What is Zener Diode:
A zener diode is a silicon PN junction device that is designed for operation in the reverse break-down region. The breakdown voltage of a zener diode is set by carefully controlling the doping level during manufacturing.
Application of Zener Diode:
A major application zener diodes is as a type of voltage regulator for providing stable reference voltages for use in power supplies, voltmeters, and other instruments. Zener diode maintains a nearly constant dc voltage under the proper operating conditions.
Zener Diode Symbol:
Zener Diode Characteristics
Zener Diode Breakdown:
Zener diodes are designed to operate in reverse breakdown. There are two type of reverse breakdown in a zener diode:
- Avalanche Breakdown
- Zener Breakdown
The avalanche breakdown occurs in both rectifier and zener didoes at a sufficiently high reverse voltage.
Zener breakdown occurs in a zener diode at low reverse voltages. A zener diode is heavily doped to reduce the breakdown voltage. This causes a very thin depletion region. As a result, an intense electric field exists within the depletion region. Near the zener breakdown voltage (Vz), the field is intense enough to pull electrons from their valence bands and create current.
Zener diodes with breakdown voltages of less than approximately 5 V operates predominately in zener breakdown. Those with breakdown voltages greater than approximately 5 V operate predominantly in Avalanche breakdown. Both types, however, are called zener diodes.
Note:Zeners are commercially available with breakdown voltages of 1.8 V to 200 V with specified tolerance from 1% to 20 %
Consider Below diagram for reference, where the reverse portion of a zener diode’s characteristic curve. As the reverse voltage (VR) is increased, the reverse current (IR) remains extremely small u to the knee of the curve. The reverse current is also called the zener current. IZ. At this point, the breakdown effect begins ; the internal zener resistance, also called zener impedance (ZZ), begins to decrease as the reverse current increases rapidly. From the bottom of the knee, the zener breakdown voltage (VZ) remains essentially constant although it increases slightly as the zener current, IZ increases.
The Next and very important characteristic of zener diode is ” Zener Voltage Regulator”
Zener diode as Voltage Regulator:
The ability to keep the reverse voltage across its terminals essentially constant is the key feature of the zener diode. A zener diode operating in breakdown acts as a voltage regulator because it maintains a nearly constant voltage across its terminals over a specified range of reverse current value.
A minimum value of reverse current, Izk, must be maintained in order to keep the diode in breakdown for voltage regulation. we can see in the above curve that hen the reverse current is reduced below the knee of the curve, the voltage decreases drastically and regulation is lost. Also, there is a maximum current, Izm, above which the diode may be damaged due to excessive power dissipation. So, basically, the zener diode maintains a nearly constant voltage across its terminals for values of reverse current ranging from Izk to Izm. A nominal zeer voltage, Vzt, is usually specified on a data sheet at a value of revere current called the zener rest current, Izt.
Zener Equivalent Circuit:
Below diagram (a) shows the ideal model of a zener diode in reverse breakdown. It has a constant voltage drop equal to the nominal zener voltage. This constant voltage drop is represented by a dc voltage source ever though the zener diode does not actually produce an emf voltage. The dc source simply indicated that the effect of reverse breakdown is a constant voltage across the zener terminals.
Diagram (b) represents the practical model of a zener diode, where the zener impedance (Zz) is included. Since the actual voltage curve is not ideally vertical, a change in zener current (ΔVz). By Ohm’s law, the ratio of ΔIz is the impedance, as expressed in the following equation.
Zz = ΔVz / ΔIz
Normally Zz is specified at Izt, the zener test current, and is designated Zzt. In most cases, we can assume that Zz is constant over the full linear range of zener current values and is purely resistive.