Simple back emf reconstruction in pulse width modulation (PWM) mode

摘要

A circuit and method for reconstructing the back emf of a floating coil of a polyphase dc motor in PWM mode is provided. The floating coil is coupled to a first capacitor through a floating phase switch that closes during a pulse produced by the PWM drive signaling the appropriate time to sample in the PWM cycle. The signal on the floating coil is sampled and the sampled signal is stored using a capacitor. After the sampling period, the stored signal is discharged (or charged) at a rate that substantially models the slew rate or slope rate of the expected back emf signal at or near the zero crossing of the back emf signal with the common tap signal. The voltage across the capacitor is a reconstruction of the actual back emf and is generated using samples of the back emf. The reconstructed back emf is compared to the center tap voltage to more accurately detect the zero crossing.