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Sessional 2 pack
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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