2 Sinusoidal Drive Measurement Window Insertion

To remove the influence from the driving signal, the BEMF must be sampled when both of the transistors in the respective half-bridge are driven low.

The complementary PWM dead-time, as depicted in Figure 2-1, provides such a window for sampling the BEMF. The dead-time is required to prevent current shoot-through in the half-bridge.

That said, the dead-time of each phase does not necessarily align. Thus, the proposed method increases one dead-time period on all six PWM signals and the dead-times are aligned, creating a sufficiently long window to measure the BEMF, as depicted in Figure 2-2.

Figure 2-1. Dead time and PWM signals illustration
Figure 2-2. Logic Analyzer Capture to Visualize the Long Dead Time
Insertion of the BEMF acquisition window brings minor drawbacks when compared to the ordinary sinusoidal drive:
  1. Because of the increased dead-time duration, the PWM duty cycle range is decreased by a proportion, dependent on the PWM frequency and the time it takes to correctly assess the BEMF.
  2. The half-bridge body diodes get hotter because of the longer dead time. When the BEMF is positive, there are more power losses.

Apart from these disadvantages, this method is a cheap alternative for Sinusoidal Sensorless Control of three-phase motors (BLDC or PMSM) and can be implemented on 8-bit MCUs as it does not require a lot of computational power in one complete PWM cycle. Usually for motor control 20 kHz PWM switching speed is used, so a complete PWM cycle duration is 50 μs.