Brushless dc (BLDC) motors, also called permanent magnet dc synchronous motors, have rapidly gained popularity because of their desirable characteristics. The BLDC behaves like a dc motor with linear relationships between current and torque and between voltage and rotational speed. However, BLDC motors offer advantages over brushed dc and induction motors, including better speed versus torque characteristics and faster dynamic response. Other advantages include high efficiency and reliability, long operating life, noiseless operation, higher speed ranges and reduced electromagnetic interference (EMI) emissions. Also note that the ratio of delivered torque vs. motor size is higher, making BLDCs useful in applications where space and weight are critical factors.

Three-phase motors driven by pulse-width modulation techniques are used in many consumer appliances, from air conditioners and garbage disposals to adjustable beds and pool pumps. These applications typically use brushless dc (BLDC) motors with power ratings from 0.25 hp (186 W) to 3 hp (2,238 W). Applications of this type require galvanic safety isolation to prevent high voltage from encroaching into user-accessible, low-voltage planes. Isolation also provides seamless level shifting for high-side gate drive, eliminating the need for high-voltage IC (HVIC) drivers.

Advanced CMOS Isolation Devices

Until recently, optocouplers and transformers have been the mainstays of galvanic isolation circuits, mostly because better technologies were not available. Designers were, therefore, forced to use optocouplers, which have reliability issues and poor timing performance, or bulky EMI-emitting transformers with the external reset circuits and duty cycle limitations that accompany them. But now, a relatively new breed of galvanic isolators fabricated in a standard CMOS process is displacing legacy isolation devices while offering significant advantages in performance, reliability, size and cost-per-channel. Unlike optocouplers and transformers, CMOS-based isolators like the Silicon labs Si86xx are far easier to integrate into larger semiconductor ICs, enabling system-level IC functions with integrated isolation.

CMOS Isolated Gate Drivers

Three-phase motor systems require three high-side/low-side IGBT transistor pairs, as well as one motor brake IGBT for a total of seven isolated driver channels. CMOS isolated gate drivers, such as Silicon Labs’ Si823x ISOdrivers, are single-package isolated drivers that provide both isolation and high-side driver level shift functions. These single-package solutions offer advantages over legacy approaches including certified isolation ratings up to 5 kVrms, isolated floating output drivers with fast (60 ns) input/output propagation delay, and built-in dead time generators and overlap protection. In addition, these isolation solutions exhibit none of the weaknesses of legacy technologies, such as optocouplers and transformers.

CMOS Isolated AC Current Sensor

Legacy current sensing solutions include current-sense transformers (CTs), Hall Effect sensors and differential amplifier/resistive shunt combinations, again due to the lack of better solutions. CTs need a cycle-by-cycle reset to prevent core saturation, and these circuits can be simple or complex, depending on the end application (Figure 3). CTs are also physically large and have relatively high series resistance and inductance, which reduces efficiency and aggravates inductive ringing.

Many appliances found in homes rely on low- or fractional-horsepower three-phase motors. These end applications frequently require galvanic isolation for safety, ground noise mitigation and/or voltage level shifting. Legacy technologies, such as optocouplers and Hall Effect current sensors, have traditionally been used for such applications. CMOS-based isolation technology has given rise to isolated gate drivers, multi-channel digital isolators and ac current sensors that offer significant advantages in performance and reliability over legacy galvanic isolation techniques.