Power electronics in electric cars

Reading time: approx. 8 minutes – Power electronics in electric cars is a key technology for the successful transformation of an entire industry. Innovative solutions for a wide variety of drive concepts are needed to help electromobility achieve a breakthrough. What is the key role played by power electronics in electric cars?

The development and manufacture of batteries for electromobility is certainly not one of Germany’s core competences, as is often quoted in the media. Here, Asian manufacturers are a real force to be reckoned with. They are forcing European car manufacturers into a dangerous dependency. The situation is quite different for another cornerstone of the regenerative drive concept: power electronics. German companies are among the world leaders in the development of power electronics for fuel cell and battery-powered electric vehicles. System-relevant components of power electronics in electric vehicles are presented below.

The inverter in the electric vehicle

Various technologies are available for driving an electric vehicle. The technically simplest variant is a mechanically commutated DC electric motor. This is supplied directly with DC and its power output can be controlled with the help of PWM (pulse width modulation) of the supply voltage. Due to this fact, a direct supply from the traction battery is also not practical for DC motors in the vehicle. Further disadvantages of such a DC machine are the maintenance required for the wearing brushes. Additionally, there is the comparatively low efficiency and the shorter service life.

In modern electric vehicles, brushless electric motors are therefore used. These include asynchronous machines or permanently excited synchronous machines, also known as brushless DC machines. However, the commutation of the supply voltage in the phases of the motor must be carried out electronically with these motors. Thus, they are generating the rotating field required for operation.

This task is performed by the so-called inverter. Three-phase inverters are the norm in drive concepts available today. They can provide the motor with three-phase current that is variable in frequency and voltage.

In this way, voltage-frequency characteristics precisely matched to the drive can be realised. Alternatively, a motor torque independent of the input voltage and rotating field frequency can be set.

The reaction of a combustion engine to accelerator pedal commands can thus be modelled electrically using appropriately stored characteristic curves. These properties are a key factor in the design of the drive system and in optimising the user experience.

When braking, the electric motor acts as a generator and supplies a corresponding alternating voltage. This can be rectified by the inverter and fed to the traction battery. This process is also known as recuperation and significantly increases the efficiency and therefore the range of an electric vehicle.

The DC/DC converter

Both fuel cells and the batteries in electric cars supply voltages beyond the 12 VDC or 24 VDC previously known in the automotive sector. In order to ensure the safety of infotainment system users, a low-voltage on-board power supply is still required in regenerative vehicles. This avoids expensive and complex new developments. Similar to the alternator coupled to the petrol engine in combustion vehicles, the DC/DC converter supplies the low-voltage system. For this purpose, the high-voltage converter converts the high voltage of the traction battery into a correspondingly lower voltage. This supplies consumers such as air conditioning, power steering, lighting, etc.

The onboard charger in the electric car

 Another important power electronics component in the electric car is the onboard charger. Charging stations are usually available in three different versions. Electric vehicles can be supplied with single-phase or three-phase alternating current as well as direct current. Due to the relatively expensive electronics, direct current charging stations are usually only found on motorways. Inner-city charging stations usually supply alternating current. However, direct current is required to charge traction batteries. This is generated by rectifying and converting the alternating current with the help of an onboard charger.

Developments in power electronics in electric cars

In recent years, semiconductor technology has undergone rapid further development. The considerable optimisation of power semiconductors has opened up completely new application scenarios. However, the stationary power electronics used primarily in industry to date rarely fulfil the high requirements of the automotive industry. There is a need to catch up here, particularly in terms of assembly and connection technology. This is necessary in order to be able to utilise the technological potential of power semiconductors in terms of temperature resistance, efficiency and power density.

Conclusion

Various components are required for a switch from fossil fuels to renewable energy sources in motor vehicles. And even if Germany and Europe are currently lagging far behind expectations in battery production and development, there is justified hope in the field of power electronics in electric cars. Competition from Asia in terms of battery production is fierce. A battery alone does not move a car. The aim must be to further advance the field of power electronics. Germany should focus on developing innovative solutions for various drive concepts.