1. Special Purpose Diodes -ZENER DIODE-
UNIVERSITI MALAYSIA PERLIS
SCHOOL OF ELECTRICAL SYSTEM ENGINEERING
Chapter 3
Special Purpose Diodes -ZENER DIODE-
ELECTRONIC DEVICES EET PERANTI ELEKTRONIK

2. OBJECTIVE
Describe the characteristic of zener diode and analyze its operation.
Explain how a zener diode is used in voltage reference, regulation and limiting applications.
Troubleshoot zener diode regulators.

3. CONTENT
Zener diode
Zener diode applications
Troubleshooting
Summary

4. 3.1 INTRODUCTION
Zener diode – silicon pn-junction device that is designed for operation in the reverse-breakdown region.
The basic function of zener diode is to maintain a nearly constant dc voltage under proper operating condition.
Typically it is used to provide a stable reference voltage for use in power supplies, voltmeter and other equipment.
Zener diode symbol

5. 3.1 INTRODUCTION
Breakdown voltage – set by controlling the doping level during manufacture.
When diode reached reverse breakdown – voltage remains constant even though current change drastically.
If zener diode is FB – operates the same as a normal rectifier diode (0.7 V).
If zener diode is RB – operates in VZ (refer datasheet).
A zener diode is much like a normal diode – but it is placed in the circuit in reverse bias and operates in reverse breakdown.

6. General diode vs. Zener diode V-I characteristic
OPEARATING REGION
General diode vs. Zener diode V-I characteristic

7. REVERSE BREAKDOWN
Two types of reverse breakdown: avalanche breakdown & zener breakdown
i) avalanche breakdown
occurs in both rectifier & zener diodes at high reverse voltage.
breakdown voltage greater than approximately 5V.
ii) zener breakdown
occurs at relatively low reverse voltage.
a zener diode is heavily doped to reduce breakdown voltage.
this causes very thin depletion region.
as a result, intense electric field exists within the depletion region.
Near zener breakdown voltage (Vz), the field have enough energy to pull electrons from their valence bands and create current.
Note: both type called zener diode (breakdown voltages of 1.8V – 200 V)

8. ZENER DIODE CHARACTERISTIC CURVE

9. BREAKDOWN CHARACTERISTIC
The reverse voltage (VR) is increased – the reverse current (IR) remains extremely small up to the “knee” of the curve.
Reverse current – called the zener current, IZ.
At that point, breakdown effect begin where zener impedance (Zz) begin to decrease as IZ increases rapidly.
At the bottom of the knee- the zener breakdown voltage (VZ) remains constant although it increase slightly as the zener current, IZ increase.
IZK – min. current required to maintain voltage regulation
IZM – max. amount of current the diode can handle without being damage/destroyed
IZT – the current level at which the VZ rating of diode is measured (specified on a data sheet)
The zener diode maintains nearly constant voltage for value of reverse current rating from IZK to IZM

10. ZENER EQUIVALENT CIRCUIT (Ideal Model)
NO zener impedance (ZZ = 0)

11. ZENER EQUIVALENT CIRCUIT (Practical Model)
Has zener impedance (ZZ)

12. ZENER IMPEDANCE (ZZ)
Since the actual voltage is not ideally vertical, the change in zener current produces a small change in zener voltage
By ohm’s law:
Normally Zz is specified at IZT.
In most cases, assume Zz is constant over full range of zener current values and is purely resistive.

13. Example 1
A Zener diode exhibits a certain change in VZ for a certain change in IZ on a portion of the linear characteristic curve between IZK and IZM as illustrated in Figure below. What is Zener impedance?

15. Exercise 1
Calculate the Zener Impedance.
Ans:2Ω

16. ZENER DIODE DATASHEET INFORMATION
The typical data sheet (refer Figure 3-7) is for a zener series, the 1N4728A – 1N4764A.

18. Absolute Maximum Ratings
The maximum power dissipation, PD is specified as 1.0W up to 50°C.
The power dissipation is derated (on datasheet) at 6.67mV for each degree above 50°C.
Maximum reverse current, IZM is not specified on datasheet but can be determined from maximum power dissipation for a given value of VZ.

19. Example 2
From the datasheet, a 1N4736A zener diode has ZZ of 3.5Ω. The datasheet gives Vz=6.8V at a test current Iz of 37mA. What is the voltage across zener terminals when:
The current is 50mA
The current is 25mA

20. When the current is 50mA
For IZ=50mA: The 50mA current is 13mA increase above the test current, IZ of 37mA
The change in voltage due to the increase in current above the IZ value causes the zener terminal voltage to increase

21. (b) When current is 25mA For IZ=25mA: The 25mA current is 12mA decrease below the test current, IZ of 37mA The change in voltage due to the decrease in current below the IZT value causes the zener terminal voltage to decrease.

22. Exercise 2
By referring to datasheet, calculate the voltage across zener terminal for a 1N4742A zener diode when:
The current is 10mA
The current is 30mA
Ans:
(a) Vz(min)=11.901V
(b) Vz(max)=12.081V

23. 3.2 ZENER DIODE APPLICATIONS
Zener diode can be used as:
Voltage regulator to provide stable reference voltage.
(a). Zener regulation with a variable input voltage.
# Ideal model
# Practical model
(b). Zener regulation with a variable load.
Simple limiter or clipper.

24. 3.2(a). ZENER REGULATION WITH A VARYING INPUT VOLTAGE
As i/p voltage varies (within limits) – zener diode maintains a constant o/p voltage
As VIN changes, IZ will change proportionally, so i/p voltage variations are set by the min. & max. current value (IZK & IZM) with which the zener can operate.
Resistor, R –current limiting resistor
V in(min) = ?
V in(max) = ?

25. Example 3 (Ideal Zener) 1N4740 10 V zener diode
Example 3 (Ideal Zener) 1N V zener diode. Find the Min and Max Input Voltage that can regulate the zener diode to maintain an approximate 10 V, Note: Zener impedance neglected (Ideal Model).
i/p vary from
? V – ? V
Maintain o/p voltage

26. Minimum current Izk
Maximum current Izm

27. Based on Data Sheet

28. Solution Example 3
From data sheet, 1N V zener diode maintain regulation from IZK=0.25mA to IZM=100mA.
So,
For min. zener current, the voltage across the 220 ohm resistor:
Since VR = VIN - VZ,
For max. zener current, the voltage across resistor is:
Thus

29. Example 4 (Practical zener)
Determine the minimum and maximum input voltage that can be regulated by the zener diode in Figure below.

30. ANS. Example 4
From the datasheet (IN4733A): VZ=5.1V, IZ=49mA ZZ=7Ω IZK=1mA Assume ZZ over the range of current values. The equivalent circuit: -Vin+VR+Vout=0 Vin = VR+Vout

31. The maximum zener current, IZM For minimum Input voltage, Vin(min):

32. For maximum Input voltage, Vin(max):

33. Exercise 3
Determine the minimum and maximum input voltage that can be regulated if1N4736A zener diode is used in Figure below.
Ans:
Vin(min)=6.774V
Vin(max)= V

34. 3.2(b). ZENER REGULATION WITH A VARIABLE LOAD
In this illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages and loads (RL) to be able to maintain its designated zener voltage.
Zener current will increase or decrease directly with voltage input changes. The zener current will increase or decrease inversely with varying loads. Again, the zener has a finite range of operation.
Note: The zener diode maintains a nearly constant voltage across RL as
long as the zener current is greater than IZK and less than IZM
RL(min) = ?
IL(min) = ?
IL(max) = ?

35. FROM NO LOAD (IL(min), RL(max) ) TO FULL LOAD (IL(max), RL(min))
When output terminal of zener diode is open (RL= ), the load current (IL) is zero.
All of the current is through the zener =>no-load condition.
When RL is connected, current is flow through zener and RL.
The total current through R is constant as long as zener is regulating.
As RL decreases, load current IL increases, IZ decreases.
Zener diode continues to regulate the voltage until IZ reaches its minimum value, IZK.
At this point, IL is maximum and full-load condition exists.
When VZ = VZ(min) = VFL
When VZ = VZ(max) = VNL

36. First step.. The change in zener voltage, The change in zener current,
Zener voltage for min and max values are:
Total current:
Maximum and minimum load current:

37. Example 5 (Ideal Zener)
Determine the minimum and maximum load current for which the zener diode will maintain regulation. What is the minimum RL that can be used? VZ=12V, IZK=1mA and IZM=50mA. Assume ZZ=0 Ω and VZ remains a constant 12V over range of current values.

38. Solution Example 5
Step 1: When IL=0A (RL= ), IZ is maximum and equal to the total current, IT.
Step 2: IZ(max) is less than IZM, so 0A is min. value for IL because zener can handle all of 25.5mA. RL can be removed from the circuit. IL(min) = 0A
Step 3: The max value of IL occurs when IZ is min (IZ=IZK)
Step 4: The minimum value of RL is:

39. Example 6 (Practical Zener)
Figure below shows a 1N4733 zener regulated circuit. Given VZ = 10 V at IZT = 10mA,IZK = 5mA, ZZ = 20  and IZM = 20mA. Determine the maximum permissible load currents and minimum value of RL.

40. Example 6..
Zener diode connected in parallel with load resistor, RL. For parallel circuit, V same but I is different.
A certain amount of I from source will flow through zener diode and RL. If I through zener diode is max, I flow through RL is min. If I through zener diode is min, I through RL is max. V for both zener and RL is the SAME.
So, when current through zener diode, IZ is min, VZ also min. On the other hand, IL will be max as well as VL max.
Now, find VZ(min) from the formula.

41. Solution Example 6 The change in zener current is: Find VZ(min):
So, VZ(min)=9.9V. Then, we need to find total current through the circuit, IZ(min). Using the formula,
Finally find IL(max) using the formula below. Given in the question, IZK=5mA. So, IL(max) is

42. When RL=RL(min), IL=IL(max), IZ=IZK, VZ=VZ(min).
Thus:

43. Example 7 (Practical Zener)
For the circuit below:
Determine VOUT at IZK and IZM.
Determine the percent of load regulation.
Calculate the value of R that should be used.
Determine the minimum value of RL that can be used.

44. From datasheet, Vz=15V IZ=17mA ZZ=14Ω IZK=0
From datasheet, Vz=15V IZ=17mA ZZ=14Ω IZK=0.25mA (a) Determine VOUT at IZK and IZM. For IZK: For IZM:

45. (b) Determine the percent of load regulation.
(c) Calculate the value of R that should be used.
To simplified, RL= ∞, IL=0, IZ=IZM=IT

46. When RL=RL(min), IL=IL(max), IZ=IZK, VZ=VZ(min)
(d). Determine the minimum value of RL that can be used.
When RL=RL(min), IL=IL(max), IZ=IZK, VZ=VZ(min)

47. Exercise 4 (practical Zener)
For the circuit below:
Determine the minimum and maximum load current for which the zener diode can maintain regulation
What is the minimum value of RL that can be used?
Ans:
IL(min) = 0 A is acceptable
IL(max) =72.25mA
RLmin=163.5Ω

48. Exercise 5 (Practical Zener)
Determine the minimum and maximum permissible load currents for a loaded zener regulator as in Figure below. Given VZ=5.1V at IZ=49mA, IZK=1mA, ZZ=7Ω and IZM=70mA.
Ans:
IL(min) = 0 A is acceptable
IL(max) =79.9mA

49. (3.3) ZENER LIMITING
Zener diode also can be used in ac applications to limit voltage swings to desired level
a) To limit the +ve peak of a signal voltage to the selected zener voltage
During –ve alternation, zener acts as FB diode & limits the –ve voltage to -0.7V
b) Zener diode is turn around
The –ve peak is by FB zener action & +ve voltage is limited to +0.7V
c) Two back-to-back zeners limit both peaks to the zener voltage ±0.7V
During the +ve alternation, D2 is functioning as the zener limiter – D1 is functioning as a FB diode.
During the –ve alternation-the roles are reversed

50. Basic zener limiting action with a sinusoidal input voltage

51. Example 8
Determine the output voltage for each limiting circuit shown below.

52. Solution example 8
When one zener is operating in breakdown, the other one is FB with approximately 0.7V across it. For figure a, during +ve cycle, D1(3.3V) FB while D2(5.1V) is zener limiter. For figure b, D1(6.2V) is zener limiter and D2(15V) is FB during +ve cycle.

53. OTHER TYPES OF DIODE Optical Diode Current Regulator Diode
Schottky Diode
PIN Diode
Step-recovery Diode
Tunnel Diode
Laser Diode

54. (3.4) TROUBLESHOOT
Although precise power supplies typically use IC type regulators, zener diodes can be used alone as a voltage regulator. As with all troubleshooting techniques we must know what is normal.
A properly functioning zener will work to maintain the output voltage within certain limits despite changes in load.

55. TROUBLESHOOT
With an open zener diode, the full unregulated voltage will be present at the output without a load. In some cases with full or partial loading an open zener could remain undetected.
no voltage dropped between
the filtered o/p of the power
supply & the o/p terminal
undetected

56. TROUBLESHOOT
With excessive zener impedance the voltage would be higher than normal but less than the full unregulated output.
The zener has failed such that its internal impedance is more than it should be.

57. SUMMARY The zener diode operates in reverse breakdown.
A zener diode maintains a nearly constant voltage across its terminals over a specified range of currents.
Line regulation is the maintenance of a specific voltage with changing input voltages.
Load regulation is the maintenance of a specific voltage for different loads.