Circuits to control and drive brushless DC (BLDC) electric motors are known. Conventionally, the circuits provide a plurality motor drive signals, each at a different phase, and each coupled to one of a respective plurality of motor windings. The motor drive signals can have one of a plurality of different signal characteristics. In a trapezoidal drive arrangement, the motor drive signals are pulse width modulated (PWM) signals, wherein average values of the PWM signals follow respective trapezoidal patterns. In a sine drive arrangement, the motor drive signals are pulse width modulated (PWM) signals, wherein average values of the PWM signals follow respective sinusoidal patterns.
Some known electric motor drive circuits are described in U.S. Pat. No. 7,590,334, issued Sep. 15, 2009, U.S. Pat. No. 7,747,146, issued Jun. 29, 2010, and U.S. patent application Ser. No. 13/271,723, filed Oct. 12, 2011 and entitled “Electronic Circuit And Method Generating Electric Motor Drive Signals Having Phase Advances In Accordance With A User Selected Relationship Between Rotational Speed Of An Electric Motor And The Phase Advances,” each of which is assigned to the assignee of the present invention.
The motor controller can have first and second parts, which can be disposed together on a common circuit board, or which can be disposed on separate circuit boards. A motor control circuit, which generates low power drive signals, forms the first part. A plurality of half bridge circuits, which receive the low power drive signals and generate higher power drive signal to motor windings, forms the second part, and is coupled to an electric motor.
Certain fault conditions in one or more of the half bridge circuits or in the electric motor can result in a braking torque upon the electric motor. Braking torque may be undesirable in many applications, for example, in an automobile electrically assisted power steering system. The braking torque would make the automobile, which would already be difficult to manually steer due to the fault and loss of electrical assist, even more difficult to manually steer due to the braking torque.
To avoid the above condition, in some applications, it is desirable to provide an isolation circuit coupled between the half bridge circuits and the motor windings. However, when current to motor windings is turned off, inductive voltage transients on the motor windings can cause the isolation circuit to fail. To reduce the likelihood of this failure mode, high power diodes have been coupled to the electric motor to directly limit excursions of the inductive voltage transients that occur when motor windings are turned off. High power diodes are large and expensive.
Thus, it would be desirable to provide low power circuit elements that can protect an isolation circuit that is coupled between half bridge circuits and motor windings.