Thanks, yes, those are great points. AYK, the series diode gives extra conduction loss.....though luckily it means you can now use a really low rdson FET , whereas without it, you have to be wary of the fact that a low rdson fet may not have enough conduction vdson to properly reverse recover the internal fet diode.
Agree about the design for edge case......
AYK, the thing with LLC, is that those edge cases involve anything that may bring about the reverse recovery......so thats overload, short cct output....undervoltage input.....startup, re-start-up, heavy load transients, and some more. Also, since even one single instance of the reverse recovery in an LLC can kill or seriously weaken the converter, the detection needs to detect the disaster before it happens, and then stop switching the fets so it doesnt happen..........to detect this (imminent_reverse_recovery) in that timescale is extremely challenging.......as discussed, infineon claims to have LLC circuitry and software that can do that ( i think they do?)...........but if it works, then it is truly amazing........without that software based proteciton, it needs the series diode to be sure of not getting reverse recovery based destruction.
....AYK, its an old technique, but i believe needs re-introduction due to the rising popularity of the "current sloshing" converters like the LLC....AYK, "current sloshers" slosh the primary current about so that the fet diode comes on just vefore the fet switchs on........others in the same category, AYK, are Phase shift full bridge and the "Half_bridge_with_big_leakage_inductance_but_no_output_inductor"...aswell as one variety of the forward converter if the leakage L in the txfmr is big enough. (Active clamp forward).
AYK, the problem with "current sloshing", is that it can get "out of phase", and the diode can get sloshed on just as the opposite fet is turned ON....literally one single instance of this can kill or seriously weaken the converter....thats why its so difficult to avoid if you dont use the series diode.
I agree about the parasitic inductance getting increased when you add the series diode......we would try and switch the LLC fet on very slowly, and wish to get away with that due to the fact that its in ZVS.......but yes, i agree, the extra parastic inductance is certainly not welcome.
But yes, and its literally amazing that the App Notes and semico's dont talk of this.......touting LLC controllers without mentioning it isn't a great idea...some of the LLC App Notes work through Ohms Law calcs with you......but then dont bother to tell you about the reverse recovery issue.
And if infineon really do have that avoid_reverse_recovery software....then the Nobel Prize wouldnt be enough for them!...or even a mention on BBC's "click" program!!