General > General Technical Chat
Hackers Can Now Trick USB Chargers To Destroy Your Devices
Zero999:
All this talk of protection to 400VAC is BS. The maxiumum output voltage of USB-C is only 20V. It's trivial to design a protection circuit to withstand that. I'd go for overvoltage protection up to 24V, just in case, but that's still easy and inexpensive to implement.
Berni:
Yeah to be fair 24V does seam like a reasonable bar for protection. But then again something that can handle multiple amps with next to no voltage drop can be pretty bulky, especially when the whole mainboard in a phone is only the size of a few postage stamps.
Also what benefit does the manufacturer get from the extra cost and effort into adding the protection? They expect there users to buy a new phone every 2 years anyway. If a doggy charger that is the fault of the user gets that down to 1 year that's even better.
Cerebus:
--- Quote from: tom66 on July 22, 2020, 10:17:21 am ---
--- Quote from: Syntax Error on July 22, 2020, 10:15:36 am ---Today's student design challenge: design a USB charging protection device and lead that is 100% safe to connect to any random public charger point. Whether that USB charger point is on a plane, a bus or attached to a cycling machine in a train station, your design will handle over voltage, over current and, provide isolation from transients, floating earths, etc. Plus, it provides a nice user friendly status LED. Don't cut and paste a design off of Google, figure it out yourself just like your classmates will have to.
--- End quote ---
There's no such circuit. Say someone designed a USB charging point that was connected directly to 3ph 415V. No protection circuit is going to protect against that unless it is self-sacrificing. And then it would need to have a relay/contactor/MOSFET to isolate it and even that has a maximum breakdown rating.
There always comes a reasonable limit for which protection circuits are designed to handle. 28V protection on a 5V USB device makes sense because it protects against the worst case of a failed cigarette-lighter charger in a 24V vehicle. Beyond that point failures are almost certainly intentional.
--- End quote ---
You've obviously never heard of load dumps.
tom66:
--- Quote from: Cerebus on July 22, 2020, 08:12:03 pm ---You've obviously never heard of load dumps.
--- End quote ---
Of course load dumps are a thing, but you'd be having a remarkably bad day if you happened to have your device connected to one while a load dump occurred, while the charger was short circuited and malfunctioning, and the charger had no load dump provision at all on its input.
The point is you need to stop at some point and engineering a phone to survive 120V from a 28V load dump given it normally charges on 5V is not going to be easy. Maybe it is something an industrial smartphone could do.
Berni:
I think its more the case that a load dump would blow up a cheap cigarette socket USB charger since most are not all that well made. And the failure mode of buck DC/DC converters is often to just let the full input voltage trough. Still a load dump is rather rare is likely to fry something else too.
But yeah its ridiculous to try and design a phone to handle that. I do think it would make sense to survive 24V if you have a USB-C port on the phone. But i can see why a lot of phone manufaturers wouldn't bother with the extra cost and the effort to find room inside the phone for an additional bulky power IC.
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