CHARACTERISTIC FEATURES OF ZENER DIODES IN CONTROL SYSTEMS

ABSTRACT

This article discusses some characteristics of zener diodes in control systems. Modern electronic equipment places stringent demands on the stability of the DC voltage of the power source. The severity of the requirements can be judged by such indicators as: low stability, in which changes in the output voltage of the power supply are 2-5%, average stability 0.5-2%, high 0.1-0.5%, very high - less than 0.1%. Such high stability of the high voltage power source cannot be achieved without a special device - a constant voltage stabilizer, switched on at the output of the power source.

АННОТАЦИЯ

В представленной статье рассмотрены некоторые характеристики стабилитронов в системах управления. Современная электронная аппаратура предъявляет жесткие требования к стабильности постоянного напряжения источника питания. Жесткостью требований можно судить по таким показателям как: малой стабильностью, при которой изменения выходного напряжения источника питания составляют 2-5%, средней стабильностью 0.5-2%, высокой 0.1-0.5%, очень высокой- менее 0.1%. Такие высокие показатели стабильности высокого напряжения источника питания невозможно получить без специального устройства – стабилизатора постоянного напряжения, включаемого на выходе источника питания.

Keywords: zener diode, characteristic, control system, parameter, power source, input, output, signal, stability, voltage, load, resistance.

Ключевые слова: стабилитрон, характеристика, система управления, параметр, источник питания, вход, выход, сигнал, стабильность, напряжение, нагрузка, сопротивление.

INTRODUCTION

A zener diode is a special diode that operates in breakdown mode. When the breakdown voltage is reached, the reverse current through the zener diode increases sharply [7]. The differential resistance in the breakdown region (the ratio of the current change to the voltage change that caused this change) is a few to hundreds of ohms. If the external resistance of the circuit is significantly greater than the dynamic resistance of the zener diode, then the effect of voltage stabilization is achieved. In international practice, a zener diode is called [1-8].

Zener diodes are used in the following applications:

- voltage stabilizers and reference voltage sources;

- amplitude limiters;

- voltage level shift circuits [3].

ALGORITHM FOR SOLVING THE PROBLEM

Semiconductor zener diodes are divided into several subclasses [5]:

- low power zener diodes (less than 1 W);

- power zener diodes (1-5 W and above);

- precision zener diodes (temperature-compensated zener diodes and zener diodes with a hidden structure) [2];

- suppressors (TVS diodes) - specialized high-speed and powerful zener diodes designed for overvoltage protection.

Unlike a diode, the main working area of a zener diode is in the breakdown region [4]. The basic circuit of a parametric voltage stabilizer using a zener diode is shown in figure 1.

Figure 1. Basic circuit of a parametric voltage stabilizer using a zener diode

Series connection of zener diodes (figure 2) of the same series is a traditional way of increasing the operating voltage and obtaining a stabilization voltage with a rating different from the ratings of commercially produced zener diodes [6]. Parallel connection of zener diodes is not used due to mismatch of current-voltage characteristics during operation [8-14].

Figure 2. Series connection of zener diodes

The main parameters of zener diodes include:

1. Stabilization voltage (3 - 200)V.

2. Minimum stabilization current I_st.min (~3 mA) - the value of the current flowing through zener diode, at which a stable breakdown occurs [10].

3. Maximum stabilization current I_st.max (~20 mA - 1A) - current value flowing through the zener diode, in which the power dissipated on the zener diode is not exceeds the permissible value [9].

4. Maximum power dissipation P_dis. = U_st. • I_st. - maximum power released at the p-n junction, in which no thermal breakdown of the junction occurs.

5. Differential resistance r_st = D U_st /D I_st voltage increment ratio on the zener diode to the current increment in stabilization mode. Characterizes the degree stability of the stabilization voltage when the breakdown current changes. In the stabilization section r_st ~ const; r_st = 0.5 - 200 Om. [12].

6. Temperature coefficient of voltage (TKU) stabilization:

,

where t₁ °C is the initial temperature.

TKU = 0.1 ...0.01%/°C

In addition to the main parameters, there are a number of parameters that describe the deviations of the stabilization voltage of a real device under the influence of various factors (stabilization voltage, differential resistance, temperature coefficient of the stabilization voltage, drift and noise of the stabilization voltage). These parameters are taken into account when constructing circuits with accuracy requirements. In some cases, with sudden changes in current, it is necessary to take into account the operating features of the device, i.e. dynamic parameters of the zener diode [13].

The stabilization mode is possible in a wide range of currents and voltages (I_st.min., I_st.max., U_st.min., U_st.max.), and within these ranges - nominal values (I_st., U_st). Typically, the minimum stabilization current is equal to the current at the output from the region where the reverse current-voltage characteristic turns, the rated current is set at 25 - 35% of the maximum. For all operating temperatures (-60…..+125⁰С), the minimum stabilization current is set at 1mA, and the maximum depends on temperature and atmospheric pressure. For example, at normal atmospheric pressure and a temperature not exceeding +35⁰C, the current should not exceed 37.5mA, and at a temperature of +125⁰C - 15mA. As the pressure decreases, the maximum currents decrease by 2 times due to weak heat removal in a rarefied environment [13]. The device stabilization voltage spread (U_st.min.,……..U_st.max.) is normalized for a current of 5mA and for various temperatures from -60⁰C to +125⁰C. At -60⁰C the voltage spread is 3.1...3.8V., at +125⁰C – 2.8...3.5V. [12].

The safe operating area of the zener diode is limited by a number of parameters, the most important of which are the maximum values of direct current, pulsed current, pn junction temperature and power dissipation. These restrictions must be met simultaneously, and failure to take into account any of them leads to the destruction of the zener diode [3,9]. The current and power limitations are known, but the temperature requires an estimate of the permissible power, and the calculated temperature of the pn junction must be greater than the maximum permissible value. Typically, such an assessment in technical documentation is presented in a graphical representation of the dependence of the permissible power on the ambient temperature T. [1,14].

CONCLUSION

Based on the above, we can give the following definition: a zener diode is a silicon diode, the internal resistance of which changes little when the current changes. The small dependence of the voltage drop on the zener diode on the flowing current is its main property, due to which the voltage on the zener diode and the load connected to it are practically maintained constant. A device that maintains the load voltage with the required accuracy when the network voltage and load resistance changes within specified limits is a voltage stabilizer , and its basis is a zener diode.

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