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I have made my own LLC converter using the UCC25600 controller chip. I can only operate the power supply at some Rmin and Rmax for the output load. Controlling the Rmin makes sense to me because you do not want to short circuit your load. However, Rmax --> inf, the LLC architecture cannot regulate the output load. I am trying to find some easy (ish) way to regulate the output voltage at an open (or light load) condition. I was thinking of putting on some onboard logic like the Pi Pico and following this logic tree enter image description here

I would love some feedback on if this is a 'good' path to go down or if there are simpler architectures that people have used?

Erik McKee
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    If you afford the power loss, just put a minimum load resistor on the output of the power supply. – SteveSh Apr 04 '23 at 23:46

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There's no need to do this, the UCC25600 already does it for you: It's called "Burst Mode".

Quoting the datasheet, section 7.4.1:

To prevent output overvoltage during this condition, the UCC25600 includes the burst-mode operation function. When the control loop demands switching frequency higher than 350 kHz, the gate driver is disabled and the power stage stops switching. When the output voltage drops, the control loop begins to demand switching frequency less than 330 kHz, the gate driver recovers and the power stage begins to deliver power again. This allows the output voltage to be regulated.

All you have to do is to set the maximum frequency limit significantly higher than 350kHz (i.e. 500kHz).

Jonathan S.
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