GaN is accelerating its application in motor drives

Brushless DC motors (BLDC) are increasingly being used in robots, power tools, home appliances, and drones. These applications require devices to be light, compact, with low torque ripple, low noise, and extremely high precision control. To meet these demands, the inverters that drive the motors need to run at higher frequencies, and advanced technology is needed to reduce the resulting higher power losses.

Gallium nitride (GaN) transistors and integrated circuits can operate at higher frequencies without significantly increasing losses, and they can significantly reduce cost, noise, size, and weight compared to silicon-based devices. For this reason, GaN has shown great potential in the field of motor drives.

At the same time, in the fast charging market, GaN has long been widely used and has proven to be sufficiently safe and reliable.

Basic advantages of GaN in motor drives

GaN has significant advantages over traditional silicon, including higher electron mobility, higher breakdown voltage and lower on-resistance. These characteristics enable GaN devices to operate at higher frequencies while reducing power losses.

High efficiency : The conduction loss and switching loss of GaN devices are significantly lower than those of traditional silicon devices. For example, TI's DRV7308 GaN IPM can increase inverter efficiency to more than 99%, while traditional IGBT solutions can only reach 97%.

High power density : GaN devices allow higher switching frequencies, reducing the size of passive components, thereby increasing the power density of the system. The DRV7308 GaN IPM has a package size of only 12mm x 12mm, which is 55% smaller than a traditional 250W IPM and can reduce PCB size by more than 65%.

Better thermal management : Due to the low power losses of GaN devices, heat sinks can be eliminated in many applications, further reducing system size and cost.

High-frequency operation : GaN devices are capable of operating at frequencies up to 3 MHz, meeting the requirements of high-precision control and low torque ripple.

Integrated design : For example, the EPC2152 GaN ePower Stage integrates two 70V, 10mΩ FETs and a self-contained half-bridge gate driver, greatly reducing common source inductance (CSI) and gate loop inductance, while TI's DRV7308 IPM includes six FETs and other protection and drive circuits.

Reduced costs: Due to the high efficiency and high power density of GaN devices, system designs can be more compact, thereby reducing the cost of PCBs and heat sinks. For example, in a 250W HVAC compressor system, using the DRV7308 GaN IPM can save more than $2 in system costs.

Eliminating Dead Time in Motor Drive Designs Using GaN

As shown in the figure, the advantages of GaN in BLDC can be divided into two parts: higher efficiency and lower dead time.

In the world of power conversion, dead time is an essential yet onerous aspect of design, forcing engineers to make compromises to ensure reliability. However, recent technological advances, particularly the advent of GaN FETs, can reduce dead time while improving motor drive performance.

Understanding Dead Time

Dead time, the delay between turning off one power device and turning on another, is critical to prevent simultaneous conduction and potential short circuits. For example, in a synchronous buck converter, having two devices conducting at the same time can result in additional losses, higher operating temperatures, or even catastrophic failure.

The controlled dead time is inserted by the controller to ensure a positive effective dead time. Calculating this dead time is a complex process that takes into account factors such as propagation delays, gate resistor value, and FET turn-on/turn-off times. GaN devices have a shorter dead time compared to silicon MOSFETs because they have no body diode reverse recovery and faster switching times.

Motor drive dead time

One major effect of dead time is increased distortion during zero current crossings. This is because during the dead time, both the high-side and low-side devices in the inverter legs are off, so the actual voltage applied to the motor depends on the sign of the current. At the zero current crossing, this voltage suddenly changes sign, creating voltage distortion that results in higher-order harmonics in the motor current waveform. These currents do not produce any useful torque, but do cause increased losses in the motor windings and reduced overall efficiency.

Comparison of the effects of dead time on zero-crossing distortion in sinusoidal motor drives

Reducing dead time in motor drives is critical to improving efficiency. By minimizing dead time, distortion is reduced, resulting in smoother current waveforms and lower losses. This ultimately improves motor efficiency and overall system performance.

Combined effects of reduced dead time and increased PWM frequency on sinusoidal motor drives

GaN enables better dead time optimization for motor drive applications:

Switching Speed : GaN FETs have much faster switching speeds compared to silicon MOSFETs. This allows for more precise control of dead time because the turn-off and turn-on times of GaN FETs are significantly shorter. This faster switching speed allows for tighter control of dead time.

Lower gate capacitance : GaN FETs have lower gate capacitance compared to silicon MOSFETs. This means they can be turned on and off faster, reducing dead time without the risk of shoot-through current. This enables more efficient operation and better optimization of dead time in motor drive applications.

Zero reverse recovery charge : GaN FETs do not have a body diode like silicon MOSFETs, eliminating the reverse recovery charge associated with silicon devices. This reduces the effective dead time required in motor drive applications because the body diode recovery time does not need to be considered.

Opportunities for using wide bandgap switching devices in motor drives

Low inductance motor

Low inductance motors have many different applications, including large air gap motors, slotless motors, and low leakage induction motors. They can also be used in new motor types that use PCB stators instead of winding stators. These motors require high switching frequencies (50-100 kHz) to maintain the required ripple current.

However, these requirements cannot be met using standard insulated gate bipolar transistors (IGBTs) as they can only achieve high power switching frequencies up to 20 kHz. The high losses when operating at these frequencies with silicon MOSFETs have created new opportunities for wide bandgap devices.

High-speed motor

Due to their high fundamental frequency, these motors also require high switching frequencies. They are suitable for applications such as high power density electric vehicles, high pole count motors, high speed motors with high torque density, and megawatt-class high speed motors. Similarly, the maximum switching frequency that MOSFETs and IGBTs can achieve is limited, and it may be possible to break through these limitations by using wide bandgap switching devices.

Harsh working conditions

There are two interesting benefits to using wide bandgap devices in motor control inverters. First, they generate less heat than silicon devices, reducing the need for heat sinking. Second, they can withstand higher operating temperatures - SiC: 600°C, GaN: 300°C, while silicon devices can only withstand a maximum operating temperature of 200°C.

Although GaN devices currently have some packaging-related issues that limit their applicable operating temperature to more than 200°C, research is underway to address these issues. As a result, wide-bandgap devices are more suitable for motor applications that may face harsh operating conditions, such as integrated motor drives in hybrid electric vehicles (HEVs), subsea and downhole applications, and space applications.

Summarizing the opportunities for using wide bandgap devices in motor drives

Challenges of Using Wide Bandgap Devices in Motor Drives

While there are clearly many attractive benefits to using wide bandgap devices in motor drive systems, there are still some challenges that need to be overcome.

Winding insulation

The first risk is related to turn-to-turn shorts, which can occur because wide bandgap devices switch at speeds that existing motor winding insulation cannot withstand. There are two potential ways to address this risk. The first is to limit the rate of voltage change (dv/dt), but this means that the full potential of wide bandgap devices cannot be fully utilized and inverter losses are increased. The second is to research and further develop new insulation materials that can withstand these switching frequencies and voltage change rates (dv/dt).

Bearing life

Faster switching speeds increase partial discharge in the motor bearings, which can reduce bearing (and motor) life, negating the benefits that can be achieved by using wide-bandgap devices. One potential way to address this is to use bearings with ceramic coatings. Unfortunately, they are very expensive and add to the cost of the motor.

Cable length

Higher switching frequencies can cause problems with signal reflections on longer cables. One way to solve this problem is to use filters, but this adds filter losses to the system. Research is currently underway on how to increase integration, such as by placing the inverter closer to the motor, to shorten the cable length and reduce the impact of signal reflections on the cable.

Summarize

The application of GaN technology in the field of motor drive has shown significant advantages and broad prospects. With its high efficiency, high power density and excellent thermal management characteristics, GaN devices not only improve the efficiency and performance of motor drives, but also significantly reduce system size and cost. Although there are some challenges in the application process, such as winding insulation, bearing life and cable length, these problems are expected to be effectively solved with the continuous advancement and innovation of technology. The introduction of GaN technology has undoubtedly brought new changes and unlimited possibilities to motor drive systems.