cupoftea
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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?
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?