GaN power devices offer advantages in industrial motor control

Motor control, particularly frequency-controlled drives, is a technology that has advanced quickly in recent years as a result of the widespread use of motors in a variety of applications and the potential for enormous energy savings. Frame-based power modules for motor control have been a significant revolution in application domains that are particularly sensitive to cost, size, and performance.

Electric-motor designs that extract higher performance from compact platforms are required for emerging electronics applications. Motor driver circuits relying on classic silicon MOSFETs and IGBTs are struggling to fulfill the new criteria. It’s becoming more difficult for designers to keep power losses under control as silicon technology approaches theoretical limits for power density, breakdown voltage, and switching frequency. The principal consequences of these constraints are reduced efficiency and additional performance issues at high operating temperatures and switching rates.

Consider a silicon-based power device operating at a switching frequency of ≥40 kHz. Under those conditions, switching losses are greater than conduction losses, with cascading effects on overall power losses. Dissipating the excess heat that is generated requires a heatsink, driving up the weight, footprint, and cost of the solution. High-electron-mobility transistor (HEMT) devices based on gallium nitride (GaN) offer superior electrical characteristics and are a valid alternative to MOSFETs and IGBTs in high-voltage and high-switching–frequency motor control applications. Our discussion here centers on the advantages that GaN HEMT transistors provide in the power and inverter stages of high-power–density electric motor applications.

Size and energy efficiency are important in motors for robotics and other industrial uses, but other factors also come into play. A GaN solution enables higher pulse-width–modulation (PWM) frequencies, while low switching losses facilitate driving permanent-magnet motors with very low inductance and brushless DC motors. These features also minimize torque ripple for precise positioning in servo drives and stepper motors, enabling high-speed motors to achieve high voltages in applications such as drones.

GaN benefits

GaN is a wide-bandgap material. As a result, its forbidden band (the energy necessary for an electron to move from the valence band to the conduction band) is substantially broader than that of silicon: roughly 3.4 eV versus 1.12 eV. Because the charges that ordinarily build up in the joints may be dissipated more quickly, the enhanced electron mobility of a GaN HEMT correlates to a faster switching speed.

GaN’s low switching losses and ability to operate at switching frequencies up to 10× greater than silicon are due to its shorter rise times, lower drain-to-source on-resistance (R_DS(on)) values, and reduced gate and output capacitance. The ability to operate at high switching frequencies allows for a smaller footprint, weight, and volume and eliminates the need for bulky components like inductors and transformers. The switching losses of a GaN HEMT transistor remain much lower than those of a silicon MOSFET or IGBT as the switching frequency increases, and the greater the switching frequency, the more noticeable the difference becomes.

In summary, GaN devices outperform traditional silicon-based power devices in several aspects, including the following:

• GaN’s breakdown field is over 10× higher than silicon (3.3 MV/cm versus 0.3 MV/cm), allowing GaN-based power devices to support 10× higher voltage before being damaged.
• Operating at the same voltage values, GaN devices exhibit lower temperatures and generate less heat. As a result, they can operate at higher temperatures (up to 225˚C and above) than silicon, which is limited by its lower junction temperature (150˚C to 175˚C).
• Due to its intrinsic structure, GaN can switch at higher frequencies than silicon and provides a low R_DS(on) and excellent reverse recovery. That, in turn, results in high efficiency with reduced switching and power losses.
• Being a HEMT, GaN devices have a higher electric field strength than silicon devices, allowing for a smaller die size and reduced footprint.

Motor control solutions

A common solution for driving an AC motor includes an AC/DC converter, a DC circuit, and a DC/AC converter (inverter). The first stage, usually based on a diode or transistor, converts the 50-Hz/60-Hz main voltage into an approximate DC voltage, which is subsequently filtered and stored in the DC circuit for later use by the inverter. Finally, the inverter converts a DC voltage into three sinusoidal PWM signals, each of which drives a single motor phase. GaN HEMT transistors are typically used for the implementation of the motor driver inverter stage, the most critical point of a high-voltage and high-frequency motor driver solution.

EPC’s EPC2152, for example, is a driver and eGaN FET half-bridge power stage IC in one package, based on the company’s patented GaN IC technology. A monolithic chip contains the input logic interface, level shifting, bootstrap charging, and gate-drive buffer circuits, as well as eGaN output FETs configured as a half-bridge. The high integration enables the compact 3.85 × 2.59 × 0.63-mm package size in a chip-scale LGA form factor. In a half-bridge topology, the two eGaN output FETs are intended to have the same R_DS(on). The use of on-chip gate-drive buffers with eGaN FETs virtually eliminates the impacts of common source inductance and gate-drive loop inductance (see Figure 1). Internal regulation of the gate-drive voltage based on feedback from the driven output FETs ensures a safe gate-voltage level while still turning on the output FETs to a low R_DS(on) state.

Figure 1: Functional diagram of the EPC2152 (Source: EPC)

Another example is the GS-065-004-1-L enhancement-mode GaN-on-silicon power transistor from GaN Systems. The properties of GaN allow for high current, high-voltage breakdown, and high-switching frequency. GaN Systems implemented its patented Island Technology cell layout for high-current die performance and yield. The GS-065-004-1-L is a bottom-side–cooled transistor in a 5 × 6-mm PDFN package that offers low junction-to-case thermal resistance. These features combine to provide very high-efficiency power switching.

Navitas Semiconductor’s NV6113 integrates a 300-mΩ, 650-V enhanced GaN HEMT, a gate driver, and associated logic, all in a 5 × 6-mm QFN package. The NV6113 can withstand a slew rate of 200 V/ns and operates at up to 2 MHz. Optimized for high-frequency and soft-switching topologies, the device creates an easy-to-use “digital-in, power-out” high-performance powertrain building block. The power IC extends the capabilities of traditional topologies (such as flyback, half-bridge, and resonant types) into switching frequencies above the megahertz band. The NV6113 can be deployed as a single device in a typical boost topology or in parallel for use in the popular half-bridge topology.

Figure 2: Navitas Semiconductor’s NV6113 typical application circuits (Source: Navitas Semiconductor)

Texas Instruments Inc. offers a wide portfolio of GaN integrated power devices. The LMG5200, for instance, integrates an 80-V GaN half-bridge power stage based on enhancement-mode GaN FETs. The device consists of two GaN FETs driven by one high-frequency GaN FET driver in a half-bridge configuration. To simplify designing with the device, TI provides the TIDA-00909, a reference design for high-frequency motor drives using a three-phase inverter with three LMG5200s. The TIDA-00909 is provided with a compatible interface to connect to a C2000 MCU LaunchPad development kit for easy performance evaluation.

GaN versus SiC

Because of properties such as energy savings, size reduction, integration options, and reliability, the use of silicon carbide (SiC) devices in motor control and electrical power control applications is a major breakthrough. Among other things, it is now feasible to employ the optimum switching frequency for the connected motor in the inverter circuit, which has significant implications for motor design.

A decrease in losses of up to 80% can be a game-changer in solutions in which active cooling to regulate semiconductor losses is a critical aspect for performance and reliability. One example is the CoolSiC MOSFET based on SiC with XT connecting technology in a 1,200-V optimized D2PAK-7 SMD package from Infineon Technologies, offering attractive thermal capabilities in a small form factor. This combination allows for passive cooling in high-density motor drive segments such as servo drives, allowing the robotics and automation industries to create maintenance-free and fanless motor inverters. Fanless solutions in automation open up new design possibilities because they save money and time on maintenance and materials. The resulting small system size makes it suited for drive integration in a robotic arm.

Compared with an IGBT with a similar rating, a higher current can be achieved with the same form factor, depending on the type of power chosen for the CoolSiC, while still maintaining a constant junction temperature that is significantly lower in the case of a SiC MOSFET (about 40–60 K) compared with an IGBT (105 K). A SiC MOSFET allows higher currents to be driven without a fan for a given device size.

Conclusion

Electric motors may be found in almost every aspect of modern civilization, from the electric equipment we use in our homes and kitchens to the automobiles we drive (including gasoline-powered, hybrid, and all-electric vehicles) and the factories that produce our smartphones. Although some motors are quite simple and others are extremely complicated, they all have one thing in common: They all need to be controlled.

Other motor applications, such as those found in today’s industrial plants, require complex motor control to deliver both highly precise and high-speed motor control activities. Traditional silicon MOSFETs and low-PWM–frequency inverters are being phased out in DC and battery-powered motor applications in favor of GaN-based, high-PWM–frequency inverters. The benefits include increased system efficiency and the elimination of large passive components — namely, electrolytic capacitors and an input inductor.