TI Lowers the Acoustics with BLDC Motor Drivers Leveraging Two Control Schemes
Distracted by the noise caused by the many appliances in your home? TI’s newest BLDC motor drivers claim to quiet appliances by up to 3.3 dBA using trapezoidal and field-oriented control.
Brushless DC motors (BLDC), replacing AC induction motors, have become increasingly common in applications for general appliances, including fans, air purifiers, washer and dryer pumps, and medical CPAP blowers.
However, the modulation of BLDC motors can generate acoustic noise, which contributes to ambient background noise, especially with open-concept floor planning being so popular for both the office and home environment.
To help aid in these noise-related issues, Texas Instruments (TI) is announcing today two new 70 W sensorless BLDC motor drivers, the MCT8316A and the MCF8316A. These drivers claim to be highly integrated, code-less chipsets that use advanced trapezoidal control and field-oriented control (FOC), respectively.
A few examples of common noise levels for appliances. Screenshot used courtesy of Texas Instruments
The overall goal of these new chips is three-fold: reducing audible noise sources coming from everyday applications, shrinking the solution board size by up to 70%, and finally, lessening the overall design time for BLDC tuning (potentially down to 10 minutes).
With these goals in mind, let's take a look at TI's latest offering.
An Overview of the MCx8316A Chips
The MCT8316A and MCF8316A operate with common electrical parameters, including a 4.5 V to 35 V operating range with up to 8A peak output. Also, these chips are designed for applications in speed-controlled motors requiring 12 V to 24 V BLDC or permanent magnet synchronous motors.
Overview of the MCx8316A chips. Screenshot used courtesy of Texas Instruments
One of the defining features of the MCx8316A family of drivers is its reduction of acoustic noise. Specifically, the MCT8316A supports 120° and 150° modulation, which improves acoustic performance through trapezoidal control schemes.
Meanwhile, the MCF8316A, which uses a FOC scheme, utilizes automatic deadtime compensation to reduce the harmonic content fed to the motors, thereby reducing acoustic noise in the audible range.
One family, two chips, and two control schemes, thus it begs the question, what is the difference between trapezoidal control and field-oriented control?
Trapezoidal Control vs. FOC in BLDC Motors
There are two general types of BLDC motors, including Hall effect sensor-driven architectures and sensorless architectures. Although more complex, sensorless architectures are the more preferred option. For sensorless operation, two control schemes emerge.
The timing diagram for the trapezoidal control scheme. Screenshot used courtesy of Texas Instruments
Trapezoidal control operates on a principle where two phases are asserted at any given time. Consequently, there is no torque produced during the zero crossings of any phase, as seen above.
The effect of this scheme results in a ripple at every 60° zero phase crossing, which results in harmonics. In 120° mode, BLDC drivers will use a High-Z state for the remaining 60° of each cycle.
Conversely, the field-oriented control scheme requires significantly more complexity in the transformation of feedback control.
Basic block diagram for field-oriented control. Screenshot used courtesy of Texas Instruments
The result of these two transforms brings direct torque control through the translation of the stator current vector from a three-phase time-variant system to a two-coordinate time-invariant system.
How the MCx8316A Chips Reduce Acoustic Noise
There are several advantages and disadvantages to either control scheme employed to control BLDC motors. Each method produces harmonic artifacts which contribute to acoustic noise.
Pros and cons for the two types of sensorless BLDC control schemes. Screenshot used courtesy of Texas Instruments
Modulation and deadtime compensation is said to be crucial to the reduction of audible noise present in the motor's movement. Precisely, the MCF8316A compensates for the deadtime between the high-side and low-side switching MOSFETs using a resonant controller to reduce the harmonic content to zero.
Enabling deadtime compensation inside the MCF8316A removes audible harmonics (left) producing a purer sinusoidal signal (right). Screenshot used courtesy of Texas Instruments
The effect on the sinusoidal output and subsequent fast Fourier transform (FFT) is shown above.
Similarly, the FFT response of a trapezoidal control scheme, like the MCT8316A, is improved by utilizing the chip's variable commutation scheme to dynamically switch between 120° and 150° by "windowing" the amount of time the motor phase is in Hi-Z mode.
All in all, this solution for noise reductions seems promising for many applications, especially for consumer products.
Although it's been a while since many workers have visited their open-concept office, it's easy to recall how distracting the amount of ambient noise can be. Perhaps, even while working from home, maybe the noises from your fridge have become your companion.
Advances in the control schemes for both trapezoidal and FOC methodologies strive to clean up the electrical spectrum of the signals driving brushless DC motors, resulting in a quieter running environment.
The advanced integration offered by the MCx8316A family is said to reduce the solution to ten components using 2 cm2, as compared to a discrete solution using twenty-eight components in 7 cm2 for a 70 W implementation.
TI's newest BLDC motor drives might replace your constant companion with a novel new one; the sound of near-silence.