The introduction of Li-ion batteries and brushless DC or BLDC motors has opened up a new market for battery powered motor driven products. You will find brushless motors powered with rechargeable batteries being used in products such as uninterruptible power supplies, wheelchairs, e-bikes and other small electric vehicles and in small tools such as leaf blowers, chainsaws and drills. To take advantage of the integration of BLDC motors with Li-ion batteries for providing power requires updated MOSFET bridge drivers.
Although batteries such as lead-acid, Ni-MH and Ni-Cd are more popular, Li-ion batteries with their high energy density offer significant advantages over other battery technologies. Li-ion batteries typically offer two to three times the energy density as compared to what other conventional battery technologies currently offer. With higher energy density, users can make do with smaller battery packs that lead to lighter and more compact hand-held tools. Wheelchairs and e-bikes can operate for longer times without any increase in the physical size of their original battery pack.
However, there are some disadvantages associated with the high energy density of Li-ion batteries. It is customary to think of batteries as voltage sources, but for Li-ion batteries, this is not the case. Li-ion batteries have a significantly high internal inductance that generates considerable ripples on its voltage as a consequence of driving the motor with PWM or Pulse Width Modulation methods. Although this can be easily offset by adding sufficient capacitance across the MOSFET bridge, there can be enclosure limitations leading to prohibitive cost increases.
Low capacitance on the MOSFET bridge can lead to significant voltage ripples. For example, the ripple voltages found on a typical 18-20V Li-ion battery under heavy load can range from 5V at minimum to 36V at the maximum. Additionally, the battery voltage is likely to fall to an abysmally low value when the motor is overloaded to a stall or locked rotor condition. Therefore, presence of a controller is necessary to decide on how to react to such extreme operating conditions.
Compared to conventional brushed DC motors, BLDC motors offer significant advantages. For example, brushes limit the speed of a brushed DC motor, but the BLDC motor has no such limitations; the design of its rotor decides its maximum operating speed or RPM. Most applications do not require the full speed of the motor and a transmission with a gear reduction is used to bring down the motor speed to the desired RPM. A BLDC motor can rotate at significantly higher RPMs compared to the speed of a brushed motor. Therefore, the required torque at the output of the device can be achieved easily with a smaller BLDC motor and a corresponding transmission gear ratio.
As BLDC motors do not have brushes, they do not produce EMI as the brushed motors do. Additionally, the absence of brushes leads to lower maintenance and an increase in the efficiency of BLDC motors. On average, a BLDC motor is 1.5 times or more efficient than a brushed motor is. However, the drive electronics adds complexity to the application of a BLDC motor, requiring ICs to reduce component count, real estate and BOM costs, especially where space is a constraint.
