EP0747910A2 - PTC resistance - Google Patents

Abstract

PTC resistor has an electrical resistor component (A) between two contact connections. Component (A) comprises (A1) a polymer matrix contg. (A2) embedded electroconductive, powdered filler in which the predominant vol. fraction consists of particles with an average dia. of less than 100 and more than 5 mu m.

Description

TECHNICAL AREA

The invention is based on a PTC resistor according to the preamble of claim 1. Resistors based on a polymer matrix and a powdery filler made of electrically conductive material with PTC behavior embedded in the polymer matrix are used as current-limiting elements in energy technology uses and serve to limit a short-circuit or overcurrent occurring in a circuit. Here, the PTC resistor is heated by the short-circuit or overcurrent to a critical temperature at which the polymer of the PTC resistor embedding the filler particles changes its phase, for example by melting, and then interrupts the current-carrying percolation paths of the PTC resistor formed by the filler particles.

STATE OF THE ART

A PTC resistor based on a polymer matrix and a powdery filler of electrically conductive material embedded in the polymer matrix is described in WO-A-91 19 297. The matrix of this resistor is made of a thermoplastic polymer, such as in particular polyethylene. Carbon black with particle sizes of up to 0.1 µm, metals such as nickel, tungsten, brass or aluminum, Borides, such as TiB ₂ , nitrides, such as ZrN, oxides, such as TiO, or carbides, such as TaC, with particle sizes of up to 100 μm are used. Due to the material composition and suitable manufacturing processes, the known PTC resistor has a specific resistance between 30 and 50 mΩ · cm in the cold conducting state and can then be loaded with relatively high nominal currents.

However, for the PTC behavior of the resistor, which is important for current limitation in a power engineering circuit, it is not only its specific resistance in the cold conducting state that is important, but also material-specific properties that are significant, which quickly when the current through the resistor rises above a limit value Limit this current without the resistance being heated to an unacceptably high level. In particular, when the switching process in the PTC resistor is inhomogeneous, this can lead to the PTC resistor forming locally overheated areas, so-called "hot spots", approximately in the middle between the contact connections. In the overheated areas, the PTC resistor switches to the high-resistance state earlier than at unheated areas. The total voltage across the PTC resistor then drops over a relatively small distance at the location of the highest resistance. The associated high electrical field strength can then lead to breakdowns and damage to the PTC resistor.

SUMMARY OF THE INVENTION

The invention, as specified in claim 1, is based on the object of providing a PTC resistor of the type mentioned at the outset, which enables particularly rapid limitation of a short-circuit or overcurrent flowing in a circuit.

The PTC resistor according to the invention is characterized in that it responds extremely quickly to a short-circuit or overcurrent and can therefore limit this current at an early point in time. The PTC resistor according to the invention absorbs relatively little energy and is largely spared from impermissibly high thermal and electrical loads. Overheated local areas are therefore generally avoided. These favorable properties result from the suitably selected and dimensioned filler.

mit δT=T_c-T, wobei T die Umgebungstemperatur ist.A short-circuit or overcurrent I (t) heats the PTC resistor to its critical temperature T _C , at which the PTC transition takes place and the current is limited. The time δt required for a homogeneous material to limit the short-circuit or overcurrent depends on the specific resistance r, the specific density d _mass and the specific heat c _{P of} the material of the PTC resistor as well as its cross section A and its length 1 between its connection electrodes . The following inequality applies: $${\text{r · (l / A) · I (t)}}^{\text{2nd}}{\text{· Δt ≥ A · l · c}}_{\text{p}}{\text{· D}}_{\text{mass}}\text{· ΔT,}$$

with δT = T _c -T, where T is the ambient temperature.

This means that the energy converted in the response period δt in the PTC resistor must be at least as large as the energy which is necessary to heat the material of the resistor from the ambient temperature T to the transition temperature T _c .

However, the energy supplied by the short-circuit or overcurrent is not converted homogeneously in the PTC resistor. The resistor has percolating current paths formed by the conductive particles. The greatest electrical resistance and thus also the greatest conversion from electrical to thermal energy takes place on electrical Contact between the individual filler particles instead. The thermal energy generated at the contact points heats the polymer embedding the filler particles. If the filler particles are relatively large, for example larger than 100 μm, relatively large gaps filled with polymer form between the individual particles. If, on the other hand, the filler particles are relatively small, only relatively small gaps filled with polymer form between the individual particles. The energy converted at the contact points can heat the polymer located in the small gaps much faster than the polymer provided in the larger gaps. The temperature T _c required to carry out the PTC transition is therefore reached more quickly with smaller filler particles. However, the major part of the filler particles must not be less than 10 µm, since otherwise the specific resistance becomes too great.

BRIEF DESCRIPTION OF THE DRAWING

Preferred exemplary embodiments of the invention and the further advantages achievable therewith are explained in more detail below with reference to drawings. Here shows:

Fig. 1

Characteristic curves of three PTC resistors, in each of which the size of a short-circuit current I [A] generated in a circuit by discharging a capacitor bank charged to 200 V, flowing through the PTC resistors and differently limited by the PTC resistors, as a function of time t [ms] is shown,

Fig. 2

Characteristic curves of five further PTC resistors, in which, according to the characteristic curves according to FIG. 1, the magnitude of the current I [A] as a function of the time t [ms] is shown, but the capacitor bank of the circuit was charged to 400 V,

Fig. 3

further characteristics of the five PTC resistors mentioned in FIG. 2, in which the energy consumption W [J] of the PTC resistors through which the current I (t) flows is shown as a function of the time t [ms], and

Fig. 4

Characteristic curves of two further PTC resistors and one of the five PTC resistors named in FIG. 2, in which, according to the characteristic curves according to FIG. 2, the magnitude of the current I (t) is shown as a function of the time t [ms].

WAYS OF CARRYING OUT THE INVENTION

A thermoplastic PTC polymer, such as, in particular, polyethylene, was mixed with electrically conductive filler powder using a process customary in the production of PTC resistors, and cuboidal resistance bodies with end faces smoothed out by lapping or polishing from the resulting mixture at elevated temperature and pressure pressed. Contact connections were soldered to the end faces. The length 1 of the resistors was in the centimeter range, the cross-sectional area A was in the square centimeter range. Typical values for 1 and A were approx. 0.5 to approx. 2 cm and 0.3 cm ² .

Polyethylene was used as the starting material for the polymer. Instead of polyethylene, depending on the application, an epoxy or some other thermoplastic or thermosetting polymer can also be used. Powdery TiB ₂ with different particle sizes was used as filler, in particular with average particle sizes between 100 and 200 µm, between 71 and 90 µm, between 63 and 71 µm, between 50 and 63 µm, between 32 and 50 µm, between 32 and 45 µm , between 10 and 30 µm, between 1 and 5 µm and with average particle sizes smaller than 45 µm, of which Particles were 10% by weight less than 4 µm, 20% by weight less than 8 µm, 50% by weight less than 15 µm and 90% by weight less than 20 µm. Instead of TiB ₂ , the filler may also be another conductive boride such as ZrB ₂ , a conductive carbide such as TiC, VC or SiC, a conductive nitride such as ZrN, a conductive oxide such as RUO ₂ or VO, or a conductive silicide, such as MoSi ₂ or WSi ₂ , and / or a metal or an alloy containing this metal, for example based on nickel, silver, tungsten, cobalt, copper, aluminum, zinc, tin or molybdenum.

In order to obtain comparable results, the individual PTC resistors were designed in such a way that they had the same cross-section A with the same chemical composition of polymer and filler and differed from one another by length 1 and above all by the size of the powdery filler.

PTC resistance samples A to N were produced with the mean filler particle sizes and filler quantities as well as geometric dimensions given in the table below.

Differences in the filler contents in samples A - D cause only a negligible change in the specific heat and therefore had no significant influence on the response behavior of the PTC resistors in the comparative tests described below. The same filler contents were selected for samples E - N.

sample Particle size [µm] Volume fraction filler [%] Cross section [cm ² ] Length [cm] A 100-200 43 0.30 2.1 B 63-71 35 0.30 1.8 C. 32-45 40 0.30 1.95 D 1-5 60 0.30 2.0 E <45 50 0.32 0.47 F <45 50 0.32 0.55 G 10-30 50 0.30 0.52 H 10-30 50 0.31 0.57 J 32-50 50 0.31 0.55 K 50-63 50 0.31 1.08 L 50-63 50 0.31 0.58 M 63-71 50 0.31 0.525 N 71-90 50 0.32 0.46

From these resistors, the cold resistance R [mΩ], the specific resistance r [mΩ · cm] and, with the help of short-circuit current tests, the maximum short-circuit current I _max [A] and the energy absorbed by the resistor when the short-circuit current was effective were determined. Joule] determined.

In the short-circuit current tests, each of the resistance samples A - N to be examined was installed in a circuit in which a capacitor bank with a capacity of 7.5 was charged as the short-circuit current source in the samples A-D to 200 V and in the samples E - N to 400 V. mF was available. The inductance of the circuit was 4.5 µH, which after short-circuiting the circuit with an ignitron led to a resonant circuit frequency of approx. 800 Hz. The resistance samples A - N were each protected from overvoltages by varistors connected in parallel. Samples E, F, H, J and L - N were protected against external flashovers by immersion in transformer oil and samples G and K were each protected by applying a silicone coating.

The results of the short-circuit current tests are shown in the figures and together with the resistance measurements in the table below.

sample Cold resistance R [mΩ] spec. Resistance [mΩ cm] I _max [A] Energy [Joule] A 140 15.4 1350 107 B 145 18.1 1300 91 C. 132 16.7 1200 60 D 226 24.7 (800) 110 E 43 6.3 3080 241 F 41 6.3 3120 242 G 39 6.7 3360 252 H 43 6.7 3360 266 J 42 8.4 3480 353 K 38 5.9 3680 388 L 42 5.9 4000 438 M 43 8.1 3960 421 N 40 7.6 4320 487

The current I _max is the highest measured current that has flowed through the respective test resistor during the short-circuit current test. The higher its value, the later the PTC transition and thus the current limitation started. Energy in joules means the energy that was absorbed by the respective test resistor in the form of heat in the period between the occurrence of the short-circuit current (t _A = 0) and its disappearance (depending on the test resistance t _E = 0.4-2 ms) . $$\text{[Energy consumption = ∫ U (t) · I (t) dt]}$$

The energy consumption is a measure of the switching behavior of the PTC resistors. The switching capacity of the PTC resistors is all the better the lower the energy consumption under comparable conditions.

The measurement results show that with a PTC resistor with comparatively large filler particles (sample A with particle sizes between 100 and 200 µm) the short-circuit current is limited relatively late and the short-circuit current is at the same time a fairly high value (I _max = 1350 [A]) reached. The energy absorbed by the resistance is also relatively large at 107 [J].

In the case of PTC resistors with smaller filler particles (samples B and C with particle sizes between 63 and 71 µm or 32 and 45 µm) the short-circuit current is sometimes limited considerably earlier (with sample C approx. 50 µs, ie approx. 25% rather than with sample A). In addition, the short-circuit current is no longer as high as with sample A (with sample C with 1200 A only about 90% of the value of sample A). In addition, the energy absorbed by the resistors when limiting the current is lower. In sample B this energy is approx. 15% and in sample C approx. 45% smaller than in sample A. The tendency to be seen from this is that the smaller the size of the filler particles, the better the current limiting capacity and the switching behavior of the PTC resistors , is not due to the small differences in the specific resistance or in the cold resistance of the individual samples, but solely to the suitable choice of filler particles.

This behavior can be seen particularly clearly from the measurements on samples E -N (FIGS. 2-4 and table), in which, in contrast to the measurements on samples AD, the charging voltage of the capacitor bank was 400 V and, accordingly, the test current was stronger increased than in the measurements on samples A - D. An extremely rapid and effective current limitation with good cold conduction properties was achieved with samples E to H, i.e. with PTC resistors in which, as in samples G and H, the filler particles 10 - 30 µm or, as with samples E and F, the filler particles were predominantly smaller than 20 or even 15 µm.

Compared to PTC resistors, which mainly contain filler particles with average diameters larger than 100 µm, PTC resistors, in which the predominant volume fraction of the filler has particles with average diameters smaller than approx. 100 µm or better smaller than approx. 70 µm, have a significantly improved switching behavior on. A particularly favorable switching behavior with low energy consumption, shorter Response time and small peak value of the current I _max carried in the resistor is achieved when the predominant volume fraction of the filler has particles with particle sizes smaller than 30 µm or even smaller than 20 µm.

However, the average size of the particles provided in the predominant volume fraction must not be chosen too small, since then among other things the specific resistance and thus also the cold resistance of a PTC resistor made from such a material increases too much. This can be seen from sample D, in which the filler particles had average particle sizes between 1 and 5 μm. In order to achieve a cold resistance comparable to that of Samples A to C at least (deviation approx. 50-60%), practically 50% more filler had to be mixed into the polymer with Sample D with a volume fraction of approx. 60% than with the others Rehearse. Current limitation through a PTC transition could not be achieved with such a resistor at a test voltage of 200 V. The limit current I _max given in the table above is only due to the high cold resistance of 226 mΩ and not due to a PTC transition.

A further improvement in the response behavior of the PTC resistor according to the invention is achieved if the filler particles are hollow or have a low mass, since particularly rapid heating of the polymer can then be achieved due to a relatively low specific heat.

Claims (7)

PTC resistor with an electrical resistance body made of composite material arranged between two contact connections with a polymer matrix and a powdered filler made of electrically conductive material embedded in the polymer matrix, characterized in that the predominant volume fraction of the filler has a fraction of particles, the middle of which Diameter is less than 100 microns and greater than 5 microns. PTC resistor according to claim 1, characterized in that the predominant volume fraction of the filler has a fraction of particles whose average diameter is less than 70 µm. PTC resistor according to claim 2, characterized in that the predominant volume fraction of the filler has a fraction of particles whose average diameter is less than 30 µm. PTC resistor according to claim 3, characterized in that the predominant volume fraction of the filler has a fraction of particles whose average diameter is less than 20 µm. PTC resistor according to one of claims 1 to 4, characterized in that the predominant volume fraction of the filler has a fraction of particles whose average diameter is greater than 10 µm. PTC resistor according to one of claims 1-5, characterized in that the filler is electrically conductive particles in the form of at least one metal boride, carbide, nitride, oxide and / or silicide and / or a metal and / or an alloy the base of the metal are provided. PTC resistor according to one of Claims 1-6, characterized in that the filler has predominantly hollow particles.

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