Both the brushless DC (BLDC) and permanent magnet synchronous motor (PMSM) require electronic commutation in order to create a rotating electromagnetic field, which is needed to turn the rotor. Both are synchronous machines, where the magnetic field and rotor move at the same speed.
The main difference between these two motor types lies in the construction of the stator windings. So, in the case of the BLDC, it is ideally commutated using a trapezoidal waveform that can be created relatively easily.
In contrast, the PMSM (the lead image), requires a sinusoidal commutation waveform that is more complicated to produce.
Although it requires a more complex waveform, the PMSM benefits from lower torque ripple and reduced audible noise. For this reason, it is often the preferred motor type in applications requiring smooth and quiet motion, such as high-end home appliances, power tools and industrial automation.
Historically, sine-wave commutation has been handled using algorithms that are implemented in microcontroller unit (MCU) firmware that require extensive optimisation and fine-tuning to work with the chosen motor and meet the requirements of the application. Also, the MCU’s performance must be adequate to execute the control algorithm up to the maximum required speed, while also handling application-level processing.
Position Sensing and Soft-Start
Unlike a brushed motor, where simply applying power will ensure the correct coils are engaged to start the motor regardless of where the rotor last stopped, starting and running a BLDC motor requires knowledge of the current rotor position. This is needed to allow the appropriate coils to be excited and the rotor to begin rotating in the correct direction. Sensors are often fitted to brushless motors to detect this position. Alternatively, a sensorless setup saves the expense and the potential reliability issues associated with sensors (such as Hall devices).
In this case, techniques are needed to move the stationary rotor in a known starting position before energising the coils. Without proper precautions, the rotor and anything attached to it could whip back in the wrong direction.
When the coils are energised, this must be undertaken in a manner that prevents PWM switching from generating excessive noise and vibration during the time where no useable back-EMF is available to determine the rotor angle. In essence, the motor control algorithm is driving the motor blind. Once sufficient back-EMF is available, the motor controller can switch to the chosen control method.
Fine-Tuning the Drive
The ability to start the motor and select the speed, however, are only a subset of the functions needed to operate properly. The motor-drive designer must have the flexibility to integrate the controller with MOSFETs of a suitable voltage and power rating for the application. They also require the ability to optimise parameters (such as acceleration, lead angle, and PWM frequency) to ensure the system will respond as required to the user’s inputs and maximise energy efficiency in all operating conditions.
Achieving MCU-less Control
The TC78B011FTG sine wave pre-driver, developed by Toshiba, relieves any need for an MCU. This parameterizable chip for sensorless three-phase brushless motor control is a pulse-width modulated (PWM) chopper that can be connected to external low-side and high-side N-channel MOSFETs, allowing for a scalable inverter implementation to match a range of different motors.
While the device provides open-loop speed control, closed-loop control that maintains the target speed unaffected by power supply or load variations, with an adjustable speed curve, is a more typical requirement. This can be achieved by configuring the precise operating mode via the I2C interface, with the option to store the settings in a non-volatile memory (NVM). As a result, suitable settings can be programmed during manufacture for circuits that do not use a microcontroller or processor.
On the other hand, the motor speed can be adjusted by writing to a register through the chip’s I2C interface and can also be determined using either a PWM input or an analogue signal.
Braking and direction, also, are controlled via register settings or external pins. The motor current and rotation speed can be read from external pins while the motor is running.
More Accurate Positioning
After powering-on, the IC retrieves the stored device configuration from its NVM. At this point, a brake sequence may be applied by shorting the appropriate coils through the motor inverter to ensure that the rotor is stationary before attempting to start rotation. Once the initialisation sequence is complete, after around 3.5ms, the driver enters idle mode with all MOSFETs turned off and awaits further instruction from the host system.
The required speed can be defined via I2C to the speed command register (SPD) or applied as a PWM or analogue signal to the SPD pin. When either of these is received, the motor start-up sequence is engaged. The process begins with a DC excitation of the motor coils that moves the rotor to the starting position. When this is completed, the forced commutation of the motor starts. At this stage, a rough electrical field is applied in 120° commutation to generate an initial back-EMF. A configurable soft-start feature is also included, which limits the current drawn when spinning up the motor. All speed control at this stage is open loop.
The system changes to sensorless control, with the current limit set for normal operation, as soon as the motor is rotating fast enough to generate a back-EMF usable for the control algorithm. Closed-loop speed control can then be engaged.
The rotor may already be rotating before power is applied, which can be caused, for example, by air passing over a fan’s blades. In this case, known as idling or windmilling the motor driver will skip the initial excitation and forced commutation steps and proceed directly with sensorless operation. In a typical application the back-EMF measurement capability can be excessively sensitive in this type of situation, causing the driver to incorrectly try to skip the first open loop start-up stages. The TC78B011FTG is able to prevent this by providing a register that allows the designer to change the minimum rotor speed considered fast enough to skip the start-up process.
Alternatively, to avoid the challenges associated with starting an idling motor, the controller can be configured to apply the braking sequence each time after leaving standby or power on, enabling the rotor to always start from a stopped state.
Greater Adjustability
To allow flexible speed control in closed-loop mode, the IC provides registers for setting the time between each step change in speed and for determining how quickly the speed changes can occur. The supported speed settings are configurable through individual control of the starting, stopping, and maximum duty cycles. The RPM associated with the start and maximum values (as shown below) can also be set and up to two speed slopes between the start and maximum RPM may be defined.
The minimum and maximum speed can be set. And two different speed slopes are available
The frequency used for the PWM output can be fixed or set to increase automatically for optimum efficiency as the motor speed increases. The available frequency range lies between 23.4kHz and 187.5kHz.
Adjusting the PWM frequency also helps designers ensure compliance with the electromagnetic compatibility (EMC) requirements relevant to the application.
There is also a register for adjusting the lead angle according to the motor’s characteristics, which helps to optimise energy efficiency and minimise audible noise. For the quietest possible operation, the lead angle can be set so that the back-EMF and motor current are in phase.
The IC contains three half-bridge pre-drivers for external N-channel MOSFETs. These can supply a gate-source (VGSS) voltage of up to 8V above the motor supply voltage and can be configured to deliver gate-source (IGSS) current from 10mA to 100mA for both high- and low-side MOSFETs.
The highest usable switching frequency may be limited by the MOSFET choice and motor used. Since the back-EMF is measured for position sensing during the off-time of the PWM, choosing a highly inductive motor, or choosing MOSFETs with low switching performance, can cause position detection to fail. To avoid this, the optimum PWM frequency can be determined by testing suitable settings under all usage conditions.
The chip also comes with safety features, including shoot-through prevention with configurable dead-time. A status register indicates abnormal conditions including excessive current draw, low charge-pump voltage, thermal shutdown, and start-up failure. An alert pin is set when any of these conditions arise. This pin is also used to indicate under-voltage and motor operation outside the pre-set maximum and minimum speeds. The controller can be programmed to await a signal from an external source after an abnormal condition is detected or attempt to restart the motor in auto-recovery mode.
Conclusion
Designers can take advantage of BLDC motors, and the smooth and quiet PMSM type in particular, without embarking on an MCU development project.
They can leverage programmable controllers that are featured for standalone operation with closed-loop control and parameterizable speed setting.
A MikroE board featuring the Toshiba TC78B011 IC plus selected MOSFETs is now available for evaluation purposes and will help to facilitate the motor system development process.
Author details: Anno Schmitz, Principal Engineer, Toshiba Electronics Europe
