across the junction, typically only about 0.2-0.3v,

but have a higher forward resistance value because of their small junction area.

Silicon Signal Diodes – These have a very high value of reverse resistance and

give a forward volt drop of about 0.6-0.7v across the junction. They have fairly

low values of forward resistance giving them high peak values of forward

current and reverse voltage.

The electronic symbol given for any type of diode is that of an arrow with a bar or line at

its end and this is illustrated below along with the Steady State V-I Characteristics Curve.

Silicon Diode V-I Characteristic Curve

The arrow always points in the direction of conventional current flow through the diode

meaning that the diode will only conduct if a positive supply is connected to the Anode,

( a ) terminal and a negative supply is connected to the Cathode ( k ) terminal thus only



allowing current to flow through it in one direction only, acting more like a one way electrical valve, ( Forward Biased Condition ).

However, we know from the previous tutorial that if we connect the external energy source in the other direction the diode will block any current flowing through it and

instead will act like an open switch, (Reversed Biased Condition) as shown below.

Forward and Reversed Biased Diode

Then we can say that an ideal small signal diode conducts current in one direction

(forward-conducting) and blocks current in the other direction (reverse-blocking).

Signal Diodes are used in a wide variety of applications such as a switch in rectifiers,

limiters, snubbers or in wave-shaping circuits.

Avalanche breakdown

Materials conduct electricity if they contain mobile charge carriers. There are two types

of charge carrier in a semiconductor: free electrons and electron holes. A fixed electron in

a reverse-biased diode may break free due to its thermal energy, creating an electron-hole

pair.

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If there is a voltage gradient in the semiconductor, the electron will move towards the

positive voltage while the hole will "move" towards the negative voltage. Most of the

time, the electron and hole will just move to opposite ends of the crystal and stop. Under

the right circumstances, however, (i.e. when the voltage is high enough) the free electron

may move fast enough to knock other electrons free, creating more free-electron-hole

pairs (i.e. more charge carriers), increasing the current.

Fast-"moving" holes may also result in more electron-hole pairs being formed. In a

fraction of a nanosecond, the whole crystal begins to conduct.

The large voltage drop and possibly large current during breakdown necessarily leads to

the generation of heat. Therefore, a diode placed into a reverse blocking power

application will usually be destroyed by breakdown, as the external circuit will be able to

sustain a large current and dump excessive amounts of heat. In principle, however,

avalanche breakdown only involves the passage of electrons, and intrinsically need not

cause damage to the crystal.

Avalanche diodes (commonly encountered as high voltage Zener diodes) are constructed

to have a uniform junction that breaks down at a uniform voltage, to avoid current

crowding during breakdown.

These diodes can indefinitely sustain a moderate level of current while on the edge of

breakdown.

The voltage at which the breakdown occurs is called the breakdown voltage. There is

a hysteresis effect; once avalanche breakdown has occurred, the material will continue to

conduct even if the voltage across it drops below the breakdown voltage.

This is different from a Zener diode, which will stop conducting once the reverse voltage

drops below the breakdown voltage.

Zener breakdown

The Zener effect is a type of electrical breakdown in a reverse biased p-n diode in which

the electric field enables tunneling of electrons from the valence to the conduction band

of a semiconductor, leading to a large number of free minority carriers, which suddenly

increase the reverse current. Zener breakdown is employed in a Zener diode.

Under a high reverse-bias voltage, the p-n junction's depletion region expands, leading to

a high strength electric field across the junction.

A sufficiently strong electric field enables tunneling of electrons from the valence to the

conduction band of a semiconductor leading to a large number of free charge carriers .

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https://en.wikipedia.org/wiki/Current_crowding

https://en.wikipedia.org/wiki/Current_crowding

https://en.wikipedia.org/wiki/Zener_diode

https://en.wikipedia.org/wiki/Zener_diode

https://en.wikipedia.org/wiki/Charge_carriers_in_semiconductors

https://en.wikipedia.org/wiki/Charge_carriers_in_semiconductors



This sudden generation of carriers rapidly increases the reverse current and gives rise to

the high slope conductance of the Zener diode.

The Zener effect is distinct from avalanche breakdown which involves minority

carrier electrons in the transition region which are accelerated by the electric field to

energies sufficient to free electron-hole pairs via collisions with bound electrons. Either

the Zener or the avalanche effect may occur independently, or both may occur

simultaneously.

In general, diode junctions which break down below 5 V are caused by the Zener effect,

while junctions which experience breakdown above 5 V are caused by the avalanche

effect. Intermediate breakdown voltages (around 5V) are usually caused by a

combination of the two effects.

This Zener breakdown voltage is found to occur at electric field intensity of

about 3×107 V/m.[1] Zener breakdown occurs in heavily doped junctions (p-type

semiconductor moderately doped and n-type heavily doped), which produces a narrow

depletion region.[2] The avalanche breakdown occurs in lightly doped junctions, which

produce a wider depletion layer. Temperature increase in the junction decreases Zener

breakdown and increases the contribution of avalanche breakdown.

Rectifier

Rectifier is an electronic device which converts the alternating current(AC) to unidirectional

current, in other words rectifier converts the AC voltage to DC voltage.

We use rectifier in almost all the electronic devices mostly in the power supply section to convert

the main voltage into DC voltage. Every electronic device will work on the DC voltage supply

only.

Many applications of rectifiers, such as power supplies for radio, television and computer

equipment, require a steady constant DC current (as would be produced by a battery). In these

applications the output of the rectifier is smoothed by an electronic filter (usually a capacitor) to

produce a steady current.

A more complex circuitry device that performs the opposite function, converting DC to AC, is

called an inverter.

Half Wave Rectifier:

The half wave rectifier is a type of rectifier that rectifies only half cycle of the waveform. This

article describes the half wave rectifier circuit working.

The half rectifier consist a step down transformer, a diode connected to the transformer and a load

resistance connected to the cathode end of the diode.

The circuit diagram of half wave transformer is shown below:


The main supply voltage is given to the transformer which will increase or decrease the

voltage and give to the diode. In most of the cases we will decrease the supply voltage by

using the step down transformer here also the output of the step down transformer will be

in AC.

This decreased AC voltage is given to the diode which is connected serial to the

secondary winding of the transformer, diode is electronic component which will allow

only the forward bias current and will not allow the reverse bias current. From the diode

we will get the pulsating DC and give to the load resistance RL.

Working of Half Wave Rectifier:

The input given to the rectifier will have both positive and negative cycles. The half

rectifier will allow only the positive half cycles and omit the negative half cycles. So first

we will see how half wave rectifier works in the positive half cycles.

Positive Half Cycle:

In the positive half cycles when the input AC power is given to the primary winding of the step

down transformer, we will get the decreased voltage at the secondary winding which is given to

the diode.

The diode will allow current flowing in clock wise direction from anode to cathode in the forward

bias (diode conduction will take place in forward bias) which will generate only the positive half

cycle of the AC.

The diode will eliminate the variations in the supply and give the pulsating DC voltage to the load

resistance RL. We can get the pulsating DC at the Load resistance.

Negative Half Cycle:

In the negative half cycle the current will flow in the anti-clockwise direction and the

diode will go in to the reverse bias. In the reverse bias the diode will not conduct so, no

current in flown from anode to cathode, and we cannot get any power at the load

resistance.



Only small amount of reverse current is flown from the diode but this current is almost


negligible. And voltage across the load resistance is also zero.

Characteristics of Half Wave Rectifier:

There are some characteristics to the half wave rectifier they are

1. Efficiency: The efficiency is defined as the ratio of input AC to the output DC.

Efficiency, Ƞ = P dc / Pac

DC power delivered to the load, Pdc = I2dc RL = ( Imax/pi ) 2 RL

AC power input to the transformer, Pac = Power dissipated in junction of diode + Power dissipated in load resistance RL

= I2rms RF + I2rms RL = {I2MAX/4}[RF + RL]

Rectification Efficiency, Ƞ = Pdc / Pac = {4/ 2}[RL/ (RF + RL)] = 0.406/{1+ RF/RL }

If RF is neglected, the efficiency of half wave rectifier is 40.6%.

2. Ripple factor: It is defined as the amount of AC content in the output DC. It nothing but

amount of AC noise in the output DC. Less the ripple factor, performance of the rectifier

is more. The ripple factor of half wave rectifier is about 1.21 (full wave rectifier has about

0.48). It can be calculated as follows:

The effective value of the load current I is given as sum of the rms values of

harmonic currents I1, I2, I3, I4 and DC current Idc.

I2 =I2dc+I21+I22+I24 = I2dc +I2ac

Ripple factor, is given as γ = I ac / Idc = (I2 – I2dc) / Idc = {( Irms / Idc2)-1} = Kf2 – 1)

Where Kf is the form factor of the input voltage. Form factor is given as

= Irms /Iavg = (Imax/2)/ (Imax/pi) = pi/2 = 1.57, ripple factor, γ = (1.572 – 1) = 1.21

3. Peak Inverse Voltage: It is defined as the maximum voltage that a diode can with stand in

reverse bias. During the reverse bias as the diode do not conduct total voltage drops across the

diode. Thus peak inverse voltage is equal to the input voltage Vs.

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SUBJECT NAME: Electronic Devices and Circuts


4. Transformer Utilization Factor (TUF): The TUF is defined as the ratio of DC power is

delivered to the load and the AC rating of the transformer secondary. Half wave rectifier has

around 0.287 and full wave rectifier has around 0.693.

Half wave rectifier is mainly used in the low power circuits. It has very low performance

when it is compared with the other rectifiers.

Advantages and Disadvantages of Half wave rectifier:

A half wave rectifier is rarely used in practice. It is never preferred as the power

supply of an audio circuit because of the very high ripple factor.

High ripple factor will result in noises in input audio signal, which in turn will affect

audio quality.

Advantage of a half wave rectifier is only that its cheap, simple and easy to construct.

It is cheap because of the low number of components involved.

Simple because of the straight forwardness in circuit design. Apart from this, a half

wave rectifier has more number of disadvantages than advantages!

Disadvantages of Half wave rectifier

The output current in the load contains, in addition to dc component, ac components

of basic frequency equal to that of the input voltage frequency. Ripple factor is high

and an elaborate filtering is, therefore, required to give steady dc output.

The power output and, therefore, rectification efficiency is quite low. This is due to

the fact that power is delivered only during one half cycle of the input alternating

voltage.

Transformer utilization factor is low.

DC saturation of transformer core resulting in magnetizing current and hysteresis

losses and generation of harmonics.

The DC output available from a half-wave rectifier is not satisfactory to make a general

power supply. However it can be used for some applications like battery charging.

Full Wave Rectifier Working & Operation

The working & operation of a full wave bridge rectifier is pretty simple. The circuit

diagrams and wave forms we have given below will help you understand the operation of

a bridge rectifier perfectly.

In the circuit diagram, 4 diodes are arranged in the form of a bridge. The transformer

secondary is connected to two diametrically opposite points of the bridge at points A &

C. The load resistance RL is connected to bridge through points B and D.



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Full Wave Bridge Rectifier – Circuit Diagram with Input and Output Wave Forms

During the first half cycle

During first half cycle of the input voltage, the upper end of the transformer secondary

winding is positive with respect to the lower end. Thus during the first half cycle diodes

D1 and D3 are forward biased and current flows through arm AB, enters the load

resistance RL, and returns back flowing through arm DC.

During this half of each input cycle, the diodes D2 and D4 are reverse biased and current

is not allowed to flow in arms AD and BC. The flow of current is indicated by solid

arrows in the figure above.

We have developed another diagram below to help you understand the current flow

quickly. See the diagram below – the green arrows indicate beginning of current flow

from source (transformer secondary) to the load resistance.

The red arrows indicate return path of current from load resistance to the source, thus

completing the circuit.

Flow of current in Bridge Rectifier

During the second half cycle

During second half cycle of the input voltage, the lower end of the transformer secondary

winding is positive with respect to the upper end.

Thus diodes D2 and D4 become forward biased and current flows through arm CB, enters

the load resistance RL, and returns back to the source flowing through arm DA. Flow of

current has been shown by dotted arrows in the figure.

Thus the direction of flow of current through the load resistance RL remains the same

during both half cycles of the input supply voltage. See the diagram below – the green


http://www.circuitstoday.com/wp-content/uploads/2009/08/current_flow_in_bridge_rectifier_1.jp


arrows indicate beginning of current flow from source (transformer secondary) to the

load resistance.

The red arrows indicate return path of current from load resistance to the source, thus

completing the circuit.

Path of current in 2nd Half Cycle

Peak inverse voltage of a full wave bridge rectifier

Let’s analyses peak inverse voltage (PIV) of a full wave bridge rectifier using the circuit

diagram. At any instant when the transformer secondary voltage attains positive peak

valueVmax, diodes D1 and D3 will be forward biased (conducting) and the diodes D2

and D4 will be reverse biased (non conducting).

If we consider ideal diodes in bridge, the forward biased diodes D1 and D3 will have zero

resistance. This means voltage drop across the conducting diodes will be zero. This will

result in the entire transformer secondary voltage being developed across load resistance

RL.

Thus PIV of a bridge rectifier = Vmax (max of secondary voltage)

Bridge Rectifier Circuit Analysis

The only difference in the analysis between full wave and centre tap rectifier is that

In a bridge rectifier circuit two diodes conduct during each half cycle and the forward

resistance becomes double (2RF).

In a bridge rectifier circuit Vsmax is the maximum voltage across the transformer

secondary winding whereas in a centre tap rectifier Vsmax represents that maximum

voltage across each half of the secondary winding.

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The different parameters are explained with equations below:

1. Peak Current

Instantaneous value of the voltage applied to the rectifier is given as

vs = Vsmax Sin wt

If the diode is assumed to have a forward resistance of RF ohms and a reverse resistance

equal to infinity, then current flowing through the load resistance is given as

i1 = Imax Sin wt and i2 = 0 for the first half cycle

and i1 = 0 and i2 = Imax Sin wt for second half cycle

The total current flowing through the load resistance RL, being the sum of currents i1 and

i2 is given as

i = i1 + i2 = Imax Sin wt for the whole cycle.

Where peak value of the current flowing through the load resistance RL is given as

Imax = Vsmax/(2RF + RL)

2. Output Current

Since the current is the same through the load resistance RL in the two halves of the ac

cycle, magnitude od dc current Idc, which is equal to the average value of ac current, can be

obtained by integrating the current i1 between 0 and pi or current i2 between pi and 2pi.

Output Current of Full Wave Rectifier

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SUBJECT NAME: Electronic Devices and Circui

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3. DC Output Voltage

Average or dc value of voltage across the load is given as

DC Output Voltage of Full Wave Rectifier

4.

5.

6.

Root Mean Square (RMS) Value of Current

RMS or effective value of current flowing through the load resistance RL is given as

RMS Value of Current of Full Wave Rectifier

Root Mean Square (RMS) Value of Output Voltage

RMS value of voltage across the load is given as

RMS Value of Output Voltage of Full Wave Rectifier

Rectification Efficiency

Power delivered to load,

Rectification Efficiency of Full Wave Rectifier


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7.

Ripple Factor

Form factor of the rectified output voltage of a full wave rectifier is given as

Ripple Factor of Full Wave Rectifier

So, ripple factor, γ = 1.112 – 1) = 0.482

Regulation

The dc output voltage is given as

Regulation of Full Wave Rectifier

Merits and Demerits of Full-wave Rectifier Over Half-Wave Rectifier

Merits –Lets talk about the advantages of full wave bridge rectifier over half wave version first.

I can think about 4 specific merits at this point.

Efficiency is double for a full wave bridge rectifier. The reason is that, a half wave

rectifier makes use of only one half of the input signal. A bridge rectifier makes use of

both halves and hence double efficiency

The residual ac ripples (before filtering) is very low in the output of a bridge rectifier.

The same ripple percentage is very high in half wave rectifier. A simple filter is enough

to get a constant dc voltage from bridge rectifier.

We know the efficiency of FW bridge is double than HW rectifier. This means higher

output voltage, Higher transformer utilization factor (TUF) and higher output power.

Demerits – Full-wave rectifier needs more circuit elements and is costlier.

Merits and Demerits of Bridge Rectifier Over Center-Tap Rectifier.

Acenter tap rectifier is always difficult one to implement because of the special

transformer involved. A center tapped transformer is costly as well. One key difference

betweencenter tap & bridge rectifier is in the number of diodes involved in construction.


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Acenter tap full wave rectifier needs only 2 diodes where as a bridge rectifier needs 4

diodes. But silicon diodes being cheaper than a center tap transformer, a bridge rectifier is

much preferred solution in a DC power supply. Following are the advantages of bridge

rectifier over a center tap rectifier.

A bridge rectifier can be constructed with or without a transformer. If a transformer is

involved, any ordinary step down/step up transformer will do the job.

This luxury is not available in a center tap rectifier. Here the design of rectifier is

dependent on the center tap transformer, which cannot be replaced.

Bridge rectifier is suited for high voltage applications. The reason is the high peak

inverse voltage (PIV) of bridge rectifier, when compared to the PIV of a center tap

rectifier.

Transformer utilization factor (TUF) is higher for bridge rectifier.

Demerits of Bridge rectifier over center tap rectifier

The significant disadvantage of a bridge rectifier over center tap is the involvement of 4

diodes in the construction of bridge rectifier.

In a bridge rectifier, 2 diodes conduct simultaneously on a half cycle of input. A center

tap rectifier has only 1 diode conducting on one half cycle. This increases the net voltage

drop across diodes in a bridge rectifier (it is double to the value of center tap).

Uses of Full wave Bridge rectifier

Full wave rectifier find uses in the construction of constant dc voltage power supplies,

especially in general power supplies. A bridge rectifier with an efficient filter is ideal for

any type of general power supply applications like charging a battery, powering a dc

device (like a motor, led etc) etc.

However for an audio application, a general power supply may not be enough. This is

because of the residual ripple factor in a bridge rectifier. There are limitations to filtering

ripples. For audio applications, specially built power supplies (using IC regulators) may

be ideal.

Full Wave Bridge Rectifier with Capacitor Filter

Output of full wave rectifier is not a constant DC voltage. You can observe from the

output diagram that its a pulsating dc voltage with ac ripples. In real life applications, we

need a power supply with smooth wave forms.

In other words, we desire a DC power supply with constant output voltage. A constant

output voltage from the DC power supply is very important as it directly impacts the

reliability of the electronic device we connect to the power supply


We can make the output of full wave rectifier smooth by using a filter (a capacitor filter

or an inductor filter) across the diode. In some cases an resistor-capacitor coupled filter

(RC) is also used. The circuit diagram below shows a half wave rectifier with capacitor

filter.

Voltage regulator

A voltage regulator is designed to automatically maintain a constant voltage level. A

voltage regulator may be a simple "feed-forward" design or may include negative

feedback control loops .

It may use an electromechanical mechanism , or electronic components. Depending on the

design, it may be used to regulate one or more AC or DC voltages.

Electronic voltage regulators are found in devices such as computer power

supplies where they stabilize the DC voltages used by the processor and other elements.

In automobile alternators and central power station generator plants, voltage regulators

control the output of the plant. In an electric power distribution system, voltage regulators

may be installed at a substation or along distribution lines so that all customers receive

steady voltage independent of how much power is drawn from the line.

Full Wave Rectifier - with Capacitor Filter

13


http://www.circuitstoday.com/wp-content/uploads/2009/08/bridge_rectifier_with_capacitor_filter.jp

http://en.wikipedia.org/wiki/Negative_feedback



http://en.wikipedia.org/wiki/Control_theory

http://en.wikipedia.org/wiki/Power_supply

http://en.wikipedia.org/wiki/Power_supply


Ripple factor in a bridge rectifier

Ripple factor is a ratio of the residual ac component to dc component in the output

voltage. Ripple factor in a bridge rectifier is half than that of a half wave rectifier.

Zener diode characteristics

The Zener diode is like a general-purpose signal diode. When biased in the forward

direction it behaves just like a normal signal diode, but when a reverse voltage is applied

to it, the voltage remains constant for a wide range of currents.

Avalanche Breakdown: There is a limit for the reverse voltage. Reverse voltage can

increase until the diode breakdown voltage reaches. This point is called Avalanche

Breakdown region. At this stage maximum current will flow through the zener diode.

This breakdown point is referred as “Zener voltage”.

The Zener Diode is used in its "reverse bias". From the I-V Characteristics curve we can

study that the zener diode has a region in its reverse bias characteristics of almost a

constant negative voltage regardless of the value of the current flowing through the diode

and remains nearly constant even with large changes in current as long as the zener

diodes current remains between the breakdown current IZ(min) and the maximum current

rating IZ(max).

This ability to control itself can be used to great effect to regulate or stabilise a voltage source against supply or load variations. The fact that the voltage across the diode in the

breakdown region is almost constant turns out to be an important application of the zener

diode as a voltage regulator

Characteristics


Figure shows the current versus voltage curve for a Zener diode. Observe the nearly

constant voltage in the breakdown region.

The forward bias region of a Zener diode is identical to that of a regular diode. The

typical forward voltage at room temperature with a current of around 1 mA is around 0.6

volts. In the reverse bias condition the Zener diode is an open circuit and only a small

leakage current is flowing as shown on the exaggerated plot.

As the breakdown voltage is approached the current will begin to avalanche. The initial

transition from leakage to breakdown is soft but then the current rapidly increases as

shown on the plot.

The voltage across the Zener diode in the breakdown region is very nearly constant with

only a small increase in voltage with increasing current. At some high current level the

power dissipation of the diode becomes excessive and the part is destroyed.

There is a minimum Zener current, Iz(min), that places the operating point in the desired

breakdown. There is a maximum Zener current, Iz(max), at which the power dissipation drives the junction temperature to the maximum allowed. Beyond that current the diode can be damaged.

Zener diodes are available from about 2.4 to 200 volts typically using the same sequence

of values as used for the 5% resistor series –2.4, 2.7, 3.0 3.3, 3.6, 3.9, 4.3, 4.7, 5.1, 5.6,

6.2, 6.8, 7.5, 8.2, 9.1, 10, 11, 12, 13, 15, 16, 18, 20, 22, 24, etc. All Zener diodes have a

power rating, Pz


From Watt’s law the maximum current is IZ (MAX) =PZ / VZ.

Zener diodes are typically available with power ratings of 0.25, 0.4, 0.5, 1, 2, 3, and 5

watts although other values are available.

Diode equivalent circuit

It is generally profitable to replace a device or system by its equivalent

circuit. Once the

device is replaced by its equivalent circuit, the resulting network can

be solved by

traditional circuit analysis technique.

CLIPPERS

A diode clipping circuit can be used to limit the voltage swing of a signal. Figure 1.17 shows

a diode circuit that clips both the positive and negative voltage swings to references voltages.

Diode clipping circuit and its output waveform.

CLAMPING

When a signal drives an open-ended capacitor the average voltage level on the output

terminal of the capacitor is determined by the initial charge on that terminal and may

therefore be quite unpredictable. Thus it is necessary to connect the output to ground or some

other reference voltage via a large resistor. This action drains any excess charge and results in

an average or DC output voltage of zero.


Diode clamp circuit and its output waveform.

A simple alternative method of establishing a DC reference for the output voltage is by using

a diode clamp as shown in figure 1.18. By conducting whenever the voltage at the output

terminal of the capacitor goes negative, this circuit builds up an average charge on the

terminal that is sufficient to prevent the output from ever going negative. Positive charge on

this terminal is effectively trapped.

CAPACITOR FILTERS

It can be seen from Figures that the waveform vout is not very smooth. For many applications

it is desired to have a much smoother DC waveform, and so a filtering circuit is used. We will

consider the filtered half-wave rectifier of Figure, and leave the filtered full-wave rectifiers

up to you to work out (not hard-see lab).


Filtered Half-wave rectifier.

The waveform produced by this filtered half-wave rectifier is shown in Figure illustrating the

ripple.

Filtered Half-wave rectified waveform.

Here, ripple is defined as the difference between the maximum and minimum voltages on the

waveform, Figure (i.e. peak-to-peak).

Filtered Half-wave rectified waveform showing Vrpp and VDC.


The (peak-to-peak) ripple factorr is defined as

whereVrpp is the peak-to-peak ripple voltage and VDC is the DC component of the ripple

waveform. T is the period of the AC source voltage: T=1/f, . For f=50 Hz (the

frequency of the AC supply in Australia), T= 20 ms.

We now explain how to calculate (approximately) Vrpp and VDC. Think of the ripple

waveform as being approximated by a triangular waveform (Figure 1.22), so that

Using symmetry. Suppose that at the beginning of a cycle the capacitor is fully charged to

Vm(out), and that the capacitor is large enough so that the time constant RLC is much larger

than T. The rate of change of vout at the beginning of the cycle t=0 is

so that at time t=T the capacitor voltage has decreased by an amount

approximately (straight line approximation). This allows one to design C for a given load and

desired ripple.

Exercise. Show that Vrpp in the case of a full-wave rectifier is given by


.

Approximate (triangular) filtered Half-wave rectified waveform

THE ZENER DIODE

There are several other types of diodes beside the junction diode. As the reverse voltage

increases the diode can avalanche-breakdown (zener breakdown). This causes an increase in

current in the reverse direction. Zener breakdown occurs when the electric field near the

junction becomes large enough to excite valence electrons directly into the conduction band.

Avalanche breakdown is when the minority carriers are accelerated in the electric field near

the junction to sufficient energies that they can excite valence electrons through collisions.

Figure 1.23 shows the current-voltage characteristic of a zener diode, its schematic symbol

and equivalent circuit model in the reverse-bias direction. The best zener diodes have a

breakdown voltage ( ) of 6-7 V.


a) Current versus voltage of a zener diode, b) schematic symbol for a zener diode and c)

equivalent circuit model of a zener diode in the reverse-bias direction.

ZENER DIODE SHUNT REGULATOR:

Zener Diode Shunt Regulator

The zener is connected in parallel(or shunt)with the load, therefore the circuit is known as

shunt regulator. A resistance (Rs) is connected in series with the zener to limit current in the

circuit. Therefore the resistance Rs is also known as series current limiting resistor.

The output voltage(VL)is taken across the load resistance(RL).For proper operation, the

input voltage(Vs)must be greater than the zener voltage(Vz).This ensures that zener operates

in the reverse breakdown region.

The input current is given by the relation,

Is =Vs-Vz / Rs


Where Vs- d.c input voltage

Vz- zener voltage

The ideal zener diode acts as a constant voltage source of voltage (Vz).However, a practical

zener diode has a finite value of resistance called zener resistance(Rz).Because of the zener

resistance, there is a voltage drop across it, which is equal to Iz.*Rz. Therefore the voltage

across the terminals of the zener diode,

VL=Vz+IzRz (1)

If the zener resistance is negligible, then the load voltage,

VL=Vz (2)

And the current through the load resistance, is given by the relation,

IL=VL/RL (3)

We know that input current is the sum of zener current and load current i.e.,

Is= Iz+ IL or Iz = Is-IL

WORKING OF ZENER DIODE SHUNT REGULATOR:

Regulation with varying input voltage:

Consider the regulator circuit. Here the load resistance (RL) is kept fixed and the input

voltage (Vs) varies within the limits. As the input voltage increases, the input current (Is) also

increases. This increases the current through zener diode, without affecting the load current

(IL). The increase in input current will also increase the voltage drop across series resistance

(Rs), thereby keeping the load voltage (VL) as constant. If the input voltage is decreased then

the input current also decreased.

Varying Input with

Fixed Load

Fixed

Load

Fixed Load Variable

Vin

RS

Vi

n

Fixed Load IZ IL

IS

RAJIV GANDHI COLLEGE OF ENGINEERING AND TECHNOLOGY IT-T32 ELECTRONIC DEVICES AND CIRCUIT Regulation with varying load resistance:

Here the input voltage (Vs) is kept fixed and the load resistance (RL) varies. The variation of

load resistance changes the current (IL)through it, thereby changing the voltage (VL) across it.

When the load resistance decreases, the load current increases. This causes the zener current

to decrease. As the result, the input current and the voltage drop across the series resistance

remains constant. Thus the load voltage (VL) is also kept constant .If the load resistance

increases, then the load current decreases. As the result of this, the zener current increases.

This again keeps the values of input current and voltage drop across series resistance as

constant. Thus the load voltage remains constant.

DISADVANTAGES:

1. The maximum load current, which can be supplied to load resistor(RL) is limited to

Iz(max)-Iz(min), which is usually of few milliamp.

2. A large amount of power is wasted in the zener diode and the series resistance (Rs)in

comparison with the load power.

3. The regulation factor and the output resistance are not very low.

TRANSISTOR SHUNT REGULATOR:

The is connected in shunt with the load resistance, the circuit is known as shunt regulator.

It may be noted from the regulator circuit, that the load voltage(VL) is equal to the sum of

zener voltage(Vz) and the base -to –emitter voltage (Vbe) of the transistor,.i.e.,

VL=Vz+ Vbe or Vbe=VL-Vz (1)

The working of a transistor shunt regulator may be understood by supposing that the

unregulated input voltage (Vs) increases. Because of this the load voltage also increases.

As a result of this and from equation (1) we find that base to emitter voltage also

increases. And the base current of the transistor (Ib) increases. Due to this, the collector

Fixed

Vin

Variable

load

RS

IZ

lo

ad

IL

lo

ad

IS


current of a transistor (Ic) also increases. This causes the input current (Is) to increase,

which in turn increases the voltage drop across the series resistance (Vrs). Consequent, the

load voltage decreases. It s true because sum of the voltage drop across series resistance

(VRS)and the load voltage (VL) s equal to the iput voltage (Vs) at all times. i.e.,

Vs= Vrs + VL or VL = Vs – Vrs …………….(2)

The above discussion may be summarized in the form of an equation as given below,

Vs = VL- Vbe or Ic –VRS - VL

Here an arrow in the upper direction shows the increase in the value, while the arrow in

the downward direction indicates the decrease in value. The transistor shunt regulator is

suitable for the load in which the load current varies from a finite minimum value to a

finite maximum value.

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