Zener-Diode

A Zener diode, or Zener diode, is a particularly doped silicon diode with a low barrier layer thickness after the American physicist Clarence Melvin Zener, who discovered the Zener effect, named. The characteristics of Zener diodes allows that they can be used in numerous circuits for stabilizing and limiting voltages.

In the forward direction, they behave like normal diodes. In the reverse direction, the differential resistance decreases above a certain reverse bias voltage, the so-called breakdown voltage, significantly. Then the voltage increases little further, even when the current increases. This can lead to thermal overload.

Breakdown effects

The breakdown voltage VBR is referred to in zener diodes Zener voltage as IP and is usually around 3 ... 100 V ( produced in exceptional cases for the range 2 ... 600 V). If now the IP applied to the diode in the reverse direction, then the current is given by the diode from the formula:

If this voltage is below 5V, so outweighs the Zener effect and thus its negative temperature coefficient.

At voltages above 6.5 V outweighs the avalanche effect, also known as avalanche effect (English: avalanche effect) is known, and thus be a positive temperature coefficient.

In the range between 5 and 6.5 V both effects act in a similar strength. Although the kink in the curve is not as sharp, but the temperature response is particularly pronounced low.

Therefore Clarence Zener suggested to divide the said first diode in general zener diodes Zener diodes ( with breakdown voltages of 5 V) and Zener diodes ( with greater than 5 V). In everyday use, the term Zener diode has established itself as a comprehensive designation of zener and avalanche diodes.

At avalanche diodes of the buckling of the characteristic is more pronounced.

When avalanche breakdown enough electrons are accelerated so much that they beat another electron from the atomic bonds. As a result there is an avalanche- like rising carrier concentration and thus a lower resistance. If the current flowing not strong enough limited, the effect leads to the breakthrough of the second kind and therefore to the destruction of the diode.

Temperature dependence

The specification of the Z- voltage UZ refers normally to 300 Kelvin. And TC is the temperature coefficient for the relative change in the Z- voltage as a function of the temperature T:

Often specified in data sheets that the temperature coefficient is performed based on the voltage in millivolts per kelvin. The conversion is as follows:

Less than 7 volts, the temperature coefficient is dependent significantly on the diode current, and therefore the indication of the load current is always required.

Common values ​​are:

The Zener voltage and the change in function of the temperature is calculated using the following formulas:

The Zener effect has a negative temperature coefficient of the avalanche effect a positive temperature coefficient. At about 5 V, both coefficients are approximately equal and cancel each other out. This voltage value is therefore particularly suitable as a reference voltage source with low temperature drift. For particularly long-term stable reference, alternatively, a series circuit of a Zener diode with 6.2-6.3 V, and a temperature coefficient of 2 mV / ° C and a normal silicon diode (or base-emitter path of a transistor ) in the forward direction with -2 mV / ° C on the same chip used, thereby cancel the temperature coefficient.

Data sheets in the current-voltage characteristics are occasionally reported, i.e., the characteristics of the diode at 25 ° C and 125 ° C.

Equivalent circuit

Rz is the differential resistance of the Zener diode; its value is a few ohms.

Operating

The operating point of a Zener diode is located in the intersection of the diode characteristic and the load resistance characteristic.

In the accompanying diagram, the characteristic curve is shown with fluctuating supply voltage. Depending on the load, different voltages Set - at full load, the minimum, at idle the highest voltage. The operating point varies between points 1 and 2 (control range ), thereby causing a corresponding variation of the Zenerstromes IZ.

The lower limit of the control range is determined by the bend of the curve and is approximately 10 % of Imax. Having a variable load resistance of the whole control range between points 1 and 3 can be used.

Application

Zener diodes are usually operated in the reverse direction. They apply at the voltage limit, the overload protection and, the most common application, in which voltage stabilization. Is common, for example, the parallel control of a voltage to other electronic circuit elements that require a stable power supply or the input voltage. Another example is the Zener. Furthermore, let Zener very good use as a generator for white noise, which is caused by the avalanche effect.

Voltage stabilization with Z- diode

Differential Voltage limitation with anti-serially arranged Z- diodes

Wherein the circuit for blocking the voltage limiting zener diode for voltages. The output voltage is in this area only from the series resistor and - in the case of voltage stabilization - the load resistor.

When the zener diode conducts with, is located at the maximum load resistance to the Zener voltage.

This leads to the following formula:

Using the example of a Zener diode with a Zener voltage of IP = 10 V would look something like this:

This results in a smoothing (limitation ) of the actual input voltage and hence a stabilization of the output voltage. These are described by the smoothing factor G and the stabilizing factor S, which will become apparent from the following formulas:

Rz is the differential resistance of the Zener diode, which should be as small as possible. With the typical values ​​rz = 5 Ω and Rv = 1000 Ω (also the ripple) reduced to 0.5 % variation of the input voltage.

Relative stabilization factor S:

The symmetric voltage limiting functions similarly to the voltage limit as described herein with only a Zener diode. However, it also limits negative input voltages on IP. However, this is a voltage drop at the second UD Z diode, which is conductive in this case. This is analogous to the voltage drop of a conventional diode.

A better option for stabilizing the voltage supply voltage regulator which can regulate the voltage is much more precise.

Identification

The housing carries a ring of zener diodes at the cathode side, as a rule. The labeling corresponds to the convention in other diodes.