Understanding Zener Diode

Zener diode is one of the primarily used electronic component in the embedded designs. Let us consider a Zener diode from a vendor and start exploring it's datasheet to understand the specifications.

The starting point of understanding Zener diode specifications is to keep in mind the forward and reverse voltage characteristics. the below diagram shows the same for Zener diode:

Zener is generally used in reverse biased mode. Zener operation is similar to normal diode under forward biased mode. There are 3 main voltages of significance in Zener diode.

• Forward Voltage
• Reverse Voltage
• Zener Voltage

Forward voltage (VF in above characteristic) is under forward bias condition. Reverse voltage is the voltage at which Zener starts conducting in reverse direction but it doesn't breakdown at this voltage. So, there is more of a reverse leakage current at this point which is not good for circuit. The Zener breaks down only at Zener Voltage (VZ). this is the actual voltage where Zener stabilizes to that voltage.

A Zener diode for an application is selected based on the below criteria:

• Power dissipation allowable by the Zener
• Zener voltage
• Operating Temperature

Zener diode considered:

Manufacturing Part Number : IN4747A

Vendor: Vishay Semi-Conductors

Specification 1: Power Dissipation in the Zener

One of the important specification of the Zener diode is Power dissipation. As we know, Power dissipation is the product of Zener voltage and current flowing through the Zener. Just by going through the specification above don't select the Zener for your for your application. Most of the fresh embedded engineers tend to do a mistake by checking this power dissipation and assume that it works for their application. The given specification in this table is only for room temperature. If your product is used in an application which is used across wide temperature range, then you might have to refer the "Power dissipation" vs "Temperature" graph that is present in the datasheet. The below graph snapshot from the Zener datasheet gives an indication to the user whether to use this or not.

Notice from the above graph that as temperature increases the permissible power drop across the Zener is reduced.

Specification 2: Zener Current

This specification gives an indication of the maximum current that Zener diode allows through it for a given Zener voltage.

Specification 3: Thermal Resistance

Thermal Resistance is an important specification for any electronics component and it determines up to which temperature a particular can be used. Follow our post on thermal resistance for more information.

Specification 4: Forward voltage

As described in the above characteristic curve and it's explanation, the forward voltage in Zener is similar to normal P_N junction diode. 

Specification 5: Zener voltage

The "ZENER VOTLAGE RANGE" mentioned in the above table is the actual Zener breakdown voltage. If your intending to use a Zener for 20V in your application, use this 1N4747A. In the characteristic curve, as per above table, VZ = 20V.

Applications of Zener diode:

• Used in stabilizing the voltage in circuits
• Voltage clipping