Every optocoupler has min and max current transfer ratio. I have read articles that said always design the circuit using the minimum current transfer ratio. However, I want to understand which parameter affect the transfer ratio. I did look at a datasheet of an optocoupler but couldn't find any information about it. Could someone please explain them to me?
The transfer ratio will vary in manufacturing from lot to lot and from device to device. This will be due to variation in LED, receiver, insulator and gap distance in manufacturing.
The minimum current transfer ratio is guaranteed and if you design to this your circuit should work. Meanwhile you need to keep an eye on the max value if there is a risk of problems due to too much current on the output side.