optocoupler in flyback design
In a closed-loop system like with a switching power supply, the optocoupler collector current will vary depending on the operating conditions. For most of the controllers configurations, regulation is ensured by pulling down the FB pin. This is easy for the PCB layout as you do not need to resort to another bias and route it to the opto.
In the below configuration, you can see that the pull-up resistance is 20 kohms and the internal \$V_{dd}\$ bias is 5 V. The maximum collector current is therefore obtained with the collector-emitter junction saturates at 150 mV or so: \$I_{c,max}=\frac{V_{dd}-150m}{20k}=242.5\;µA\$

This collector current will then be reflected in LED current by an \$I_F\$ value depending on the optocoupler current transfer ratio or CTR. Now it depends on several aspects on how you want to select your CTR:

you do not want to operate the LED at too high a current because its life-time depends on it. Fortunately, with a 250-µA collector current worst-case, you will operate the LED at a very small current and its dynamic resistance \$r_d\$ will affect the overall gain. Unfortunately, in this low-current operating mode, the CTR is extremely poor and values of 100-200 given by the data-sheet often drop to 20-30%.

The below table excerpted from the SFH615A shows how the CTR drops at two different LED currents:


if you want to push crossover and do not want to be bothered by the low-frequency optocoupler pole, then you will select low CTR grades. They have a smaller parasitic capacitance compared to the large CTRs ones but obviously impose larger circulating currents. If you select the large CTR grade, you will see how poor the dynamic response will be.

if you look for the smallest standby power at no load, you want to minimize the LED current and will shoot for the largest CTR. For a 19-V output, assume a CTR of 30-40%, then the 250-µA collector current becomes a 833-µA LED current which, multiplied by 19 V gives 16 mW lost in regulation. When brought back to the primary side, it can easily be the double. Now choose a higher CTR grade having around 100% despite the low LED current and the regulation power drops to (250u/1)*19 = 4.75 mW which is much lower than before.


The PC817 is a popular reference and I have seen also the SFH615A quite often in high-volume designs.