Analysis of Circuit Containing Zener Diode Part 2

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Zener diode is the type of the diode that is designed for special purpose which is regulation. By the regulation we mean, maintaining constant voltages across its terminals. So Zener diode is designed in such a way that it has the ability to provide the constant voltages that is called (Zener voltages). Just like small signal diodes, Zener diode operates normally in the forward region but in the reverse region Zener goes into breakdown. Two types of breakdown occur in Zener diode:
Avalanche breakdown
Zener breakdown
Zener diodes usually provide voltage regulation from 2V to 1000v.The doping level in Zener diode is set high while designing which causes a very thin depletion region. As we know that

E= v/d

Where v is equal to applied voltages, d is the width of depletion region and E is the produced electric field. So, according to this formula the thin depletion region causes strong electric field to exist between its terminals. This strong electric field in reverse mode causes the Zener breakdown. While the avalanche breakdown happens because of the collision of high energy electrons. Diodes having breakdown voltages are said to operate in Zener breakdown region while those having breakdown voltages greater than 5v are said to operate in avalanche breakdown.

Characteristic curve
If we have the closer look of characteristic curve of Zener diode, we will notice its characteristic curve can be divided into three regions: forward region, reverse leakage region, breakdown region.

Forward Region
By the forward region we mean when the diode is forward biased. So it acts just like the normal diode causing approximately voltage drop of 0.7V. In this region it can be used as a rectifier diode.

Reverse Leakage Region
This region can be identified from 0v to knee point (before breakdown point). In this region very small current flows through the Zener diode and usually diode is not operated specifically in this region.

Breakdown Region
As mentioned in the introduction, this region is very important and the property of maintaining constant voltages is utilized in different applications. There is the specific range of the reverse current where the Zener provides the regulation i.e., from to . causes the reduction in Zener voltages while causes increase in marked as shown in the figure below.


Fig 1: characteristics curve of Zener diode
Applications of Zener diode
Regulation
Two types of the regulation that can be achieved through the zener diode are:

line regulation
load regulation
The detail of these regulations is given below.

1.1 Line Regulation
In line regulation, we have to determine the range of the input voltages for which zener will be providing constant voltages across its terminals. Almost all electronic devices of our daily use need constant supply of voltages so any change in the line voltage may cause some damage to our expensive devices. That’s here we can use zener diode. Zener provide the line regulation by controlling the current through it.

1.2 Load Regulation
By load regulation means if we change different loads whose current rating is different then how zener will maintain constant voltages. We generally do that by considering from no load (infinite resistance) to full load condition (very small resistance).

Both regulation phenomenon are explained in figure below.

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