Class D Amplifier Power Supply Design -> 12,5 A @ +/-80V = 1000W

[selfservice]

You can insert a RC network into the current sense circuit.
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you mean a filter? to reduce the current spikes and prevent sg3525 going in "protection mode"

 

[ChocoHolic]

ChocoHolic!

There is a wire with isolation qualified to >4 kV strength, and it has only ~0,2 mm thick isolation. I have it, but I don't know its name. It's yellow.

Sounding like the triple isolated TEX-E wire of Furukawa.
If yes: Good stuff.

 

[ChocoHolic]

how to workaround? optocoupler? that's require a massive redesign...

There are two traditional ways:
a) optocoupler
b) proper isolated auxilary winding

Optocoupler is usually resulting in better regulation properties and
is less critical in design, because the isolation is insured by the semiconductor manufacturer.
Try to find and use some high spec types, which offer high isolation and allow large creepages.
http://www.vishay.com/docs/83540/83540.pdf