FSAE Integrated High Voltage Distribution Board and Enclosure

In this project, I designed and integrated a PCB for our FSAE car which safely discharges the high voltage capacitors in our inverter from 360V to <60V, controls the TSAL (large high voltage indicator light), and creates safe high voltage measuring points. This project was a really great experience because I got to learn Altium and get a much understanding of circuit board design.

Fully fabricated and soldered board

The fun part of this project balancing all the pros and cons of moving parts around on the board and changing the board shape to best integrate into the rest of the car, as well as getting a strong introduction to PCB design in Altium. 

Altium screenshot of the board design

There are a lot of constraints that went into this design, the main ones are listed here:

• High voltage spacing rules from the FSAE rulebook and online resources
• The enclosure is close to a wiring harness which needs to be easily accessible and serviceable
• The board needs to be accessible so it can be tested and replaced if something goes wrong
• The enclosure is next to the motor controller which also needs to be easily serviceable 
• We already have carbon fiber chassis molds, this adds a strict limit to the overall length of the enclosure because the chassis geometry cannot change
• High voltage wires have strict bend radii that must be followed
• The shielding on high voltage wires needs to be grounded
• The enclosure must be fully sealed because the entire car gets sprayed down by 100s of gallons of water at the competition to prove that it's safe to run in the rain

The internal walls in the enclosure are required to keep LV and HV wire service loops from touching each other. This is required by the FSAE rules and easy to design in with a 3d printed enclosure

Side view of the enclosure next to our battery packs and inverter. I chose to mount it under the metal inverter mounting plate for easy access to low resistance grounding points for cable shielding, and simple waterproofing for the connectors

The 3d printed box is sealed with face seal O-rings between the enclosure and the mounting plate, and the enclosure and the base piece. Wires come out through cable glands and grommets

I ultimately decided on a two-layer board with components on one side due to the simplicity of the design and the decreased design time. The tradeoff to this is that the board ended up a little larger than originally intended and I had to move some of the mechanical components around to make it fit. 

The board can be disconnected to be tested or replaced quickly

I designed the bottom half of the enclosure to drop down so that the board can be disconnected or accessed. This design is with the expectation that the first revision of the board won't work as intended, and we'll need to modify components and maybe make another iteration of the board. 

Distinct high voltage and low voltage sections of the PCB are required by FSAE rules. High voltage and low voltage communicate using the two optoisolators in the top right

I was fortunate that the schematic for the TSAL (Tractive System Active Light) control circuit was already designed in a previous year so I did not have to build it from scratch. This circuit makes a red light flash at 3Hz when high voltage is present at the inverter and otherwise turns on a green light. 

The rest of the schematic is fairly simple; it just needs to discharge the capacitors in the inverter when the relay is triggered. There are also two large resistors that provide safe high voltage measuring points outside of the enclosure. 

Board schematic. I included test points in this revision of the board to make debugging easy

This project was amazing to work on. I have a much better appreciation for the electronics design process and learned a lot about what goes into the design of a circuit board. Its also been interesting working through iterations of this circuit board and balancing improvements to the board with the mechanical geometry of the enclosure and high voltage wiring. In many ways, it's been really useful because I'm not constantly trying to communicate specific mechanical requirements to the electronics team members, and I'm able to evaluate all the pros and cons of different designs much better. 

Thoughts on Altium:

Learning Altium has been a challenge. The software is much more powerful than KiCad, the other PCB design software that I've used. That being said KiCad felt a lot more intuitive and the process for making a small easy board was a lot more streamlined. 

This design is currently still in progress but I'm doing a design review this week and testing a prototype board by the end of the semester!

Appendix: 

Quick Matlab script I wrote to calculate the RMS current of our laps and justify dropping down to gauge 6 wire (better bend radius and lighter than gauge 4 wire, but significantly more resistance). The smaller wire size has made this design much easier especially inside the enclosure