Those who go on a longer trip with their pedelec usually think about more range. Possible options are a second battery or charging at a charging station. Taking your own charger with you is probably not on everyone’s mind. It is often simply too heavy and too big. Perhaps this will change in the foreseeable future.
The desire for lighter, more compact devices that provide sufficient power is at least in line with the work of researchers at the University of Stuttgart. There, scientists from the Institute of Robust Power Semiconductor Systems (ILH) and the Institute for Power Electronics and Electrical Drives (ILEA) have now presented a corresponding prototype. The charger for ebikes and e-scooters is intended to set new standards in terms of performance and compactness.
More than a simple black item
From the outside, chargers look rather unspectacular. From a scientific point of view, however, they hold more than just one challenge in the endeavour to fundamentally develop them further. Those involved in the project in Stuttgart were determined to keep the dimensions small and the weight low. Anything else, in their view, would not be a real advantage over previous chargers. Such an objective has consequences. Additional cooling elements such as fans are no option right from the start.
With the contact pins as a reference, one can guess how small the charger of the researchers from Stuttgart turns out to be.
Basically, the only remaining starting point is the electrotechnical circuit. This should be as small as possible and at the same time work as efficiently as possible. For this, it is always an advantage if a device serves a relatively limited purpose. Unfortunately, however, chargers are much more complex for example than an ordinary power supply unit. In the so-called state of charge, they can generate voltages and currents of many different strengths. This is necessary to charge the battery not only quickly but also gently.
In addition, it must ensure safe handling. The housing must not heat up too much during operation, so that we can touch it safely at any time.
Silicon no longer the sole means of choice
In a first step, the researchers tackled the problems mentioned with elaborate 3-D simulations. In numerous designs, they created the interior of the charger in different ways and moved components back and forth. At some point, a design crystallised in which the components were optimally distributed. Optimal in this case meant that there was as little temperature rise in the charger as possible, which was distributed as evenly as possible, ensuring the best possible performance.
This is what the optimal structure of the charger looks like.
The real key in the development of the prototype, however, lay in the use of a material for the semiconductor components that is still relatively rarely used. At the University of Stuttgart, gallium nitride (GaN) was used. It replaced the much more common silicon, so to speak. The result is a charger whose volume could be reduced by half, according to the researchers. Solutions already on the market serve as a comparison. Including the housing, it measures 75.5 millimetres in length, 60.5 millimetres in width and 33 millimetres in height. At the same time, it is just as powerful. In the case of the prototype, this means 150 watts, which corresponds to a power density of about 1.6 kilowatts per litre. The output current of four amps also seems perfectly adequate. With its weight of just 300 grams, the prototype is almost twice as light as Bosch’s current smallest charger, the Compact Charger.
Promising perspective
Gallium nitride is one of the compound semiconductors consisting of two or more elements. It has a higher efficiency than silicon, enables higher switching frequencies and reaches lower temperatures under full load. Consequently, power supplies designed with it can be smaller. In addition, the compound is considered robust and non-toxic. Experts consider applications ranging from communications technology, the automotive sector and space to medicine to be realistic.
However, there is still a need for research in other areas. For example, scientists around the world are looking into the question of how GaN single crystals of consistently high quality can be grown. Up to now, crystal defects on an atomic scale and inclusions have impaired the use of the material. In Stuttgart, the researchers have apparently been able to work with high-quality material. Let’s see if and when the development makes it to series production.
Pictures: University of Stuttgart, ILH
