One very important thing to keep in mind here is that you must make sure that you never get a Vgs higher than the max Vgs spec on the high-side FETs. How fast do you need the high-side FETs to turn on? If you don't need very strong gate drive, I'd recommend just using a photovoltaic optocoupler (basically an LED and a solar cell in a DIP package). Some examples are the international rectifier parts starting with PVI (for example, PVI5033). These can turn the FET off quite fast but cannot turn it on very fast (for example, a FET with 30nC of gate charge would take about 30 milliseconds to turn on with this device). If you need very fast turn on, you could probably do what other posters suggested (putting one of these in parallel with the bootstrap cap - you would have to leave it turned on all the time to keep the cap charged). Sean On Thu, Jun 5, 2008 at 8:14 AM, Gerhard Fiedler wrote: > gardenyu wrote: > >> Besides, if I can have this additional power supply, how can I turn on >> this supply to charge the boostrap cap? I don't want another output from >> microcontroller saying we want a full duty cycle, I just want to turn >> on the supply with the 100% PWM output, if it appears. But, if PWM is >> below 100%, it won't turn on. How can I realize that? > > One possibility is to have an (one) additional supply voltage that is a bit > higher than the highest gate voltage you need, switch it with opto couplers > and limit the gate voltage at each FET. > > Gerhard > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist