Brushed DC motors have been the predominate choice in power tool design, such as drills, for decades. However, adoption of Brushless DC (BLDC) in power tools is rapidly proliferating due to better efficiency and the availability of cost-effective BLDC motors. Today, in fact, most high-end power drilling tools use BLDC motors.
The typical current rating of a power tool is much higher compared to popular consumer BLDC applications, such as drones and toys. Power drills must be capable of handling various kinds of material during operation, for example, when drilling a hole in a typical household wall, the drill bit first meets the relatively soft drywall, then the much harder wooden stud.
This means that the load torque can vary considerably during operation. A Hall sensor is thus needed to provide the rotor position in real-time. Considering the high current rating, a discrete topology with external MOSFETs is typically used. Since motor current is very high, a small shunt resistor (about 1-5mΩ) is used for current sensing. Although the noise level is quite high when such a small resistor is used, layout and other techniques are necessary to find right the compromise between power dissipation and optimum measurement.
While it’s true that a dedicated current sensing IC can provide flawless motor phase current feedback, this is not a cost-effective solution for power tools. A power tool BLDC motor control block diagram is shown in the figure below.
Advanced 3-phase BLDC driver + STM32
- Industrial automation
- Battery powered home appliances
- Drones and aeromodelling
- Power tools
- STM32 Cortex M0 + 3-phase gate driver
- Fully compatible with STM32 ecosystem
- 12V LDO and 3.3V DC/DC regulators integrated
- FOC and 6-step sensorless/sensored algorithms
- 48MHz, 32k Flash and 4k SRAM
- 12-bit ADC
- Fully protected (UVLO, short circuit, OCP, OTP)
- Compact design with 7 x 7mm QFN
- Extended temperature range: -40°C to +125°C
- 3 op-amps and 1 comparator
- FW boot loader support
- VS = 6.6V to 45V, I = 600mA driving capability
- 4 op-amps and 1 comparator
- VS = 45V, I = 600mA driving capability