COMPONENT DESIGN FEATURE
SWEPT SIDEWAYS
How a brushless motor has augmented power and performance in integrated circuits
Compensating on the volume of components in a circuit to improve efficiency is happening more commonly within the industry - to accommodate a larger battery, for example. Toshiba explains how its design of DC motors could improve their output
P
erhaps surprisingly, there are still many applications today where brushed DC motors are used. Despite their low efficiency and the electromagnetic interference they generate, they are simple to integrate and control. In many cases, such as simple children’s toys or the electric mirrors of a vehicle, this proves them to be a good match to the application. There is the question of quality and long-term efficiency, however, as there are plenty of applications where motors run for long periods of time or even continuously: it is here that brushed DC motors have made way for new, brushless solutions. Brushless DC motors (BLDC) do away with the mechanical commutation, turning instead to an electronic control system that energises the motor’s coils in the correct order, at the right moment, to keep the rotor turning at the desired number of revolutions per minute. This results in a significant improvement in efficiency, but at the cost of a more complex electronic control system. Whereas a conventional DC motor is commutated at precise moments due to the angular position of the commutator, the angular position of the BLDC motor has to be submitted to the control electronics by some other means. Often, this is implemented using one or more sensors based upon Hall technology. This precise, electronically controlled commutation delivers impressive increases in efficiency compared to DC motors. However, using the angular
position, as provided by the Hall sensors, to control the moment of commutation alone, does not deliver the full efficiency that such motors can achieve. As the rotational speed increases, the phase difference between the voltage applied and the current drawn also gradually increases. This is caused by the impedance of the motor windings; the phase difference results in a reduction in drive efficiency. If it were possible to bring these back into phase with one another, regardless of rotation speed, efficiency can be improved by several percentage points. The method of achieving
this is simple enough to describe: actively modify the moment of commutation (changing the lead angle) to bring voltage and current back in phase. Many approaches to this calculate a rough lead angle value for various motor RPMs, applying them as the rotation speed changes. But the lead angle is also affected by variations in the motor, due to production methods. For the optimal approach, the electronic control mechanism must actively match itself to the motor during operation. This is where the Intelligent PhAse Control (InPAC) technology from Toshiba comes in. By integrating InPAC into the
/ ELECTRONICS
The TC78B016FTG - a brushless motor controller with an integrated pre-driver
electronics that monitor rotor angle, as well as the current drawn by the motor, the internal control mechanism can apply the appropriate lead angle across the entire range of rotational speeds required by the application. This is achieved without experimentation in the laboratory during development, or applying tuning parameters during production. InPAC is available across a wide range of single-chip solutions suitable for both 1-phase and 3-phase brushless motors. Next generation products can also be used with an external power stage for more flexibility. Devices such as the TC78B016FTG provide 3-phase sine-wave PWM drive output for BLDC control applications, driving a wide range of external MOSFETs to meet the challenges of these applications across a wide range of power levels. Supporting input voltages of up to 40V, and drive currents of up to 3A, it is a fully integrated solution. Rotation speed is determined by either an analogue or PWM input signal, with provisions made for electronic phase difference correction in the case of a mechanical shift of the hall sensor. For low power applications, there is also the TC78B025FTG, a fully integrated driver version, which is suitable for applications that operate up to 16V/3.5A.
Toshiba
www.toshiba.co.uk ELECTRONICS | FEBRUARY 2020 17
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