Motor controllers with silicon carbide based power semiconductors increase the efficiency and power density of new energy commercial vehicle drivetrains.

The electro-mobility market has started to take off, the total electric vehicle stock – this comprises battery electric and plug-in hybrid vehicles – exceeded two million units during 2016. Especially in China this market development is heavily pushed by sophisticated public funding programs, consequently the country took the global lead with more than one million registered vehicles. Even more impressive are the achieved market volumes of new energy commercial vehicles (“NEV”) in Chinese mega cities: the commercial NEV market share has been 34% in 2016 or 17,1000 vehicles. Thereof 133,000 city busses have been put on city roads – this is more than 90% of the global market share. In 2016 the Chinese government established their last funding program which cuts the incentives year by year and will be fully removed in 2021. Consequently the pressure to continuously improve the cost effectiveness will continue and catalyse the ongoing market penetration.

Figure 1: SKAI2-HV system

Besides batteries and electric motors power electronic equipment is the third key technology to electrify vehicles. The ongoing task is to further reduce material -use, -weight and -cost of these building blocks. Efficiency and power density are key indicators for these challenges.

SEMIKRON, one of the leading suppliers of power electronic equipment, provides products for vehicle electrification since more than 25 years. Starting with material handling applications as forklifts in the 90ies SEMIKRON meanwhile equipped more than 800 000 forklifts, 50000 city busses and 60000 passenger cars with power electronic products. Since 2013 SEMIKRON offers the SKAI2HV motor controller platform which is dedicated for electric and hybrid commercial vehicle powertrains between 50kW and 300kW (Fig 1).

Figure 2: A demonstration of the potential loss reductions resulting from the introduction of silicon carbide-based power semiconductors instead of silicon devices

The introduction of SiC power semiconductors offers the opportunity of significant efficiency increase. Fig. 2 illuminates the potential achievements which can be realized by usage of SiC power semiconductors instead of silicon devices. 1) represents the standard 1200V SKAI2HV as the reference with all built-in power semiconductors based on silicon. 2) shows 50% less losses with built-in SiC freewheeling diodes. 3) shows the improvement achieved with SiC MOSFETs. 4) uses SiC MOSFETS which are assembled in an optimized low inductance assembly which will be available with the next SKAI HV generation; here the achievement will be in the range of 85% reduction. 5) uses SiC MOSFETS and – in addition – SiC freewheeling shottky diodes.

The significant reduction of losses allows shrinkage of the motor controller and the required cooling efforts. The overall powertrain efficiency can be increased in the range of several percent which allows a higher vehicle mileage with the same battery size. Power semiconductors based on SiC will never step below the cost level of silicon based devices. However, due to the potential improvements the complete powertrain with SiC devices might show a better cost effectiveness in the future.

Figure 3: The next-generation SKAI HV with DPD technology replaces the wire bond connection with a sintered joint, increasing power cycling capability by more than 300%.

Furthermore SiC based power semiconductors allow a significant increase of the junction temperature compared to silicon devices. Today this capability is limited by the power semiconductor assembly technology. SKAI2HV comprises single side sintered silicon based power semiconductors with Aluminium-Copper bonds on the top sides, which represents today the top end of power semiconductor assembly technology for high volume production. SKAI HV next generation will comprise SEMIKRON´s recently presented “direct pressed die technology - DPD” which increases the power cycling capability by more than 300% (Fig 3) towards SKAI2HV. With DPD the wire bond connection will be replaced by a sintered joint.
Obviously the potential of SiC based power semiconductors and DPD will contribute to further reduce of material -use, -weight, - and cost in electric vehicle drivetrains. SEMIKRON will contribute to these developments in order to support the migration towards electro-mobility.