powerpower supplyusb
I'm looking to see if assembling my own USB hub is an option. I need to charge 20 devices via Micro USB at once. Data transfer is not needed.
Is anything stopping me from getting a 5V 20A power supply and wiring up 20 Micro USB cables in parallel? Does any protection circuitry need to be added?
Thanks!
My understanding of the your design is that the entire device is on a single PCB, is within a single enclosure, and is connected to the host by a single USB cable. You've integrated a hub onto the PCB to allow both the devices to communicate with the PC. The following answer will hinge on these assumptions, if it's made of several separate devices connected by disconnectable cables then that changes things.
In this case, I suggest that you simply configure the hub to enumerate as a high-power device, and share the resulting 500 mA among the whole board. Interestingly enough, TI's ganged-port sample schematic shows the devices all connected together, even when using their power management IC:
The incoming 5V power supply line (highlighted in blue, as it's one of two nets that we're interested in on this complicated schematic) is connected to a TPS2041 power management IC (a generous description, it's really just a FET that shuts down when it detects 500mA of current being passed). However, each of the inputs are shorted together, and each of the outputs are shorted together as well, and then distributed to each of the downstream ports (the net shown in red).
Basically, they're doing overcurrent protection for all of the downstream sections in a single IC. They have no way of detecting whether they have three low-power (100mA) units, a single high-power unit, or two low-power units and one 300 mA unit. All these options are acceptable based on this reference design. You wrote:
According to the USB specification, a bus-powered hub can provide only one unit per downstream port while drawing max 5 units...
but, to directly answer your question, this design from Texas Instruments (a USB group member and major implementor) shows that you only have to guarantee that the total current is less than 5 units.
To solve your problem, the rules state (taken from the excellent USB in a nutshell document):
High power bus powered functions will draw all its power from the bus and cannot draw more than one unit load until it has been configured, after which it can then drain 5 unit loads (500 mA Max) provided it asked for this in its descriptor.
If you can guarantee that your driver stage will not begin drawing current until the device has been configured (which might be as simple as a timed delay in the host controller), you can simply wire everything together. Because your entire circuit is on a single PCB and has no user-accessible downstream ports, you can probably also leave out the TPS2041 and simply design the system to not require more than 500 mA of current in any state.
Another benefit of enumerating as a high-power device is improved input voltage specifications. When you have enumerated as a low-power device, the host is only required to produce 4.40 V at the upstream port (which will be lower at your device due to the resistance of the cable). When you have enumerated as a high-power device, the specification guarantees that you'll get 4.75 V, which is more likely to be within the operating range of any 5V components you may be using.
The problem with this approach is, as with every other "connecting two power supplies", that the voltages are not the same. The nominal voltage is 5V - yes, but the actual voltage is different. One might be 5.05V and the other 4.8V. Both are in the spec for USB 5V.
But if you connect those two together, one will start to drive the other because it has a higher voltage.
Why the normal Y-Cables work? You plug them into the same device which has a single 5V source, so the voltage is really the same. Only each port is current limited.
And I see another possible question coming:
But there are USB hubs which use an external power supply, which is also a different voltage than the original port - how does that work?
Right, but those use either completely the external power supply or have some switching logic inside to switch from bus-powered to self-powered mode. For example the TPS2070 from TI.
So I would not connect two seperate power supplies to a single cable as long as I don't know the exact inner workings and can be sure that nothing will go wrong. It might work (if the things are reverse current protected for example) or it might not.
But I'm not an expert on USB power, so maybe I'm completely wrong.
Best Answer
Not really. If you want something simple, just put a 2-3 A resettable polyfuse on each VBUS wire on your board, and use good grade full-sized micro-B cables. The simplest way to provide your devices with up to 1.5 A, short D+ and D- wires together and leave them floating (in each cable). This connection will inform attached devices that the power source is "Dedicated Charging Port", aka DCP (which is an adoption of Chinese Federal Standard). Most devices understand this "charging signature" and will take about 1 A from the ports (up to 1.5). So you might need a 30-Amp power supply.
If your devices are not using "fast charging", you can leave D+ and D- unconnected, and devices will take no more than 500 mA (if they are good-behaving devices).