1. ZENER DIODES 1Department of ECE / EEE

2. Zener diode zA 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. 2Department of ECE / EEE

3. Zener diode 3Department of ECE / EEE

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

5. VOLTAGE REGULATION zA 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 5Department of ECE / EEE

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

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

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

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

10. ZENER DIODE RATINGS zA zener spec sheet typically provides - the maximum power rating Pz - the nominal zener voltage Vz at test current Iz T - the maximum DC zener current Iz M 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. 10Department of ECE / EEE

11. ZENER DIODE CIRCUIT EXAMPLE zIn a zener diode voltage regulator circuit, V DC(IN) = 20V, Vz = 9V, Rs = 82 , R L = 100  Q: Calculate I L. A: I L = V L /R L = V Z /R L = 9V/100  = 90 mA 20V 82  9V 100  11Department of ECE / EEE

12. ZENER DIODE EXAMPLE (CONT’D) Q: Calculate I T. A: I T = V Rs /R s = [V DC(IN) - Vz]/R s = [20 - 9]/82  = 134 mA 20V 82  100  9V 12Department of ECE / EEE

13. ZENER DIODE EXAMPLE (CONT’D) Q: Calculate Iz A: Iz = I T - I L = 134mA - 90mA = 44mA 20V 82  9V 100  13Department of ECE / EEE

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

15. ZENER DIODE EXAMPLE (CONT’D) Q: Suppose load changes from 100  to 200 . Find new I L, Iz, I T. Is Iz safe ? A: I L = V L /R L = V Z /R L = 9V/200  = 45 mA 200  9V20V 15Department of ECE / EEE

16. ZENER DIODE EXAMPLE (CONT’D) A: I T = V Rs /R s = [V DC(IN) - Vz]/R s = [20 - 9]/82  = 134 mA (no change) Iz = I T - I L = 134mA - 45mA = 89mA Since Iz M = 55mA, zener is not safe. 9V 200  20V 16Department of ECE / EEE

17. ZENER DIODE EXAMPLE (CONT’D) Q: Suppose load changes from 100  to 80 . Find new I L, Iz, I T. Is Iz safe ? A: I L = V L /R L = V Z /R L = 9V/80  = 112.5 mA 9V 80  20V 17Department of ECE / EEE

18. ZENER DIODE EXAMPLE (CONT’D) A: I T = V Rs /R s = [V DC(IN) - Vz]/R s = [20 - 9]/82  = 134 mA (no change) Iz = I T - I L = 134mA - 112.5mA = 21.5 mA Since Iz M = 55 mA, zener is safe. 9V 80  20V 18Department of ECE / EEE

19. ZENER DIODE EXAMPLE (CONT’D) Q: Suppose zener regulator circuit has no load connected, is Iz safe ? A: I T = V Rs /R s = [V DC(IN) - Vz]/R s = [20 - 9]/82  = 134 mA (no change) Iz = I T = 134 mA this is very Unsafe 9V 20V 19Department of ECE / EEE

20. ZENER DIODE EXAMPLE (CONT’D) Q: In the previous example, suppose V DC(IN) changes from 20V to 30V. Is Iz safe ? A: I L = 9V/100  = 90 mA I T = [30-9]/82  = 256 mA Iz = I T - I L = 256 mA - 90 mA = 166 mA Very Unsafe 30V9V 82  100  20Department of ECE / EEE

21. ZENER DIODE EXAMPLE (CONT’D) Q: Suppose V DC(IN) changes from 20V to 18V. Is Iz safe ? A: I L = 9V/100  = 90 mA I T = [18-9]/82  = 110 mA Iz = I T - I L = 110 mA - 90 mA = 20 mA Safe 18V9V 100  82  21Department of ECE / EEE

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