In practice, pure sinusoidal driving voltage is rarely used because the efficiency of generating voltage for each motor terminal is low compared to grounding. A better method is to generate a sinusoidal differential voltage between phases, with a phase offset of 120 ? Perform reversing. The implementation method is to change the PWM duty cycle (as well as the driving voltage) relative to ground using a "saddle" diagram (instead of a sine) (Figure 3). Subsequently, the phase current of the driving motor follows a pure sine wave variation of the interphase voltage.

    The saddle plot method has two advantages: firstly, the maximum differential voltage generated is higher than the voltage that can be generated by a pure sine signal, thus the given input torque and speed are also greater. Secondly, 1/3 of the time output of each terminal is zero, further reducing switching losses in the power stage.

    One complication of the sine control method is that it precisely controls the duty cycle based on the motor angle necessary to form a saddle voltage input. This even becomes more difficult during high-speed rotation. The main challenge is that the motor position can only be accurately determined six times per revolution, and one of the magnetic poles of the rotor passes through one of the three Hall sensors. For example, the commonly used solution for FRDM-KE04Z is to multiply the motor angular velocity by partial T and assume that the motor velocity is constant, thereby estimating the motor angle ("mtrAngle") between Hall sensors.

    Then use a query table to determine the PWM duty cycle for a specific angle. In FRDM-KE04Z, the query table provides the duty cycle for each angle of motor rotation (actual 384 increments).

    This type of method utilizes the incidental effects of using saddle plots. Special note: Since the voltage value of a specific phase is zero within one-third of the time, there is no need to query during this period, which requires less processor resources and allows for the use of more common low-cost MCUs in applications.

    The disadvantage of this method is that when the motor accelerates rapidly during the startup phase, the motor speed interpolation between Hall sensors is likely to be inaccurate. This can lead to uneven torque response.

    A common solution adopted by the BD62011FS fan motor controller of ROHM Semiconductor to address this issue is to start the motor in trapezoidal control mode and switch to sinusoidal control after reaching a specific speed (usually 5-100 Hz), resulting in higher interpolation accuracy.

    Rohm‘s equipment is mainly aimed at controlling BLDC motors equipped with Hall sensors. The chip adopts PWM control and sinusoidal commutation logic of high-voltage and low-voltage MOSFETs. It can operate within the input range of 10 to 18 V and provides an output range between 2.1 and 5.4 V (up to 1 W). The target applications include air conditioning, water pumps, and white goods.

    Another design challenge is the phase delay between the given phase driving voltage and the generated sine wave current, which typically occurs in non compensated BLDC motors. The motor can operate normally, but its efficiency will decrease, which will first defeat the goal of achieving a sinusoidal control scheme. The reason for this low efficiency is not the phase delay between the driving voltage and phase current, but rather the phase delay between the phase current and the sinusoidal back electromotive force.

    Fortunately, many driver chips, including ON Semiconductor‘s LV8811G power MOSFET driver, allow designers to introduce a leading phase angle in the sinusoidal driving current, ensuring that its peak is consistent with the peak of the back electromotive force. The leading phase angle is usually set to increase linearly with the input voltage, which determines the motor speed (Figure 4).

    LV8811G is a three-phase BLDC motor driver controlled by a single Hall sensor and adopts sinusoidal control. Both direct PWM pulse input and DC voltage input can be used to control motor speed.

    When using LV118811G, designers can set the initial conditions through the voltage divider resistors on pins PH1 and PH2: the speed at which the phase angle begins to lead and the gradient of the leading phase angle slope. Afterwards, the internal logic of the chip determines the leading phase angle of the given speed based on a predetermined formula.