Full sessional pack ii

Full Sessional pack II

Muhammad Faseeh

Diode ApplicationsFull wave Rectifiers

Week 07

Lecture 12

Peak Inverse voltage in Centre-tapped Full-wave Rectifier

The Bridge Full-Wave Rectifier

The Bridge Full-Wave Rectifier

Ideal diode

Practical diode

Power Supply Filters and regulators

Filters in Half wave rectification

Capacitor discharging - filter

Charging of capacitor- filter

Diode Applications and Types

Week 07

Lecture 13

Bridge Rectifiers

Bridge rectifier

Ripples and ripple factor

Ripple factor (r) is an indication of the effectiveness of the filter,

r = Vr (pp) / VDC

The lower the ripple factor, the better the filter.It can be lowered by increasing the value of filter capacitor or increasing the load resistance.

Surge Current in the Capacitor-Input Filter

Diode Clippers

• These circuits are also called limiters

• Used to clip off portions of signal voltages above or below certain levels.

• Half wave rectifier can also be called as a clipper circuit

Example of diode limiters

Example of diode limiters

Finding Vp(out)

Biased Limiters - Positive

Biased Limiters - Negative

Positive limiter with modification

Negative limiter with modification

Diode Applications – Diode Clampers

Lecture 15

April 04, 2013

Diode Clampers

• A clamper adds a dc level on an ac voltage.

• Prevents the signal from exceeding certain defined magnitude by shifting its dc value.

• They are also called dc restorers.

Positive Clamper operation

• Consider the first negative half cycle of the input voltage.

• When the input voltage initially goes negative, the diode is forward-biased, the capacitor get charged

• ??

Positive Clamper operation

Working and operation• The capacitor is now charged to Vp (in) – 0.7 V.

• Just after the negative peak, the diode is reverse biased because the cathode is held near Vp (in) – 0.7 V by the charge on capacitor.

• The capacitor can only discharge through the high resistance of RL.

Operation and working

• So, from the peak of one negative half cycle to the next, the capacitor discharges very little.

• This discharged amount depends on the value of RL.

• For good clamping action, the RC time constant should be at least ten times the period of the input frequency.

Negative Clamper operation

Diode Types

• Key Terms• Zener Diode

• Zener Breakdown

• Varactor

• Light Emitting Diode (LED)

• Photodiode

• Laser

Zener Diode

• A zener diode is a silicon pn junction device that is designed for operation in reverse-breakdown region.

• A major application • A type of voltage regulator for providing stable reference voltages

for use in power supplies, voltmeters etc.

General diode VI Characteristics

Zener Breakdown

• Zener diodes are designed to operate in reverse breakdown.

• The two types of reverse breakdown in a zener diodes are avalanche and zener.

Zener BreakdownAvalanche Breakdown

Occurs in both rectifier and zener diodes at a significantly high reverse voltage.

Zener Breakdown

Zener breakdown occurs in a zener diode at a low reverse voltages. • A zener diode is heavily doped to reduce the

breakdown voltage. This causes a very thin depletion region. As a result a very intense electric field exists

within the depletion region.

• Near the zener breakdown voltage (Vz), the field is intense enough to pull electrons from their valence

bands and create current.

Zener Breakdown

• Zener diodes with breakdown voltages of less than approx 5V operate predominately in zener breakdown.

• Those with breakdown voltages greater than approx 5 V operate in avalanche breakdown.

Zener Summary

• Both types are called Zener diodes.

• They are commercially available with breakdown voltage of 1.8 V to 200 V with specified tolerances from 1% to 20 %.

Reverse Characteristics of Zener

Equivalent Circuit

Zener Impedance (Zz)

Answer

Diode Types

Lecture 17

Temperature Coefficient

• This is the percent change in zener voltage for each 0C change in temperature.• e.g., a 12 V zener diode with a positive temperature coefficient of

0.01% / 0C will exhibit a 1.2 mV increase in Vz when the junction temperature increases one degree centigrade.

• ∆Vz = Vz * TC * ∆T

• Vz is the nominal zener voltage at 25 0C, TC is the temperature coefficient, and ∆T is the change in temperature.

Temperature Coefficient

• A positive TC means the zener voltage increases with an increase in temperature or decreases with a decrease in temperature.

• A negative TC means that the zener voltage decreases with an increase in temperature or increases with a decrease in temperature.

Temperature Coefficient

• In some cases, the temperature coefficient is expressed in mV/ 0C rather than as %/0C. • For these cases,

• ∆Vz = TC * ∆T

Practice Problem

Problem:

Zener Power Dissipation

• Zener diodes are specified to operate at a maximum power called maximum dc power dissipation, PD(max).

• For example IN746 zener is rated at a PD (max) of 500 mW and

IN3305A at PD (max) of 50W.

• The dc power dissipated is determined by

• PD = VZIZ

Power Derating• The max power dissipated of a zener diode is

typically specified for temperature at or below a certain value (500C for example).

• Above the specified temperature, the maximum power dissipation is reduced according to a derating factor.

• The derating factor is expressed in mW/0C.

• The maximum derated power can be determined with the following formula:

• PD (derated) = PD(max) – (mW/0C) ∆T

Problem

Solution

Zener-From No Load to Full Load

• When RL = ∞, load current is 0 and all the current is through the zener; this is a no load condition.

• When RL is connected, current gets divided between zener and RL.

Optical Diodes

• Two types of optoelectronic devices – the light emitting diode (LED) and the photodiode

• LED• Light emitter

• Photodiode • Light detector

LED

• When the device is forward biased, electrons cross the pn junction from the n-type material and recombine with holes in the P-type material.

• When the recombination takes place, the recombining electrons releases energy in the form of heat and light.

• A large exposed surface area on one layer of the semiconductor material permits the photons to be emitted as visible light.

• (electroluminescence process)

LED

• Various impurities are added during the doping process to establish the wavelength of the emitted light.

• The wavelength determines the color of the light and if it is visible or infrared (IR)

Operation of LED

Spectral output curve

Typical LEDs

The Photodiode

• A device that operates in reverse bias, where

Iλ is the reverse current.

• The photodiode has a small transparent window that allows the light to strike at the pn junction.

• Recall, when reverse biased, a rectifier diode has a very small reverse leakage current. The same is true for a photodiode.

The Photodiode

• A photodiode differs from a rectifier diode in that when its pn junction is exposed to light, the reverse current increases with the light intensity.

• When there is no incident light, the reverse current, Iλ, is almost negligible and is called dark current.

• An increase in the amount of light intensity, expressed as irradiance (mW/cm2 ), produces an increase in the reverse current.

General graph of Photodiode

Photodiode biasing and symbol

Typical photodiode characteristics

Finding resistance…

• Reverse current = 1.4 micro Ampere

• Reverse-bias voltage = 10 V

• Irradiance = 0.5 mW/cm2

• R = VR / Iλ

• 10 V / 1.4 μ A = 7.14 MΩ

• Find resistance at 20 mW/cm2 , current 55 μ A at VR = 10 V

Operation of photodiode

VARACTOR DIODES

• They are also known as variable-capacitance diodes because the junction capacitance varies with the amount of reverse-bias voltage.

• They are specifically designed to take advantage of this variable-capacitance characteristic.

• These devices are commonly used in electronic tuning circuits in communications systems.

Capacitance and Varactor

• A varactor is a diode that always operates in reverse-bias and is doped to maximize the inherent capacitance of the depletion region.

• The depletion region, widened by the reverse bias, act as a capacitor dielectric because of its nonconductive characteristics.

• The p and n regions are conductive and acts as the capacitor plates.

Reverse-biased varactor diode

Operation…

Electronics 1Lecture 18

Current Regulator Diode

• Often called constant-current diode.

• Rather than maintaining a constant voltage, as the zener diode does, this diode maintains a constant current.

• Forward bias operation.

• Current = Ip

Characteristic curve

Schottky Diode

• Used primarily in HF and fast-switching applications.

• Also called hot-carrier diodes.

• A schottky diode is formed by joining a doped semiconductor region (usually n type) with a metal such as gold, silver or platinium.

• Rather than a pn junction, there is a metal-to-semiconductor junction.

• The forward voltage drop is typically typically around 0.3 V.

Internal construction of schottky diode

Schottky…

• Majority carriers only , there are no minority carriers.

• Hence no reverse or leakage current.`

A Zener-Regulated DC Power Supply

Power Supply schematic

A Zener-Regulated DC Power Supply-Full load