Hi,
We need a “rough” isolated rail of 45-53V at 200w. Vin is PFC output. It Undulates from 370v to 430v at the usual 100hz.
We wish to simply use an half bridge LLC converter switching at f0 all the time. We will make the NS/NP equal to 0.5 *(vin/vout).
..No feedback…who needs feedback? In fact, feedback in an LLC converter makes the LLC more self-destructable…because during a sudden heavy load transient, the feedback error amplifier would command a lower operating frequency…..and its lower operating frequencies which bring about overcurrents in an LLC converter……so scrub the feedback…its not needed, would you agree?
……….Actually the FSFR2100XS LLC controller looks like its good for this. It can provide a constant switching frequency of 100khz (94khz to 106khz) if a 5.2k resistor is connected to its RT pin. (94khz to 106khz).
Another resistor R_START could be placed in parallel with it and switched in only at startup, so that the frequency was f0*1.5 for a soft start. During this soft start, it would be ensured that the secondary loading was off.
Once the Vout gets up to 48V, then the R_START resistor would be switched out…leaving the controller to gormlously switch away at 100khz.
I really cant see any point in having the TL431 based error amplifier….i mean, it wouldn’t be needed…so just leave it out…would you agree?
The only threat would come from output overload……but that would cause the primary side sense resistor to trip the controller out…so even that’s not a problem.
Why doesn’t the datasheet suggest doing it like this?
The LTspice sim (attached) of it shows that even when the secondary is suddenly no_loaded , there is no serious overvoltage at vout….it just harmlessly goes up a few volts.
(when the load suddenly goes from no_load to full_load, then , there is no serious undershoot in vout, and no overvoltage on the resonant capacitor which would be 1kv rated.
“Dumb oscillator” switching away and does the job great…where’s the gotcha?
FSFR2100XS datasheet
https://www.onsemi.com/pub/Collateral/FSFR2100XS-D.pdf