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Special Purpose Diodes

Special Purpose Diodes. Chapter 5 & Lecture 5 & 6. -V Z (Volts). 4. 6. 2. 0. 20. 40. 60. -I Z (mA). V Z. 80. 100. 120. 140. Graph of zener current versus voltage. A zener diode voltage regulator. R S. Power supply. V S. R L. V Z.

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Special Purpose Diodes

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  1. Special Purpose Diodes Chapter 5 & Lecture 5 & 6

  2. -VZ (Volts) 4 6 2 0 20 40 60 -IZ (mA) VZ 80 100 120 140 Graph of zener current versus voltage

  3. A zener diode voltage regulator RS Power supply VS RL VZ This circuit will regulate when the Thevenin voltage facing the zener diode is greater than the zener voltage. RL VTH = VS RS + RL

  4. RS Power supply VS RL VZ Assuming the zener is conducting: VZ VS - VZ IZ = IS - IL IS = IL = RS RL

  5. A zener waveshaping circuit R Power supply RL VZ + 0.7 V -VZ - 0.7 V

  6. Power supply RZ Zener diode second approximation VZ

  7. Applying the second approximation RS RZ Power supply VS RL VZ The deviation in load voltage from the ideal case: DVL = IZRZ

  8. The zener regulator also reduces ripple RS RZ Power supply VR(in) RL VZ Assuming that both RL and RS >> RZ: RZ VR(in) VR(out)@ RS

  9. Zener diode ratings • Maximum power = PZ(max) = VZIZ(max) • Available tolerances: 20, 10 and 5 percent • Zener resistance, RZT, increases at the knee of the characteristic curve • A derating factor such as 3.2 mW per degree for temperatures above 50 degrees Celsius is typical.

  10. V 1 kW -20 -30 -10 0 RS Q1 VS -10 mA Q2 -20 VS = 20 V 30 V -30 What happens to VZ when VS varies from 20 to 30 volts? Load lines provide a graphical solution.

  11. IS = VS - VD RS RS Power supply VS Light-emitting diode The typical voltage drop for most LEDs is from 1.5 to 2.5 V.

  12. RS Power supply VS Photodiode Photodiodes are reverse biased and conduct in the presence of light.

  13. R2 R1 Signal source Power supply V1 V2 Optocoupler combines an LED and a photodiode.

  14. Vin Vout Tails High-frequency rectification Charge storage can cause poor performance at high frequencies. Vin Vout

  15. Vin Vout Hot-carrier rectifier Schottky diodes eliminate tails at high frequencies. Vin Vout

  16. Less reverse bias (more capacitance) More reverse bias (less capacitance) Varactor symbol

  17. A varistor diode can be used to protect line-operated equipment from voltage surges.

  18. RS Power supply VS Current-regulator diode RS can vary over a wide range and the current stays the same.

  19. Other diode types • Laser: emits coherent light • Step-recovery: snaps off when reverse biased • Back: conducts better when reverse biased • Tunnel: has a negative resistance region

  20. Other diode applications • Laser: CD players, communications • Step-recovery: Frequency multipliers • Back: Small-signal rectifiers • Tunnel: High-frequency oscillators

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