I'm trying to design a capacitor discharge circuit for a hobby rocket launching system. In some cases we have long wire runs with multiple rockets so a CD circuit seemed the way to go. I'm trying to use an IGBT and the Avago ACPL-312U driver. What I didn't realize is that an IGBT can't be used as a high side switch. Is this correct? Can I get some suggestions on how I can configure the circuit to use the IGBT for switching 100 volts?
You can use a IGBT for the high side switch.
But you need a supply with a voltage higher than 100V to drive your IGBT. I usually do it with a separated and isolated supply just for the gate driver (a small switched supply), there are some on chip solutions for that (Power Convertibles HPR 105 for example). The supply's GND is tied to the IGBT emitter. Then you have a voltage to drive your IGBT.