Zener Diode. Nafees Ahamad Asstt. Prof., EECE Deptt, DIT University, Dehradun Website: By: Nafees Ahamad, EECE Deptt, DITU 1

Transcription

1 Zener Diode Nafees Ahamad Asstt. Prof., EECE Deptt, DIT University, Dehradun Website: By: Nafees Ahamad, EECE Deptt, DITU 1

2 Zener Diode A Zener diode is like an ordinary silicon PN junction diode except that it is properly doped so as to have a sharp break down voltage. A Zener diode is always reverse connected i.e. it is always reverse biased. A Zener diode has sharp reverse breakdown voltage, called Zener voltage V z. When forward biased, its characteristics are just those of ordinary diode. When a diode reaches reverse breakdown, its voltage remains almost constant even though the current changes drastically, and this is key to the Zener diode operation. 2 By: Nafees Ahamad, EECE Deptt, DITU

3 Zener Diode Actual Zener Diode OR By: Nafees Ahamad, EECE Deptt, DITU Representation 3

4 V-I Characteristic 4 By: Nafees Ahamad, EECE Deptt, DITU

5 V-I Characteristic As the reverse voltage (V R ) increases, the reverse current(i R ) remains extremely small up to the knee of the curve. Reverse current is also called Zener current(i z ). At knee point the breakdown effect begins, the internal Zener resistance (Z Z ) begins to decrease. The reverse current increase rapidly. The Zener breakdown (V Z ) voltage remains nearly constant. By: Nafees Ahamad, EECE Deptt, DITU 5

6 Zener Diode Impedance By: Nafees Ahamad, EECE Deptt, DITU 6

7 Zener Diode Impedance The zener impedance, Z Z, is the ratio of a change in voltage in the breakdown region to the corresponding change in current: What is the zener impedance if the zener diode voltage changes from 4.79 V to 4.94 V when the current changes from 5.00 ma to 10.0 ma? Z Z VZ 0.15 V I 5.0 ma Z 30 W 7 By: Nafees Ahamad, EECE Deptt, DITU

8 Zener Diode as Voltage Regulation A Zener diode can be used as voltage regulator to provide a constant voltage. Consider following circuits Voltage regulator Circuit Equivalent Voltage 8 regulator Circuit By: Nafees Ahamad, EECE Deptt, DITU

9 Zener Diode as Voltage Regulation E 0 is the desired constant output voltage. Series resistance R absorbs the output voltage fluctuations. It may be noted that Zener will maintain a constant voltage V z (=E 0 ) across the load so long as the input voltage does not fall below V z. Now two possible cases are By: Nafees Ahamad, EECE Deptt, DITU 9

10 Zener Diode as Voltage Regulation 1. input voltage increases. It is clear that output voltage V z = E 0 will remain constant. The excess voltage is dropped across the series resistance R. Total current I will increase so I z will increase but I L will be same By: Nafees Ahamad, EECE Deptt, DITU 10

11 Zener Diode as Voltage Regulation 2. Input voltage is constant but load resistance R L decreases It will increase load current I L, I z will decrease but I will be the same. Voltage drop across R, =E i E 0 Current through R, I = I z + I L So by Ohm s Law R = E i E 0 I Z +I L By: Nafees Ahamad, EECE Deptt, DITU 11

12 Zener Diode as Voltage Regulation It maintains constant voltage over a range of reverse current values. A minimum reverse current I ZK must be maintained in order to keep diode in regulation mode. Voltage decreases drastically if the current is reduced below the knee of the curve. Above I ZM, max current, the Zener may get damaged permanently. 12 By: Nafees Ahamad, EECE Deptt, DITU

13 ON & OFF States of Zener Diode If voltage across Zener diode V z If voltage across Zener Diode < V z => ON state => OFF state By: Nafees Ahamad, EECE Deptt, DITU 13

14 Solving Zener Diode Circuits The first step is to determine whether the Zener is in the on state or off state. Next, the Zener is replaced by its appropriate model and unknown quantities are determined. By: Nafees Ahamad, EECE Deptt, DITU 14

15 Solving Zener Diode Circuits 1. E i and R L fixed. The first step is to find the state (ON or OFF) of Zener diode for this remove Zener diode and find open circuit voltage By: Nafees Ahamad, EECE Deptt, DITU 15

16 Solving Zener Diode Circuits If V Vz On state If V < Vz Off state (i) On state. Referring to circuit shown in Fig. By: Nafees Ahamad, EECE Deptt, DITU 16

17 Solving Zener Diode Circuits (ii) Off state. Referring to the circuit shown in Fig By: Nafees Ahamad, EECE Deptt, DITU 17

18 Solving Zener Diode Circuits 2. Fixed E i and Variable R L : Note that there is a definite range of R L values (and hence I L values) which will ensure the Zener diode to be in on state. By: Nafees Ahamad, EECE Deptt, DITU 18

19 Solving Zener Diode Circuits (i) R Lmin and I Lmax : By: Nafees Ahamad, EECE Deptt, DITU 19

20 Solving Zener Diode Circuits (ii) R Lmax and I Lmin : If the load resistance exceeds this limiting value, the current through Zener will exceed I ZM and the device may burn out. By: Nafees Ahamad, EECE Deptt, DITU 20

21 Solving Zener Diode Circuits 3. Fixed R L and Variable E i : There is a definite range of E i values that will ensure that Zener diode is in the on state. By: Nafees Ahamad, EECE Deptt, DITU 21

22 Solving Zener Diode Circuits (i) E i (min): By: Nafees Ahamad, EECE Deptt, DITU 22

23 Solving Zener Diode Circuits (ii) E i (max): By: Nafees Ahamad, EECE Deptt, DITU 23

24 Questions: Book: Principles of electronics by V.K Mehta S. Chand & Company Pvt. Ltd. Q1 For the circuit shown in Fig., find : (i) the output voltage (ii) the voltage drop across series resistance (iii) the current through Zener diode. By: Nafees Ahamad, EECE Deptt, DITU 24

25 Question 1 Solution: Remove the Zener diode and find out voltage across its ends V>V z (=50) V, So Zener will be in ON condition By: Nafees Ahamad, EECE Deptt, DITU 25

26 Question 1 Now replace Zener diode with its equivalent circuit i.e. 50 V battery By: Nafees Ahamad, EECE Deptt, DITU 26

27 Question 1 (i) O/P voltage (ii) Voltage across R = Vz = 50 Volts = 120- V z = = 70 Volts (iii) I L = V z /R L = 50/10000 = 5 ma I = Voltage across R /R = 70/5000 = 14 ma So by KCL I = I z + I L => I z =I I L = 14-5 = 9 ma By: Nafees Ahamad, EECE Deptt, DITU 27

28 Question 2 Q2. A 7.2 V Zener is used in the circuit shown in Fig.and the load current is to vary from 12 to 100 ma. Find the value of series resistance R to maintain a voltage of 7.2 V across the load. The input voltage is constant at 12V and the minimum Zener current is 10 ma. By: Nafees Ahamad, EECE Deptt, DITU 28

29 Question 2 The voltage across R is to remain constant at =4.8 V as the load changes from 12 to 100 ma. The minimum Zener current will occur when the load current is maximum. So By: Nafees Ahamad, EECE Deptt, DITU 29

30 Question 3 Q3. The circuit of Fig. uses two Zener diodes, each rated at 15 V, 200 ma. If the circuit is connected to a 45-volt unregulated supply, determine : (i) The regulated output voltage (ii) The value of series resistance R By: Nafees Ahamad, EECE Deptt, DITU 30

31 Question 3 When the desired regulated output voltage is higher than the rated voltage of the Zener, two or more Zeners are connected in series as shown in above Fig. However, in such circuits, care must be taken to select those Zeners that have the same current rating. By: Nafees Ahamad, EECE Deptt, DITU 31

32 Question 4 Q4. Over what range of input voltage will the Zener circuit shown in Fig. maintain 30 V across 2000 Ω load, assuming that series resistance R = 200 Ω and Zener current rating is 25 ma? Ans: 33 V to 38 V By: Nafees Ahamad, EECE Deptt, DITU 32

33 Thank you? By: Nafees Ahamad, EECE Deptt, DITU 33