gene heskett wrote: > To start with mosfet/hexfet's have one very non-endearing feature. Because > they are effectively the perfect equivalent to a vacuum tube triode, they > also share a attribute known as miller effect in the tube world. This > makes them look as if there is a large capacitor from drain to gate, which > is in fact the gates input capacitance, which for some devices can exceed > 25,000pf, but since a goodly amount of that is drain-gate capacitance, it > is then multiplied by the transconductance of the device, some of which > exceed 25 mhos! It can't switch any faster that the gate driver circuit > can charge or discharge this amplified capacitance. So the gates driver > stage, in addition to being floated at the src point for the 2 bugs at the > top of the bridge, must itself be capable of delivering that current fast > enough to achieve the switching speed desired. That could easily be 100 > amps, for that 10 to 15 nanoseconds. > I just redesigned my brushless servo amp due to this problem. The IR triple half-bridge driver chips only put out 200 mA of gate drive. The single half-bridge chips I use in my brush amp is good to 2 A. So, I changed to using 3 chips to get more drive to the FET gates, so I could have shorter dead time without risking shoot-through.
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