[krapht]

Yes, and that's why the USB spec puts the burden of protecting against electrical shorts on the controller. The device side shouldn't have to worry about trying to shunt 100 Watts of power somewhere.

I mean this kind of protection is way beyond typical ESD protection due to the power involved. It would add a lot of cost to do this, maybe even a dollar or two in raw material.

[mikeash]

If you stop the current from flowing, you don't have to shunt any power anywhere.

You'd have to open the circuit in the face of 5V of potential hooked up backwards, not really a tall order.

[krapht]

I don't want to be a broken record, but you're right, not a tall order, but no it's unreasonable on account of cost. It's impossible for USB C to be hooked up backwards by its mechanical design, so why guard against it? The high-side transistor to prevent reverse polarity faults could be 3 to 4 cents in manufacturing cost! It's also not trivial to make sure that transistor trips fast enough to prevent all your low-voltage ICs from being destroyed.

[dh997]

Most likely cheap consumer devices wouldn't get it, but prosumer, enterprise and industrial gear should think twice about cutting corners on peripherial protection circuits and make sure to test vcc ground reversal and application to data pins... because commercial engineering boils down to risk management.

[kevin_thibedeau]

> t's impossible for USB C to be hooked up backwards by its mechanical design, so why guard against it?

Because cabling can fail in other ways. A short could be caused by mechanical damage to the cable anywhere along its length. It would be nice to have a host that doesn't burn up in those scenarios seeing as they are already required to protect the bus.

[krapht]

Like other comments have mentioned, a short is not a voltage reversal. The methods to alleviate them are different. As the connector is polarized, the only way to get a voltage reversal is for something seriously strange to happen either at the power supply end or the cable.

If it is at the power supply end, all bets are off. If your USB charger fails short prepare for mains voltage to absolutely fry anything and everything connected to it and a negative voltage is the least of your worries. In the cable, a miswiring is seriously negligent and I see no reason why it should be designed around.

[mikeash]

Sure, but the magnitude of that cost matters a lot. You were saying a dollar or two in raw materials to protect against 100W, now you're saying three or four cents. A few cents to protect hundreds of dollars in equipment from electrical faults seems worthwhile to me.

[teraflop]

That makes sense from the consumer's point of view. But imagine you're a manufacturer, producing USB device controller chips by the million. Would you really be willing to spend tens of thousands of dollars to protect against a failure mode that can only happen due to some other company's extreme negligence?

[mikeash]

Yes, but of course it doesn't matter what I think.

It's not abnormal to add protection that isn't needed when everything works properly. As noted elsewhere, the USB spec already requires resettable overcurrent protection, for example.

[knughit]

Tens of thousands of dollars total is far less than it costs a company just to moot the question.

[krapht]

Sorry I was not clear in my comments. The dollar solution is for some type of crowbar circuit. This is the most robust solution.

The 3-4 cents is for specifically, reverse polarity protection when somehow, a -5V differential is applied to device power inputs.

This cheaper solution only narrowly addresses this particular power fault, and no others.

[mikeash]

I see! A dollar per port for robust protection against a wide variety of faults sounds pretty good to me, but I know that sort of stuff doesn't sell well when people compare primarily on price.

Do I assume correctly that a fuse for overcurrent protection wont act quickly enough to save the hardware in the event of reverse polarity?