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Power electronics in 800-volt electric cars

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It’s a topic that can come up at international engineering conferences or in meetings among friends: Discussions on electric cars seldom focus on the driving aspect but rather on charging the batteries, and in particular the amount of time this takes
Dr. Marco Denk, December 07, 2018
Dr. Marco Denk is responsible for advanced engineering of power electronics at ZF. The mechatronics technician previously worked on power semiconductors for his doctorate project.
When it comes to refueling, expectations seem to depend on habit. Gasoline or diesel-powered vehicles can be refueled for an additional range of 600 kilometers within minutes. Even if electric cars will probably never attain this charging time, an ultra-fast charging infrastructure should significantly reduce waiting times in the future. For instance, the ultra-fast charging network set up by German automobile manufacturers seeks to provide a charging capacity of up to 350 kW. Theoretically, this would allow for a large 90-kWh battery to almost be completely charged in just 15 minutes. In practice, however, the charging times are slightly longer. Nonetheless, refueling will become increasingly faster.

Such high charging capacities call for a new on-board supply system. In order to ensure power losses are kept to a minimum during charging with direct current, the charging voltage should match the battery voltage and thus also the voltage of the entire propulsion system. In this sense, the 800-volt on-board supply system will increasingly become the future standard for high-performance electric vehicles. At the same time, all components of the propulsion system will have to run on voltage that is twice as high as current solutions. The new voltage level is especially relevant for power electronics, and in particular for the semiconductors installed in these systems. These semiconductors control the electrical current between the battery and the motor. Consequently, the new ones must be designed to manage a voltage that is twice as high.

Decisive factors are the conduction and switching losses, the permissible operating temperatures, the switching times and the reverse voltage. The latter is defined as the voltage that completely stops the electrical current in the semiconductor at a given amperage. As long as the semiconductor material is not modified, the reverse voltage is proportional to the thickness of the semiconductor. It is, however, not a good idea to simply use thicker semiconductors for higher voltages. This is because the internal resistance increases the greater the thickness is, which leads to correspondingly higher power losses. These losses are disproportionately high if only low amounts of drive power are required, such as for driving in urban areas.

Due to this situation, ZF has decided to use a new semiconductor material in its next generation of power electronics. Instead of the pure silicon that has been used thus far, the semiconductors used in the next decade will be based on silicon carbide (SiC). In the grid structure of the substance used in the different phases, each silicon atom is linked by covalent bonds with four carbon atoms and vice versa. As a semiconductor material, silicon carbide has a relatively large band gap. The valence electrons have a stronger bond to the nucleus, far more than is the case with pure silicon. As a result, a silicon carbide semiconductor can manage ten times the reverse voltage level than a silicon semiconductor of the same thickness. A thickness of about 0.1 millimeters is enough to create a power bridge in an 800-volt propulsion unit, while the internal resistance remains considerably lower. Initial calculations show that, depending on the operating cycle, expected losses in power electronics can be reduced by between five and ten percent. This means an electric vehicle with the same battery can travel a greater distance.

Currently, ZF is testing the first SiC semiconductors that meet the special mechanical and thermal requirements in the vehicles. Our experts in Auerbach and Bayreuth have observed that there may be premature failures in semiconductors if they only follow the manufacturer's instructions. To develop highly efficient power electronics, a system supplier like ZF must develop a deeper understanding of semiconductor physics and possess the ability to conduct detailed tests on its own. And we are well on our way to achieving this.