Back in the late 90s, a mechanical engineer on the MIT solar car team was explaining the latest high-efficiency motor controllers for brushless DC motors, and m

Back in the late 90s, a mechanical engineer on the MIT solar car team was explaining the latest high-efficiency motor controllers for brushless DC motors, and my degree is also in Mechanical Engineering, so this could be way off. However, as explained to me, the highest efficiency brushless DC motor controllers in the late 1990s used a phase-locked loop to get some of the off periods in the square wave in the power conversion stage to be in sync with the switches between coils in the motor. Transistors don't switch instantly, and a good deal of your power loss is during the transition between fully off and fully on. By synchronizing the two parts of the motor controller, you get the coil changes to happen when that part of the controller is seeing dips in its power input, so you get less loss.