zener diode.pdf

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PHYS 162 - Chapter 3 Special-Purpose Diode

Prepared By: Syed Muhammad Asad – Semester 102 Page

Figure 1 Zener diode symbol and V-I charaterist

Figure 2 Reverse characteristics of zener diode

CHAPTER 3

SPECIAL-PURPOSE DIODES

3-1 THE ZENER DIODE

- Zener diodes are commonly used for voltage regulation, that is, they maintain a

constant voltage at the output.

- It is designed to work in the reverse breakdown region.

- The breakdown voltage is adjusted by controlling the level of doping.

- The V-I characteristic and zener diode symbol are shown in Figure 1.

3.1.1 Zener Breakdown

- There are two types of breakdowns in zener diodes.

o Zener Breakdown

o Avalanche Breakdown

-

Avalanche breakdown occurs at high voltages typicallymore than 5 V.

- Zener breakdown occurs at low voltages typically less

than 5 V.

3.1.2 Breakdown Characteristics

- As the reverse voltage VR increases from 0V, the

reverse current IR remains extremely small.

- The reverse current starts to increase rapidly when the

VR reaches the knee point.

- The current at this point is called the zener knee

current IZK.

- The breakdown effect starts at this point.

- After this, the current starts to increase as the zener

impedance ZZ decreases.

- From the knee, the zener voltage VZ remains almost

constant.

Zener Regulation

-

Keeping the voltage constant across its terminal isthe main advantage of zener diode.

- The minimum zener current required maintaining

voltage regulation is IZK.

- The maximum zener current allowed before it is

damaged is IZM.

- The zener voltage VZ, specified in the datasheets is the voltage at the zener test current IZT.





Figure 3 Ideal zener diode

Figure 4 Practical zener diode

3.1.3 Zener Diode Model or Equivalent Circuit

- Zener diodes are modeled in two approximations.

o Ideal Zener Diode

o Practical Zener Diode

Ideal Zener Diode

- Ideal zener diode has constant voltage drop equal to

the nominal zener voltage VZ.- This drop is represented as a voltage source even

though the zener diode does not produce any voltage.

Practical Zener Diode

- The practical diode includes the zener

impedance ZZ.

- Since the zener voltage is not constant, a

change in zener current Δ, produces a

small change in the zener voltage Δ.- Then the zener impedance is given by

=Δ

Δ.

- It is best to avoid the zener diode to

operate at the knee current because the

impedance changes rapidly in this area.

- The impedance is usually very small and

can be neglected in calculations.

NOTE: REFER EXAMPLE 3-2 PAGE 111

3.1.4 Temperature Coefficient

- Temperature coefficient () describes the percent change in zener voltage for each degree Celsiu

change in temperature.

- The formula for calculating the change in zener voltage Δ is given by

Δ = × × Δ

- A positive temperature coefficient means the zener voltage will increase with increase in temperatur

or decrease with decrease in temperature.

- A negative temperature coefficient means the zener voltage will decrease with increase i

temperature or increase with decrease in temperature.

3.1.5 Zener Power Dissipation and Derating

- Zener diode are specified to operate at the maximum DC power dissipation PD(max).

- This is given in datasheet of the diode.

- The power dissipated by a zener diode at any zener current IZ is given as = .

Power Derating

- The PD(max) of a zener diode is specified at a certain value, for example 50°C.





Figure 5 Zener voltage regulation

- Above this temperature, the maximum power dissipation is reduced according to a derating facto

(DF).

- Derating factor (DF) is expressed in mW/°C.

- The maximum derated power can be expressed as = − × Δ.


3.1.6 Zener Diode Datasheet Information

NOTE: You will find the datasheet of the zener diode at the end of the notes.

- The most popular zener diode series is 1N4728A-1N4764A.

Absolute Maximum Ratings

- The maximum power dissipation, PD is given as 1W at 50°C.

- The power derate factor (DF) is given as 6.67mW/°C above 50°C.

- The maximum reverse current IZM is not given but can be found for any VZ as

=

Electrical Characteristics

- For each device, minimum, typical and maximum zener voltages VZ are given.

- The zener voltages are measured at zener test current IZ which are given in the datasheet.

- Maximum zener impedance, ZZ is given at IZ.

- Minimum zener impedance, ZZK, at the knee of the characteristics curve is given at IZK.

3-2 ZENER DIODE APPLICATIONS

- Zener diode can be used as a type of

voltage regulator.

3.2.1 Zener Regulation with a Varying

Input Voltage (No Load Condition)

- Zener diode regulators are not very

efficient so they are limited to

applications that require low current

to the load.

- Figure 5 illustrates the concept of

zener voltage regulation.

- The zener diode will regulate the

output voltage provided > and

< < .

- As the input voltage VIN increases, IZ

will increase and there will be very

small change in the output voltage

+ Δ.

- As the input voltage VIN decreases, IZ will decrease and there will be very small change in the outpu

voltage − Δ.





Figure 6 Zener regulation with variable load


3.2.2 Zener Regulation with a Variable Load

- The zener diode will maintain a constant voltage across the load resistor RL as long as zener current

in within IZK and IZM, i.e., < < .

3.2.3 From No Load to Full Load

No Load- When the output terminal of a zener diode are

open it mean = ∞.

- All the current goes to the zener diode and the

load current is zero. This is no load condition.

Full Load

- When a load resistance RL is connected, some current IL goes to RL.

- As the load resistance decreases, the load current IL increases and zener current IZ decreases.

- The zener diode will regulate the voltage until IZ is equal to IZK.

- At this point, load current IL is maximum and this is full load condition.

NOTE: REFER EXAMPLE 3-6 & 3-7 PAGE 117-119

Figure 7 Basic zener diode limiter

Zener Limiting

- Zener diodes can be used in AC applications as AC voltage limiters.- Circuit in Figure 7(a) will limit the forward voltage to VZ while in the negative cycle, the zener will ac

as a forward biased diode and limit the negative voltage to 0.7V.

- Circuit in Figure 7(b) will act in the opposite manner to the circuit in (a).

- Back to back zener diodes as in Figure 7(c) will limit both peaks to ± ± 0.7.

- In the positive cycle, D1 acts like a forward biased diode while D2 acts like zener diode so the peak wi

be limited to + 0.7V. In the negative cycle, D2 acts like a forward biased diode while D1 acts lik

zener diode so the peak will be limited to − − 0.7V.





Figure 9 LED

Figure 10

Electroluminescence in LED

Figure 8 Varactor


3-3 THE VARACTOR DIODE

- The junction capacitance of diodes change with reverse bias.

- Diodes designed to be used as voltage controlled capacitors

are called varactors.

- They always work in reverse bias.

- The depletion region acts as dielectric and p and n-types act

like capacitor plates.

3.3.1 Basic Operation

- The capacitance of a material can be determined by plate area A, dielectric constant and plate

separation d and is expressed as

=

- As the reverse bias increases, the depletion region becomes wide and increases the plate separatio

d . This decreases the capacitance.- As the reverse bias decreases, the depletion region becomes narrow and decreases the plat

separation d . This increases the capacitance.

3-4 OPTICAL DIODES

- Two types of optical diodes are discussed in this section.

o Light-Emitting Diode (LED) – These are light emitters.

o Photodiodes – These are light detectors.

3.4.1 The Light-Emitting Diodes (LED)

- Figure 9 shows the symbol of an LED.

Basic Operation

- We know that the free electrons in the n-type have high energy.

- When the LED is forward biased, electrons cross the pn junction and

recombines with the holes in the p-type.

- When these high energy electrons recombine with holes, they release energy

in the form of photons.

- The emission of these photons is called electroluminescence.

- The doping determines the wavelength of the emitted photons.

LED Biasing

- The forward voltage VF across an LED is higher than silicon rectifier diodes

(typically between 1.2V to 3.2V).

- Reverse breakdown is lower than silicon rectifier diodes (3V to 10V).





Figure 11 LED operation

- Figure 11(a) shows a typical LED

circuit. The graph in Figure 11(b)

shows that power of light output is

directly proportional to the

forward current IF.

Applications of LED

1. Seven – Segment Display- LED’s are widely used in various

applications like indicator lights,

read out displays etc.

- A very common application of LED’s is the seven-segment display shown in Figure 12.

- Each segment is an LED.

- By forward biasing selected combination of segments, any decimal digits can be formed.

2. Remote Controls

- Infrared LED’s are commonly used in remote controls.

- Infrared LED emits beam of invisible light. - Each button corresponds to an electrical code which is converted to a light code and transmitted

through the LED.

- The receiver recognizes the code and takes the required action.

Figure 12 7-segment display





Figure 14 LED array and circuit

Figure 13 Counting and control system 3. Industrial Application

- Infrared LED’s can be used in industry for various applications. Figure 13 shows a ball counting system

- Each ball passing through the tube interrupts the IR beam emitted by the infrared LED.- This is detected by the detector and the circuit counts each ball.

- When a set number of balls are counted, the flow of balls in stopped in order for the next empty box

to be set in the proper place.

3.4.2 High Intensity LED’s

- These LED’s produce high light output than ordinary LED’s.

Applications

1. Traffic Lights

- Arrays of tiny LED’s form the red, yellowand green lights in a traffic signal.

- LED arrays shown in Figure 14 have 3

advantages compared to ordinary bulbs.

o Brighter light

o Longer life (years vs. months)

o Less energy consumption (about

90% less)

- The array is connected in series-parallel

through limiting resistors.- The lens in front of the LED (Figure 15) is

used to properly direct light towards the viewer.

- Different color LED’s require different forward voltages to operate.

- Referring to Figure 16, red LED requires 2V, blue requires between 3V-4V, green requires between

2.5V-3V.





Figure 15 Lens directing LED light

Figure 16 V-I characteristics of visible light LED


2. Display LED’s

- LED’s are widely used message boards and

large-screen television.

- Full-color displays use small groups of high

intensity red, green and blue LED’s. This

group is called a pixel (Figure 17).

- A typical screen contains thousands of such

pixels (Figure 18).

- Mixing of the primary colors red, green and

blue (RGB) in various amount can produce

any color (Figure 17).

Figure 17 RGB concept





Figure 18 Display formed by RGB pixels

3.4.3 The Photodiode

- A photodiode is a device that works in the reverse bias.

- It has small window that allows light to strike the pn junction.

- The reverse current increases when the pn junction is exposed

to light (Figure 19).

- When there is no light on the pn junction, the reverse current

is minimum and is called dark current.

- Increase in the amount of light produces an increase in the

reverse current .

3-5 OTHER TYPES OF DIODES- There are other types of diodes used for special purpose and applications. They will be discussed in

this section. Some of these are

o The Laser Diode

o The Schottky Diode

o The PIN Diode

o The Tunnel Diode

o Current Regulator Diode

3.5.1 The Laser Diode

- Laser stands for l ight

amplification by stimulated

emission of r adiation.

- Laser light is monochromatic meaning it consists of single light.

Construction

- The pn junction is formed by two layers of doped gallium arsenide (Figure 20).

- The length of the pn junction is related to the wavelength of the laser.

- There is highly reflective surface on one end of the pn junction and partially reflective on the other.

Figure 19 Reverse bias operation of photodi

Figure 20 Laser diode symbol and construction




Prepared By: Syed Muhammad Asad – Semester 102 Page 1

Figure 22 Tunnel diode

Operation

- The laser diode is forward biased by external voltage source.

- As electrons move through the junction, they recombine with holes.

- This recombination releases photons.

- These photons strike atoms and release other photons.

- This process increases as the forward current is increased and more and more photons are released

due to avalanche effect.

- At the certain point, some of these photons strike the highly reflective surface and move along the

depletion region and pass through the partially reflective end of the junction (Figure 20c).

- Each photon produced is identical in wavelength, phase and frequency.

The Schottky Diode

- Schottky diodes (Figure 21) are high current diodes used in high

frequency and fast switching applications.

- A schottky diode is formed by joining a doped n-type

with a metal such as gold, silver of platinum.

- It means it has a metal-to-semiconductor junctionrather pn junction.

- The forward voltage drop is 0.3V.

- There are only majority carriers with no reverse leakage current.

- The metal conductor has many conduction band electrons and n-type is also heavily doped.

- When forward biased, the n-type electrons move across to the metal region and rapidly loss energy.

- The process is very fast which makes schottky diodes ideal for fast switching application.

The PIN Diode

- The pin diode consists of heavily doped p and n region

separated by intrinsic (i) region (Figure 22a).

- In reverse bias, the pin diode acts like a constant

capacitance (Figure 22b).

- When forward biased, it acts like a current-controlled

variable resistance (Figure 22c).

The Tunnel Diode

- The tunnel diode (Figure 23)

exhibit a characteristic known as

negative resistance.

- This makes it useful in oscillator

applications.

- The p and n region of the tunnel diode are heavily doped.

- This means the depletion region is very narrow and results in large reverse current effectively having

no breakdown effect.

Figure 23 PIN diode

Figure 21 Schottky diode symbol and structure




Figure 24 Current regulator diode

Current Regulator Diode

- The current regulator diode keeps a constant current rather than constant voltage as in the case of

zener diode.

- The current regulator works in the forward bias and the forward

current remains constant for forward voltage range from 1.5V to

6V.

- The constant forward current in called the regulator current IP.

- This device should never be operated in reverse bias.

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