Kristen Parrish

Storage temperature rating is a key metric used to determine power-module reliability, but this metric isn't always as it seems. In order to be sure that you're selecting the correct module for your devices, you need to be sure that the module manufacturer is independently verifying the stand-alone inductor component used in the power module. Inductor qualification tests are not standardized across companies and quantities, and ratings that appear on data sheets can be inconsistent. In this blog, I will explore TI's high-temperature storage (HTS) testing of modules as well as the individual testing of the inductors.

What Is High-temperature Storage Testing?

Accurate HTS testing begins with exposing the device to a rating temperature for 1,000 hours, and then checking for consistent characteristics before and after the exposure. Our power modules are rated to a storage temperature of 125C or above, meaning that the devices are guaranteed to maintain performance after exposure to high temperatures.

Going the Extra Mile to Ensure Reliability with Your Power Module.

In addition, the module qualification regimen pre-screens module inductor components to ensure that they will meet a storage temperature requirement of 150C as stand-alone components as well. Although inductor data sheets often cite a storage temperature, we have found that some inductors on the market that are rated to a specified storage temperature aren't suitable for long term reliable usage. The inductors are not usable because thermal aging in the material occurs after exposure to high temperatures. This well-known phenomenon may not have been a concern if your devices are used below 65C, but it has a significant effect on devices made to operate at temperatures above 85C. You can observe the change in the material by thermally measuring the losses of the inductor under test currents, or by using the inductor in a switching converter and noting a drop in efficiency, especially in wide V_in or high-output-current devices.

Although these two test methods may be complex, they are worthwhile to complete. Without qualifying the inductors before prototype and production, redesigns occur and cause delays, financial losses and frustration. You can also determine thermal aging with two straightforward measurements, which TI uses to qualify and select inductor components.

Let's Look at the Data.

Figure 1 proves thermal aging's impact on inductors by showing two simple measurements that indicate HTS failure. Samples from four different inductor vendors are evaluated at 150C with measurements every 168 hours, showing significant drops in both quality factor (Q) at the switching frequency, and shunt or core resistance measured without biasing the device. The inductors that fail this test demonstrate high losses post-exposure when used in a buck-converter application, in spite of a lack of any visually observable changes, or change to direct current resistance (DCR) or L₀. This impact would be more significant in a wide V_in device and/or a device with a high output current simply due to the increased leakage current flowing through the now-reduced shunt resistance of the inductor.

In the evaluation shown in Figure 1, feedback from TI enabled one vendor to sample a second device, using an improved magnetic material that did not demonstrate thermal aging (vendor A2 in Figure 1), which was then chosen for the TI module. The details of the exact revision made to the device are generally proprietary, but it is likely that the revised device had a different material formula than the first device.

Conclusion

We have developed an independent inductor testing regimen for every inductor component selected for use in a power module. The HTS test is a critical aspect of a reliability evaluation, enabling us to assess whether an inductor may exhibit significant core losses once exposed to high temperatures for an extended period.

This test is significant, since the deficiency may not be obvious when measuring only the inductance and DCR before and after exposure. Qualifying the inductor component by itself is critical to ensuring a reliable switching converter. When purchasing a module from us, you can avoid revisions and reliability issues later in the design cycle because our rigorous testing ensures that our parts are reliable at every temperature.

Additional Resources

• Watch the video: https://training.ti.com/high-temperature-storage-stress-testing-power-modules