Full sessional pack ii


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Full sessional pack ii

  1. 1. Full Sessional pack IIMuhammad Faseeh
  2. 2. Diode ApplicationsFull wave RectifiersWeek 07Lecture 12
  3. 3. Peak Inverse voltage inCentre-tapped Full-wave Rectifier
  4. 4. The Bridge Full-Wave Rectifier
  5. 5. The Bridge Full-Wave Rectifier
  6. 6. Ideal diode
  7. 7. Practical diode
  8. 8. Power Supply Filters and regulators
  9. 9. Filters in Half wave rectification
  10. 10. Capacitor discharging - filter
  11. 11. Charging of capacitor- filter
  12. 12. Diode Applications andTypesWeek 07Lecture 13
  13. 13. Bridge Rectifiers
  14. 14. Bridge rectifier
  15. 15. 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.
  16. 16. Surge Current in the Capacitor-Input Filter
  17. 17. Diode Clippers• These circuits are also called limiters• Used to clip off portions of signal voltagesabove or below certain levels.• Half wave rectifier can also be called as aclipper circuit
  18. 18. Example of diode limiters
  19. 19. Example of diode limiters
  20. 20. Finding Vp(out)
  21. 21. Biased Limiters - Positive
  22. 22. Biased Limiters - Negative
  23. 23. Positive limiter with modification
  24. 24. Negative limiter with modification
  25. 25. Diode Applications – Diode ClampersLecture 15April 04, 2013
  26. 26. Diode Clampers• A clamper adds a dc level on an ac voltage.• Prevents the signal from exceeding certaindefined magnitude by shifting its dc value.• They are also called dc restorers.
  27. 27. Positive Clamper operation• Consider the first negative half cycle ofthe input voltage.• When the input voltage initially goesnegative, the diode is forward-biased, thecapacitor get charged• ??
  28. 28. Positive Clamper operation
  29. 29. 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.
  30. 30. 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.
  31. 31. Negative Clamper operation
  32. 32. Diode Types• Key Terms• Zener Diode• Zener Breakdown• Varactor• Light Emitting Diode (LED)• Photodiode• Laser
  33. 33. 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.
  34. 34. General diode VI Characteristics
  35. 35. 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.
  36. 36. 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.
  37. 37. 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.
  38. 38. 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 %.
  39. 39. Reverse Characteristics of Zener
  40. 40. Equivalent Circuit
  41. 41. Zener Impedance (Zz)
  42. 42. Answer
  43. 43. Diode TypesLecture 17
  44. 44. 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.
  45. 45. 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.
  46. 46. Temperature Coefficient• In some cases, the temperature coefficient is expressed in mV/ 0C ratherthan as %/0C.• For these cases,• ∆Vz = TC * ∆T
  47. 47. Practice Problem
  48. 48. Problem:
  49. 49. 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
  50. 50. 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
  51. 51. Problem
  52. 52. Solution
  53. 53. 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.
  54. 54. Optical Diodes• Two types of optoelectronic devices – the light emitting diode (LED) andthe photodiode• LED• Light emitter• Photodiode• Light detector
  55. 55. 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)
  56. 56. 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)
  57. 57. Operation of LED
  58. 58. Spectral output curve
  59. 59. Typical LEDs
  60. 60. 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.
  61. 61. 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.
  62. 62. General graph of Photodiode
  63. 63. Photodiode biasing and symbol
  64. 64. Typical photodiode characteristics
  65. 65. 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
  66. 66. Operation of photodiode
  67. 67. 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.
  68. 68. 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.
  69. 69. Reverse-biased varactor diode
  70. 70. Operation…
  71. 71. Electronics 1Lecture 18
  72. 72. 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
  73. 73. Characteristic curve
  74. 74. 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.
  75. 75. Internal construction of schottky diode
  76. 76. Schottky…• Majority carriers only , there are no minority carriers.• Hence no reverse or leakage current.`
  77. 77. A Zener-Regulated DC Power Supply
  78. 78. Power Supply schematic
  79. 79. A Zener-Regulated DC Power Supply-Full load