Using new silicon carbide (SiC) diodes, Siemens and its research partners have succeeded in increasing the power of frequency converters by almost ten percent. In the recently ended project MV-SiC, these diodes were tested in the sort of commercial converters used for large drives. SiC diodes reduce the complexity of the system, and because they have lower losses, they also increase energy efficiency. Another result is that the switching frequency of converters can be increased by approximately a third, which boosts the performance and speed range of the drives. Siemens managed the project, and it was funded by the German Federal Ministry of Education and Research as part of the program Power Electronics for Raising Energy Efficiency.
As the name implies, silicon carbide (SiC) is a compound of silicon and carbon. Apparently someone figured out that this particular compound is significantly better than silicon for high-power/high-voltage semiconductor devices.
Silicon carbide has three major advantages over silicon:
- higher thermal conductivity
- higher critical breakdown field
- wider bandgap
The higher the thermal conductivity, the higher the current density, and the wider the bandgap, the lower the leakage current at high temperatures. The high critical breakdown field allows a given voltage rating to be maintained while minimizing device thickness, and less thickness equals less on-state resistance..
The bottom line is that if you’re using a transistor for switching and you might be dealing with seriously high currents or voltages, a silicon carbide mosfet is worth a look.
Silicon carbide (SiC) devices provide benefits such as higher power density, lower cooling requirements, and lower overall system cost in applications such as inverters, motor drives, and battery chargers.
Although a SiC device is more expensive than a silicon device, the system level benefits, particularly at 1,200V, outweigh the higher device cost. At or below 600V, the advantages over silicon are negligible. To reap the benefits of a SiC die, it must be packaged and driven by specially designed gate drivers.
Advantages Of Sic Over Silicon
Typically, silicon carbide loses only 1% of its energy during the reverse recovery phase. Because there is no tail current, the turnoff is much faster and the losses are much lower. Because there is less energy to dissipate, SiC devices can switch at higher frequencies, increasing efficiency.
Because of SiC’s higher efficiency, smaller size, and lighter weight, it can produce a higher-rated solution or a smaller design with lower cooling requirements.
Silicon’s performance degrades with increasing temperature, whereas SiC is far more stable. To maintain specification at higher temperatures, silicon devices are typically over-specified at room temperature. Because SiC is much more stable over higher temperatures and does not require significant derating, a SiC device with half the current rating will typically perform the same job as a silicon IGBT.
SiC operates at voltages well above 10kV, far exceeding what is currently available. There are SiC devices with ratings of 1,200V and 1,700V available. With issues like arcing, creepage, and clearance, packaging has become the limiting factor, not semiconductor technology.
Lower Losses
Conduction losses are the most significant sources of energy loss in a SiC module. SiC has a low gate charge as a wide bandgap material, which means it requires far less energy to make the device switch.
Due to the dramatic improvement in reverse recovery energy and tail current, diode switching losses are virtually eliminated. Switch conduction losses are resistive and, as a result, are comparable in both technologies. Next-generation SiC processes promise even more advancement. Also, since the values of the components in the transformer LC filter become significantly lower at higher frequencies, the magnetics become smaller and lighter.
SiC also has a 10x longer mean time to failure (MTTF) than silicon and is 30x less susceptible to radiation and single event failure. However, because SiC has a lower short circuit tolerance, a fast-acting gate driver is required.
A standard, hard turn off transition (left) and a softer stepped transition, which will reduce the di/dt. Higher frequency switching is usually not advantageous in low-speed applications. In such cases, the cost premium of a SiC device and the additional design considerations are unjustifiable, making a silicon IGBT the more logical solution.
Furthermore, SiC is in short supply. At 600V/650V, there is a scarcity of SiC devices, and those that are available are mostly discrete components.
During the design process, a silicon IGBT necessitates less RFI mitigation. It is not necessary for a high-performance gate driver to manage the turn-off or to react quickly in the event of a short circuit to protect the device.
Gate Drivers
A specially designed gate driver is required for SiC devices. A gate driver designed for driving silicon IGBTs cannot support the switching speed of a SiC device, nor will it be able to provide the rapid fault response time required to protect a SiC device in the event of a short circuit.
They also require different drive voltages than silicon IGBTs. Voltage rails are frequently asymmetric, and a few volts of negative voltage is usually required to turn the device completely off.
Another consideration is that SiC modules necessitate augmented turn-off. Higher frequency/harder switching, combined with lower internal losses, causes current spikes and ringing.
To manage the impact of sudden current changes and reduce ringing, augmented or ‘soft’ turn-off employs intermediate voltage steps. Because of the dampening effect of internal losses, silicon devices suffer less.
Packaging Issues
Packaging technology has now become the primary constraint – even for SiC optimised packages – as a result of the performance improvements provided by SiC.
SiC packages are typically smaller, lower profile, and more thermally efficient than silicon packages, though they must be designed with a symmetrical layout to reduce loop inductance. Where dies are mounted in traditional packages designed for switching at lower frequencies with less stringent rise and fall time requirements, the advantages of SiC are not realized.
Moreover, because of the effects of wave propagation, non-symmetrical designs perform poorly at higher frequencies. To realize the benefits of the technology, SiC-specific packaging and gate drivers must be used, making SiC an excellent choice for new system designs.
