Gerhard Fiedler wrote:
> 
>It doesn't have to be
> terribly fast; I'm doing the PWM with the low side drivers -- but I need
> the lowest Rdson I can get, so I really need >12V Vgs. 


Since you are only switching direction with the top fets, a switched 
capacitor charge pump should work.  You set it up with a large-ish 
reservoir cap and that's that.

If you happen to also have an RS-232 interface chip like a MAX232 IC 
already in the design, you can borrow the +10V it already produces and 
pump this up to 20 with a few parts.

Or if you have a raw input supply of something like 12 (you must already 
have this somewhere for your bridge) feed a pump cap from 12V via a 
Schottky and lift it up to the 12V rail and dump through another 
Schottky into your big reservoir cap.

Big N-Fets gate capacitance is on the order of 10nF; a reservoir cap of 
only 10uF gives you 1000:1 coulomb storage.

In all these scenarios, you must determine if the Vgs-max can be 
exceeded during startup, shutdown, fault, etc, and provide a zener clamp 
as appropriate.

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