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 electronic 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.
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.
Although the voltage reference diode is normally referred to as a Zener diode, there are two different breakdown mechanisms that can occur:
- Zener effect: This effect predominates below 5.5 volts.
- Impact ionisation: This effect predominates above 5.5 volts.
The result of both breakdown effects is the same and design engineers do not need to design their circuits differently in any major way. The main difference is that the two effects have different temperature coefficients.
Zener diode symbol
To differentiate a Zener diode from a normal signal diode the circuit symbol is modified slightly. The Zener diode symbol has a small "tag" applied to the bar of the diode symbol to identify its function.
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.
The Zener diode utilises the same basic structure as an ordinary diode, but the concept of operation of the reverse breakdown effects are not normally wanted or used for normal diode operation.
The Zener diode structure is optimised to ensure the required performance - this entails some differences to the structure of an ordinary diode.
Zener diode theory and operation
There are two effects that can be used in Zener diodes. One is called Zener breakdown, and the other, impact or avalanche 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.
- Zener breakdown effect: Zener breakdown effect is the one from which the diode gains its popular name. It is the quantum mechanical effect tunnelling effect, but when applied to the voltage reference diode, it retains the Zener name after the man who discovered it.
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. Clarence Zener in 1934 from whom it gains its name. - Impact ionisation: Impact ionisation is very different to Zener breakdown and it 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 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 two reverse breakdown effects in the diode have very similar characteristics, but they are not the same. In most cases it is possible to ignore the difference between the two effects and use a diode in the same manner.
Diode operation
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 Circuits & Applications
Zener diodes are used in many circuits in a variety of ways.
The most common Zener diode circuit is one in which the Zener diode is used as a voltage reference element. This type of circuit uses the constant voltage as a reference in one of a variety of forms of power supply circuit.
There are other Zener diode circuits and applications. They can be used to limit voltages, preventing surges from damaging electronics circuits.
Simple Zener diode ciruit for voltage regulator
When used in a regulator cirrcuit, 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.
Simple circuit of a Zener diode shunt regulator
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.
Where:
V1 is the input voltage
V2 is the Zener diode voltage
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 circuit for PSU with series transistor
The very simple shunt regulator shown above is not particularly efficient and is not practicable for many higher current applications. The solution is to utilise a Zener diode circuit that uses a series pass transistor. A simple circuit is shown below and here the transistor is used as an emitter follower.
Zener diode circuit for a simple regulated power supply
When utilising this circuit, the current required from the Zener resistor potential diver should be calculated. This is the emitter current from the transistor divided by the gain.
When choosing the Zener diode voltage, it should be remembered that the emitter voltage will be lower than the Zener voltage by the amount of the base-emitter voltage - 0.6 volts for a silicon transistor.
Zener diode circuit for overvoltage protection
Another form of Zener diode circuit is an overvoltage protection circuit. While power supplies are normally reliable, the effects of the series pass transistor or FET can be catastrophic if it fails by forming a short circuit. In this case the full unregulated voltage would be placed onto the circuits using the regulated power. This could destroy all the chips being powered.
One solution is to use a crowbar circuit. When this form of circuit detects an overvoltage situation it fires an SCR. This quickly holds down the output voltage and in the instance shown, it blows a fuse that disconnects the input source power.
SCR overvoltage crowbar circuit
The circuit operates by firing the SCR when the overvoltage is detected. The Zener diode is chosen to have a voltage above the normal operating voltage - sufficient margin not to fire under normal operating conditions, but small enough to allow current to flow quickly when the fault condition is detected.
Under normal operating conditions the output voltage is below the reverse voltage of the Zener diode and no current flows though it and the gate of the SCR is not fired.
However, if the voltage rises above the allowed voltage, the Zener diode will start to conduct, the SCR will fire and the fuse will be blow.
Circuit tips
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.
Zener IV characteristic
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, but it is in the reverse direction where its specific performance parameters can be utilised.
Zener diode IV characteristic
Major Zener diode specifications explained
When looking at the specification sheet for a Zener diode there are several parameters that will be included. Each details a different element of its performance and is required to ensure it operates correctly within any circuit.
- 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, often following the E24 ranges, although not all diodes are bound by this convention.
Values generally start at around 2.4 V although not all ranges extend as low as this. Values below this are not available. 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. - Current : 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. - Zener resistance Rz: 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.
Zener diode resistance
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. - Power rating: All Zener diodes have a power rating that should not be exceeded. 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 at 20°C, but larger varieties are available with much higher dissipation levels. Surface mount varieties are also available, but generally have lower 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, 5W. Values for surface mount devices may be around 200, 350, 500 mW with occasional devices extending up to 1 W. - 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 applications, 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 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.
Zener diode temperature characteristic - Junction temperature: 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 normal design, 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. Carer must be taken to ensure that individual items do not become too hot despite the ambient temperature outside the equipment.
- Package: Zener diodes are specified in a variety of different packages. The main choice is between surface mount and traditional leaded devices. However the package chosen will often define the package heat dissipation level.
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