A 100 kW Bidirectional DC Fast Charger: Architecture, Design and Performance

Multiport architecture for megawatt charging

The Oak Ridge National Laboratory (ORNL) has proposed a multiport concept that illustrates such an architecture for megawatt charging. It connects multiple charging points, stationary energy storage systems, and photovoltaic installations via a shared electrical infrastructure. The entire charging station has a capacity of 10 to 15 MW; individual multiport units are designed to transmit power outputs of between 1 and 5 MW. A converter for heavy-duty electric vehicles has a rating of 1.2 MW and can supply either a single charging point with the full power or three charging points, each with 400 kW. 

The concept provides a DC/DC stage in a dual-active-bridge topology for each charging point. A DAB consists of two actively controlled full bridges which are coupled to one another via a high-frequency transformer. The primary-side bridge generates a high-frequency AC voltage from the DC link voltage. Following galvanic isolation, the second bridge performs active rectification and provides a DC voltage on the vehicle side.

Since both sides have active power switches, energy can, in principle, be transferred in both directions. This means that a DAB can charge a vehicle battery and also transfer energy from a vehicle or stationary battery back into the shared DC system. Actively controlled semiconductors also enable synchronous rectification. During this process, the transistors perform the function of passive rectifier diodes, thereby reducing conduction losses.

Power transmission in a DAB is controlled via the phase shift in the time domain between the square-wave voltages of both bridges. The switching frequency can remain constant. This simplifies the design of the high-frequency transformer and the filters, as the main switching and interference components occur within defined frequency ranges. This control structure can also accommodate operation under varying loads.

From the three-phase mains supply to the DC intermediate circuit

On the mains side, an active three-phase power factor correction (PFC) stage handles the AC/DC conversion. This stage regulates the currents of the three mains phases, making them as sinusoidal and in phase with the respective voltages as possible. This improves the power factor while reducing the harmonic content of the input current. At the same time, the PFC stage stabilizes the DC bus voltage for the downstream DC/DC converters.

A T-type neutral-point-clamped topology is ideal for this active mains stage. This three-point circuit can connect a phase terminal to three different voltage potentials. Compared with a two-stage circuit, this results in smaller voltage steps. This reduces current ripple and facilitates filtering. Depending on their position within the circuit, individual semiconductors only need to switch a portion of the total intermediate circuit voltage. 

As charging power increases, integrated silicon carbide (SiC) power modules are becoming increasingly important. These modules combine several power switches of a given topology within a single package, enabling short electrical connections between the semiconductors. At the same time, their substrates establish a defined thermal link to the heat sink. This integration reduces parasitic inductances and aids the dissipation of heat generated by the semiconductors.

As charging power increases, cooling also becomes a key aspect of the system design. In addition to the power modules, active cooling of the cables and connectors may be necessary for very high charging currents. To facilitate this, a closed-loop liquid circuit pumps coolant through pipes running parallel to the power cables and on to the connector. The heat absorbed there is transported back to the charging station and transferred to the cooling system.

These requirements result in a two-stage, modular design. An active T-NPC stage connects the system to the three-phase mains supply and generates a regulated DC link voltage. Galvanically isolated DAB converters adapt this voltage for the vehicle battery and enable bidirectional energy flow. SiC power modules and liquid cooling help manage electrical and thermal loads.

Simulation of a separate 50 kW T-NPC input stage with a SiC power module yields an efficiency of 98.96 per cent at an input voltage of 400 V AC, an output voltage of 800 V DC, and a switching frequency of 20 kHz. At a heat sink temperature of 90 °C, the junction temperature reaches a maximum of 125.34 °C, with losses of 523.9 W reported for the module. It should be noted that these values are derived from a simulation and are different from the measurements on the 100 kW demonstrator described below. 

Assembly and commissioning

Testing of the 100 kW power unit begins with an initial prototype test of the individual circuit boards. This involves checking the component values and the quality of the solder joints, among other things. This is followed by commissioning tests to verify the functionality of the circuit boards. 

When commissioning the PFC stage, the necessary components are installed first and the laboratory equipment is prepared. Testing then takes place in both rectifier and inverter modes. During this process, operational behaviour and efficiency are recorded. The DAB stages undergo a similar commissioning process. 

In inverter mode, the operating points could not be measured at battery voltages of 800 V and 850 V. Above 750 V, the centre point of the DC intermediate circuit became unbalanced. Summary efficiency measurements of the complete system were carried out at a transmitted power of 50 kW and a three-phase mains voltage of 480 V, and at most of the investigated operating points, the measured efficiency was above the specified minimum value. Reducing the gate resistances of the SiC modules in the PFC and DAB stages is suggested as a way of improving efficiency further. This adjustment is planned to optimize the system further.