Analog systems and applications — lecture-XII.

Zener diode and voltage regulation, Lecture-XII.

This article belongs to a series of lectures on analog electronics, the paper goes by the name “Analog Systems and Applications” for the physics honors degree class. All lectures of this series will be found here. This is the 12th lecture of the series. The lecture was delivered on 13th February 2018. The present web version has many new additional concepts though compared to the offline hand made notes.

Today we will discuss the Zener diode. Zener diodes get applications in various facets of electronics and electrical circuits. But we will focus on the most basic application of the same in voltage regulation.

Diode IV characteristics.

In our lecture-VII we discussed the current-voltage (IV) characteristics of the diodes, under forward and reverse bias, in detail. Lets recapitulate. The IV characteristics looked like how its depicted in the following diagram.

IV characteristics of a pn junction diode in forward and reverse bias conditions.

IV characteristics of a pn junction diode in forward and reverse bias conditions.
IV characteristics of a pn junction diode in forward and reverse bias conditions.

We see that at the Zener level (shown by arrow with reference to reverse breakdown voltage in the diagram, above) the resistance of the diode falls suddenly to a small value and reverse current gains by a large amount. This may produce a large amount of heat and damage the diode. But there are a class of diode which are designed so as to operate in the reverse breakdown mode and they are known as ZENER diode.

Diodes as voltage regulators.

We see that the voltage across such diodes remains practically constant at this breakdown voltage. This enables such diodes to act as voltage regulators in electrical circuits. How much reverse current can flow in the diode is determined by the power rating (maximum power) and Zener breakdown voltage (Vz) of a given diode.

The breakdown voltage on the other hand is determined by the properties of the p and n regions of the diode, at the junction (depletion or space-charge layer) viz doping. A highly doped material would have a lower breakdown voltage and vice versa. The Zener region of any IV characteristics curve can therefore be controlled by appropriate doping. The Zener voltage can range from anywhere between less than 1 V to a few 100 volts.

The general diode’s reverse current is called a reverse saturation current. Its based on the minority carriers; free electrons in p-side and holes in n-side are responsible for such a small (micro-amp) current. There is high resistance thats met by free carriers at the pn junction or depletion region. This is called saturation current as such a current can’t be increased by increasing the applied potential. What happens then that at a particular maximum value of voltage there is a small resistance, and a large current that flows in the diodes. (in Zener diodes).

Mechanisms behind breakdown or Zener voltage.

The reverse breakdown occurs when there is a large applied potential (large value of reverse bias). The breakdown can be ascribed to two different processes. The Zener breakdown and the Avalanche breakdown. Zener breakdown is caused by a Field emission process and Avalanche breakdown is caused by a collision-ionization process.

Field emission.

Mr Zener (this diode must have been named after a scientist and not a politician, right?) hypothesized the mechanism of Field Emission as follows. When reverse bias potential is increased this means there is an increase in the value or magnitude of the associated electric field. Electric fields are homewreckers. They break the marital vow between the valence electrons and the core of the atoms. Such vows are known as covalent bonds in the language of scientists. A large number of valence electrons that come out of this covalent bonding are now conduction electrons. They can carry large amount of electric current under the influence of the electric field. This type of breakdown is known as Zener breakdown.

Ionization by collision.

We have already discussed in one of our earlier lectures in this series how forward bias decreases the pn junction (or depletion layer) width as well as height. This also implies a reverse bias increases the same, the increase in height of the barrier being akin to an energy hill, making it more difficult for the free carriers to change sides at the layer thereby failing to carry any electric current.

The reverse bias also increases the width of the depletion layer. At some trade-off point the carriers travel a longer path and gain high momentum as the electric field is high as well. Such high momenta carriers can knock off valence electrons in the atoms in their path. These primary new electrons can create new secondary electrons through the same process of collision and ionization and gradually their number accumulates to a large value producing whats called an avalanche of free carriers. This is known as Avalanche breakdown as this produces a large reverse current in the diode corresponding to the critical value of voltage known as breakdown voltage.

Zener breakdowns are evinced in narrow junctions and Avalanche breakdowns are evinced in wider junctions. In case of Zener breakdown the breakdown voltage decreases with temperature. Its the opposite for Avalanche breakdown. The IV characteristics of Zener breakdown is very sharp where as it increases gradually and is not as sharp in the case of Avalanche breakdown.

Voltage regulation circuits and numerical.

Zener diode regulator.

Lets begin with an appropriate diagram that depicts the Zener diode (also breakdown diode or avalanche diode) as a voltage regulator.

Zener diode as a voltage regulator, needs a current limiting resistor in series and a load resistor in parallel.

Zener diode as a voltage regulator, needs a current limiting resistor in series and a load resistor in parallel.
Zener diode as a voltage regulator, needs a current limiting resistor in series and a load resistor in parallel.

The resistor RS is connected to the Zener diode in series. Its function is to regulate or limit the maximum current that can flow through the Diode, with a given voltage source. The voltage source VS is connected across the combination of the current limiting resistor and the diode. The output voltage which is now stabilized is taken from across the Diode. The Zener diode is connected so that it operates under reverse bias condition. Obviously the voltage source must have a potential which is larger than the breakdown voltage. (VS > VZ)

There are two contrary situations with similar consequence as far as exceeding the power rating of the diode is concerned. When no load resistor is connected (across diode) load current is zero and diode dissipates its maximum power. But when current limiting resistor is small but a large load resistor is connected this will increase the power dissipation of the diode. So appropriate current limiting resistor must be selected.

The voltage across the load is always same as that across the diode. In other words: VL = VZ. The Zener current value must be between a maximum and minimum that keeps it within the Zener breakdown region. Only then the stabilization is effective. This can be seen in the IV characteristics diagram and as we already discussed in this lecture, the maximum value depends on the power rating of the device.

Numerical

Now lets solve an example problem that illustrates the Zener diode as a voltage regulator.

A 5 V stabilized power supply is required to be produced from a 12 V DC power supply input source. The maximum power rating PZ of the Zener diode is 2 W. Using the circuit above, calculate 

1. The maximum current flowing through the diode.
2. The minimum value of the series resistor, RS. 
3. The load current IL if a load resistor of 1  is connected across the Zener diode.
4. The Zener current IZ at full load.
click to see or hide answer to the above problem

1. maximum current = watts/voltage = 2W / 5 V = 400 mA.
2. minimum value of series resistor: RS = (VS – VZ) / IZ = (12 – 5) volt /400 mA = 17.5 Ω.
3. load current IL if load resistor of 1 is connected across diode. IL = VZ/RL = 5 V /1000 ohm = 5 mA.
4. Zener current at full load. IZ = IS – IL= ( 400 – 5 ) mA = 395 mA. .

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