ON Semiconductor – Efficient, reliable – and now simple to implement: a new approach to BLDC motor design

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By Thomas Britnell
Field Applications Engineer, Future Electronics (Germany)

The Brushless DC (BLDC) motor is the most efficient and reliable motor type in widespread use today. But while its performance and characteristics are attractive, it is complex to operate in most applications, and this has discouraged some design engineers from considering its use.

The semiconductor industry is now, however, providing more and more support to designers making the shift from brushed to brushless motor designs. In particular, they are providing integrated motor driver ICs which reduce the complexity of both the hardware and software elements of the design. This article describes the application of one of these devices, the LV8907 from ON Semiconductor.

Benefits of BLDC motor technology
Most electric motors used today are more or less identical to the first electric motors developed in the 19th century: they still include a commutator and mechanical brushes. As long ago as 1889, however, Nikola Tesla demonstrated that a three-phase brushless motor could increase energy efficiency by up to 20% compared to brushed motors. By eliminating mechanical brushes and slip rings, brushless motors also significantly increase system reliability.

They also operate more quietly, and are not prone to brush arcing (sparking), which can disturb electrical equipment such as phones, radios and wireless  communication networks. Arcing also generates ozone and carbon dust and can be a potential source of fire when exposed to inflammable substances.

The appeal of brushed motors, of course, is that they are simple to implement. A DC voltage drives the motor: vary the voltage and the speed varies proportionately. In a BLDC motor, the commutator is eliminated, and this results in a much more complex system. In order to drive a BLDC motor efficiently the three motor phases have to be switched in turn with three external half bridges for uni-directional rotation, as shown in Figure 1.

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Fig. 1: Half bridge for driving a BLDC motor

The rotating electric field is created by synchronising six PWM signals; the field needs to be matched to the speed and position of the rotor in order to drive the motor efficiently. This requires feedback about the motor’s position. This can be provided by a position sensor. The position can also be calculated without the need for a sensor, using back-emf measurement.

When an electric motor turns, the moving permanent magnets induce a back-electromagnetic force (back-EMF) voltage in the coils which can be detected at the motor’s terminals, as shown in Figure 2.

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Fig. 2: Two phases of a BLDC motor’s operation. The back-EMF ramp may be seen in the green and yellow signals, superimposed on switching signals before and after the PWM driving state

Complex hardware and software requirements
It is obvious from the previous sections that, compared to brushed DC motors, a brushless motor drives requires more components and a more complex circuit design. In developing the LV8907 motor drive controller, however, ON Semiconductor has attempted to make the implementation of a BLDC motor-control system almost as easy as that of a brushed motor.

The LV8907 is a 12V, automotive-compatible BLDC motor controller for stand-alone applications without a microcontroller. Once the system parameters have been loaded into the IC, an integrated state machine handles the sensorless brushless motor control. A speed feedback signal is included, as well as an internal speed-control algorithm with PI controller.

This means that the only input required to run a motor using the LV8907 is a single, low-frequency PWM signal – the same input as for a simple brushed motor. This PWM signal is either translated into the motor voltage directly, or used to select a programmed motor speed from nine internal speed registers. The driver IC will then apply this speed in a speed-control loop independent of load and supply-voltage variations.

An internal regulator is included to provide power to a small 5V or 3.3V microcontroller, in case a more complex user-interface protocol needs to be implemented. This MCU can interface with the LV8907 via the SPI interface, and can use the integrated LIN transceiver and watchdog. In this configuration, the MCU performs the interface and application tasks while the LV8907 handles the motor commutation.

The LV8907 also includes a wide range of safety features to protect the application. These include:
• Short-circuit and over-current protection
• Over-temperature protection in the IC and MOSFETs
• Over- and under-voltage protection
• Rotor-blockage detection

Summary
In addition to making BLDC motor control easy by eliminating the need for software implementation, the LV8907 also reduces hardware requirements as it includes the pre-driver, charge pump, the current-sense circuit as well as an optional linear regulator and LIN transceiver.

Thus all that is required to drive a BLDC motor is a PWM input, six power FETs, a shunt resistor and a few external capacitors and resistors.

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