A capacitor pump of +12V referenced to the center of the bridge allows the top FET to turn on. I tried it at +170V top, 0V bottom and it works great, but agree the 700V would exceed its ratings.
Why generate a split source to power the inverter? I've not seen that done before. It seems like the cost of the copper and the extra DC link capacitor outweighs the cost of a couple of extra transistors.
If you're custom-winding a transformer to achieve this, can't you just add a tap at +/- 350V and another at +/- 360V to allow a simple level shifter transistor to turn on top FET?
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