Research note
Thermoelectrical degradation processes in NTC thermistors for in-rush current protection of electronic circuits

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Abstract

Degradation processes, caused by the extreme values of current pulses or thermal burn-in, are studied in thermistors with negative temperature coefficient of resistance. The drift of thermistor resistance at 25°C is observed. These changes depend on the value and the number of current pulses. The first 100 cycles are the basic ones for the future exploitation parameters of thermistors. Similar effect, rise of thermistor resistance, was observed after burn-in. The investigated thermistors are used for in-rush current protection of electronic circuits.

Introduction

Thermistors with negative temperature coefficient of resistance (NTC thermistors) are used in electronics as elements for suppression of in-rush current, relay delay, temperature control and sensing, fan control, etc. [1], [2], [3]. Low costs of manufacturing, reliability at normal conditions of exploitation (when the temperature of ceramics is lower than the critical degradation temperatures for ceramics and contact materials), simplicity are the main advantages of these devices.

At the same time the problem of minimization of thermistor dimensions (at the guaranteeing of maximum exploitation load) is actual up to now. In this connection the problems of reliability, lifetime of thermistors, determination of degradation mechanisms in ceramics, interaction of contact material and ceramic matrix, resistance to the attacks by heat, etc. are very important.

Section snippets

Experimental

The effect of cyclic switching of extreme impulse current loads (according to the method of accelerated degradation of thermistors [4]) on the power disk NTC thermistors, used for in-rush current protection of electronic circuits, is studied in this work. The changes of thermistor parameters are caused by the influence of electric fields and high temperatures, appearing due to the current heating up.

Power NTC thermistors, which are used for investigations, are manufactured from the

Results and discussion

Percent change in resistance ΔR/R0 of NTC thermistors in dependence on the number N of cycles of current impulse supply (the average data for all investigated samples) is presented in Fig. 2.

Three sections for the ΔR/R0(N) curve can be seen clearly. The first section (∼0–200 cycles) is characterized by the sharp rising of ΔR/R0 value up to 3–5%. The second section (∼200–700 cycles) has the relatively stable values of resistance, and the third one (more than 700 cycles) reveals the new sharp R

Conclusions

Process of degradation of NTC thermistors has complex character and is, in considerable degree, determined by the total number of applied current impulses or thermal cycles. Initial rise of resistance is caused probably by oxidation process or modification of ionic distribution. It must be verified by additional investigations. Thermistor destruction is connected with electrical breakdown of ceramic matrix on the edges of contact land and has the main role for the further changes of

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