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Posted on 15 March 2019

PTC Thermistors for Overcurrent Protection

Epcos PTC thermistor

 

 

 

 

 

 

 

As to their possibilities of application, PTC thermistors (Positive Temperature Coefficient thermistors) can be divided in the following manner:

1. By Function

PTC thermistor functions

2. By Application

PTC thermistor applications

Ceramic PTC thermistors are used instead of conventional fuses to protect loads such as motors, transformers, etc., or electronic circuits against overcurrent. They not only respond to inadmissibly high currents but also respond if a preset temperature limit is exceeded. Thermistor fuses limit the power dissipation of the overall circuit by increasing their resistance and thus reducing the current to a harmless residual value. In contrast to conventional fuses, they do not have to be replaced after elimination of the fault but resume their protective function immediately after a short cooling down time.

As opposed to PTC thermistors made of plastic materials, ceramic PTC thermistors always return to their initial resistance value, even after frequent heating/cooling cycles.

PTC thermistor fuse connected in series with the load

Figure 1. PTC thermistor fuse connected in series with the load

IL → Load Current

Operating states of a PTC thermistor for overcurrent protection

Figure 2 illustrates the two operating states of a PTC fuse. In rated operation of the load the PTC resistance remains low(operating point A1). Upon overloading or shorting the load, however, the power consumption in the PTC thermistor increases so much that it heats up and reduces the current flow to the load to an admissible low level (operating point A2). Most of the voltage then lies across the PTC thermistor. The remaining current is sufficient to keep the PTC in high-resistance mode, ensuring protection until the cause of the overcurrent has been eliminated.

Operating states of a PTC thermistor

Figure 2. Operating states of a PTC thermistor: a) Rated operation; b) Overload operation

RL → Load resistance
IS → Switching current
IR → Rated current
RPTC → PTC resistance

Considerations on the rated current IR

An essential parameter for the function and selection of a PTC thermistor fuse is the rated current. It is mainly a function of:

  • PTC dimensions
  • PTC temperature
  • PTC resistance
  • Heat dissipation

Very often, high rated currents are required. Higher rated currents with unchanged resistance are obtained through larger thermistor dimensions (see figure 3) or by raising the reference temperature. Favorable conditions for high rated currents can be achieved by making the best possible use of the cooling effect of the environment. Manufacturers can contribute to good heat dissipation by producing thermistors with large surfaces and making them as thin as possible. The user can enhance the heat dissipation effect by further measures (e.g. cooling fins) so that protective ratings of more than 200 W per component can be achieved.

PTC thermistor volume versus rated current

Figure 3. Influence of the PTC volume V on the rated current at given resistance RPTC

Another mechanism for controlling the rated current is the PTC resistance itself. To keep the difference between rated and switching current as small as possible, PTC thermistor fuses are only produced in narrow resistance ranges. In practice, this leads to PTC types with tolerances of 25% and tighter so that the protective function is also possible in applications with only slight differences in current between rated operation and overload.

Another quantity affecting the rated current is the ambient temperature at which the PTC thermistor is operated.  Figure 4 illustrates this relationship.  An increase in ambient temperature means that the PTC thermistor reaches the temperature causing it to trip with much less power consumption. A cooler environment has the opposite effect, i.e. power consumption and rated current rise.

PTC rated current versus ambient temperature

Figure 4. Standardized rated current IR versus ambient temperature TA (measured in still air) Parameter: Tref1 < Tref2 < Tref3

Switching time versus switching current

The dynamic heating behavior of the PTC thermistor is determined by the specific heat capacity of the titanate material, which is approximately 2.7 Ws/Kcm3. At short switching times less than 5s with commonly used overcurrent protection devices, heat dissipation through the surface and lead wires is virtually negligible. Almost the entire electrical dissipation is consumed to heat up the ceramic material, to increase the temperature above the reference temperature and thus to produce a stable operating point on the R/T characteristic. When dissipation increases with rising difference between device temperature and ambient, only a small amount of excess energy remains for heating the component and the result is the switching time curves as a function of switching current shown in figure 5.

PTC thermistor switching time versus current

Figure 5. Switching times tS of some PTC thermistors (parameter: different geometries) versus switching current IS (measured at 25° C in still air)

 

For more information, please read:

Interruption of DC Fault Currents

Fuse Placement in Typical Converter Circuits

Overvoltage Limitation for Power Transistors

Overvoltage Protection with Varistors

 

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