Benefits of SiC MOSFETs in Motor Drive Applications

Electric motors are an important topic when considering sustainability. In the US, three-phase motors over one horsepower consume almost 30% of all grid energy according to a 2021 study. However, the International Energy Agency (IEA) sees this as an opportunity as the potential 70% energy savings are the largest of any sector.

With this increased focus on sustainability, engineers are looking for ways to capture the potential energy savings. As well as ensuring motors are matched to the task, inverter technology is being used for speed control to make the drive more efficient.

Until recently, Insulated Gate Bipolar Transistors (IGBTs) had been the preferred choice for motor inverters. These devices are constructed like bipolar transistors with a MOSFET-like gate structure and can handle over 6kV at thousands of amperes – enough for even the largest motors.

Over the years innovations such as advanced packaging and Kelvin source pins to improve gate drive, switching and EMI have kept IGBTs relevant. However, in the sub-600V space, silicon MOSFETs have replaced IGBTs in many applications. Above 600V, silicon MOSFETs were generally considered unsuitable so here, IGBTs remained the preferred solution, despite their limitations.

Since the advent of wide-bandgap (WBG) silicon carbide (SiC) MOSFETs, higher voltage applications are now able to replace IGBTs with MOSFETs.

The primary drivers for making the change are that SiC MOSFETs offer

• Lower switching losses
• Higher switching frequencies
• Better thermal conductivity than equivalent IGBT devices.

Thermal management is critical in motor drives as IGBTs can suffer from thermal runaway. This presents challenges as designers strive to design more compact, lighter inverters that can be co-located with the motor itself. SiC MOSFETs have a higher thermal conductivity, making them ideal for compact, highly integrated designs. In all cases, thermal management is simpler – and in some cases, it is not required at all.

IGBTs turn off relatively slowly as VCE takes time to stabilize. Energy during this time is wasted and counts as a loss, while the slow switching limits the maximum operating frequency of an IGBT.

On the other hand, SiC MOSFETs switch rapidly, reducing turn-off losses by at least 60%. With a >50% improvement in turn-on losses as well, Toshiba research concluded that moving from IGBTs to SiC MOSFETs in real world conditions can reduce total switching losses by 66%.

Consequently, SiC MOSFET based inverters can operate at frequencies around 50–200kHz compared to the 15–20kHz of IGBT based designs. This allows for the size (and cost) of magnetic components to be significantly reduced, delivering a more compact and efficient design.