Overview

Inductors change their characteristics depending on the operating temperature.

DC resistance, inductance, and saturation characteristics are the main characteristics that change, and the amount of change varies depending on the core material used.

Temperature change of DC resistance

Copper wire is used for most inductors, and the DC resistance value is determined by the electrical resistivity, wire diameter, and length as shown in formula (1). A temperature change occurs due to the temperature coefficient of copper in the value.

ρ: Resistivity

S: Cross-sectional area

ℓ: Length

The resistance value due to temperature change can be calculated by formula (2).

RT: Resistance value when temperature changes to T​

Rt: Resistance value when temperature is t​

αt: temperature coefficient of resistance

Consider the effect of using a temperature coefficient of resistance of 0.00393 @20 °C at 100 °C.

DC resistance increases by about 31% as shown in formula (3).

Note that this change will increase losses and heat generation.

Figure 1 shows the results of a comparative simulation of the temperature rise value of an inductor rated at 1.8A using Wurth Elektronik 's RedEXPERT for verification. It can be confirmed that the temperature rise value is also about 1.3 times.

Figure 1: Comparison of inductor temperature rise due to changes in operating temperature

change in inductance

Inductance can be obtained from the iron core performance (shape, material, etc.) and the square of the number of turns, as shown in formula (4).

AL value: Value determined by core performance (magnetic permeability, cross-sectional area, magnetic path length)

N: number of turns

When the temperature changes, the magnetic permeability changes, so the inductance also changes accordingly. However, the amount of change varies greatly depending on the core material used for the inductor, and the change is large for inductors that use ferrite.

Fig. 2 shows an example of the magnetic permeability and temperature characteristics of a ferrite core.

The magnetic permeability increases as the temperature rises, decreases when it approaches the Curie temperature, and becomes 0 when it reaches the Curie temperature.

With this example of characteristics, the inductance will change by about ± 40% in the range of-40 °C to 105 °C with normal temperature as the reference value.

Fig. 2: Example of ferrite core permeability vs. temperature characteristics

In the case of metal cores, which are frequently used in inductors like ferrite cores, this change is small, so there is no need to consider it that much.

Change in saturation characteristics

Regarding the saturation characteristics of the core material, the saturation magnetic flux density (Bs) of the ferrite system also decreases at high temperatures, so the characteristics deteriorate.

What needs more attention is that even if the current is the same, the inductance will drop at high temperatures.

Figure 3 shows the electrical characteristics of Wurth Elektronik 's 74477810 using a ferrite core.

Figure 3: 74477810 electrical characteristics

From Wurth Elektronik 74477810 datasheet​

In the characteristics of Fig. 3, Isat is specified as |​ ​⊿L/L|<10% at 2.2A, so it can be seen that it drops to 9μH at 2.2A.

However, this is a regulation at room temperature, and in a high temperature environment, the saturation magnetic flux density drops, so the inductance drops more at high temperatures.

Figure 4 compares the DC superimposition characteristics of this inductor at 20 °C and 100 °C.

Figure 4: Changes in DC superimposition characteristics due to temperature conditions of 74477810​

It can be seen that the inductance of 9​ ​μH at 20°C at 2.2A has decreased to about 6​ ​μH at 100 °C.

Metallic cores have little temperature change, so there is no need to consider changes due to temperature conditions.

Fig. 5 shows changes due to temperature conditions when using a metallic core.

Figure 5: Changes in DC superimposition characteristics depending on the temperature conditions of the metallic core

​Wurth Elektronik​ ​HP​

Summary

Inductors change their characteristics depending on the operating temperature.

When the temperature is high, the heat generated by the DC resistance increases, and the inductance value and DC superimposition characteristics of products using a ferrite core change.

Information on simulation tools

Please click the image above. Link to the simulation tool "RED EXPERT" page.

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