The role of silicon carbide in next-generation industrial motor drives

The International Energy Agency (IEA) estimates that electric motors consume more than 45% of the world’s electricity. Therefore, it is critical to find ways to maximize the energy efficiency of their operation. More efficient drives can be smaller and closer to the motor, reducing the challenges of long cables. This will make practical sense from an overall cost and continued reliability perspective. The emergence of wide bandgap (WBG) semiconductor technology is expected to play a major role in achieving new benchmarks in motor energy efficiency and form factor.

Using WBG materials such as silicon carbide (SiC) can create products that outperform silicon (Si) counterparts. While there are a variety of important opportunities for using this technology, industrial motor drives are gaining the most interest and attention.

SiC’s high electron mobility enables it to support faster switching speeds. These faster switching speeds mean that corresponding switching losses will also be reduced. Its dielectric breakdown field strength is almost an order of magnitude higher than that of silicon. This enables thinner drift layers, which translates into lower on-resistance values. In addition, since SiC has a thermal conductivity three times that of Si, it is much more efficient at dissipating heat. Therefore, it is easier to reduce thermal stress.

Traditional high-voltage motor drives use a three-phase inverter with Si IGBTs integrated with anti-parallel diodes. The three half-bridge phases drive the corresponding phase coils of the inverter to provide a sinusoidal current waveform, which then runs the motor. The energy wasted in the inverter will come from two main sources - conduction losses and switching losses. Replacing Si-based switches with SiC-based switches can reduce both losses.

Instead of using an anti-parallel silicon diode, SiC Schottky barrier diodes can be integrated into the system. While silicon-based diodes have a reverse recovery current that causes switching losses (as well as generating electromagnetic interference, or EMI), SiC diodes have negligible reverse recovery current. This allows switching losses to be reduced by up to 30%. Since these diodes generate much lower EMI, the need for filtering is not as great (resulting in a smaller bill of materials). It should also be noted that reverse recovery current increases the collector current during turn-on. Since the reverse recovery current of the SiC diode is much lower, the peak current through the IGBT during this period will be smaller, thereby improving the reliability level of operation and extending the life of the system.

Therefore, if you want to improve the driving efficiency and extend the working life of the system, it is obviously beneficial to migrate to SiC Schottky. So why do we take a further approach? If SiC MOSFET is used to replace the IGBT responsible for the actual switching function, the improvement in energy efficiency will be more significant. Under the same operating conditions, the switching loss of SiC MOSFET is five times lower than that of silicon-based IGBT, and the conduction loss can be reduced by half.

Other benefits associated with the WBG solution include significant space savings. The superior thermal conductivity offered by SiC means that the required heat sink size will be significantly reduced. With a smaller motor drive, engineers can mount it directly on the motor housing. This will reduce the number of cables required.

ON Semiconductor now offers engineers IGBTs co-packaged with SiC diodes. In addition, we have SiC MOSFETs rated at 650 V, 900 V, and 1200 V. With products like these, it is possible to revolutionize motor drives, improve energy efficiency parameters, and make implementation more streamlined.