How does silicon carbide enable motor drive?

The most important development in power electronics in recent years has been the rise of so-called wide bandgap (WBG) materials, namely silicon carbide (SiC) and gallium nitride (GaN). The properties of WBG materials promise to enable smaller, faster and more efficient power electronics. WBG power devices are already having an impact on a wide range of applications and topologies, from ordinary power supplies and chargers to solar power generation and energy storage. SiC power devices have been on the market longer than GaN and are generally used in higher voltage, higher power applications.

Electric motors account for a significant portion of the total power in industrial applications. They are used in heating, ventilation and air conditioning (HVAC), heavy robotics, material handling and many other functions. Improving the energy efficiency and reliability of motor drives is an important way to reduce costs. SiC is increasingly being used in high-power industrial drives. SiC's unique properties make it a preferred power electronics material for these challenges.

SiC Material Characteristics

SiC is a semiconductor material with a larger band gap (3.26 eV) than silicon (1.12 eV) and many favorable properties for power electronics.

SiC has a dielectric breakdown strength 10 times higher than silicon. One of the most important functions of a power electronic switch is to maintain high voltage. Due to its high dielectric strength, SiC can support high voltages through the device over a shorter distance. This distance is also the drift region between the channel and drain contacts in a vertical device. A shorter drift region reduces the resistance of the device and directly results in lower power losses.

The wide bandgap also reduces the number of thermally excited carriers, resulting in fewer free electrons and lower leakage current. In addition, the leakage current is small and stable over a wider temperature range than traditional Si devices. This makes SiC MOSFETs and diodes a more efficient choice for high-temperature applications.

SiC has a thermal conductivity three times higher than silicon, which allows for better heat dissipation. Heat dissipation of power electronics is an important part of system design. The thermal conductivity of SiC allows the switch to operate at lower temperatures and thermal stress.

Finally, SiC's electron saturation speed is twice that of silicon, which allows faster switching. Faster switches have lower switching losses and can operate at higher pulse width modulation (PWM) frequencies. In some power conversion topologies, higher PWM frequencies allow the use of smaller, lighter and cheaper passive components, which are often the largest and most expensive part of the system.

The process of making SiC wafers (the raw material for semiconductor devices) is more challenging than making Si wafers. While silicon ingots can be pulled from a melt, silicon carbide ingots must be grown in a vacuum chamber by chemical vapor deposition. This is a slow process, and it is difficult to grow with an acceptable number of defects. SiC is a relatively hard and brittle material (usually used for industrial cutting), so special processes are required to cut wafers from the ingots.

ON Semiconductor has multiple supply agreements for SiC substrates, which ensures that production capacity is available to meet the growth in SiC demand. In addition, we are developing our own internal supply of SiC substrates.

Figure 1: Wide bandgap advantage

Improved three- phase inverter

The three-phase inverter is a traditional solution for variable speed high voltage motor drive, with silicon IGBTs co- packaged with anti-parallel diodes to support motor current commutation. The three half-bridge phases drive the three-phase coils of the inverter to provide a sinusoidal current waveform and drive the motor.

There are several ways to improve system performance with SiC. The energy wasted in the inverter consists of conduction losses and switching losses. SiC devices affect both loss mechanisms.

It is becoming more common to replace anti-parallel silicon diodes with SiC Schottky barrier diodes. Si reverse diodes have reverse recovery current, which increases switching losses and generates electromagnetic interference (EMI).

The advantage of SiC diodes is that there is almost no reverse recovery current, which can reduce switching losses by up to 30% and may reduce the need for EMI filters. Similarly, reverse recovery current increases the collector current when turning on, so SiC diodes reduce the peak current flowing through the IGBT, thereby improving system reliability.

The next step to improve inverter efficiency is to completely replace IGBTs with SiC MOSFETs. SiC MOSFETs can reduce switching losses by 5 times, further improving efficiency. The conduction losses of SiC MOSFETs can be half of those of Si IGBTs with the same rated current, depending on the choice of device.

The increase in energy efficiency results in less heat dissipation. Designers can then reduce costs by shrinking the cooling system or eliminating active cooling altogether. Smaller motor drives can then be mounted directly on the motor housing, reducing cables and motor drive cabinets.

WBG devices switch very quickly, which reduces switching losses but creates other challenges. The higher dv/dt generates noise and can cause stress on the insulation of the motor windings.

One solution is to use a gate resistor to slow down the switching, but then the switching losses rise back to IGBT levels.

Another solution is to place a filter on the motor phase. The filter size shrinks as the PWM frequency increases, offering a trade-off between heat dissipation and filter cost.

Fast-switching power devices cannot tolerate stray inductance and capacitance in the inverter circuit. So-called “parasitic” inductances can cause voltage spikes due to high transients generated during switching. To eliminate parasitic effects, ensure that the printed circuit board (PCB) is laid out correctly. All power loops and traces should be short and devices should be closely spaced. Even the gate drive loops should be carefully minimized to reduce the possibility of unwanted device turn-on due to noise.

Power modules integrate multiple devices together in the right topology for motor drive (among others), providing a faster solution with low parasitic inductance and optimized layout. Power modules reduce the number of components that need to be connected to the heat sink, saving PCB area and simplifying thermal management.

ON Semiconductor's Solution

ON Semiconductor offers a growing portfolio of SiC devices for a wide range of applications.

Our SiC diodes are available in 650 V, 1200 V and 1700 V versions in TO-220, TO-247, DPAK and D2PAK packages. We also co-package SiC diodes with IGBTs to obtain hybrid solutions that balance performance and cost.

Our SiC MOSFETs are available in 650 V (newly released!), 900 V and 1200 V versions in 3-lead and 4-lead packages, and we are developing a three-phase inverter module based on SiC MOSFETs.

Finally, we offer non-isolated and galvanically isolated gate drivers designed specifically for SiC switches to form a comprehensive solution.

Figure 2: ON Semiconductor’s new 650 V SiC MOSFET

Summarize

The fast switching and lower losses of SiC devices make them an important solution for efficient, integrated motor drives. As mentioned above, system designers can reduce the size of the motor drive and place it closer to the motor to reduce costs and improve reliability. ON Semiconductor offers a wide and growing range of devices and systems for SiC motor drives suitable for a wide range of industrial applications.