Ripples and ripple factorRipple factor (r) is an indication of the effectiveness of the filter,r = Vr (pp) / VDCThe lower the ripple factor, the better the filter.It can be lowered by increasing the value of filter capacitor or increasing the loadresistance.
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 capacitordischarges 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 ofthe input frequency.
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 powersupplies, voltmeters etc.
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 BreakdownOccurs in both rectifier and zenerdiodes at a significantly high reversevoltage.Zener BreakdownZener breakdown occurs in a zenerdiode 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 intenseelectric field exists within the depletion region.•Near the zener breakdown voltage (Vz), the field is intense enoughto pull electrons from their valence bands and create current.
Zener Breakdown• Zener diodes with breakdown voltages of less than approx 5V operatepredominately in zener breakdown.• Those with breakdown voltages greater than approx 5 V operate in avalanchebreakdown.
Zener Summary• Both types are called Zener diodes.• They are commercially available with breakdown voltage of 1.8 V to 200 Vwith specified tolerances from 1% to 20 %.
Temperature Coefficient• This is the percent change in zener voltage for each 0C change intemperature.• e.g., a 12 V zener diode with a positive temperature coefficient of 0.01% / 0C willexhibit a 1.2 mV increase in Vz when the junction temperature increases one degreecentigrade.• ∆Vz = Vz * TC * ∆T• Vz is the nominal zener voltage at 25 0C, TC is the temperature coefficient, and ∆T is the change intemperature.
Temperature Coefficient• A positive TC means the zener voltage increases with an increase intemperature or decreases with a decrease in temperature.• A negative TC means that the zener voltage decreases with an increase intemperature or increases with a decrease in temperature.
Temperature Coefficient• In some cases, the temperature coefficient is expressed in mV/ 0C ratherthan as %/0C.• For these cases,• ∆Vz = TC * ∆T
Zener Power Dissipation• Zener diodes are specified to operate at a maximum power called maximumdc 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 specifiedfor temperature at or below a certain value (500C for example).• Above the specified temperature, the maximum power dissipationis 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
Zener-From No Load to Full Load• When RL = ∞, load current is 0 and all thecurrent is through the zener; this is a no loadcondition.• When RL is connected, current gets dividedbetween zener and RL.
Optical Diodes• Two types of optoelectronic devices – the light emitting diode (LED) andthe photodiode• LED• Light emitter• Photodiode• Light detector
LED• When the device is forwardbiased, electrons cross the pn junctionfrom the n-type material and recombinewith holes in the P-type material.• When the recombination takes place, therecombining electrons releases energy inthe form of heat and light.• A large exposed surface area on one layerof the semiconductor material permits thephotons to be emitted as visible light.• (electroluminescence process)
LED• Various impurities are added during the dopingprocess to establish the wavelength of theemitted light.• The wavelength determines the color of thelight and if it is visible or infrared (IR)
The Photodiode• A device that operates in reverse bias, whereIλ is the reverse current.• The photodiode has a small transparent window that allows the light to strike at thepn junction.• Recall, when reverse biased, a rectifier diode has a very small reverse leakage current. The same istrue for a photodiode.
The Photodiode• A photodiode differs from a rectifier diode in that when its pn junction is exposed tolight, the reverse current increases with the light intensity.• When there is no incident light, the reverse current, Iλ, is almost negligible and iscalled dark current.• An increase in the amount of light intensity, expressed as irradiance (mW/cm2), produces an increase in the reverse current.
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
VARACTOR DIODES• They are also known as variable-capacitance diodes because the junctioncapacitance varies with the amount of reverse-bias voltage.• They are specifically designed to take advantage of this variable-capacitancecharacteristic.• These devices are commonly used in electronic tuning circuits incommunications systems.
Capacitance and Varactor• A varactor is a diode that always operates in reverse-bias and is doped tomaximize the inherent capacitance of the depletion region.• The depletion region, widened by the reverse bias, act as a capacitordielectric because of its nonconductive characteristics.• The p and n regions are conductive and acts as the capacitor plates.
Current Regulator Diode• Often called constant-current diode.• Rather than maintaining a constantvoltage, as the zener diode does, this diodemaintains a constant current.• Forward bias operation.• Current = Ip
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 (usuallyn 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.