Can Cascodes Provide the Stepping Stone into SiC Needed for EV Success?

  • Jun 21, 2018
  • UnitedSiC

By Jonathan Dodge, P.E., Senior Applications Engineer at UnitedSiC

If electric vehicles are to deliver their full potential for decarbonizing transportation, the automotive industry needs a cost-effective and low-risk way to adopt silicon carbide technology

The International Energy Agency (IEA) says electric vehicles (EVs) will make up 25% of vehicles on the road by 2025 (figure 1). Looking slightly further ahead, the Clean Energy Ministerial (CEM) forum wants to see sales of new EVs account for 30% of the global car market by 3030, with its EV30@30 campaign.

Figure 1. IEA prediction for EV deployments. (source: International Energy Agency)

Let’s be clear, however; neither the CEM nor the IEA is an EV enthusiast lobby. Both are interested in decarbonizing the transportation sector. And EVs may not be the only solution. Competition could come from an unexpected source, following Mazda’s recent announcement of new gasoline engine technologies, including Homogeneous Charge Compression Ignition (HCCI), that could cut CO2 emissions to levels comparable with EVs when calculated on a well-to-wheel basis.

In the longer term, we know that oil reserves are finite, so gasoline’s time will end, but right now the CEM acknowledges that car makers have struggled to make EVs broadly acceptable to the buying public. It’s supporting the 30@30 agenda with other campaigns, like Advanced Power Plant Flexibility (APPF), which should increase the availability of electricity generated from renewable sources.

Better EVs Needed

In addition to such high-level initiatives, there is a case for better EVs, including more efficient vehicles that can travel further per charge, or run smaller, lighter, lower-cost batteries that can go far enough for most users and take less time to recharge. It is critical to convert that stored energy as efficiently as possible, and one technology that can help is road ready. Silicon carbide (SiC) power FETs are known to have lower switching losses and reduced on-state losses, compared to ordinary silicon counterparts. They can also survive higher operating temperatures, enabling better reliability and simpler thermal management.

Any EV or hybrid contains multiple power modules that could benefit from the increased efficiency made possible by SiC technology. In addition to the main traction inverter, these include the on-board battery charger (OBC), inductive charging converter and auxiliary converters supplying systems such as lighting, climate control, power steering, fluid pumps and infotainment.

There are, however, a couple of barriers standing in the way of widespread SiC technology, and they are significant for the traditionally cautious automotive industry: the risk associated with new and untried electrical architectures; and the cost premium for the SiC devices themselves.

It is true that the cost of SiC devices is reducing, as fabrication processes become more mature and as wafer sizes are increased – both resulting in higher production yield. Costs can be reduced from a system perspective too, not only thanks to simplified thermal management but also by taking advantage of higher switching frequencies to reduce the size and cost of associated capacitive and inductive components.

To fully exploit SiC’s superior efficiency and robustness, however, the automotive industry needs to overcome the risk inherent in significantly changing proven silicon-based converter designs.

Familiar Barriers to Overcome

The latest SiC cascodes provide the push the industry needs. They can be used almost directly in existing silicon-converter topologies that are proven to be effective and reliable, so eliminating the risks associated with substantial redesign. Whereas today’s IGBT-based inverters or converters are typically switched at frequencies in the 8-12kHz range, a relatively modest increase in switching frequency – to a maximum of, say, 50kHz to maximize efficiency, especially at light loads – allows passive components to be up to 80% smaller, bringing valuable savings in BOM cost, size, and weight.

Smaller components bring other advantages, too. The inductor, for example, can be a planar type device with stamped metal terminations, compatible with automated assembly processes – unlike the larger bobbin-wound inductors typically used at lower frequencies, which have flying leads and are usually reliant on manual assembly.

“Package” of Incentives

The roadmap for SiC devices is exciting, as vendors like UnitedSiC prepare to introduce new power modules containing multiple SiC chips that will enable converter makers to replace large numbers of discrete devices in leaded packages such as TO-247. These modules will bring reductions in BOM costs while also helping to save electrical connection costs, mechanical assembly costs, and simplify cooling.

All together, these enticements could be enough for the automotive industry – including Tier 1 module makers as well as car brands themselves – to put SiC devices at the center of their strategies to create the better-performing EVs the world needs to continue decarbonizing transportation and ensure sustainable mobility for future generations.