Automotive
How GaN HEMT devices can be used to maximise the power density of OBC chargers
By Matthias Kasper, lead principal engineer, and Jon Azurza, senior staff engineer, Infineon Technologies A
s sales of electric vehicles (EV) grow, the performance requirements for onboard chargers (OBCs) are becoming increasingly
demanding with original equipment manufacturers looking to minimize the size and weight of these components to help improve vehicle range. Currently, the level of power density which can be achieved in OBCs constructed using silicon devices is typically 2 kW/L. It has been suggested that wide bandgap devices will make it possible to achieve power densities even beyond 6kW/L, which would represent a 3x improvement. Infineon used this figure as the target level of power density in the design of a three-phase 11 kw OBC using GaN HEMTs. In this article, we consider the design challenge, the topology selection process, and the features of the final design.
Charger specifications The main challenges when attempting to realize a high-power density OBC are the wide input and output voltage range specifications. Grid voltages vary around the world, and this adds complexity to
Figure 1: Topology combining Vienna rectifier with DABs
the design of a PFC rectifier stage. The output voltage range is dependent on the EV battery voltages, and this represents yet a further challenge for the DC/DC stages. Balancing these requirements means choosing between hard-switching losses or increasing RMS currents, neither of which is desirable. In addition, as more renewable forms of electricity begin to supply the grid, the peak power trimming provided by connected EVs is considered as a means which the grid can
be stabilized. This further complicates the design task because the OBC now also requires bidirectional power processing. A final design specification is that OBCs must be able to operate from single- and three- phase grid supplies. Assuming that the available current in each phase is limited by a fuse, a 230 Vrms (line-to-neutral) phase with a 16 A-rated fuse can provide 3.6 kW of single- phase power and 11 kW of three-phase power. While this is possible for a traditional three-phase system (e.g.,
a Vienna Rectifier), for the same OBC to operate in the USA, a 240 Vrms split single-phase grid interface with a 40A fuse would be required for 9.6 kW of power.
Selecting a topology
An existing 10 kW EV-charger with a power density of 10 kW/L was used as a starting point in the quest to maximize the power density of the OBC design. The specifications for this charger are shown in Table 1.
Figure 2: PCB of GaN-based EV charger solution
18 February 2023 Components in Electronics
The final charger design (Figure 1) used a Vienna rectifier PFC with a regulated split DC-link, to which four cascaded Dual Active Bridges DC/DC converters (DAB) with 600 V rated GaN HEMTs (IGOT60R042D1) were connected, in order to provide output voltage regulation. By their very nature, DABs are bidirectional topologies which can
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