Auch ohne Schnellladestation ist der Auto-Akku im Eiltempo wieder voll: Dank On-Board-Charger. Foto:
Fraunhofer IZM has succeeded in combining some of the latest achievements in the field of power electronics for the next generation of on-board chargers. The result: twice the charging power with half the volume, plus bidirectional and machine-manufactured. A low-cost solution and a signpost for the shortcut to the future.
If you take your electric car to a fast charging station, you can fully charge your battery in 15 to 30 minutes. One reason for this is that fast-charging stations offer high power, some up to 350 kW. Secondly, they supply the energy in the form of direct current, as required by the car battery. This means that the battery can be charged directly without the need for a charger in the car. The situation is different with AC-based charging options, which are much more widespread. These include standard household sockets with single-phase alternating current with up to 3 kW power, which can be found in almost every garage. On the other hand, e-cars can be charged at charging points in public areas or at an in-house wallbox using 3-phase three-phase current with up to 22 kW. This means that many models can be fully charged in four hours. However, the majority of the current e-fleet is only designed to accept a maximum of 11 kW – due to its charger, the built-in on-board charger (OBC). In addition, the existing OBCs consist of several discrete components, including large coils, some of which have to be manufactured and assembled by hand and ultimately take up a lot of space. For many car models, an upgrade from 11 to 22 kW is available – by installing a second or larger OBC module, which doubles the already large space requirement and drives up the price. In addition, most OBCs only work in one direction, namely for charging the car battery. They cannot feed the electricity from there back into the grid or use the large vehicle battery as home storage for your own solar system. This means that the storage potential of car batteries cannot be used for the envisaged energy transition.
Sine-amplitude converter – over 1 MHz clock rate thanks to gallium nitride semiconductors
In order to circumvent these limitations, Fraunhofer IZM developed several components and combined them in a small space. One of these components is a sine-amplitude converter (SAC) – a resonant high-frequency transformer that initially ensures galvanic isolation of the vehicle battery from the supply network. This isolation is necessary because capacitors in the vehicle electrical system cause low-frequency earth currents, which in turn would trigger an RCD in the circuit and make operation impossible. However, the real progress of the SAC is made possible by the gallium nitride semiconductors (GaN) used – novel and powerful semiconductors with a wide bandgap, better known as wide-bandgap semiconductors. They make it possible to switch the transformer on and off at a clock frequency of 1.3 MHz, i.e. 1.3 million times per second. Oleg Zeiter from Fraunhofer IZM, who played a leading role in the development of the OBC, explains: “These high clock frequencies allow us to design the components in a completely different way.” This affects one other component in particular: the PFC choke.
PFC choke – flat coils from the machine
Another central component in an OBC is the so-called power factor correction converter (PFC). It forms the interface to the supply network and stabilizes the AC voltage on the input side in sinusoidal form at – depending on the network – 50 or 60 Hz. This requires chokes – a very bulky component in previous OBCs, which also causes high production costs. Fraunhofer IZM has now developed a flat PFC choke based on a printed circuit board, with four magnetically coupled windings on a common ferrite core. This has the great advantage of cost-effective machine production and saves a lot of space. Although the planar design with PCB only allows lower inductances, these are no obstacle for the PFC, which is built with SiC switches and clocked at 140 kHz. “Because we can clock so quickly, we are able to handle the low inductance,” says Oleg Zeiter. “If we only switch the current on for a very short time, it doesn’t even reach the high currents, even with low inductance. The short switching sequences make it possible.”
Thanks to these clever construction and connection techniques, Fraunhofer IZM was finally able to develop an OBC that reduces the volume of such devices to 3 liters, halving the volume compared to conventional chargers, but doubling the charging power from 11 to 22 kW. “In principle, we now only need one large circuit board. Thanks to our packaging solutions, everything else only needs to be applied to this circuit board by the machine,” says Oleg Zeiter. In this way, manufacturing costs can be significantly reduced.
However, the list of advantages of the new OBC does not end there: The module is compatible with 400 and 800 volt batteries and has an efficiency of over 97 percent. Last but not least, the new OBC allows electricity to flow in both directions, i.e. from the battery to the grid. This homework for the energy transition has therefore already been completed by research and development. European funding from the ECSEL JU (Electronic Components and Systems for European Leadership Joint Undertaking) initiative in the Horizon 2020 framework program for research and innovation has provided a tailwind.
