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Zener Diodes

Written by Andrew Levido

A Zener diode is a diode with a well-defined reverse breakdown characteristic that is often used as a voltage references or for voltage regulation and for clamping in electronic circuits. Its forward voltage characteristics is similar to a conventional diode in that it begins to conduct when the forward voltage reaches around 0.65 V. Unlike conventional diodes which do not conduct when reverse biased (at least until some high breakdown voltage is reached), Zener diodes exhibit a controlled breakdown characteristic at some defined reverse voltage.

Figure 1 shows the V-I characteristic of a typical example. You can see that at some reverse voltage Vz, the device begins to conduct, maintaining a relatively constant reverse voltage. Zener diodes are typically available with a Vz value of between 2.4 V and around 100 V.

Figure 1
The V-I curve of a typical Zener diode shows a typical forward bias characteristic. Unlike a conventional diode the Zener diode also begins to conduct at some specified reverse voltage Vz, maintaining a relatively constant reverse voltage over a range of current. The limiting factor on reverse current is the power dissipation in the diode. This is why Zener diodes are specified by both breakdown voltage and power dissipation.

This breakdown characteristic occurs because of two different mechanisms – the Zener effect at low voltages and the avalanche effect at higher voltages. The Zener effect (shown in Figure 2a) is caused by electrons in the junction region being pulled out of the covalent bonds under the influence of the electric field. This creates free carriers and a resulting reverse current. This effect typically dominates in Zener diodes up to about 5.6V.

Figure 2
This breakdown characteristic of Zener diodes is due to the Zener effect and the avalanche effect. The Zener effect is caused by electrons in the junction region being pulled out of the covalent bonds under the influence of the electric field creating free carriers and a resulting reverse current. The avalanche mechanism occurs when free electrons are accelerated by the electric field to the point that they have sufficient energy to knock further electrons free These electrons in turn are accelerated due to the field, creating further collisions and yet more free carriers in a cascading fashion.

Above this level, the avalanche mechanism gradually takes over. In this scenario, shown in Figure 2b, free electrons are accelerated under the electric field to the point that they have sufficient energy to knock further electrons free from the silicon atoms. These electrons in turn are accelerated due to the field, creating further collisions and yet more free carriers in a cascading fashion.

One interesting thing to note is that Zener effect has a negative temperature coefficient, while the avalanche effect has a positive temperature coefficient. As both these effects are present in Zener diodes around 5V its possible to have these temperature effects cancel out by careful selection of Zener and its operating conditions.

Figure 3 shows an extract from the data sheet for the BZX79 series of Zener Diodes. This family of diodes is rated for 400 mW and is available with voltage ratings between 2.4 V and 75 V, and in ±2% or ±5% voltage tolerances. You can see the temperature coefficient swings from negative to positive around 5V. If you can choose a diode in this range, you can almost eliminate any voltage drift with temperature by choosing the right Zener current.

Figure 3
This is an extract from the data sheet for the BZX79 series of Zener Diodes. You can see that the temperature coefficient of the breakdown voltage is negative for diodes below about 5V and positive for those above. If we choose a Zener in this voltage range and pay attention to its operating conditions, we can virtually eliminate any drift in voltage with temperature.

Figure 4 shows the temperature coefficient of a selection of Zeners vs current. You can see that the 5V1 Zener will have almost zero tempco at 10 mA, and the 4.7 V Zener will have similarly have nearly zero tempo at about 19 mA. Of course these are typical graphs and you may need to tweak the current for each device if you need very precise regulation.

Figure 4
This graph, extracted from the BZX79 data sheet, shows the temperature coefficient of Vz versus Zener current. You can see that by choosing the right current, the 4V7 and 5V1 Zeners can operate at a point where the tempco is zero.

Using Zener diodes as voltage regulators is fairly straightforward as shown in Figure 5. These devices are shunt regulators, which means that the maximum Zener diode current and power dissipation occur when the load is open circuit. These are given by the first two equations in the figure. When a load is applied, current is diverted away from the Zener. The minimum load resistance (maximum load current) is given by the third equation. This is the point where the Zener current just drops to zero.

Figure 5
A Zener diode is a shunt regulator, in parallel with the load. This means the maximum Zener current and power dissipation occur when the load current is at its minimum. In the worst case, where the load current is zero, the Zener voltage and current are given by the first two equations. As the load current increases, current is diverted away from the Zener diode. At the point where this current drops to zero, the Zener will drop out of regulation. The last equation describes the load resistor value at which this occurs.

REFERENCES
Nexperia. “BZX79 Series – Voltage Regulator Diodes.” Accessed March 6, 2023. https://www.nexperia.com/products/diodes/zener-diodes/series/BZX79-SERIES.html.

Electronic Circuits and Diagrams-Electronic Projects and Design. “PN Junction Breakdown Characteristics-Avalanche & Zener Breakdown,” August 25, 2009. https://www.circuitstoday.com/pn-junction-breakdown-characteristics.

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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.

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Zener Diodes

by Andrew Levido time to read: 4 min