CCS Failure Question / Discussion

[HaulingAss]

I am no Tesla design engineer so this is speculative; Yes there are two parallel 24A units. I speculate/expect these to work in parallel each take a portion of the charge so _not_ one takes it up to 24A, then the other kicks in.

Obviously, Tesla would design the way the AC charging works with peak efficiency as a top priority. Unless there is something unusual about the hardware that makes it more efficient to engage all MOSFETs, even at very low charge amperages, I would expect the highest efficiency would result from leaving the unneeded MOSFETs idle, until the requested charge current required them to be activated.

The failure mode supports this (when one 24 amp bank of MOSFETs fails, the charge current can still be 24 amps).

The efficiency reason is the strongest one, because obviously Tesla could build in fall-back mode for failure of one bank of MOSFETs that would then fully load the remaining good bank.

MOSFETs are not as efficient when lightly loaded. If loading all MOSFETs lightly for low amperage charging reduced charging efficiency by even 10% (it's likely a much bigger hit than that), it would be utterly foolish to design the system so they were all lightly loaded whenever charging at lower amperages.

A third possibility is the MOSFETs from bank one and two are "interleaved" when charging at low amperage. In other words, each bank is contributing to charging below 24 amps, but the individual MOSFETs in each bank are not activated until the active MOSFETs are fully (or nearly fully) loaded. But this sounds like a more complex and expensive circuit design with no clear benefit.

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[yohst]

Obviously, Tesla would design the way the AC charging works with peak efficiency as a top priority. Unless there is something unusual about the hardware that makes it more efficient to engage all MOSFETs, even at very low charge amperages, I would expect the highest efficiency would result from leaving the unneeded MOSFETs idle, until the requested charge current required them to be activated.

I have worked/designed with relative high power MOSFETs (10Amp, not 24) a few years back. They are not the low efficient at low load devices anymore. Plenty of designs which work fine at low loads/good efficiency. Switching frequency is way higher, much lower Rds on resistance and lower gate charges.

It is more likely Tesla has the MOSFETs in parallel and adjusts switching frequency for most loads unless perhaps dropping to a 120V/10A charge.

But we're just speculating and any Tesla designer would giggle at our writings here.

 

[mongo]

The DCAC is two separate identical modules. Each has a single mosfets for each switch node on the DC side. The AC side has pairs for each. The design appears to be a CLL / LLC resonant type with split capacitance so it may have a zone of best operation with soft switching (ZVS, ZCS).

 

[PhilEsq]

I have ~22k miles and no PCS issue (yet) although my PCS is on the "bad" list. My home charger is 60->48A 240VAC and I can charge at work at 240VAC 30A.
I rarely supercharge as the other 2 are free. I *can* set these down to 24A charge rates if needed.

I'm aware that the recommendation to avoid/delay CCS failure is a lower charge current (24A)... Do we have any paperwork to back that up? Sounds like all we know is that there are a bunch of potential FET issues cold solder joints in affected lots/parts. I can trim back charging rate but quite honestly I'd rather fail fast and wait as opposed to drag my feet and hit 50K miles and then be past warranty. I'm interested in hearing thoughts on this as I don't want indefinitely be feel like I need to charge at 24A ...... ...... ......

Check your alerts. I didn't know I had a problem until I looked in alerts to see if there was anything when FSD suddenly turned off unexpectedly. I didn't see anything about FSD but I saw a pile of alerts about my windshield washer fluid being low. Under all of those there were several alerts about the truck not being able to charge. I have an appointment next week at Tesla.

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