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Steady state analysis for thermal performance

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Hi:

My issue for the thermal analysis is that the system always reports an unrealistic junction temperature that is far below the limit, which is shown in the first picture, in other cases it might display a unrealisitically high temperature.

I have already set the initial temperature of all components as 50 degrees so there should not be a negative temperature. However, the thermal circuit I simulated in the normal way will successfully converge to around 150 degrees and will never reach that high junction temperature, so I am confused about the difference between the two different simulation results. May I ask which part of my design or PLECS setting is wrong? Thanks.

The normal simulation result is in the second picture. My thermal circuit and target component is shown in the third picture.

Haochen Tao
asked Mar 9, 2022 by Haochen Tao (18 points)
Are you able to post your model here as well or send it to support@plexim.com? Then we will be able to debug what's happening in your model.
Thank you. I have attached the file below.
Do you also have the "mosfet_description.xml" and "diode.xml" files?
Yes, here they are. I assume the circuit has already achieved ZVS so the switching loss is 0. I also assume that the on-resistance does not change with temperature.

1 Answer

0 votes

Haochen,

I was able to reproduce your error based on the model you posted. In this case there are some changes to the model settings (solver and Steady-State Analysis) that are required for this to converge at the proper temperatures.

First, you should be first tuning the Relative Tolerance setting to a smaller value and not the Max Step Size, to improve speed and accuracy. The step size setting only limits the largest step the solver can take, but not influences its performance in between any two steps, whereas the tolerance setting instructs it how to check and determine accuracy of each step. I have changed these from what you had of Max step size = 1e-7 and Relative tolerance = 1e-3 (the default value) to Max step size = 1e-3 (back to the default value) and Relative tolerance = 1e-5, as shown below:

Further, because the initial thermal states (temperature) are so far from the final values, unless you use initial temperature values closer to the final ones (than 50C like you have now), you need to add significantly more Initial cycles. I found that 200 worked well:

With these changes to the model the Steady-State Analysis should now work as you expect, but I attached my version of your model for reference.

Let me know if you have any questions.

Best,

Kris

answered Mar 9, 2022 by Kris Eberle (1,575 points)
Thank you very much for the help, I think what you pointed out is right where the issure lies in my model. Really appreciated for it.

Regards

Haochen Tao
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