View Single Post
Old 01-18-2011, 01:23 AM   #4229 (permalink)
EcoModding Apprentice
Join Date: Nov 2010
Location: Annapolis
Posts: 159
Thanks: 0
Thanked 32 Times in 27 Posts
What motivated the choice of the IRFP4668, vs the higher power and lower on resistance of the IXFX230N20T? The lower gate charge?

Have you considered using the a dedicated SR driver chip such as the IR11672AS or ZXGD3101T8CT? They sense the gate-drain voltage to emulate a diode, even tracking the PWM frequency to anticipate the transition. There are many similar chips out there, but the two above have a good combination of few external components, handling the full voltage without a voltage divider or input clamp, and having strong gate drivers. I haven't found ones in a through-hole package -- these are at least big enough to hand solder.

The gate drives for SR controllers might initially seem a little weak, but remember that the Miller charge is actually helping rather than hurting. And the intrinsic/body diode is still working, even if the gate is slow to turn on. The only performance critical region is the turn-off: an early turn-off lets the full body diode capacitance build, while a late turn-off allows shoot-through current. A perfect turn-off has the gate region narrow with a short carrier lifetime.

The neat thing is that you can add this circuit with only connections to SR MOSFET pins and a "bootstrap" diode connection to the low-side gate driver power supply. Note that you want the power supply V- to come from close to the low-side MOSFET drain, and SR MOSFET 'source sense' connected to the high side source pin. They are opposite ends of the same wire, but that wire carries a lot of current.
  Reply With Quote