I often see this in practical SMPS designs. An optocoupler & voltage reference pair is used for informing the controller section about the output voltage level. However, when I read the datasheet of the optocoupler in the circuit, I see that the given signal characteristics "rise time" and "fall time" are higher than the switching period of the controller circuit.
In the schematics, it says that the switching frequency of this circuit is
250kHz, which means the switching period is
4us. In the optocoupler datasheet, the rise time is given as
4us (typical) and
18us (maximum). The fall time is
3us (typical) and
18us (maximum). The signal has to rise and then fall, so it takes typically
36us at maximum) for a pulse. An engineer must make his design considering the worst case circumstances, so we should take the maximum pulse time as
36us, which is
9 times the switching period.
Wouldn't it mean that the controller will respond to an output change 9 periods late? Is this much latency acceptable, doesn't it cause output voltage level controlling problems? Or, is there anything I'm missing here? Can you please explain me how does this kind of SMPS designs work with all these slow optocouplers?