An electric car is composed of an energy source (batteries, usually lithium batteries), an energy conversion device with a cooling circuit (inverter by the CEA), a motor (by Brusa), a cooled transmission (by Punch) coupled to the drive shaft and connected to the axle of the car. The powertrain (inverter + motor + transmission + cooling system) must be as compact as possible. The batteries are not part of the drivetrain here. The objective is to have the most efficient, reliable, smallest possible drive train. In fact, the miniaturization of systems is one of the most worrying issues of the 21st century.
Today, electronics mostly uses silicon (Si) based components. Silicon is a plentiful resource on earth and therefore is not expensive. However, Si based components are still facing almost 20% of energy losses. To achieve better performance in both efficiency and size, new components are emerging on the market:
- The Silicon Carbide
- The Gallium nitride
|Silicon Carbide||Gallium Nitride|
|Energy performance||Medium energy performance (less losses than Si based components, reaching up to 94% energy efficiency)||Better energy performance (less losses than state-of-art components, reaching up to 98% energy efficiency)|
|Size and power density||Medium chip surface with the same current/voltage rating, reaching a higher power density than Si based component (about 25W/inch3)||It has a smaller chip surface with the same current/voltage rating, reaching a higher power density than state of art component (ModulED’s power density : >25W/inch3, below the GaN theoretical performance limit)|
|Thermal properties||Has interesting thermal capacities||Has more degraded thermal properties; however, has GaN is generating less losses, it compensates itself|
CEA Leti laboratory is oriented on GaN technology. Within the framework of ModulED, we proposed to design an inverter based on GaN component, a less mature technology than SiC (already integrated in Tesla cars thanks to its ST microelectronics partnership), to highlight its performances. Moreover, by allowing frequency rise, GaN component can increase the power density and thus reduce the size of the large passive components (inductance and capacitor) that contribute to the volume of the power electronics system.
A major impact for ModulED is the reduction of powertrain size. In fact, the electronics must be integrated into the powertrain, which must be as compact as possible. This becomes possible with the use of the integrated GaN based components, smaller than Si based components. The electronics have been designed to ease integration and disassembly as modularity is a key point of the project. This allows, in case of failure, to change only the defective parts, instead of changing an entire organ.
A second impact for ModulED is energy consumption optimization:
Figure 1: ModulED's strategy for energy consumption optimization
This also allows for a better reliability of the electronic system (components are less stressed, therefore less risk of breakage).
ModulED GaN inverter
Thus, the latest CEA’s developments (see Figures 2 and 3) allowed the consortium to propose a first version of the GaN inverter.
Figure 2: Half bridge + serial switch PCB prototype
Figure 3: Experiment full bridge – Single Phase system
By validating its performances at lab scale, the inverter has reached TRL4 and is planned to be tested in real environment by the end of the ModulED project. Today, the current product reaches performances beyond state of art thanks to the following breakthrough results:
- High adaptability: inverter’s topology allows to put motor phases in series or parallel;
- Reconfigurable machine, increasing fault tolerance management;
- Higher efficiency thanks to the latest GaN semiconductor, generating lower losses. Testing proves that GaN inverter with 6 phases full bridges has reached very high efficiencies. In fact, in standard 3 phase half-bridge topology, the inverter’s efficiency with silicon-based components is limited to 97%. With a heavier 6-phase topology, the consortium proved that a near 99% efficiency can be reached.
Figure 4: Efficiency vs. torque/speed of the electric machine
The ModulED’s topology is needing 4 times more components than standards architectures. It generates a reduced power capacity compared to traditional power supply topology. However, thanks to GaN properties, the ModulED inverter has reached the same efficiency than conventional inverters. This is an interesting first step and a world’s first for such a GaN technology embedded into a vehicle.