Silicon carbide – a material that developers dream about

Dr. Bernd Laska is head of development for MoComp traction converters at the Nuremberg location. He’s at the forefront of efforts to further develop rail vehicles and their drive trains and to meet the ambitious energy efficiency targets of both vehicle manufacturers and customers. The key is silicon carbide – SiC for short – a new semiconductor material with excellent properties that is especially beneficial when used for designing traction converters. We asked him about the prospects for this new technology. 

First successful applications

The versatility of silicon carbide technology and its tremendous advantages can already be seen in the Avenio trams used for passenger rail service by the urban transport company Münchner Verkehrsgesellschaft (MVG) and in Siemen’s first deployment of the new Mireo Plus B battery-powered train for the Landesanstalt Schienenfahrzeuge Baden-Württemberg (SFBW) in Ortenau. SiC technology is particularly beneficial for regional trains, with their innovative battery hybrid drive for use on rail sections with or without an overhead contact line. It rreduces weight to a minimum, optimizes performance, and boosts the efficiency of multiple units in terms of mileage and range. We wanted to discuss the practical implications of this technology with the man who, together with his team, has been a key player in the development of SiC technology and its qualification for use in rolling stock.

Dr. Laska, silicon carbide isn’t a new material. Why is it now causing such a sensation in semiconductor technology?

That’s right. Because of its high hardness, SiC is traditionally used, for example, to manufacture abrasives, although not in an especially clean composition. What’s relatively new is its use as a semiconductor material in power electronics. The excellent semiconductor properties of high-purity SiC have been known since the 1960s. The higher breakdown field strength (wide band gap material) means that its properties are far superior to those of silicon. Nevertheless, the design of appropriate semiconductor elements requires SiC monocrystals. These crystal structures are much more expensive to manufacture than silicon monocrystals, which can be pulled from a melt with large dimensions. SiC monocrystals, on the other hand, are deposited at high temperatures from the gas phase and built up layer by layer. In the early days, the insufficient yield from monocrystal production was extremely high, but significant progress has been made over the last few years and quality improvements in material manufacturing have been achieved. But despite the high effort and the associated high costs, it definitely pays off. The material opens up outstanding possibilities for manufacturing innovative and highly energy-efficient components, thanks to its extraordinary electrical properties. For example, its use in traction converters provides new opportunities for optimizing energy efficiency throughout the traction system.

Changing one component can have a positive impact in many other areas. That’s why we at MoComp take the perspective of a vehicle manufacturer when developing our components.
Dr. Bernd Laska, Head of R&D Power Electronics (Auxiliary Converters & Traction Converters), Siemens Mobility GmbH

Why do you find this material to be so exciting?

The main benefit is SiC’s higher breakdown field strength. This semiconductor material can be used to implement components with a higher usable field strength, making it possible to produce much thinner semiconductor layers and thus reduce forward voltage and improve switching properties. At the same time, when this type of power semiconductor device is switched on, it behaves like an ohmic resistor, which makes it easily scalable in terms of its current carrying capacity. That’s why converters with SiC components are low-loss and permit high clock frequencies, which in turn benefits other traction components like traction motors and transformers.

But that can’t be all?

You’re right. Another benefit is that a lower power loss always means less heat to be dissipated. Which brings us to the next point: SiC components provide the same output but are more compact and lighter – and more efficient. And because there isn’t much heat to be dissipated, it’s also possible to take simplified component cooling into account. In some applications, smaller heat exchanger units can be used and operated at a lower cooling air flow rate. This reduces weight and space requirements and helps reduce the vehicle’s aerodynamic drag because the air turbulence that’s normally caused by air flows from more powerful heat exchange units is reduced. If a smaller heat exchanger unit is used, the auxiliary converter for its power supply can also be smaller, which again saves space and reduces weight.

And if I have a small cooling fan, won’t that also make it more pleasant for passengers standing next to a stationary train?

Passengers will experience the new technology as quiet drive systems, which means quiet trains. This has multiple reasons. On the one hand, the heat exchange units are smaller and quieter, with fanless air cooling at the extreme end. On the other hand, the high switching frequency of SiC semiconductors also ensures that the noise induced by the converter – particularly in the traction motors – is significantly reduced, which makes it more pleasant. This means that a train departing from a platform or station is much quieter than today’s vehicles. As mentioned earlier, the switching frequency is an important lever for building better on-board motors and transformers – better means more energy-efficient, lighter, and quieter.

So we shouldn’t look at the traction converter in isolation when it comes to optimizing the vehicle?

That’s right, changing one component can have a positive impact in many other areas. That’s why we at MoComp take the perspective of a vehicle manufacturer when developing our components. This is especially true in the area of energy efficiency. What matters to customers is that they consume and pay for as little energy as possible for vehicle operation. The diagram shows the potential savings when operating a typical regional train. You can see that with an energetically optimized drive system, the actual energy absorption from the grid is nearly 10 percent less than that of today’s vehicles, thanks to the use of SiC components. The diagram also shows which components are making a significant contribution to energy savings. Besides the converter, these are mainly assigned to the traction motors and transformers.

How reliable are these numerical values?

The potential savings correspond to realistic estimates but, of course, they’re highly dependent on the individual utilization characteristic of the vehicle. Basically, I’d say that the figures incorporate the innovation potential that is known today and that MoComp consistently takes into account in system optimization. At the same time, however, SiC technology is still in a relatively early stage of its development, which means that we can still hope for a few innovations and a cost reduction for the components that are relatively expensive today. For now, our customers are impressed by the higher energy efficiency, reduced weight, and well-known extreme reliability, as well as reduced noise generation inside the vehicle. And these are the areas where we’re currently seeing the technology’s greatest potential. Depending on the application and reference traction system, we’re achieving a weight and volume reduction on the converter and traction system level of between 10 and 25 percent and are striving to increase energy efficiency by up to 13 percent. We also want to provide passengers with quiet vehicles based on quiet traction motors and optimized cooling systems. It’s in response to these many challenges that we’re continuing to develop SiC technology in traction converters.