Originally Posted by
SystemTheory
Also I note your comment on the correlation of efficiency with the power advantage. It shows up clearly in the graph John just posted.
One reason may be the power dissipation in a MOSFET has two components, one for the DC power loss and one for the switching loss at high frequency. If an ESC has MOSFETs with lower DC on resistance they tend to have a much larger device capacitance and thus greater switching losses with increasing PWM frequency. This tradeoff would partly explain the correlation. John or another Dyno expert might be able to test this hypothesis or pull the ESC specification for Rds(on) and see if lower value jives with getting the earlier advantage in efficiency/power.
Yes, for power MOSFETs, less Rds-on = more gate charge, etc. I've designed DC-DC converters with a real measured 92% efficiency and MOSFET choice is critical -- when choosing MOSFETs, you typically have to consider this tradeoff. That's one reason to use dedicated gate drivers rather than just straight logic signals. I've never made a working brushless ESC but I've read white papers on it, some of our hobby ESCs for aircraft at least do let you set the PWM frequency or choose from a number of ranges.
As for switching MOSFETs, you have the PWM and in brushless you also have the phase switching. Once the ESC is out of 'soft start' and the RPMs are sufficient, the PWM might be at full duty cycle (esp. if you're at full throttle) so at this point the only switching going on is phase switching. You can see the transition if you hook up a scope on one of the brushless poles. I only have a Novak Havoc and a super stock (13.5) motor, but when I ramped the throttle, slowly, letting the motor spin up freely, I could observe the PWM on the scope and it went to full duty cycle around 70% throttle.