Understanding Zener Diode Datasheet Specifications

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Datasheets for electronic components state a variety of different parameters or specifications for the particular component and in this case for Zener diodes or voltage reference diodes - these parameters define the performance of the diode within certain limits and investigating them forms an integral part of any electronic circuit design process.

When selecting a suitable Zener voltage reference diode for any given position in a circuit, it is necessary to ensure that it will fulfil its requirements. Understanding the datasheet specifications is key to selecting a suitable device.

There are many different parameters that are seen in the Zener diode specifications given in datasheets. Some of the more important ones are given below.

Zener diodes can be used for a host of purposes in circuit designs, but in particularly they find many uses in power supply applications where they can supply a stable reference voltage.

In fact, although these diodes are more commonly called Zener diodes, many rely on a different form of breakdown and therefore these electronic components are referred to as voltage reference diodes.

Zener IV characteristics

The IV characteristic of the Zener / voltage reference diode is the key to its operation. In the forward direction, the diode performs like any other semiconductor diode with its turn on voltage of around 0.6V for a silicon diode.

However it is in the reverse direction where its specific performance parameters can be utilised as it has a very flat breakdown voltage which can be used in many electronic circuit designs from voltage regulators to voltage limiters and many more.

The Zener diode has a normal forward characteristic where the current rises after the initial turn-on voltage is reached. This is typically 0.6 volts for silicon diodes - virtually all Zener diodes are silicon diodes.

As the voltage rises in the reverse direction, initially very little current flows. Only once the reverse breakdown voltage is reached, does current flow as seen in the diagram. Once the revere breakdown voltage is reached, the voltage remains relatively constant regardless of the current flowing through the diode.

Zener diode specifications

When looking at the specification sheet for a Zener diode there are many parameters about these electronic components that will be included. Each parameter details a different aspect of the Zener voltage reference diode performance.

Looking at each different characteristic it is possible to understand the performance of the diode and ensure it will operate correctly in any given electronic circuit design.

To achieve the required circuit performance, each component within the design must be able to operate together to give the required overall performance. This obviously includes the performance of the Zener diode, and understanding its performance parameters are key to selecting the required component.

The specifications for Zener diodes, like any other electronics component are normally available on the website of the manufacturer. Also electronic component distributors often have details of the specifications of components, or sometimes a link tot he specification on the manufacturer's website.

It is also worth noting for components that can be obtained from several manufacturers, that the specifications may vary slightly between manufacturers. For any critical parameters it is wise to use the actual figures from the manufacturer whose product is being used.

Often a second source and further sources may be required to give some level of insurance against a particular supplier or manufacturer ceasing their operation and the part becoming obsolete. In this case the specification parameters for all manufacturers should be closely checked to ensure they meet the requirements for the particular electronic circuit design.

Voltage Vz

The Zener voltage or reverse voltage specification of the diode is often designated by the letters Vz. Voltages are available over a wide range of values, normally following the E12 and E24 ranges, although not all diodes are bound by this convention.

In some cases the electronic components with E12 values can be slightly cheaper and they may be more widely available than those in the ranges like the E24 range.

Values generally start at around 2.4 V although not all ranges extend as low as this. Values below this are not available because the diodes do not break down below this voltage. Ranges may extend top anywhere in the region of 47 V to 200 V, dependent upon the actual Zener diode range. Maximum voltages for SMD variants are often around 47 V.

There are twice as many values available in the E24 range as in the E12, giving a much greater choice of values. This can be of benefit in some instances as more exact values can be chosen, reducing the requirement for adjustment where the exact value is not met.

Current specification

The current, IZM, of a Zener diode is the maximum current that can flow through a Zener diode at its rated voltage, VZ.

Typically there is also a minimum current required for the operation of the diode. As a rough rule of thumb, this can be around 5 to 10 mA for a typical leaded 400 mW device. Below this current level, the diode does not break down adequately to maintain its stated voltage.

It is best to keep the Zener diode running above this minimum value with some margin, but without the likelihood of it dissipating too much power when the Zener needs to pass more current.

Power rating

All Zener diodes have a power rating that should not be exceeded so accordingly this is an important specification. In fact different series of Zener diodes or voltage reference diodes are defined by the power they can dissipate.

This defines the maximum power that can be dissipated by the package, and it is the product of the voltage across the diode multiplied by the current flowing through it.

For example many small leaded devices have a dissipation of 400mW or 500 mW at 20°C, but larger varieties are available with much higher dissipation levels.

Surface mount varieties are also available, but generally have lower power dissipation levels in view of the package size and their ability for heat removal.

Common power ratings for leaded devices include 400mW (most common), 500 mW, 1W, 3W, 5W and even 10 W. 50w versions are even available, but these are often stud mounted to ensure that the diode can be mounted onto a heatsink to remove the heat dissipated.

Values for surface mount devices may be around 200, 350, 500 mW with occasional devices extending up to 1 W.

The use of high power Zeners will increase costs as a result of the larger devices themselves being more expensive as well as the additional hardware required to mount the devices and remove the heat. This is on top of the increased power consumption.

Sometimes alternative methods can be used so that lower power Zeners employed and efficiency is improved, although it may be necessary to balance this against increase complexity.

Zener resistance Rz specification

The IV characteristic of the Zener diode is not completely vertical in the breakdown region. This means that for slight changes in current, there will be a small change in the voltage across the diode. The voltage change for a given change in current is the resistance of the diode. This value of resistance, often termed the resistance is designated Rz.

The inverse of the slope shown is referred to as the dynamic resistance of the diode, and this parameter is often noted in the manufacturers' datasheets. Typically the slope does not vary much for different current levels, provided they are between about 0.1 and 1 times the rated current Izt.

Voltage tolerance

With diodes being marked and sorted to meet the E12 or E24 value ranges, typical tolerance specifications for the diode are ±5%. Some datasheets may specify the voltage as a typical voltage and then provide a maximum and minimum.

Temperature stability:

For many circuit designs, the temperature stability of the Zener diode is important. It is well known that the voltage of the diode varies according to temperature. In fact the two mechanisms that are used to provide breakdown within these diodes, i.e Zener breakdown and impact ionisation have opposite temperature coefficients, and one effect dominates below about 5 Volts and the other above. Accordingly diodes with voltages around 5 V tend to provide the best temperature stability.

It can be seen in the example given that there is a noticeable difference between the specification for the Zener diode reverse voltage at 0°C and 50°C. This needs to be taken into account if the circuit and equipment in which the Zener diode is to be used is subject to temperature change.

Junction temperature specification

In order to ensure the reliability of the diode, the temperature of the diode junction is key. Even though the case may be sufficiently cool, the active area can still be very much hotter. As a result, some manufacturers specify the operating range for the junction itself.

For most electronic circuit designs, a suitable margin is normally retained between the maximum expected temperature within the equipment and the junction. The equipment internal temperature will again be higher than the temperature external to the equipment. The temperatures of the overall component and in particular, the junction can be calculated knowing the cooling, temperature of the surroundings, etc.

Care must be taken to ensure that individual electronic components do not become too hot despite there being an acceptable ambient temperature outside the equipment. Typically a good margin is left to ensure that the reliability of the whole electronic design is not impacted by runing the voltage reference diode too hot. If the temperature is high, then its reliability will fall and this could have a significant impact on the overall equipment reliability.

Package

Zener diodes are specified in a variety of different packages. The main choice is between surface mount devices and the traditional leaded ones.

Whether surface mount or leaded, the package chosen will often define the heat dissipation level. In some cases where the diode is capable of very high levels of dissipation, the package may have an arrangement where it can be bolted to a heatsink.

Whatever the requirements and dissipation levels, the choices available will be detailed in the datasheet specification.

Example Zener diode datasheet characteristics

To give some idea of the datasheet characteristics to be expected from a Zener diode a real example is provided below. The main parameters that would be needed in a circuit design are given.

• BZY88 leaded Zener diode   This diode is described as a miniature Zener diode for regulated power supply circuits, surge protection, arc suppression and other functions in a variety of areas. The 5V1 (5.1 Volt) version has been taken as an example.

The datasheet parameters given for this common Zener diode / voltage reference diode give a useful indication of specification given for electronic component.

Although they are only for a small diode and used in many lower power elements of an electronic circuit design, the same sorts of data are given for other Zener diodes and voltage reference diodes with similar or higher power dissipation levels, etc.

Zener diodes / voltage reference diodes are a form of electronic component that are widely used in many electronic circuit designs, and in particular in various forms of power supply design. These electronic components are cheap and widely available from all electronic component distributors. This makes them ideal for inclusion in many circuit designs.

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