I use a fast optocoupler(6n137) to interface two circuits. When ON signal comes to input, the output transistor immediately turns on without a significant delay. But when OFF signal comes to input, the output transistor of the optocoupler doesn't turn off immediately. It takes 6-7us to turn off. How can i reduce this time. The load is an igbt driver and optocoupler has a delay capability of sub microsecond at ON and OFF.
All depends on type of load. If load has 200-300pF capacitance then you will have turnoff delay abt. 5-6us. Optocoupler, specified delay time is valid foe 350ohm paralleled with 15pF.
It's the recommendation of the igbt gate driver manufacturer so it's not related to the 6n137. On the other hand it can be reduced but not as low as to 330ohm, i think.
Or you can use circuit similar to that below, where the optocoupler's transistor is Q1..
Ather option without resistors, diodes etc is two transistors NPN at the top and PNP at the bottom, with both bases connected together as input and both emitters connected togeter as output, collector of NPN goes to (+) and collector of PNP goes to GND.
Regards, IanP