“Brushless DC motors (BLDC) replace AC induction motors and are becoming more and more common in general appliances, including fans, air purifiers, washing machine and dryer pumps, and medical CPAP blowers.
Are you distracted by the noise caused by many electrical appliances in your home? TI’s latest brushless DC motor driver adopts trapezoidal and field-oriented control, which can reduce electrical noise by 3.3 dBA.
Brushless DC motors (BLDC) replace AC induction motors and are becoming more and more common in general appliances, including fans, air purifiers, washing machine and dryer pumps, and medical CPAP blowers.
However, the modulation of brushless DC motors can also generate noise, especially the open concept is becoming more and more popular in office and home environments.
In order to solve these noise-related problems, Texas Instruments (TI) has announced the launch of two new 70 W sensorless brushless DC motor drivers, MCT8316A and MCF8316A. These drives are highly integrated and code-free, and use advanced trapezoidal control and field-oriented control (FOC) respectively.
Several examples of common noise levels in electrical appliances.
The goals of these new chips include three aspects: reducing the source of acoustic noise in daily applications, reducing the solution size by 70%, and reducing the overall design time for BLDC tuning (which may be reduced to 10 minutes).
Let us see how TI’s latest products achieve these three goals.
MCx8316A chip overview
MCT8316A and MCF8316A support an operating range including 4.5 V to 35 V, with peak output up to 8A. These chips are designed for speed control that requires 12 V to 24 V brushless DC motors or permanent magnet synchronous motors.
MCX836A chip overview.
An important feature of MCX836A series drivers is reduced noise. Specifically, MCT8316A supports 120° and 150° modulation, and improves acoustic performance through a trapezoidal control scheme.
MCF8316A adopts the FOC scheme, using automatic dead time compensation to reduce the harmonic content fed to the motor, thereby reducing the noise in the audible range.
One series, two chips, two control schemes, so two popular controls can be flexibly realized
Introduction to Trapezoidal Control and FOC Control in Brushless DC Motor
There are generally two types of brushless DC motors, including Hall-effect sensor drive structure and sensorless structure. Although more complex, sensorless architecture is the more preferred choice. For sensorless operation, two control schemes have emerged.
The sequence diagram of the trapezoidal control scheme.
The working principle of trapezoidal control is that there are two phases at any given time. Therefore, as shown above, no torque is generated during the zero-crossing period of any phase.
This scheme will cause ripples at every 60° zero-phase crossing, thereby generating harmonics. In 120° mode, the BLDC driver will use the high Z state for the remaining 60° of each cycle.
The FOC is more complicated in the conversion of feedback control.
The output of the three-phase inverter produces a sinusoidal signal, which is sampled and fed by Clark transform and Park transform.
The basic block diagram of field directional control FOC.
Both of these transformations realize direct torque control by converting the stator current vector from a three-phase time-varying system to a two-coordinate time-invariant system.
How does MCX836A chip reduce noise
The two control schemes used to control brushless DC motors have their own advantages and disadvantages. Each method generates harmonics and thus noise.
Comparison of the advantages and disadvantages of two sensorless brushless DC control schemes.
Modulation and dead time compensation are essential to reduce motor motion noise. MCF8316A uses a resonance controller to compensate the dead time between the high-side and low-side switching MOSFETs to reduce the harmonic content to zero.
Enabling dead time compensation inside the MCF8316A can eliminate audible harmonics (left), resulting in a purer sinusoidal signal (right).
The effect on the sine output and subsequent Fast Fourier Transform (FFT) is shown above.
Similarly, the FFT response of trapezoidal control schemes (such as MCT8316A) is improved by dynamically switching between 120° and 150° through the “windowing” of the motor phase in the high-Z mode by using the chip’s switchable scheme.
All in all, this noise reduction solution seems very promising in many applications, especially in consumer products.
Write at the end
Although many employees have not worked in the office for some time, it is easy to recall how environmental noise can be distracting. Perhaps, even if you work at home, the noise of the refrigerator will also interfere with your work.
The progress of the control scheme of the trapezoid and FOC method is dedicated to removing the fluctuation of the signal driving the brushless DC motor, thereby producing a quieter operating environment.
The advanced integration provided by the MCX836A series reduces the solution to 10 components with an area of only 2 cm², while a discrete solution will use 28 components with an area of 7 cm².
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