Emerging electronics applications require electric motor designs that squeeze higher performance from ever more compact platforms. Designers are hard-pressed to meet the new requirements with motor driver circuits based on traditional silicon MOSFETs and IGBTs. As silicon technology reaches theoretical limits for power density, breakdown voltage, and switching frequency, it gets tougher for designers to contain power losses. The main effects of these limitations are sub-optimal efficiency and additional performance problems at high operating temperatures and high 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 heat sink, 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.
Gallium nitride is a wide bandgap (WBG) material. As such, its forbidden band (corresponding to the energy required for an electron to pass from the valence band to the conduction band) is much wider silicon’s: about 3.4 electron-volts, compared with 1.12 eV for silicon. The higher electron mobility of a GaN HEMT translates into a greater switching speed, since the charges that normally accumulate in the joints can be dispersed more quickly. The faster rise times, lower drain-to-source on resistance (RDS(on)) values, and reduced gate and output capacitance achievable with GaN all contribute to its low switching losses and ability to operate at switching frequencies up to 10 times higher than silicon. Reducing power losses brings additional benefits, such as more efficient power distribution, less heat dissipation, and simpler cooling systems.
The possibility of operating at high switching frequencies enables solutions with reduced footprint, weight, and volume, avoiding the use of bulky components such as inductors and transformers. Figure 1 shows the trend lines for conduction and switching losses for power devices built with silicon and gallium nitride technology as switching frequencies rise. For both materials, conduction losses remain constant and switching losses mount. But as the switching frequency increases, the switching losses of a GaN HEMT transistor remain significantly lower than those of a silicon MOSFET or IGBT, and the higher the switching frequency, the more marked the difference becomes.
GaN HEMTs’ main advantages over traditional silicon devices are:
- a higher slew rate (dV/dt of 100 V/ns or more), which in turn supports faster switching rates, thereby reducing switching losses;
- a near-zero reverse recovery charge (because GaN HEMTs do not have an intrinsic body diode, no anti-parallel diodes are required, and power losses and electromagnetic interference [EMI] effects are reduced);
- full operation at higher temperatures (up to about 300 °C) without affecting the switching capabilities;
- higher breakdown voltage (above 600 V);
- switching losses, at a given switching frequency and motor current, that are 10% to 30% those of a silicon MOSFET; and
- greater efficiency, smaller footprint, and lower weight.
All of these characteristics favor the use of GaN HEMT devices in the design of drivers for high-voltage and high-frequency electric motors. With GaN HEMTs, designers can build electric motors that achieve the same output characteristics as a silicon-based design but in a more compact size and with lower power absorption.
HIGH-PERFORMANCE ELECTRIC MOTOR DRIVE
Low-voltage, low-inductance, high-rotation–speed brushless motors require driver circuits with typical switching frequencies between 40 kHz and 100 kHz, capable of minimizing losses and variations in motor torque. A common solution for driving an AC motor, shown in Figure 2, includes an AC/DC converter, a DC circuit (represented in Figure 2 by the capacitor), 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 pulse-width–modulated (PWM) signals, each of which drives a single motor phase.
The DC circuit filters the voltage and current coming from the AC/DC converter, suppresses voltage transients that could damage the inverter transistors, reduces inductive currents that could damage the inverter transistors, stabilizes the voltage supplied to the load, and improves overall efficiency. The capacitor must operate under particularly critical conditions, such as high slew rate and high voltage peaks. The designer should therefore select the capacitor carefully to ensure the required high-voltage features — choosing, for example, a base metal electrode (BME) capacitor.
INTEGRATED POWER SOLUTIONS
Referring back to Figure 2, 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. Today, several integrated devices based on GaN technology are commercially available.
For example, 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.
Texas Instruments has 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.
Figure 3 is a system block diagram of the three-phase GaN inverter. The red broken line defines the boundaries of the TIDA-00909. Each of the three inverter half-bridges employs an integrated 80-V, 10-A GaN half-bridge module (LMG5200); a 5-mΩ phase current shunt; and a differential current sense amplifier (INA240) with a gain of 20 V/V and a midpoint voltage of 1.65 V, which is set by the 3.3-V reference (VREF3333).
The commercial availability of wide-bandgap semiconductors, including GaN HEMTs, allows designers to create efficient and reliable inverter stages for driving high-power–density electric motors. Compared with silicon-based MOSFETs and IGBTs, GaN HEMTs allow for more efficient, more compact, lighter-weight, more economical drivers. ■