Hi,
So with a quick opamp. I need to detect overcurrent and and turn off IGBTs
An OPAMP is an amplifier, it has an analog output.
To detect "= logic output" you should use a comparator.
(Although OPAMP and comparator share the same schematic symbol, they are designed for different operating conditions. It has been discussed several times here in ths forum)
I'd say for your overcurrent system a comparator is essential, an Opamp only is needed if the analog signal is too low for the comparator to work correctly.
What exactly do you mean with "soft turn off"? .. in a discussion about "short circuit detection".
One question is, how do I determine if the spike is passing or is the short circuit ?
I don't understand what you mean with "passing".
"Short circuit" usually is meant when the outputs of a device are connected with (too) low impedance. This usually causes higher current than at normal operation. You (the designer) has to determine the behaviour of current vs time at worst case normal operating conditions.
Then you need to determine worst case short circuit conditions.
In my eyes a "short circuit" system needs to safely run at normal conditions (with headroom) but needs to switch OFF the output at overcurrent condition. Overcurrent threshold as well as timing needs to be determined (by you) in a way, that it protects your circuit from being destroyed.
For fast reaction I recommend to use a flipflop to trigger and hold the overcurrent condition. It should be cleared by software.
Klaus