No, I wouldn't agree. How do you power the secondary side comparator in the event of an output short?
Thanks..ie, making it so that the absence of a signal means its gone short.
Not quite, as this results in a classic "chicken or the egg" kind of problem at startup.
I am not sure if you were referring to this but the problems i see for flyback bias coil in s/cct is that due to leakage L, it often doesnt collapse fully in output short circuit.
Well, you yourself didn't specify whether the "isolated offline smps" was a flyback or forward type, hence I tried to cover both in my reply.
That said, overcurrent protection using the bias winding voltage as a proxy for main winding overcurrent isn't perfect, it's just cheap and expedient, which tend to be unspoken design criteria for an offline 100-200W PSU these days. If you expect to need to deal with prolonged hard shorts on the main output of a DCM flyback then you will either need to make the primary side current limit circuit capable of operating from the meager current of the mains-derived startup circuit, or else use a separate power supply to either operate it or a conventional current sense circuit on the secondary. The separate rectifier on the bias winding trick is more geared to dealing with overloads and "soft" shorts, and even then it can really only address the latter by putting the controller IC into hiccup mode.
Also, i am not sure if you were speaking of this either but shorting the output of an offline flyback results in high staircasing of primary current due to the lack of secondary voltage to discharge the secondary coil, also the LEB time on the primary controller means there's unfortunately an on time of the primary which allows the unfortunate staircasing, hence the need for "timeout" overload protection (s/cct protection).
Staircasing of the primary current is always a problem in peak current mode control and/or when leading edge blanking is used on the switch current sense signal. Paradoxically, this problem is worse when synchronous rectification is used (and sometimes even with Schottkys) because the forward drop of the rectifier is so low. Again, if you expect to have to deal with a hard short on a flyback (which I define as demanding more than 10x nominal load current) then you might very well need to use alternative means of supplying the overload protection circuit. There is another technique that can be used which is kind of kludgy and dirty: oversize the rectifier on the main secondary and intentionally give that winding a higher than ideal leakage inductance; the latter will limit fault current while the former will better tolerate it. I suspect a lot of commercial flybacks under 300W or so use these techniques, whether intentionally or not, to improve their robustness.