I've tried lots of things so far, to a point I have it working. But it's not working as smoothly or as nice as I would like, so I dont think there is anyway out of driving the high side also. - but how???

I'm planning on using the same NPN IGBT and driver on the top side as I do on the low side, this should keep switching time consistant. (Also I have a tonne of 600V 70A IGBTs)

The question is, what's the best way to generate 12 or so volts above my 300V rail, also - what's the best way to trigger the drivers with such a huge potential across them.

It all has to be done quite quickly and presisely, for eg at the moment I'm using an opamp for current limiting, however the slew rate is far too lesuirly. It's taking 5us to respond, from what I've observed that's going to have to be cut right down a hundred fold to 50ns or so. So an optocoupler is just not an option (25uS typical fall time)

Any ideas on large potential high side driving?