Tesla DCDC Converter - Design Insights

byJeff Cooke 4 May 2017

"Houston, we have a problem". I've been messing around with the DCDC converter from the Tesla 2015 70D. In this configuration the DCDC is a simple device. It steps down the 400v from the battery pack to provide the 12v supply for the low voltage devices in the car. In earlier Teslas, the DCDC converter and the front HV junction box were combined. One unit would do DCDC and also feed the PTC cabin heater, coolant heater, and AC compressor. In the 2015 70D the DCDC is standalone and the HVJB connects to the DCDC, PTC, Heater, and AC compressor.

This should be a dumb device, much like the PTC should be dumb, and the AC should be dumb. You tell them to switch on, you tell them how hard to 'run', and they dutifully switch on and run.

What I see in the Tesla units (too numerous to list) is either design brilliance or complete lunacy. They have added smarts to devices that are normally pretty dumb. Lets take the PTC heater as an example... it is a special type of resistor - put current through it and it gives off heat, as it heats the resistance increases. Why, why , why would such a thing have to listen for CAN traffic from the rest of the car?

Tesla DCDC Topdown

Tesla is a technology company, and I suspect they have designed these devices much like they would develop modules or classes in software. The class embeds data, logic, and communication to 'hide' that complexity from the rest of the system. You can change out one class for another, because the interfaces are well defined. Some of this approach has found its way into the Tesla devices, they listen and speak to the other devices in the car. Sounds great if you're a software engineer, but there's a bust... what happens when the units begin to break?

For the most part, software classes don't break. Assuming you can continue to compile the code you can keep re-using classes for decades - SOFTWARE DOESN"T WEAR OUT, but units inside cars do. In about 5-10 years from now Tesla Model S cars are going to be junk. They are going to be junk because unless you get the specific unit from month X in Year Y, with revision Z of the loaded software, and maintenance kit W, you're AC, or your heater, or your drive unit, or a very long list of things won't work. There's a very short list of people willing to drive a car without a functioning AC system. Some of the faults won't allow the car to drive at all. Tesla's are going to be no more usable that a Rolls Royce with a problem. No matter how simple the problem, the replacement cost of the part is more than the remaining value of the vehicle. There will be Teslas sitting in driveways that probably still look good, but will be no better than a boat anchor as regards their capacity for transport.

I don't think Tesla engineers did this to be a pain to EV converters and hackers. I like the adage "Don't assume malice when incompetence will suffice.". Elon let the software engineers go nuts with the systems on the Model S and they've done what software engineers do. I'll give credit to the Detroit crowd here. They've learned from experience how to mass produce, service, and engineer cars for the long-term.