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L99ASC03 Fully-Integrated Motor Driver IC for Three-Phase Brushless Motors Slide 9

To overcome the limitations of the conventional method, a method is proposed to avoid the neutral point voltage. The PWM signal is applied on high side switches only, and the back-EMF signal is detected during the PWM off time. Shown here is a example, if phase A and B are conducting current, phase C is floating. In this step, the high side switch of phase A is PWM, and the bottom side switch of phase B is on all the time. When the PWM is “on”, the current flows through the high side switch to winding A and B. When the PWM is “off”, the current will freewheel through the bottom side diode of the phase A. During the “off” time, two terminals are tied to Ground, and if the engineer ignores the voltage drop on the diode and switch, the neutral point voltage is only a function of the back-EMF of phase A and B.

PTM Published on: 2015-04-06