Electronics > Projects, Designs, and Technical Stuff

USB C PD

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artag:
It's not so much the components I need to implement it that's a problem (though those can be interesting, thanks). It's the philosophy or user expectation that I'm concerned about.

The big advantage I see of USB C PD is that I can use a generic supply yet provide more than USB 2 offers. It's possible, with careful reading of the specs, for an informed user to replace a custom power supply or an over-specced USB 2 power supply.

Yet it appears that although USB C PD offers through negotiation all the capability I need, it doesn't  guarantee it. It's still possible for a naive user to replace the power supply with another USB C supply and it will fail to meet the required specs (eg it might offer only 5 & 15V, not 20V). And all I can do is silently fail.

The appliance is an enhanced table lamp (a PD sink). It doesn't have a screen or other UI where it could report an inability to charge. The best it might manage - and this would require some otherwise unnecessary circuitry - is to give some sort of gas-gauge error indication such as a low red light.

janoc:

--- Quote from: artag on July 24, 2020, 12:11:40 pm ---The appliance is an enhanced table lamp (a PD sink).

--- End quote ---

If it is a table lamp, then why to bother with USB chargers in the first place?  :-//

I know, everyone has one but is the complexity (PD + DC-DC converter), risk of users using incompatible chargers (not every USB C charger is PD or 20V capable), risk of someone connecting this to their laptop ("plug fits so it should be safe and work, right?") worth it?

A small off-the-shelf power brick at the right voltage will solve all these problems.

USB is really meant to support and power computer peripherals. While it can be used and abused for other things, it doesn't mean it always makes sense to do so.

artag:

Table lamp is a bit disparaging, but gives an accurate idea of the function. It does contain USB parts that may need reprogramming, a battery that needs charging, etc. It would have a buck converter even if run from a brick. All that is irrelevant though, especially in the case of USB PD which is very much aimed toward charging.

It could use a power brick, yes.

I dislike them. There are a thousand varieties with different capabilities and connectors. Using the wrong one can cause damage. Most people don't know how to choose the right one. 

If I can buy a generic item that can safely and cheaply be replaced by something off the shelf in any country, why would I use a brick ?

ejeffrey:

--- Quote from: artag on July 24, 2020, 03:29:06 pm ---
Table lamp is a bit disparaging, but gives an accurate idea of the function. It does contain USB parts that may need reprogramming, a battery that needs charging, etc. It would have a buck converter even if run from a brick. All that is irrelevant though, especially in the case of USB PD which is very much aimed toward charging.

It could use a power brick, yes.

I dislike them. There are a thousand varieties with different capabilities and connectors. Using the wrong one can cause damage. Most people don't know how to choose the right one. 

If I can buy a generic item that can safely and cheaply be replaced by something off the shelf in any country, why would I use a brick ?

--- End quote ---

That's fine, but you really should have provision to warn when an incompatible USB source is attached.  You aren't really avoiding the compatibility issue just replacing it with a different one.  You should be avoiding the possibility of damage -- if the other parts in your system are compliant.  If you enable charging at 3A/5V there is still the possibility that someone will use a non-compliant "high speed" USB A->C cable that illegally reports 3A charging capacity which can damage cheap USB A chargers and potentially your device if the charger does something stupid on overload.  Using a 16 W power delivery profile to require 9V or higher voltage will prevent that, at the cost of not being able to run from the USB-C ports on a lot of computers which can do 5V/3A but not higher voltages.

You will probably want to use an off the shelf controller such as the STUSB4500 mentioned above.  USB type C w/ power delivery is rather complicated with lots of possible configurations.  A pre-programmed controller is going to have its implementation fairly well vetted.  At least so one hopes.

fordem:

--- Quote from: artag on July 24, 2020, 12:11:40 pm ---Yet it appears that although USB C PD offers through negotiation all the capability I need, it doesn't  guarantee it. It's still possible for a naive user to replace the power supply with another USB C supply and it will fail to meet the required specs (eg it might offer only 5 & 15V, not 20V). And all I can do is silently fail.

--- End quote ---

I accept that this was not a scientific approach, but at the time I was just being curious.

I have several USB C powered devices, and the USB C chargers that were supplied with them - one of these is a Samsung wireless charging pad and a second is a Lenovo ThinkPad - the ThinkPad uses a 20V 65W charger, which will power the Samsung charge pad, however the Samsung charger (rated at 5V, 2.0A; 9V, 1.67A or 12V, 2.1A) will not power the ThinkPad - more to the point, the ThinkPad does not report any errors, it just does not recognize that a charger has been connected.

So as you say, it appears the spec may offer the capability you need, but does not guarantee it.

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