Driving 3-Phase Full-Wave Brushless Motors: Sinusoidal Commutation PWM Driving with Sensors

In succession to the previous article on “120° Commutation Linear-Current Driving “, in this article “Sinusoidal Commutation PWM Driving” is explained.

As indicated in the previous article, commutation methods for 3-phase full-wave brushless motors include 120° commutation and sinusoidal commutation. Compared with 120° commutation, sinusoidal commutation is superior with respect to control precision, efficiency, and noise; however, such systems are complex, and square-wave driving is superior where cost is concerned.

Driving 3-Phase Full-Wave Brushless Motors: Example of a Sinusoidal Commutation PWM Driving Circuit Using Sensors

In sinusoidal commutation, driving is performed by control and driver circuits designed for 3 phases, each of which has a driver comprising a high side and a low side. Below is an example of a sinusoidal commutation PWM driving circuit block using sensors and of an input/output waveform diagram.

In basic operation, signals from the three Hall sensors are input to the inputs of Hall amplifiers, and after waveform synthesis, the signals are converted into PWM signals using comparators and triangular waves, which drive the motor coils via MOSFETs in the output stage. The equivalent voltages of the PWM signals A1 to A3 are sinusoidal commutation waveforms shifted in phase by 120°. In 120° commutation, commutation is performed using square waves that are on for 120° and off for 60°; in driving by sinusoidal commutation, on the other hand, 180° commutation is used, with sine waves increasing from zero to the maximum value, so that operation is smoother and low-noise operation is obtained. In addition, PWM driving contributes to improved efficiency.

Driving 3-Phase Full-Wave Brushless Motors:
Example of Sinusoidal Commutation PWM Driving Waveforms Using Sensors

Waveform examples are used in a detailed explanation; to begin with, however, PWM conversion of sine waves is explained.

We begin with the H1P/H1N input channel in the above block diagram as an example. The Hall amplifier output H1 is shaped into the sine wave M1, shown in purple in the diagram, by a waveform synthesis circuit. M1 and a triangular wave from the triangular wave oscillator are input to a comparator, and a square wave P1 (comparator output) having a certain pulse width is output as the comparison result. P1 passes through a level shift/simultaneous-on prevention circuit to control the gate of the output stage MOSFET, becoming the PWM output that drives the motor coil. Such PWM conversion using a comparator and a triangular wave is a widely used technique that is employed in many circuits such as PWM generation for switching regulators.

Next, let’s look at the input and output waveforms.

The Hall element inputs H1P/H1N to H3P/H3N are differential signals from the Hall elements, and result in H1 to H3 sine wave outputs shifted 120° respectively (see the Hall element voltage waveforms in the waveform diagram).

Hall element voltages pass through the waveform synthesis circuit to become M1 to M3. Each of the waveforms is generated with phase leading the original signal by 30° or more (see the synthesized waveforms). This is called an advance angle, and is explained in detail separately.

As explained previously, M1 to M3 are compared with triangular waves by the comparators and converted into PWM signals (see the comparator waveforms P1 to P3).

The voltage waveforms applied to the motor coils are PWM signals, but in the waveform diagram, equivalent voltage waveforms are shown. We see that driving is performed by the equivalent of sine waves.

The coil currents are of course sinusoidal. By providing an advance angle in the waveform synthesis circuit, the coil current waveforms are always kept 30° ahead of the Hall element voltages (H1 to H3). This series of control is called advance angle control.

In the case of 120° commutation, spike noise due to turn-on/turn-off of coil currents is observed in the coil voltage waveforms. In driving by sinusoidal commutation, however, 180° commutation is used and there is no turn-on/turn-off, so that spike noise does not occur.

Next, advance angle control will be explained.