Hi Conor, The LLC topology is typically limited to a 2:1 variation in the output voltage. Here is a lower power/lower voltage reference design ( PMP23454 ) which supports trickle charge from 0-12.5V and full charge from 12.5 to 22.5V using the UCC256601. The UCC256601 is easier to design with as its new feature IPPC (Input Power Proportional Control) makes it easier to avoid entering burst mode when the voltage is low, but the current is high. Another option for this power level could be the UCC28950 phase-shifted full-bridge. What is nice about this is that you can parallel power stages to help improve the efficiency and reduce output ripple. I hope this answers your question.
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