Circuit for Improved back EMF detection

摘要

A system and method of advancing the commutation sequence of a brushless DC motor is provided. The motor having a plurality of coils, each of the coils coupled together at one end to a common center tap and coupled at an opposite end, through a respective coil tap, to both a source voltage and ground via selectively actuateable switches having diodes coupled in parallel therewith. The motor operates in a pulse width modulation (PWM) mode having PWM-on states and PWM-off states. During PWM-off states, a coil tap voltage from the coil tap of a floating phase is provided to a preconditioning circuit. The preconditioning circuit adjusts the floating phase coil tap voltage to compensate for an amount of voltage substantially equal to an amount of voltage by which a voltage at the center tap deviates from zero. The preconditioning circuit further includes sharpening circuitry for amplifying the adjusted floating phase coil tap voltage. A signal output from the preconditioning circuit is provided to a zero-crossing detection circuit for detecting zero-crossing and determining when to advance in the commutation sequence.