Gene Heskett wrote: > > The controller in question in this case isn't an h-bridge, its simply a full > wave bridge rectifier that has as a DC load, a mosfet in series with the > motor, and the motor has a free-wheeling diode for when the mosfet is turned > off. This is considerably simpler than an h-bridge.
No filter capacitor at all on the output of the bridge rectifier? > Back here at the ranch though, going up to a 254 volt single phase (commonly > called 220 power) doubles the power available, and probably doesn't need > exceptionally higher rated mosfet voltages because most of the ones I've > looked up are available in 800 to 1000 volt ratings also. MOSFET on-resistance goes up significantly as you increase voltage. When you go over 500V the slope gets ridiculous. A package that could easily hold a 60 or 75 amp MOSFET at 60 or 100V will barely hold a 3 A MOSFET at 1000V. You do NOT want to use MOSFETs at high voltage. That is why IGBTs were invented - they don't have the dramatic loss of performance at high voltage. But they have their own issues - slower and more switching loss for example. > You mentioned capacitance, and I'm puzzled as to where, other than > un-avoidable circuit strays, these might have to be figured into the overall > picture. Can you point out something I've missed? I would have expected a rather large filter cap at the output of the bridge rectifier. If the circuit is a one-quadrant one (which seems to be what you are describing), maybe it isn't needed - since you can't extract any energy from the motor. But I'd still expect something there, if only to handle switching spikes from stray inductance. Regards, John Kasunich ------------------------------------------------------------------------- This SF.net email is sponsored by the 2008 JavaOne(SM) Conference Don't miss this year's exciting event. There's still time to save $100. Use priority code J8TL2D2. http://ad.doubleclick.net/clk;198757673;13503038;p?http://java.sun.com/javaone _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
