Zener Voltage Reference Diode

- a summary or tutorial covering the essentials of the Zener diode or voltage reference diode used in many power supply and other circuits.

This overview of the different types of diode is split into several pages, each addressing a different type of diode or diode technology:

Zener diodes are widely used as voltage reference diodes in electronics circuits. Zener diodes allow simple voltage regulator circuits to be made, and in addition to this they are cheap and easy to manufacture.

Zener diodes have been available for many years, and nowadays they are widely used in many areas of electronics circuits. Their obvious use is within power supply regulators, but they can be used as a reasonably stable reference voltage in many electronics circuits. In addition to this, they can be used to remove peaks in waveforms that may not be required. In one specific instance they can be used to remove spikes that may damage a circuit or cause it to overload.

Although the term Zener diode is widely used to describe diodes used as voltage references, the Zener effect that gives them their name is used in all diodes as seen later. Accordingly they should probably more correctly be termed voltage reference diodes.

In view of the many applications for Zener diodes, they are used in many areas of electronics circuits - not just in power supplies which is probably their most obvious used.

Zener diode basics

Zener diodes or as they may sometimes be called, reference diodes operate like an ordinary diode in the forward bias direction. They have the normal turn on voltage of 0.6 volts for a silicon diode. However in the reverse direction their operation is rather different. For very low voltages, like a normal diode they do not conduct at all. However once a certain voltage is reached the diode "breaks down" and current flows. It can be seen by looking at the curves for Zener diodes that the voltage is almost constant regardless of the current carried.

To differentiate a Zener diode from a normal signal diode the circuit symbol is modified slightly. The Zener diode has a small "tag" applied to the bar of the diode symbol to identify its function.

When used in a circuit, the Zener diode must have the current entering it limited. If a perfect voltage source was placed across it, then it would draw excessive current once the breakdown voltage had been reached. To overcome this the Zener diode must be driven by a current source. This will limit the current to the chosen value.

In a practical circuit, the simplest form of current source is a resistor. This will limit the current taken by the Zener diode and ensure that the operating position of the diode remain approximately constant.

The value of the series resistor is simple to calculate. It is simply the voltage across the resistor, divided by the current required. The level of Zener current can be chosen to suit the circuit and the Zener diode used.

This form of regulator circuit is known as a shunt regulator, where the regulating element in the circuit is placed in parallel with the load. The voltage appearing across the load is controlled by the Zener diode allowing a portion of the current to flow through the Zener and bypass the load to maintain the voltage across it. Shunt regulators are normally seen as being very inefficient for large levels of power, but for low power levels they are very effective. The Zener diode can be used as a shunt regulator to produce a stable reference voltage, which can then be used by a series regulator to produce the required stable voltage output. This technique is effectively used in analogue regulated power supplies.

Zener diode structure and operation

There are two effects that can be used in Zener diodes. One is called Zener breakdown, and the other, impact ionisation. The Zener effect predominates below 5.5 volts whereas impact ionisation is the major effect above this voltage.

The two effects are totally different, although they produce almost identical effects.

1. Zener effect:   The Zener effect occurs in a totally different manner. Under most conditions electrons are contained within atoms in the crystal lattice. In this state they are in what is called the valence band. If a large electric field is placed across the semiconductor this may be sufficient to pull the electrons out the atom into what is called the conduction band. When they are free from the atom they are able to conduct electricity, and this gives rise to the name of the conduction band. For them to pass from the valence band into the conduction band there must be a certain force to pull them free. It is found that once a certain level of electric field is present a large number of electrons are pulled free creating allowing current to suddenly start to flow once a certain reverse voltage is reached. The Zener effect was first proposed by Dr. Carl Zener in 1934 from whom it gains its name.


2. Impact ionisation:   Impact ionisation occurs when a high electric field is present in a semiconductor. Electrons are strongly attracted and move towards the positive potential. In view of the high electric field their velocity increases, and often these high energy electrons will collide with the semiconductor lattice.
   
   When this occurs a hole-electron pair is created. This newly created electron moves towards the positive voltage and is accelerated under the high electric field, and it to may collide with the lattice. The hole, being positively charged moves in the opposite direction to the electron. If the field is sufficiently strong sufficient numbers of collisions occur so that an effect known as avalanche breakdown occurs. This happens only when a specific field is exceeded, i.e. when a certain reverse voltage is exceeded for that diode, making it conduct in the reverse direction for a given voltage, just what is required for a voltage reference diode.

The reverse conduction effects, in common with many other aspects of semiconductor technology are subject to temperature variations. It is found that the impact ionisation and Zener effects have temperature coefficient in opposite directions. The Zener effect which predominates below 5.5 volts exhibits a negative temperature coefficient. However the avalanche effect which is the major effect above 5.5 volts has a positive temperature coefficient. As a result Zener diodes or voltage reference diodes with reverse voltages of around 5.5 volts where the two effects occur almost equally have the most stable overall temperature coefficient as they tend to balance each other out for the optimum performance.

Zener diode circuit precautions

The Zener diode is a very flexible and useful circuit component. However, like any other electronics component, there are a few hints and tips which enable the best to be made of the Zener diode. A number are listed below.

• Choose correct voltage for best stability:   In applications where stability with temperature changes is required, the Zener voltage reference diode should be chosen to have a voltage of around 5.5 volts. The nearest preferred value is 5.6 volts although 5.1 volts is another popular value in view of its proximity to 5 volts required for some logic families. Where different levels of voltage are required, the 5.6 volt Zener can be used and the surrounding electronics can be used to transfer this to the required output value.
• Buffer the Zener diode circuit with an emitter or source follower:   To keep the voltage from the Zener diode as stable as possible, the current flowing through the Zener diode must be kept constant. Any variations in current drawn by the load must be minimised as these will change the current through the Zener diode and cause slight voltage variations. The changes caused by the load can be minimised by using an emitter follower stage to reduce the current taken from the Zener diode circuit and hence the variations it sees. This also has the advantage that smaller Zener diodes may be used.
• Drive with constant current source for best stability:   Another way of improving the Zener stability is to use a good constant current source. A simple resistor is adequate for many applications, but a more effective current source can provide some improvements as the current can be maintained almost regardless of any variations in supply rail.
• Ensure sufficient current for reverse breakdown:   It is necessary to ensure that sufficient current is passed through the diode to ensure that it remains in reverse breakdown. For a typical 400 mW device a current of around 5 mA must be maintained. For exact values of minimum current, the datasheet for the particular device and voltage should be consulted.
• Ensure maximum limits of current are not exceeded for the Zener diode:   While it is necessary to ensure sufficient current is passed through the Zener diode, the maximum limits must not be exceeded. This can be a bit of a balancing act in some circuits as variations in load current will cause the Zener diode current to vary. Care should be taken not to exceed the maximum current or the maximum power dissipation (Zener voltage x Zener diode current). If this appears to be a problem, an emitter follower circuit can be used to buffer the Zener diode and increase the current capability.

When used to their best, Zener diodes can provide very high levels of performance. They often exceed the performance required, but in view of their ease of use and low cost, they provide a very effective option to use.

Summary

The Zener diode or voltage reference diode is widely used throughout electronics circuits. The Zener diodes or reference diodes can be used as discrete devices, or they may be used within integrated circuits. As such Zener diodes provide an essential building block for many circuits - one which could not easily be overcome if they were not available for some reason.

Further pages from this tutorial
Page [1] > [2] > [3] > [4] > [5] > [6] > [7] > [8] > [9] > [10] > [11] > [12] > [13] > [14] >