Technical Discussion | Wunderbox Design by Lucid Design

Today let’s talk about Lucid Air’s central onboard charging unit, Wunderbox.

Since the voltage of Lucid Air is very high, 924V. Here are several functions:

● Low-power DC power distribution part: including contactors and fuses, mainly connecting PTC, electric compressor and DC-DC

● 800V to 400V: This is mainly used to power ptc and electric compressors

● Bidirectional high-power charging: 19.2 kW. The interesting thing here is that the boost part and the subsequent 400V DC input are reused

● DC charging boost circuit: 50kW For old US DC charging piles, the voltage can be increased

▲Figure 1. Wunderbox designed by Lucid has many interesting features

▲Figure 2. Lucid Air’s Wunderbox design

Part 1: An overview of Wunderbox

This multifunctional system carries many functions, including 3 pairs of high-power connectors (DC charging socket, battery system and inverter) and 4 pairs of low-power connectors (AC socket input, DCDC output, PTC output and electric compressor output).

▲Figure 3. Lucid's Wunderbox high-voltage connector is a lot

In the explanation video, we can see that the most impressive thing is that it actually contains four parts:

●High-power circuit: Considering two parts, AC input and 400V DC input, the DCDC boost circuit can be reused in two parts.

● Control panel: Several charging modes need to be considered here, including AC, DC high-power fast charging and DC low-power fast charging.

● High-voltage power distribution: Equipped with high-voltage contact area, input fast charging input, AC input, inverter output and battery input & output

● Low power distribution circuit: This also includes a small circuit. Since both the PTC and the electric compressor are 400V

▲Figure 4. The whole consists of four parts

On the new CCS fast charging pile, it is relatively simple to bypass it directly. If a 400V pile is available, consider using the DCDC boost part of the AC charger. Here, a 50kW boost circuit is considered from the beginning. Currently, the world needs to make the old DC pile compatible at 800V. Each company has its own way to consider compatibility with AC piles. Lucid is the only one.

Figure 5. Several methods of high-power charging

For the AC part, SiC main control circuit is considered to improve efficiency on the one hand and V2X on the other. In this regard, the size of Lucid's battery is large enough, so a lot of SiC Mosfet is configured here. In the previously disclosed information, Lucid and ROHM cooperated, and the improved performance of SCT3040K and SCT3080K at high frequency and high temperature SiC MOSFET helped Lucid reduce the design size and reduce power loss. The active rectification and PFC power factor correction circuits can operate at high switching frequencies, thanks to the application of SiC MOSFET. The subsequent 50kW boost circuit also needs to use SiC circuits to achieve this function.

▲Figure 6. Main control part of the AC part

In the design, due to the above considerations, the entire design needs to be configured with relatively more fuses, and multiple circuits in it need to consider electrical insulation protection.

▲Figure 7. It contains a lot of fuses

Part 2: The future of OBC

We have made many attempts in China, mainly discussing the use of small DC charging, gradually removing the onboard charger from the car. For 800V models, we need to consider compatibility with previous charging piles and the ability to charge on low-power fast charging piles. There are currently two common solutions:

● Dedicated boost circuit: Start with Porsche and make a simple boost device

● Reusing motor windings: as part of the boost circuit

Lucid has considered high-power SiC circuits here. Since the AC is nearly 20kW, the higher short-term power is considered in the boost part, and the 50kW boost circuit design is directly undertaken. This actually provides a new path for OBC. Based on SiC, higher-power products can actually be made.

▲Figure 8. Lucid’s design considerations

summary:

I think Lucid is also a way for high-end cars. After all, the 800V system does need to be considered, whether it must be fully integrated. It is indeed a good idea to consider AC and 400V DC input together, and both bidirectional outputs are considered.