Ch02

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Ch02

  1. 1. Chapter 2: Diode Applications Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved.
  2. 2. Slide 1 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Load Line The Load Line plots all possible current (I D ) conditions for all voltages applied to the diode (V D ) in a given circuit. E/R is the maximum I D and E is the maximum V D . Where the Load Line and the Characteristic Curve intersect is the Q point , which specifies a particular I D and V D for a given circuit.
  3. 3. Slide 2 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Diode Approximations In Forward Bias: Silicon Diode: V D = .7V Germanium Diode: V D = .3V In Reverse Bias: Both diodes act like opens V D = source voltage and I D =0A
  4. 4. Slide 3 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Diode in DC Series Circuit: Forward Bias <ul><li>The diode is forward biased. </li></ul><ul><ul><ul><li>• V D = .7V (or V D = E if E < .7V) [Formula 2.4] </li></ul></ul></ul><ul><li> • V R = E – V D [Formula 2.5] </li></ul><ul><li>• I D = I R = IT = V R /R [Formula 2.6] </li></ul>
  5. 5. Slide 4 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><ul><ul><li>Diode in DC Series Circuit: Reverse Bias </li></ul></ul></ul><ul><li>The diode is reverse biased. </li></ul><ul><ul><ul><li>• V D = E </li></ul></ul></ul><ul><li>• V R = 0V </li></ul><ul><li>• I D = I R = I T = 0A </li></ul>
  6. 6. Slide 5 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><ul><ul><li>Diode in any DC Circuit </li></ul></ul></ul><ul><li>Solve this circuit like any Series/Parallel circuit , </li></ul><ul><li>knowing V D = .7V (or up to .7V) in forward bias </li></ul><ul><ul><ul><li>and as an open in reverse bias. </li></ul></ul></ul><ul><ul><ul><li>V D1 = V D2 = Vo = .7V </li></ul></ul></ul><ul><ul><ul><li>V R = 9.3V </li></ul></ul></ul><ul><li>Diodes in parallel are used to limit current: </li></ul><ul><li>I R = E – V D = 10V - .7V = 28mA </li></ul><ul><li>R .33k  </li></ul><ul><li>I D1 = I D2 = 28mA/2 = 14mA </li></ul>
  7. 7. Slide 6 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><ul><ul><li>Diodes in AC Circuits </li></ul></ul></ul>The diode only conducts when it is in forward bias, therefore only half of the AC cycle passes through the diode.
  8. 8. Slide 7 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><li>Diodes convert AC to DC in a process called rectification . </li></ul><ul><li>The diode only conducts for one-half of the AC cycle. The remaining half is either all positive or all negative. This is a crude AC to DC conversion. </li></ul><ul><li>The DC Voltage out of the diode : </li></ul><ul><ul><ul><li> V DC = 0.318Vm [Formula 2.7] </li></ul></ul></ul><ul><li>where V m = the peak voltage </li></ul>Half Wave Rectifier
  9. 9. Slide 8 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. PIV (PRV) <ul><li>Because the diode is only forward biased for one-half of the AC cycle, it is then also off for one-half of the AC cycle . It is important that the reverse breakdown voltage rating of the diode be high enough to withstand the peak AC voltage. </li></ul><ul><ul><ul><ul><li>PIV (PRV) > V m [Formula 2.9] </li></ul></ul></ul></ul><ul><li>PIV = Peak Inverse Voltage; PRV = Peak Reverse Voltage </li></ul><ul><li>V m = Peak AC Voltage </li></ul>
  10. 10. Slide 9 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Full Wave Rectification The rectification process can be improved by using more diodes in a Full Wave Rectifier circuit. Full Wave rectification produces a greater DC output.
  11. 11. Slide 10 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><li>There are two Full Wave Rectifier circuits: </li></ul><ul><ul><ul><li>• Bridge Rectifier </li></ul></ul></ul><ul><li>• Center –Taped Transformer Rectifier </li></ul>Full Wave Rectifier Circuits
  12. 12. Slide 11 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Bridge Rectifier Circuit Four diodes are required. V DC = 0.636 Vm [Formula 2.10]
  13. 13. Slide 12 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Operation of the Bridge Rectifier Circuit For the positive half of the AC cycle: For the negative half of the AC cycle:
  14. 14. Slide 13 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Center–Tapped Transformer Rectifier Circuit Two diodes and a center-tapped transformer are required. V DC = 0.636(V m ) Note that V m here is the transformer secondary voltage to the tap.
  15. 15. Slide 14 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Operation of the Center–Tapped Transformer Rectifier Circuit For the positive half of the AC cycle: For the negative half of the AC cycle:
  16. 16. Slide 15 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Note: V m = peak of the AC voltage. Be careful, in the center tapped transformer rectifier circuit the peak AC voltage is the transformer secondary voltage to the tap. Rectifier Circuit Summary
  17. 17. Slide 16 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Diodes “clip” a portion of the AC wave. The diode “clips” any voltage that does not put it in forward bias. That would be a reverse biasing polarity and a voltage less than .7V for a silicon diode. Clipper Diode Circuit
  18. 18. Slide 17 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. By adding a DC source to the circuit, the voltage required to forward bias the diode can be changed. Variations of the Clipper Circuit
  19. 19. Slide 18 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. By taking the output across the diode, the output is now the voltage when the diode is not conducting. A DC source can also be added to change the diode’s required forward bias voltage. Changing Output Perspective
  20. 20. Slide 19 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Clipper Circuits Summary
  21. 21. Slide 19 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Clipper Circuits Summary
  22. 22. Slide 20 A diode in conjunction with a capacitor can be used to “clamp” an AC signal to a specific DC level. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Clamper Diode Circuits
  23. 23. Slide 20 A diode in conjunction with a capacitor can be used to “clamp” an AC signal to a specific DC level. Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Clamper Diode Circuits
  24. 24. Slide 21 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. The input signal can be any type of waveform: sine, square, triangle wave, etc. You can adjust the DC camping level with a DC source. Variations of Clamper Circuits
  25. 25. Slide 22 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Summary of Clamper Circuits
  26. 26. Slide 23 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. The Zener is a diode operated in reverse bias at the Zener Voltage (V z ). Zener Diode
  27. 27. Slide 24 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><ul><ul><li>Determine the state of the Zener: </li></ul></ul></ul><ul><ul><ul><ul><li>if Vi  Vz , then the Zener is biased “on” ; the Zener is at Vz </li></ul></ul></ul></ul><ul><li>if Vi < Vz , then the diode is biases “off” ; Vz = Vi </li></ul><ul><li>For Vi  Vz: </li></ul><ul><ul><ul><li>The Zener voltage </li></ul></ul></ul><ul><ul><ul><ul><li> [Formula 2.16] </li></ul></ul></ul></ul><ul><ul><ul><li>The Zener current I Z = I R - I L [Formula 2.18] </li></ul></ul></ul><ul><ul><ul><li>The Zener Power P Z = V Z I Z [Formula 2.19] </li></ul></ul></ul><ul><li>For Vi < Vz: </li></ul><ul><li>The Zener acts like an open. </li></ul>Zener Calculations
  28. 28. Slide 24 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Zener Calculations
  29. 29. Slide 25 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><li>The size of the load resistor affects the current in the Zener. </li></ul><ul><li>•  RL is too large </li></ul><ul><ul><ul><li>Not enough current through the Zener and it is biased “off”. </li></ul></ul></ul><ul><li>The minimum current for a Zener is given as IZK in the data sheets. </li></ul><ul><ul><ul><ul><ul><li> [Formula 2.25] </li></ul></ul></ul></ul></ul><ul><ul><ul><ul><ul><li> [Formula 2.26] </li></ul></ul></ul></ul></ul><ul><ul><ul><li>•  RL is too small </li></ul></ul></ul><ul><li>Too much current in the Zener and it avalanches and is quickly destroyed. </li></ul><ul><li>The maximum current for a Zener is given as I ZM in the data sheets. </li></ul><ul><ul><ul><ul><ul><li>[Formula 2.21] </li></ul></ul></ul></ul></ul><ul><li>[Formula 2.20] </li></ul>Load Resistance in a Zener Circuit
  30. 30. Slide 26 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. <ul><li>Voltage multiplier circuits use a combination of diodes and capacitors to step up the output voltage of rectifier circuits. </li></ul><ul><ul><ul><li>•  Voltage Doubler </li></ul></ul></ul><ul><li> •  Voltage Tripler </li></ul><ul><li>•  Voltage Quadrupler </li></ul>Voltage Multiplier Circuits
  31. 31. Slide 27 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Doubler <ul><li>This half-wave voltage doubler’s output can be calculated as </li></ul><ul><ul><ul><li>V out = VC2 = 2V m </li></ul></ul></ul><ul><li>V m = peak secondary voltage of the transformer. </li></ul>
  32. 32. Slide 28 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Operation of a Voltage Doubler Circuit The 1 st capacitor charges up to V m during the positive half of the cycle, then the 2 nd capacitor charges up to V m in the same polarity as the 1 st capacitor, finally the output is the sum of the voltages across both capacitors: Vout = 2V m
  33. 33. Slide 29 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Voltage Tripler and Quadrupler Circuits By adding more diode-capacitor networks the voltage can be increased.
  34. 34. Slide 30 Robert Boylestad Digital Electronics Copyright ©2002 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. Practical Applications of Diode Circuits <ul><ul><li>Rectifier Circuits </li></ul></ul><ul><li>Conversions of AC to DC for DC operated circuits </li></ul><ul><li>Battery Charging Circuits </li></ul><ul><ul><li>Simple Diode Circuits </li></ul></ul><ul><li>Protective Circuits against </li></ul><ul><li>Overcurrent </li></ul><ul><li>Polarity Reversal </li></ul><ul><li>Currents caused by an inductive kick in a relay circuit </li></ul><ul><ul><li>Zener Circuits </li></ul></ul><ul><li>Overvoltage Protection </li></ul><ul><li>Setting Reference Voltages </li></ul>

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