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Chapter 2

Chapter 2. Diodes. Diode. Diode - A two-electrode (two-terminal) device that acts as a one-way conductor. pn-junction diode – A pn junction with a lead connected to each of its semiconductor materials. Forward Biased pn-Junction Diodes.

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Chapter 2

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  1. Chapter 2 Diodes

  2. Diode • Diode - A two-electrode (two-terminal) device that acts as a one-way conductor. • pn-junction diode – A pn junction with a lead connected to each of its semiconductor materials.

  3. Forward Biased pn-Junction Diodes • The component is biased so that the cathode is more negative than the anode. • The voltage across the diode exceeds its barrier potential. • The diode conducts fully when VF is approximately 0.7 V (for silicon) or 0.3 V (for germanium). • The value of IF depends on the circuit voltage and resistance values.

  4. Reverse Biased pn-Junction Diodes • The component is biased so that the cathode is more positive than the anode. • The voltage across the diode is approximately equal to the applied voltage (V ). • The diode current is approximately 0 A (as indicated by the ammeter).

  5. Diode Models • Model – A representation of a component or circuit that demonstrates one (or more) of its characteristics. • There are three diode models: • The ideal model • The practical model • The complete model

  6. Ideal Diode Model • This model represents the diode as a simple switch that is either open (not conducting) or closed (conducting). • Used in the initial analysis and troubleshooting of a diode circuit.

  7. Practical Diode Model • This model includes the diode characteristics that are considered when: • Performing a mathematical analysis of a diode circuit. • Determining whether a given diode can be used in a given circuit or application.

  8. Complete Diode Model • This model is the most accurate of the diode models. • The complete diode model includes several diode characteristics that are generally considered as a part of: • Circuit development (engineering) • Special-case circuit analysis • Explaining the differences between predicted and measured circuit values.

  9. Ideal Diode Characteristics • When reverse biased (open switch), the diode: • Has infinite resistance. • Blocks current. • Drops the applied voltage across its terminals. • When forward biased (closed switch), the diode: • Has no resistance. • Does not limit current. • Has no voltage drop across its terminals.

  10. Forward Voltage (VF) • Knee Voltage (VK) – The voltage at which device current suddenly increases or decreases. • The practical diode model assumes that IF = 0 A until VF = VK. Once the diode begins to conduct, VF 0.7 V regardless of the value of IF.

  11. VF and Circuit Analysis

  12. Percentage of Error • Indicates the difference between a calculated value and its corresponding measured value.

  13. Peak Reverse Voltage (VRRM ) • VRRM is the maximum reverse voltage that won’t force a pn junction to conduct. • When VR > VRRM , diode reverse current (IR) increases rapidly as the depletion layer breaks down. • VRRM is a diode parameter that can be found on the component spec sheet.

  14. Forward Current and Power • Average Forward Current (I0 ) – Themaximum allowable value of dc forward current for a diode. • Forward Power Dissipation ( PD(max) ) – The maximum possible power dissipation of the forward-biased diode. • I0 and PD(max) are diode parameters that are listed on the component spec sheet.

  15. Component Substitution • A substitute diode can be used in a circuit if: • Its VRRM rating is at least 20% greater than the maximum reverse voltage produced by the circuit. • Its I0 rating is at least 20% greater than the average (dc) value of IF generated by the circuit. • Its PD(max) rating is at least 20% greater than the value of PF required by the circuit.

  16. Complete-Model Diode Curve

  17. Bulk Resistance ( RB ) • RB causes VF to increase when IF increases. • DVF in the complete diode curve is produced by IFRB.

  18. Diode Reverse Current (IR ) The low-level current through a reverse biased pn junction, made up of: • Reverse saturation current (IS) – A current caused by thermal activity in a reverse biased diode. • Surface leakage current (ISL) – A current along the surface of a reverse-biased diode.

  19. Temperature Effects on IF and VF • When temperature increases: • IF increases (at a specified value of VF) • VF decreases (at a specified value of IF)

  20. Temperature Effects on IR • IR increases with increases in temperature.

  21. Zener Diode • Zener diode – A diode designed to operate in the reverse breakdown region of its operating curve. • A large change in zener current (DIZ) produces a relatively small change in zener voltage (DVZ).

  22. Zener Current

  23. Zener Current Ratings

  24. Zener Impedance (ZZ ) • The zener diode’s opposition to any change in current.

  25. Zener Equivalent Circuits

  26. Light-Emitting Diode (LED) • LED – A diode that emits light when properly biased.

  27. Series Current-limiting Resistor (RS )

  28. Diode Resistance Test

  29. Meter Diode Test Function

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