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LLC Resonant converter frequency

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I was reading up on LLC resonant converters and I see all of them that are designed for around 1-kW operate at very high frequencies (up to 1-Mhz). I was just wondering if there is a reason why they don't operate in the region of around 50 to 200-kHz and if it is possible to design one that operates around that frequency. Are they only switching that fast to reduce the transformer size and to achieve a better packing factor or is it a requirement for resonant converters to switch that fast?

This is just a general question for someone who has worked on these things before.
 
I have not designed such a converter but have seen them mentioned on and off for decades. Most likely the reason they show up more frequently in high power applications is that they are (generally) more efficient and generate less EMI. If I recall correctly the control of such converters are tricky compared to the ones with which we are more familiar and that might be the reason they are not popular in lower power applications.
 

crutschow

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I see no intrinsic reason they couldn't operate at a lower frequency.
But as you noted, the higher frequency allows for smaller, lower weight magnetics for a given output power.
Generally you operate a converter at as high a frequency as you can without sacrificing significant efficiency, to minimize the size of the magnetics.
 
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