Notes on Thermal Aging
in Inductor Cores

Coilcraft’s selection of materials makes thermal aging a non-issue
The phenomenon known as thermal aging in iron powdercores has been documented by the electronics industry
in recent years.

Thermal aging presumably has existed in iron powdercores for a long time, but it was not much of a concern whenelectronics operated in environments at or below 65°C.
But the proliferation of electronics into higher-temperatureenvironments of 85°C and above, along with the popularityof molded inductor styles that may contain iron powder,
has brought new focus to this phenomenon.

The phenomenon is particular to iron powder materials.
Neither ferrites nor other powder types like molypermalloy(MPP) are similarly impacted. Thermal aging refers to thecharacteristic of iron powder cores where the core loss
increases dramatically when the core is exposed to high­temperatures over some period of time. The increased coreloss leads to less efficient performance and can lead to a cycleof increased self-heating, in turn causing further increased
loss and so on. In a worst-case scenario, if unchecked, thiscycle can result in a devastating thermal runaway.

Iron powder core materials have a proven record of reliable
performance and value and are the core material of choice
for a wide range of applications, even in high-temperatureenvironments if suitable materials are used. Coilcraft XGL,XEL, XAL, XFL, PFL and XPL families (a.k.a. Ltra) use
such materials and are not subject to the thermal aging
phenomenon, as seen in the following figures. Core loss
was measured after exposure to a variety of time and hightemperature combinations. Figure 1 shows the stability ofthe material used in Coilcraft iron powder core inductors,
with core loss stable after aging at 165°C for 20,000 hours.
(Less than 5% change in two years at high temperature.)

Making this problem even more troublesome is the fact
that, depending on the operating temperature, the timerequired to trigger thermal aging may be hundreds or
thousands of hours — something that falls beyond normal
production line testing. Thus, gone undetected, the problem could lead to a potential field failure issue.

1000%
900%
800%

Core loss

700%
600%
500%400%
300%
200%
100%0%

02468101214161820Time (1000s of hours)
Figure 1. Coilcraft material shows little change after constant exposure to 165°C for two years.
Document 1192-1

Revised 05/16/19

155°C
1000%
900%
800%
700%

Core loss

The following figures show that other iron powder core
inductors can exhibit dramatically degraded performance.
Figure 2 shows stable core loss at 85°C for Coilcraft
XAL7030-102 and two other commercially available ironpowder core inductors. At 125°C, as shown in Figure 3,
the core loss remains constant for the Coilcraft inductor,
but shows a slight, increasing trend for the other inductors.

Core loss

500%
400%

Coilcraft

1000%

300%

Other - #1

900%

200%

800%

100%

700%0%

85°C

Other - #2

0600%
500%

500

1000

Other - #1
Other - #2

200%

1500
2000
Hours

2500

3000

3500

Figure 4. At 155°C the other inductors show
dramatic core loss.

Coilcraft

400%
300%

165°C
1000%

100%

900%0%
0500

1000

1500

2000

2500

3000

3500

800%

Hours

125°C
1000%
900%

Core loss

700%

Figure 2. At 85°C core loss is stable for all three inductors.

600%
500%
400%

Coilcraft

300%

Other - #1
Other - #2200%800%

100%

700%

Core loss

600%0%
0600%
500%
400%

Coilcraft

300%

Other - #1

100%0%0500

1000

1500

2000

2500

3000

1000

1500
2000
Hours

2500

3000

3500

Figure 5. At 165°C one inductor shows an increase incore loss of nearly 1000%.

Other - #2

200%

500

3500

Hours

Figure 3. At 125°C only the Coilcraft inductor
remains constant.
Figures 4 and 5 demonstrate the dramatic core loss
increase caused by thermal aging. Both of the otherinductors exhibit greater than five time increases in coreloss after only 1500 hours at 155°C. The increase is evenmore dramatic for inductors aged at 165°C. In this case,one of the inductors has a core loss increase of nearly 10
times its original performance.

Details of the exact materials used by Coilcraft and otherinductor manufacturers are undisclosed, being proprietary
information, so it is not feasible to make exact predictions
on the lifetime effects of thermal aging. Nor is it feasible
to make definitive statements of suitability for use in any
particular application. The information provided here isbased on the test results described with the single intent
of informing about the thermal aging phenomenon in ironpowder cores.
For a more detailed explanation of thermal aging in ironpowder cores, visit the Micrometals page on Thermal Aging:www.micrometals.com/file-manager/files/shares/Thermal
Aging Core Loss.pdf.

Document 1192-2

Revised 05/16/19