After some discussion and a thorough reading of the various threads here, I did an internet deep dive on SiC usage in EVs, and where AEHR’s test products fit. Some of this comes from this IET (Institution of Engineering and Technology) reseach paper, which is pretty dense:
Some from a LinkedIn post and comment thread:
Some from this article:
Some from three analyses of Tesla’s claim to reduce SiC usage by 75% in future vehicles:
A GaN vs SiC comparison article here:
SiC Failure Rate analysis from Wolfspeed and others:
And even a little bit from:
OK, references out of the way, here’s my investor-oriented summary:
- All EVs have inverters, which convert the DC output of a battery pack to an AC output voltage that drives the AC electric motor. The inverter controls the magnitude and frequency of that AC power, which can easily exceed 200kW (200,000 watts).
- Tesla pioneered the usage of SiC (Silicon Carbide) in its inverters for the Model 3 in 2017. Before then, almost all EVs/hybrids used IGBTs (Insulated Gate Bipolar Transistors) to control their high power traction motors.
- The advantages for SiC include higher efficiency (5%-15% overall), smaller package size, less produced heat, and longer life (about 80% longer). In particular, higher efficiency means the vehicle achieves a greater range with the same battery, or can reduce costs by having a smaller battery with the same range. Smaller packaging helps with aerodynamics and interior/cargo space.
- SiC can also be used in vehicle On-Board Chargers. The advantages here are similar: higher efficiency (less of the input power is wasted), smaller packaging size (28%) and weight (24%) in the vehicle, and longer life. However, this application doesn’t have the price-offsetting factor of potentially reducing battery size. That said, in 2019 Tesla changed their On-Board Chargers to use SiC, and that resulted in higher charge rates at Level 3 chargers.
- SiC can also potentially be used for DC to DC converters. An EV’s main system runs at either 400 or 800 volts today. But, just about everything except the traction motor and AC compressor runs at 12 volts. There are some advantages here, but since this isn’t a big power draw compared to making the vehicle move, efficiency gains are probably not worth the cost.
- A note on GaN. While inverters can use GaN, the technology isn’t yet there for use in EVs. First is what appears to be an inherent limitation of about 650volts. This is fine for most EVs that use 400 volt systems, but there are already EVs from Porsche, Lucid, Hyundai and McLaren that use 800 volt systems. It seems more likely that GaN usage in EVs will start with the On-Board Charger, but again for automotive cost and availability are very real concerns. I will note that today you can buy GaN “wall warts” for charging your USB devices today. They are smaller/lighter and can put out enough to charge a laptop and other devices simultaneously. I own a few.
- Earlier this year, Tesla’s VP for Powertrain Engineering, Colin Campbell, made a claim that Tesla will reduce its use of SiC per vehicle by 75% in the future. This sent AEHR and other SiC related stocks down, but they recovered pretty quickly.
- What I’ve gathered is that Tesla wanted to make sure that their untested SiC-based inverter didn’t fail, and so they used more SiC chips (48 total, with pairs connected in parallel to reduce current through each), and running them well below their maximum temperature range. It’s possible that in later designs (Model 3 as well as the later Model Y not to mention “refresh” versions of Models S and X), Tesla was able to reduce SiC usage based on millions of miles of real-world testing. Separately, teardown expert Munro reported that Tesla has reduced the number of temperature sensors on some of its boards, presumably based on its analysis of temperature data. It’s possible that Tesla has temperature sensors on its inverter module and since Tesla vehicles upload data back to Tesla, they could have done an analysis of what temperatures the inverters run at, even when pushed, and made internal design changes.
- Tesla used what is know as “Gen 2” SiC chips in its first Model 3 vehicles. We are now at the 3rd generation of SiC MOSFETs, which have higher current/temperature ratings. This alone could count for 50% of the Tesla target of 75% reduction is SiC count.
- Two other scenarios for SiC count reduction have been discussed. The first is that Tesla’s upcoming mass-market vehicle will have lesser performance characteristics, and thus won’t need to handle as much power, thus reducing the need for parallelizing chips to reduce current/temps. Another is speculation that since Tesla already uses multiple motors/inverters in its products today (both “dual motor” and the top-end Plaid’s “Tri-motor” vehicles), that Tesla would actually revert back to regular Silicon for the motors that kick in on high power demands. The idea is that there’s a main motor that handles everyday driving, and that uses SiC for efficiency. But when the driver puts their foot down, the secondary/tertiary motors that then get engaged don’t need to be that efficient since they’re hardly used. Presumably, a 6% reduction in 50% of the power wouldn’t be missed, and overall range is still preserved by the main motor still using SiC. I don’t know what to say about either of these.
- Although it’s been 6 years since Tesla first used SiC, you can still buy vehicles today that aren’t using this technology. That’s going to change as these vehicles simply aren’t as competitive in the market.
- One of the main advantages of AEHR’s testing is that it’s done at the wafer level. This is important, because once the chip is packaged into its device, swapping them out is expensive, if even possible at all. And even if you had a low failure rate, with dozens and dozens of these chips per inverter, the overall rate of failure is multiplied many times over. I don’t know the failure rates, but consider that if the failure rate for SiC chips were 1/10,000, then with 48 chips about 5 of every 1,000 inverters you build will fail. That’s not acceptable, and I believe the failure rate is worse than 1/10,000.
Adding one more bullet point here:
- A third speculation for Tesla reducing its SiC usage is from adopting a new type of SiC architecture called a “Trench.” Apparently, the existing designs are all “Planar” architecture. The Trench architecture has a higher power density, so fewer chips would be required. One site thought this “unlikely” as it breaks from the current norm, but then Tesla broke norms by using SiC in the first place.
Here’s a tweet on this architecture change ramification:
So, it may be more likely than some SiC insiders think. What this means for AEHR is beyond my knowledge, unfortunately.