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Calculating Switching and Conduction Losses of a Resonant Converter

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I'm using Probe along with periodic average block to calculate conductions losses & Probe with Periodic Impulse Average block to calculate switching losses of a CLLC Resonant converter in Plecs. For both the averaging blocks, averaging time should be specified as its internal input, which I think should be equal to switching period. But the resonant converter is operated by closed loop frequency control and switching frequency is calculated online. So is there any way to specify averaging time of averaging blocks as external input? If not how can I specify the averaging time of averaging blocks, since switching period is unknown?
asked Nov 20, 2020 by Tharuka (20 points)

1 Answer

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Best answer

The averaging blocks in the PLECS library do require periodicity and cannot be adapted to have a variable averaging time as an external input.

You could consider averaging over a longer period to approximate the losses, assuming a settled system. Of course, you’d need to find the right balance between the averaging time and the accuracy (because you’ll invariably capture a non-integer number of cycles), but perhaps you can determine “ballpark” losses this way.

For more accurate estimation of variable-frequency conduction losses you can probably process the loss waveform to use the edges as calculation triggers such that two successive rising edges mark the starting and ending of your averaging period.

If you want to fully engineer a solution for switching losses, I believe you would need to integrate the loss calculation into your controller design. The "Periodic Impulse Average" is a masked subsystem around a C-Script. One could add a "Period" input and replace the fixed-step task (for calculating the outputs and resetting the internal counters) with a discrete-variable task. However, unless you update the task hits for this discrete-variable task in the exact same way as the modulator, there is the risk that the switching cycle and the loss calculation will drift apart over time.

I hope this helps.


answered Nov 20, 2020 by Kris Eberle (1,607 points)
selected Nov 20, 2020 by Tharuka
Thanks Kris. Your answer is helpful.