Testing offline flyback with various CTR opto's

[cupoftea]

Hi,
Just testing a NCP1239 Offline flyback (24V, 20W) with opto’s with CTRs from 50% to 600% to check stability with all of them.

Ayk, its impossible to buy a “600% optocoupler”. You can only get “300-600%” and any part may or may not actually be near 600%.
So what do you say about using two 300-600% optos with the diodes in series and the emitter_collectors in parallel? Surely then I am bound to get at least an effective 600% CTR part ? ( i dont mind if effective CTR is >600%)

At the other end, to get a 50% CTR opto (or lower CTR) ….i suppose you could pick a 40-80% opto, and bleed off half of the collector current with a transistor that gets switched on by the opto’s collector?

The NCP1239 makes it awkward to actually measure what CTR opto you have..because the opto pullup resistor is 25k, and it “hangs down” from a 4v3 rail inside the NCP1239…..so the max collector current that you can get is just 172uA.

When using a IS357D (300-600%) opto in the actual flyback in lightish load, I measured 183uA in the opto diode, and 120uA in the opto collector…so that’s just 66% CTR. But its because, ayk, its at such low current. I wonder what actual “nominal CTR” this opto really is?

 

[cupoftea]

So, you wish to design and test an offline , Current mode Flyback. You want it to be stable with any opto with nominal CTR between 50% and 600%. The flyback is in CCM at low mains, with max of 0.57 Duty cycle, and DCM at high mains.

But when you buy an opto, you can only buy say “50 to 600%” or say “160 to 320%”, or say “300 to 600%” , or “40 to 80%”.

So how do you guarantee that you actually test your flyback with a nominal 50% (or less) opto, and also a nominal 600% (or more) opto?

..The answer is that you buy two “300 to 600%” opto’s, and put them in the test circuit 1 attached.
You then test that for stability.
You then buy one “40 to 80%” opto, and put it in the circuit 3 attached, and check for stability.

Would you agree, this is the best method?

PDF and LTspice sim attached