How to Use Zener Diodes
Rod Elliott (ESP)

All diodes will do this, but usually at voltages that are unpredictable and much too high for normal voltage regulation tasks. There are two different effects that are used in Zener diodes ...

• Impact ionisation (also called avalanche breakdown)
• Zener breakdown

Below around 5.5 Volts, the zener effect is predominant, with avalanche breakdown the primary effect at higher voltages. While I have no intention to go into specific details, there is a great deal of information on the Net (See References) for those who want to know more. Because the two effects have opposite thermal characteristics, zener diodes at close to 5.5V usually have very stable performance with respect to temperature.

Figure 1 - Zener Diode Conduction

So, what's missing? The important part that is easily missed is that the slope of the breakdown section is not a straight line. Zeners have what is called 'dynamic resistance' (or impedance), and this is something that should be considered when designing a circuit using a zener diode.

The actual voltage where breakdown starts is called the knee of the curve, and at this region the voltage is quite unstable. It varies quite dramatically with current, so it is important that the zener is operated above the knee, where the slope is most linear.

Some data sheets will give the figure for dynamic resistance, and this is usually at around 0.25 of the maximum rated current. Dynamic resistance can be as low as a couple of ohms at that current, with zener voltages around 5 - 6V giving the best result. Note that this coincides with the best thermal performance as well.

This is all well and good, but what is dynamic resistance? It is simply the 'apparent' resistance that can be measured by changing the current. This is best explained with an example. Let's assume that the dynamic resistance is quoted as 10 ohms for a particular zener diode. If we vary the current by 10mA, the voltage across the zener will change by ...

V = R * I   = 10Ω * 10mA = 0.1V (or 100mV)

So the voltage across the zener will change by 100mV for a 10mA change in current. While that may not seem like much with a 15V zener for example, it still represents a significant error. For this reason, it is common to feed zeners in regulator circuits from a constant current source, or via a resistor from the regulated output. This minimises the current variation and improves regulation.

Manufacturers' data sheets will often specify the dynamic resistance at both the knee and at a specified current. It is worth noting that while the dynamic resistance of a zener may be as low as 1.5 ohms at 25% of maximum current, it can be well over 500 ohms at the knee, just as the zener starts to break down. The actual figures vary with breakdown voltage, with high voltage zeners having very much higher dynamic resistance (at all parts of the breakdown curve) than low voltage units. Likewise, higher power parts will have a lower dynamic resistance than low power versions (but require more current to reach a stable operating point).

Finally, it is useful to look at how to determine the maximum current for a zener, and establish a rule of thumb for optimising the current for best performance. Zener data sheets usually give the maximum current for various voltages, but it can be worked out very easily ...

I = P / V   where I = current, P = zener power rating, and V = zener voltage rating.

For example, a 27V 2W zener can carry a maximum continuous current of ...

I = 2 / 27 = 0.074A = 74mA (less a small 'fudge factor', and at 25°C)

As noted in the 'transistor assisted zener' app note (AN-007), for optimum zener operation, it is best to keep the current to a maximum of 0.7 of the claimed maximum, so a 27V/2W zener should not be run at more than about 47mA. The ideal is probably about 25% of the maximum (17mA close enough, although reference 3 says 18.5mA), as this minimises wasted energy and ensures that the zener is operating within the most linear part of the curve. If you look at the zener data table below, you will see that the test current is typically between 25% and 36% of the maximum continuous current. The wise reader will figure out that this range has been chosen to show the diode in the best possible light, and is therefore the recommended operating current .

While none of this is complex, it does show that there is a bit more to the (not so) humble zener diode than beginners (and many professionals as well) tend to realise. Only by understanding the component you are using can you get the best performance from it. This does not only apply to zeners of course - most (so called) simple components have characteristics of which most people are unaware.

Remember that a zener is much the same as a normal diode, except that it has a defined breakdown voltage that is far lower than any standard rectifier diode. Zeners are always connected with reverse polarity compared to a rectifier diode, so the cathode (the terminal with the band on the case) connects to the most positive point in the circuit.

Figure 2 - Zener Diode AC Clamp

In the first case, both zener diodes will conduct as conventional diodes, because the zener voltage can never be reached. In the second case, the actual clamped voltage will be 0.65V higher than the zener voltage because of the series diode. 12V zeners will therefore clamp at around 12.65V - R1 is designed to limit the current to a safe value for the zeners, as described above.

The important thing to remember is that zener diodes are identical to standard diodes below their zener voltage - in fact, conventional diodes can be used as zeners. The actual breakdown voltage is usually much higher than is normally useful, and each diode (even from the same manufacturing run) will have a different breakdown voltage.

1. I_zt = zener test current
2. R_zt = dynamic resistance at the stated test current
3. R_z = dynamic resistance at the current shown in the next column
4. Leakage current = current through the zener below the knee of the zener conduction curve, at the voltage shown in the next column
5. Peak current = maximum non-repetitive short term current (typically < 1ms)
6. Continuous current = maximum continuous current, assuming that the leads at 10mm from the body are at ambient temperature

Figure 3 - Zener Diode Temperature Derating

Like all semiconductors, zeners must be derated if their temperature is above 25°C. The above graph shows the typical derating curve for zener diodes, and this must be observed for reliability. Like any other semiconductor, if a zener is too hot to touch it is hotter than it should be. Reduce the current, or use the boosted zener arrangement described in AN-007.

1   Reverse Biased / Breakdown Discussing the phenomenon when the diode is reverse biased/breakdown. Bill Wilson
2   RadioElectronics.com - Summary of the zener diode
3   Data Sheet Archive - BZX2C16V Micro Commercial Components 2 Watt Zener Diode 3.6 to 200 Volts.