Drive circuit for a three-phase brushless motor

Abstract

A three-phase brushless motor drive circuit switches its three-phase outputs by producing drive signals displaced in phase relative to one another. In this drive circuit, a reference clock is formed from the signals obtained by comparing the output voltages appearing across the coils of a three-phase motor individually with a voltage at a terminal common to the three phases. This reference clock is subjected to edge detection by an edge detection circuit. The output voltages include noises generated when the outputs are switched. The noise components produced when the reference clock is subjected to edge detection are reduced by a noise reduction circuit. A resulting edge detection output, which no longer includes noise components, is used to trigger an RS flip-flop to form a pulse-train signal. This pulse-train signal is used to control the charging and discharging of a capacitor. A sawtooth voltage appearing across this capacitor is compared with a predetermined reference voltage by a voltage comparator, and, in accordance with a comparison output, the displacement in phase of the pulse-train signal is determined and the pulse-train signal is distributed to produce separate signals for the three phases.