Does propagation delay in an opto isolator sum with switching time in a FET?
Of course, the delay is summed as well the propagation times, like rise fall. If you need very high switching frequency, then optocouplers may not be the good option. MAny applications use IR21xx gate drivers without galvanic separation, or you can use pulse transformers, or magnetic couplers instead of optocouplers. In your case you have tr and tf 100ns, which will determine the of switching loss, the propagation delay can be taken into account to set proper trigger delay, to avoid the cros conduction.  

Or perhaps you want to acheive nonreasonable high switching frequency, please explain what is the device used for. 

This is a cheap alternative used in many welders op to 50kHz (DT&lt;50%).






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