Cordless power tools are becoming the primary choice in both the professional and consumer markets for low-to-medium applications (Fig. 1). Workshop tools include screwdrivers, saws, drills, rotary hammers, and sanders. In the garden, there are clippers, grass trimmers, leaf blowers, and even chainsaws.
In battery-powered tools, the search for longer battery life has led to the increasing adoption of brushless dc motors (BLDCs) to provide drive power. Compared to their brushed-motor counterparts, BLDCs have higher efficiency, higher torque-to-weight ratio, lower maintenance, higher reliability, and lower noise.
In the midst of all this goodness, there’s just one (tiny) fly in the ointment: Controlling a BLDC requires considerably more electronic circuitry than its less-efficient, noisier, short-lived, slower, and less-powerful predecessors.
A brushed dc motor has a wound armature (rotor) placed between the poles of a permanent or electromagnet (stator) and uses the brushes to mechanically switch current to the armature and cause it to rotate. In contrast, a BLDC has a wound stator and a permanent-magnet rotor assembly. The BLDC controller uses rotor position information from a sensor to electronically switch the stator windings in the correct sequence to maintain rotation of the magnet assembly.
Accomplishing this requires a more complex design. Figure 2 compares the drive circuits of a typical brushed and brushless motor. In contrast to the single gate driver and power FET of the brushed motor, the BLDC motor requires six FETs arranged into three half-bridge pairs, plus a gate driver for each FET. In a space-constrained application such as a power tool, this can tax the ingenuity of the packaging engineer, especially since power FETs have traditionally come in bulky packages such as TO-220, DPAK or D2PAK.