Use of Zener Diode as Voltage Regulator | breakdown

We have learned about the Semiconductor materials and some semiconductor devices like p-n junction diode, Bipolar junction Transistor (BJT) etc. There are variety of semiconductor diodes and they have different purpose. Sometime, we need a device which can give a constant output voltage at any value of input. This will protect the devices from the fluctuating voltage supply. Zener diode is that kind of device. In this article, we are going to discuss the basic things on Zener diode and its use as a Voltage regulator.

Contents in this article:

  • Zener diode
  • Breakdown of diode
  • Avalanche Breakdown
  • Zener Breakdown
  • Differences between Avalanche and Zener breakdown
  • Symbol of Zener diode
  • I-V curve of Zener Diode
  • Use of Zener diode as Voltage Regulator
  • Load Regulation
  • Line Regulation

Zener Diode:

Zener diode is a special type of p-n junction diode with high doping concentration to operate at breakdown region. It is used as a voltage regulator to fix the output voltage at a constant value across an electronic circuit. Due to high doping, width of depletion layer of Zener diode is very small.

 Normal diodes cannot tolerate high reverse bias voltage. But Zener diode has high power dissipation capability to handle high current both in forward and reverse bias.

Breakdown of Diode:

In reverse bias, a very small current flows through a diode due to minority carriers. This reverse bias current is almost independent of reverse bias voltage. But, if we increase the reverse bias voltage, after a certain value of the reverse voltage the reverse current suddenly increases to a very high value. This exponential growth of the reverse current is called the Breakdown of the diode and the value of reverse bias voltage at which the breakdown occurs is known as the Breakdown Voltage.

  There are two types of breakdown of diode- Avalanche Breakdown and Zener Breakdown.

Avalanche Breakdown:

When a high reverse bias voltage is applied across a lightly doped p-n junction diode, the minority carriers flow with high kinetic energy and collide with crystal atoms. This breaks the covalent bonds of semiconductor atoms and a pair of free electron produces from each covalent bond. These free electrons will have sufficiently high kinetic energy to break other covalent bonds. In this cumulative process a large number of carriers appear even in reverse bias condition. This phenomenon of breakdown of diode is called Avalanche Breakdown.

Zener Breakdown:

For a highly doped p-n junction diode, the width of depletion layer becomes very small. Therefore, even at small reverse bias voltage the electric filed across the junction becomes so strong that the electric field can tear out the electrons from covalent bonds by applying strong coulomb force. In this way large number of carriers produce in the crystal and the reverse current through the junction becomes very high. This process of breakdown of diode is called Zener Breakdown.

Doping concentration in Zener Diode is very high and it attains the breakdown region at a very small reverse bias voltage. The breakdown of a Zener diode is the Zener breakdown.

Differences between Avalanche breakdown and Zener breakdown:

Normal Diodes cannot work in the breakdown region of diode. But Zener diode is a special type of diode that can work in breakdown region. Zener diode can handle relatively high reverse bias voltage and high temperature. Also, Zener diode can work similarly as normal p-n junction diodes in forward bias condition.

Symbol of Zener diode:

The symbol of the Zener diode is as below.

symbol of Zener diode
symbol of Zener diode

I-V characteristics of Zener diode:

In Current vs Voltage characteristics of Zener diode one can see that forward bias condition is same as that of a normal diode. But in reverse bias, Current increases exponentially after the Zener breakdown voltage (Vz).

I-V characteristics curve of Zener diode
I-V characteristics curve of Zener diode

Uses of Zener diode:

The main purpose of Zener diode is to fix the output voltage at a constant value. That means a Zener diode works as Voltage regulator in electronic circuit.

Use of Zener diode as Voltage regulator:

Voltage regulator is a device or combination of circuit elements which can fix the output voltage of a supply at a particular constant value.

Zener diode is a special type of diode to operate in breakdown region of the diode. The voltage across the Zener diode in breakdown region becomes almost independent of the current through it. This makes Zener diode useful as a voltage regulator.

The circuit diagram of a voltage regulator using Zener diode is as below. Here, Vi is the input voltage, RL is the load resistance, Vz is the Zener breakdown voltage of the diode and Ri is the current limiting resistance.

Zener diode Voltage regulator circuit
Zener diode Voltage regulator circuit

If Ii, Iz and IL be the input current, Zener current and load current respectively, then

Ii = Iz+ IL …..….. (1) and output voltage, Vo = ILRL = Vz  ………….. (2)

There are two ways to use Zener diode as a voltage regulator, these are

  • Load Regulation
  • Line Regulation

Load Regulation using Zener Diode:

If the output voltage is regulated by varying the load resistance RL , keeping the input power supply voltage (Vi)constant, then the regulation is called the Load regulation.

Since RL is varying, then load current (IL ) also varying. Again, the input current Ii is constant as the input voltage Vi is not varying. Then from equation (1) we observe that Zener current balances any change of load current. If Load current increases, Zener current decreases and vice-versa. Since, output voltage Vz is independent of Zener current then output still remains constant at Vz.

Line Regulation using Zener Diode:

If the output voltage is regulated by varying the input voltage Vi, keeping the load resistance RL constant, then the regulation is called the Line regulation.

Since RL is constant, then load current IL also remains same. Now, input current Ii varies due to variation of input voltage Vi. Then from equation (1) we can see that any change of input current is balanced by Zener current. Again, Zener voltage is almost independent of Zener voltage then output remains same at Vz.

Note: To use Zener diode as a voltage regulator it is necessary to have Zener voltage (Vz) less than the equivalent input voltage. Also, the Zener current should not exceed the maximum rating.

This is all from the use of Zener diode as voltage regulator. If you have any doubt or query on this topic, feel free to ask me in the comment section.

Thank You.

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