Electronic – Under what conditions does an optocoupler work fastest

high speedoptimizationopto-isolatorswitching

I want to use the optocoupler FODM452R2 in my design. I want this optocoupler to run as fast as it can (i.e.; work with minimum propagation delay and least rise/fall times). What is the optimum R1, R2 resistor values and magnitudes of V1, V2 voltages?

I know that when BJTs saturate they turn off very slowly, so I have to avoid saturation. I also know that BJTs don't respond fast when the resistor value is too high in common emitter configuration. So there is a trade off. The value of R2 must be chosen in a way to optimize the BJT's switching speed.
Is there an optimum value for R1 as well? Or, is the smaller the better without damaging the LED? On the other hand, does the value of V2 have any effect on the switching performance?

The datasheet doesn't directly answer my question, or the answers are encoded in it in a way I can't understand. It only gives the variation in propagation delay with ambient temperature.


simulate this circuit – Schematic created using CircuitLab

Best Answer

The key to maximum speed appears to be Figure 8, the frequency response, plotted at different values of RL (R2 in your circuit), and the fastest is with RL=100 ohms, where full amplitude is maintained up to 1 MHz corresponding to rise and fall times below 0.5 us, correlating reasonably well with Figure 7.

Now at the conditions for measuring Fig8, the LED current is 16mA, so R1 can be calculated to give 16mA from your supply voltage and the LED Vf. Not especially critical or interesting.

But the current transfer ratio (p.4) is guaranteed to be between 20% and 50%, giving somewhere between 3mA and 8 mA through RL/R2, or 0.3 to 0.8V pk-pk. On a 5V supply such a low value of RL keeps the phototransistor well out of saturation, but needs a gain stage to amplify the signal developed across RL to recreate logic levels.

If that's not fast enough, a different device or different approach may be necessary.