On-board chargers are a win-win solution

Vicor has demonstrated a modular approach to "virtual batteries" that can solve the problem of DC fast charging for electric vehicles.

Many existing DC fast chargers use 400V battery packs rather than 800V versions. 2020, there are approximately 400,000 publicly available DC fast chargers worldwide, but only 2% support 800V vehicles. In Europe, for example, only 400 of the 40,000 charging stations support 800V.

This incompatibility can be addressed by using compact, efficient and bi-directional power modules for on-board charging, says Haris Muhedinovic, Vicor's chief automotive senior field applications engineer.

Installing new DC fast charging stations with a wide voltage capability of 250 to 920V is one solution, but it requires a significant investment of time and money. Another approach is to upgrade a 400V charging station to support 800V, which also presents a number of challenges. Charging at speeds in excess of 150kW is not always available, and charging times can be slower than what would be expected for 800V.

Instead, adding on-board charging with modular DC-DC virtual batteries offers flexibility and 99 percent efficiency, Muhedinovic says, which allows for faster adoption and eliminates the need for a bonus investment in charging infrastructure.

Incompatibilities between 800V batteries and 400V chargers can be resolved with battery virtualization. Using this technique, the charger can see the 400V battery on one side of the on-board charger even if the 800V battery is connected to the other side. This approach starts with the battery voltage and adapts it to the voltage range accepted by the charging station.

The NBM bi-directional module creates a virtual battery for DC fast charging.

Vicor NBM bi-directional modules support tens of kilowatts with power densities of 550kW/L and 130kW/kg. These products feature a resonant sinusoidal amplitude converter topology with zero voltage and current switching that reduces power losses by up to 50%. The fixed-ratio conversion simplifies the power architecture using separate modules, while still maintaining up to 99% efficiency throughout the supply chain.

This comes from using higher frequencies with silicon transistors to reduce the size of the magnetic material. The resonant architecture takes into account the inductance and resistance of the components in the module to optimize efficiency.

Connecting a battery to one side of the NBM module will immediately virtualize the other side of the battery, dividing or multiplying the voltage or current by a constant factor to extend the voltage range of the charging station from 250 to 460 V and from to 500 to 920 V. This can increase the number of suitable charging points, making the electric vehicle compatible with any DC charging station.

The module can also be used to maximize the efficiency of the power supply system, as it can integrate traction batteries to provide higher efficiency for low RPM driving. For example, city driving requires lower RPMs and the efficiency of the 800V traction inverter drops by more than 15%.

The module can supply half the battery voltage to the inverter in this auxiliary way, cutting switching losses in half and extending driving range. This is another advantage of how an integrated, modular power supply approach optimizes the power supply network so that peak efficiency can be partially maintained using DC-DC converters.

Modular packaging technology simplifies assembly and manufacturing of EV powertrains and also provides power supply flexibility and scalability. Designers can use the same module configuration to scale charging power from 50 to 150kW without requiring additional qualifications and certifications.

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