Traditionally, determining appropriate component temperature was based on a combination of datasheet information and derating strategies. This method has since been proven outmoded since it does not factor in actual failure models and degradation mechanisms, resulting in expensive designs and/or products that lack optimum reliability.
Today, power supply engineers have access to finite element analysis, computational fluid dynamics and a host of other thermal simulation tools, yet the prevalent thermal testing question remains: how hot is too hot?
For magnetic electronics components like transformers and chokes, the answer lies in physics-based reliability testing.
Challenges for magnetics
Magnetics are often the component technology that is least considered when concerns about temperatures arise during design review and analysis. Pair that with customized magnetics, like transformers, that typically don’t come with a heat rating and determining how hot is too hot for magnetics gets murky.
There are three key issues of concern:
- Saturation current: Ferrite material has a soft saturation curve that tends to obscure as a material starts to saturate, causing temperature variance. Saturating magnetic material will not damage magnetic components, but it will appear to be shorted to the electronic circuit and can cause circuitry failure. Debugging is difficult because the transformer or inductor seems fine at room temperature.
- The maximum temperature rating being erroneously equated to the Curie temperature: The Curie temperature ranges between 100°C and 300°C, yet core loss usually reaches a minimum at temperatures between 50°C and 100° Depending on the ferrite design, structure and cooling, the magnetic can go into thermal runaway, meaning if the core temperature starts on the correct side of the minimum but core loss increases over time, higher temperatures and higher core loss results.
- Thermal aging: When cores – especially powder iron cores – are exposed to elevated temperatures, the binding agents undergo thermal aging. Factors like core material, peak AC flux density, operating frequency, core geometry, copper loss and core temperature all also impact thermal aging of magnetics. As thermal aging progresses, the eddy current loss increases significantly as does core loss. Increased core loss eventually results in higher core temperatures and magnetic component failure.
Mitigating the effects of temperature on magnetics
There are several things power supply engineers can do to mitigate the effects of higher temperatures on magnetics, including:
- Using low loss core material in the device
- Introducing higher frequency with total turns reduction while maintaining turns ratio
- Using Litz wire to reduce coil heating
- Applying varnish with vacuum evacuation to facilitate improved thermal performance and allow for washing
Physics-based reliability testing
While the issues are clear, power supply engineers have been challenged in resolving them due to a lack of effective tools.
Derating or MTBF miss the real temperature driven risks, leaving reliability-based physics as a viable alternative. Degradation behavior is predicted and tradeoff analyses are performed with validated algorithms that use environmental, material and architectural information for accurate guidance and prediction on the performance of the power supply.
DfR Solutions’ Sherlock Automated Design Analysis™ Software bridges the gap in design tool capabilities. Sherlock uses Physics of Failure (PoF) to combine standard design information with comprehensive embedded databases to arrive at the inputs necessary to complete complex calculations in streamlined software architecture. Sherlock better predicts failures and guarantees product reliability before prototyping, speeding the design and testing process and getting products to market faster.
Find out more about physics-based reliability testing and Sherlock in our webinar, Introduction to Physics of Failure. Click the button below to download the slide presentation now.