The word Ratings means the various information about that particular device is given on the nameplate of the device or data sheet provided by the manufacturer. This datasheet shows the ratings of different diodes which include Zener voltage, the tolerance range of Zener voltage, Zener current limits, the Maximum Power Dissipation, Maximum Operating Temperature, Maximum Zener Resistance or Impedance, etc.
Some of the important ratings of the Zener diode are discussed below in detail.
Zener Voltage (Vz)
The voltage at which a Zener diode breaks in the reverse bias condition known as Zener Voltage. In fact, it is the voltage at which a Zener diode is to operate. Commercially available Zener diode is having Zener voltage ratings from 3 volts to 200 volts. The value of breakdown or the Zener voltage depends upon doping. More the doping lesser the breakdown voltage.
Tolerance of Zener Diode
The range of voltages about the breakdown voltage in which a Zener diode conducts in the reverse direction is called tolerance.
In fact, during manufacturing, it is very difficult to have exact doping for every Zener of the same type. Therefore, breakdown voltages of Zener of the same number also differ slightly. This range of breakdown voltages for the same type of Zener diode is described as tolerance.
For example – consider a particular type of Zeners marked 9 V, 10% tolerance. These Zener diodes may have a breakdown voltage from 8.1 V (9 – 0.9) to 9.9 V (9 + 0.9) instead of exactly 9 V for all.
Effect of Temperature on Zener Diode
The breakdown voltage of the Zener diode depends upon operating temperature. It decreases with the increase in junction temperature. This is because of the increased reverse current (i.e., increase in minority carriers) that flows with increasing temperature. The decrease is about 2Mv/⁰C rise in temperature.
Power Ratings (PZM) of Zener Diode
The maximum power which Zener diode can dissipate without any damage is known as its Power Rating. Commercially available Zener diodes have power ratings from ¼ W to more than 50 W. The power rating is the product of maximum current IZM which a Zener diode can handle and the rated or operating voltage of a zener diode (VZ).
Maximum Current Ratings (IZM)
The maximum value of current which a Zener diode can handle at its rated voltage without damage is known as its Maximum Current Rating.
Zener Resistance (RZT)
The opposition offered to the current flowing through the Zener diode in the operating region is known as Zener Resistance (RZ) or Zener Impedance (ZZ).
When a Zener diode is operated in the breakdown region, an increase in current produces a slight increase in voltage. This shows that a Zener diode has a small AC resistance called Zener resistance.