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Power Electronics for Perfect E-Drives

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Tags: ZeroEmissions, Efficiency, Motorsport, Emobility
In electric vehicles, the power electronics are decisive for the efficiency of the drive. ZF gained early experience in racing with the new silicon carbide semiconductor material, which now also provides tangible benefits in series production.
Stefan Schrahe, March 31, 2021
Stefan Schrahe has been writing about everything four-wheeled for three decades now. In his leisure time, he enjoys traveling by bike - though he also prefers motorized ones.
Lovers of exclusive engines are familiar with this. In the early days of electronic engine management systems, the designers did everything right on paper and designed their four, six or eight-cylinder engines according to the latest findings. But a puzzling torque dip, poor response during the transition from part-load to full-load operation or exorbitant fuel consumption often spoiled the driving pleasure. The "mapping" of mixture preparation, injection and ignition timing often separated the wheat from the chaff. After all, it's the fine-tuning that turns an ambitious design into the perfect engine.

It's hardly any different with electric motors. The power electronics determine how it feels, how efficiently it works and how spontaneously it carries out the driver's commands. Their task is to send as many electrons as possible from the drive batteries into the windings of the electric motor without generating too much wastage from the internal resistance of the lines or internal switching losses. Ideally, the almost massless elementary particles travel at the speed of light along this path. This also explains the completely different, very direct response behavior of electric engines compared to combustion engines with liquid or gaseous fuels.
The crucial components here are the power semiconductors. Similar to how fuel injectors regulate the amount of fuel, they stand in the way of electrons, at least in the idle state. The semiconductor only clears the way when voltage is applied or an electric field is generated.

Much to regulate: Electron flow and voltage

Much to regulate: Electron flow and voltage

The power electronics must precisely meter the inflow of electrons in order to adapt the output of the electric motor to the driving situation. But the control center of the powertrain has another important task, because the drive battery, usually a lithium-ion battery, can only absorb and deliver direct current. Modern e-drives in cars, however, operate with alternating current. The power electronics convert battery direct current into drive alternating current and vice versa. This is because when the vehicle brakes and the electric motor operates as a generator, the alternating current generated in the process must first be rectified before it is stored in the battery.
"Formula E is a test laboratory for us, from which experience and findings are continuously incorporated into series development."
— Dr. Otmar Scharrer, Senior Vice President R&D E-Mobility, ZF Friedrichshafen AG

Power electronics for 400-volt architectures have long been part of the ZF product portfolio in production vehicles. The technology group also supplies this central element for drives with increased voltage. "We are currently working on the start of series production for various 800-volt projects," says Dr. Otmar Scharrer, who is responsible for the Development of Electric Drives in the Electrified Powertrain Technology division at ZF. "For several models of a Chinese manufacturer, we supply the complete electric driveline, including power electronics. And for a European sports car manufacturer, ZF is contributing the power electronics for a high-voltage application." Further series start-ups are already on the horizon.

Higher efficiency thanks to silicon carbide

Higher efficiency thanks to silicon carbide
All power semiconductors used in electric cars to date are based on pure silicon. This has many advantages. For example, the same processes can be used to manufacture the silicon crystals, and for their further processing, as for computer chips. However, as the voltage increases – keyword 800 volts – the semiconductors become larger and larger, while at the same time their efficiency decreases during driving.
Therefore, ZF is using a new technology for the first time: Instead of silicon transistors, components made of silicon carbide are used in power electronics. In this material, each silicon atom is bonded to four carbon atoms and vice versa. Carbon atoms are not only smaller than their silicon counterparts, but also bind the free electrons more tightly to themselves. This allows the same voltage to be processed with chips that are ten times thinner. The semiconductors of an 800-volt drive are only about 100 micrometers thick and offer significantly lower internal resistance.
By using silicon carbide, the same voltage can be processed on chips that are ten times thinner and offer significantly lower internal resistance.

Less switching resistance - more range and efficiency

Less switching resistance - more range and efficiency
Internal switching losses can be reduced even further with silicon carbide. This is particularly important because the power electronics have an enormously high energy throughput during electric driving and recuperation. Low losses have a direct effect on the efficiency of the entire electric powertrain and thus on the range. With an unchanged battery size, an electric vehicle with silicon carbide power electronics can drive about five-to-seven percent further. Thanks to modern power electronics, such as those developed by ZF at the Bavarian site in Auerbach, the drives of battery-electric vehicles already achieve an efficiency of more than 95 percent in the WLTP cycle.

Similar to the 800-volt technology, the innovative chips come from racing: ZF used silicon carbide power electronics in Formula E for the first time. Dr. Otmar Scharrer adds, "For us, Formula E is a test laboratory from which experience and findings are continuously incorporated into series development. Because there, too, responsiveness and efficiency are enormously important criteria that determine the competitiveness of a powertrain."

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