Hi,
First you may want to consider the combination of LLC and sync rects…
The main problem with synchronous rectifiers is that when the SMPS goes into light load, then the output current can potentially reverse and build up to ruinously high levels, due to the sync rects turning on when you’d rather they wouldn’t……..with a two transistor forward, the build up of reverse current is delayed by the output inductor. This is why the two tran forward is quite good for use with sync rects.
The LLC converter, as you know, has no output inductor….and so when having sync rects, and in light load, the reverse current can build up much quicker with an LLC…this is why the LLC is not quite so suited to sync rects as a two tran forward.
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...That FAN7688 of Hulk's looks great.
I see it uses secondary side controller...which means it will need an isolated offline flyback bias supply to provide power to it....also, it needs the current sense transformer.
There are other fairchild LLC chips which can mean you can do it from the primary side with less "extras" needed so to speak.
Though i confess that secondary side control is much better from a transient response point of view.
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Also, in the schem on page 10 of Hulk’s kindly supplied LLC app note….
The primary side fet drives show a Schottky at D2 and D8.
….Ive never seen this before…but suppose it must be for sweeping out the minority carriers out of the PNP so that it can turn off quicker (?)
Also, the resistors at R1 and R12…they are obviously for preventing ringing at the fet gate….but why put them inside the base-emitter loop?...i mean , you dont really want to overvoltage the Base-emitters of those PNPs and you’ve more chance of doing that with R1 and R12 where they are. I think theyd be better off outside the base emitter loop…though I have to confess that where they are, they probably allow a slightly faster turn off of the PNP.
FAN 7688App note
https://www.onsemi.com/pub/Collateral/febfan7688_i00250a.pdf