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ZENER DIODES

ZENER DIODES. Zener diode.

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ZENER DIODES

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  1. ZENER DIODES Department of ECE / EEE

  2. Zener diode • A Zener diode is a type of diode that permits current not only in the forward direction like a normal diode, but also in the reverse direction if the voltage is larger than the breakdown voltage known as "Zener knee voltage" or "Zener voltage". The device was named after Clarence Zener, who discovered this electrical property. Department of ECE / EEE

  3. Zener diode Department of ECE / EEE

  4. ZENER DIODES • The simplest of all voltage regulators is the zener diode voltage regulator. • A zener diode is a special diode that is optimized for operation in the breakdown region. Department of ECE / EEE

  5. VOLTAGE REGULATION • A voltage regulator circuit automatically maintains the output voltage of a power supply constant, regardless of - a change in the load - a change in the source voltage Department of ECE / EEE

  6. ZENER DIODE CIRCUIT The zener diode is typically connected reverse biased, in parallel with the load. Resistor Rs limits current to zener. Department of ECE / EEE

  7. ZENER DIODE CHARACTERISTICS • In the forward region, the zener diode acts like a regular silicon diode, with a 0.7 volt drop when it conducts. Department of ECE / EEE

  8. ZENER DIODE CHARACTERISTICS • In the reverse bias region, a reverse leakage current flows until the breakdown voltage is reached. • At this point, the reverse current, called zener current Iz, increases sharply. Department of ECE / EEE

  9. ZENER DIODE CHARACTERISTICS • Voltage after breakdown is also called zener voltage Vz. • Vz remains nearly a constant, even though current Iz varies considerably. Department of ECE / EEE

  10. ZENER DIODE RATINGS • A zener spec sheet typically provides - the maximum power rating Pz - the nominal zener voltage Vz at test current IzT - the maximum DC zener current IzM Example: - 1N752 has a power rating of 500mW, a nominal zener voltage of 5.6V at a test current of 20 mA, a maximum zener current of 80 mA. Department of ECE / EEE

  11. ZENER DIODE CIRCUIT EXAMPLE • In a zener diode voltage regulator circuit, VDC(IN)= 20V, Vz = 9V, Rs = 82, RL = 100 Q:Calculate IL. A:IL = VL/RL = VZ/RL = 9V/100  = 90 mA 82 100 20V 9V Department of ECE / EEE

  12. ZENER DIODE EXAMPLE (CONT’D) Q:Calculate IT. A:IT = VRs/Rs = [VDC(IN) - Vz]/Rs = [20 - 9]/82  = 134 mA 82 100 20V 9V Department of ECE / EEE

  13. ZENER DIODE EXAMPLE (CONT’D) Q: Calculate Iz A: Iz = IT - IL = 134mA - 90mA = 44mA 82 100 20V 9V Department of ECE / EEE

  14. ZENER DIODE EXAMPLE (CONT’D) Q:If zener Pz = 500mW, is Iz safe ? A:IzM = Pz / Vz = 500 mW / 9V = 55 mA Since circuit Iz = 44 mA (< 55mA), zener is safe Department of ECE / EEE

  15. ZENER DIODE EXAMPLE (CONT’D) Q: Suppose load changes from 100  to 200 . Find new IL, Iz, IT. Is Iz safe ? A: IL = VL/RL = VZ/RL = 9V/200  = 45 mA 20V 9V 200 Department of ECE / EEE

  16. ZENER DIODE EXAMPLE (CONT’D) A: IT = VRs/Rs = [VDC(IN) - Vz]/Rs = [20 - 9]/82  = 134 mA (no change) Iz = IT - IL = 134mA - 45mA = 89mA Since IzM = 55mA, zener is not safe. 200 20V 9V Department of ECE / EEE

  17. ZENER DIODE EXAMPLE (CONT’D) Q: Suppose load changes from 100  to 80 . Find new IL, Iz, IT. Is Iz safe ? A: IL = VL/RL = VZ/RL = 9V/80  = 112.5 mA 20V 9V 80 Department of ECE / EEE

  18. ZENER DIODE EXAMPLE (CONT’D) A:IT = VRs/Rs = [VDC(IN) - Vz]/Rs = [20 - 9]/82  = 134 mA (no change) Iz = IT - IL = 134mA - 112.5mA = 21.5 mA Since IzM = 55 mA, zener is safe. 80 20V 9V Department of ECE / EEE

  19. ZENER DIODE EXAMPLE (CONT’D) Q:Suppose zener regulator circuit has no load connected, is Iz safe ? A:IT = VRs/Rs = [VDC(IN) - Vz]/Rs = [20 - 9]/82  = 134 mA (no change) Iz = IT = 134 mA this is very Unsafe 20V 9V Department of ECE / EEE

  20. ZENER DIODE EXAMPLE (CONT’D) Q: In the previous example, suppose VDC(IN)changes from 20V to 30V. Is Iz safe ? A:IL = 9V/100  = 90 mA IT = [30-9]/82  =256 mAIz = IT - IL = 256 mA - 90 mA = 166 mA Very Unsafe 82 30V 9V 100 Department of ECE / EEE

  21. ZENER DIODE EXAMPLE (CONT’D) Q: Suppose VDC(IN) changes from 20V to 18V. Is Iz safe ? A:IL = 9V/100  = 90 mA IT = [18-9]/82  =110 mAIz = IT - IL = 110 mA - 90 mA = 20 mA Safe 82 18V 9V 100 Department of ECE / EEE

  22. The End … Thank You … Department of ECE / EEE

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