In terms of structure, high-speed optocouplers are different from ordinary optocouplers. The structure of high-speed optocouplers is a photosensitive diode + amplification drive circuit, while the structure of an ordinary optocoupler is a phototransistor (+ amplification drive circuit). The response speed (rise and fall time) of the photosensitive diode is on the nanosecond level, and the response speed (rise and fall time) of the phototransistor is on the microsecond level. It does not mean that ordinary optocoupler can be high-speed when it works in the linear region. Its inherent response time is so limited that it cannot be fast even if it wants to. In addition, if the ordinary optocoupler works in the linear region, it will also be limited by the cut-off frequency Fc (Cut-offFrequency) parameter. The Fc of the ordinary optocoupler is basically around 50KHz (test conditions VCC=5v, IC=5ma, RL=100R , Fc becomes smaller as RL increases. When RL=1K, Fc is about 10KHz), like TLP521, Fc is about 50KHz, PC817, Fc is about 80KHz, CNY117, Fc is about 250KHz. Of course, some ordinary optocouplers can indeed reach speeds like 500KHz when increasing the drive current (to 200MA)/reduce the load resistance (to 500OHM)/optimizing the drive pulse.
