© University of Stuttgart / ILH

Ultra-compact battery charger for electric bikes and scooters enables smallest sizes and short charging times

Electromobility is now shaping the lives of many people, and e-bikes and e-scooters are an essential part of this. The batteries used there are becoming smaller and lighter, the efficiency of the electric powertrain is increasing and the range is increasing. Chargers, however, have not yet been able to keep up with this development. Researchers at the Institute for Robust Power Semiconductor Systems (ILH) and the Institute for Power Electronics and Electrical Drives (ILEA) at the University of Stuttgart have now developed a charger for e-bikes and scooters that sets new standards in terms of performance and compactness.

A stronger battery, but at the same time shorter charging times and chargers that are light and take up no space in the backpack – that’s about the dream of every e-biker. In order to make it come true, researchers from the ILH and the ILEA have developed the prototype of a plug-integrated charger in a joint project. For this purpose, currently available chargers for e-bikes and e-scooters were first analyzed, measured, examined and compared with chargers available on the market for laptops, for example.

The power electronic topologies (arrangements) of the circuits used in the charger were then compared to find and select the most suitable topology for this application. The challenge here is that chargers for rechargeable batteries have a wide operating range in contrast to a simple power supply unit. Therefore, various voltages and currents must be set via the so-called state of charge in order to charge the battery as quickly as possible and at the same time as gently as possible. This places great demands on an electrotechnical circuit, which should be as small as possible and at the same time efficient.

Avoid performance degradation due to hotspots

During operation, the charger must not exceed a certain maximum temperature at any point. In addition, it must be completely closed – so it is cooled only by passive convection. Under these conditions, the efficiency of the circuit that converts the single-phase mains voltage from the socket into a DC voltage must be calculated with the aid of complex three-dimensional simulations. The arrangement of the components is optimized to ensure an ideally distributed surface temperature while maintaining electrical integrity and avoiding electromagnetic interference.

GaN as a game changer in power electronics

Until now, the semiconductor components of the voltage converters in commercially available chargers were made of silicon. In research, on the other hand, the material gallium nitride (GaN) aroused great interest several years ago. Recently, the first mobile phone chargers have become available that rely on chips made of GaN and promise high performance (up to 120 watts) in a small package. The components offer clear advantages in almost all relevant parameters – but must first be mastered in application at high power levels.

The Stuttgart researchers took up this challenge and built six specially designed power stages based on GaN semiconductors into the prototype. In order to operate them reliably on the highly integrated boards, in-depth knowledge of the technology and detailed electrical and mechanical simulations are required. The final prototype of the two institutes achieved the targeted goals: In terms of volume, it is only half the size of the previously available chargers, and this with the same power (over 150 W), which corresponds to a power density of approx. 1.6 kW/liter.