I have a project that I’m working on that has me a little stumped, and need advise.
Its a small USB 5v powered device/widget, the 5v supply is limited to 500mA as per USB spec. But there will be situations that I may require more than 500mA. So I’m looking for the best way to isolate the USB 5v from a second 5v source, wall wort for example, while allowing both power sources to share the load of the device, The additional 5v is for supplemental power to ease the burden of the primary USB power.
I had thought of adding a diode, but the trouble with that is the voltage drop across the diode is such that the supplemental voltage is lower than the USB voltage and therefore the device does not draw any current from it (Both power sources equalling 5v starting points.
The other idea is to use a P-chan fet, Source is connected to the external power source, the Drain connected to the device, gate connected to gnd. Now this works as I had hoped, when the external supply is connected I see a drop in current from the USB connection, so the current from the two power sources are both powering the device.
But I was not sure if the fet is going to give me the isolation that I was looking for, am I heading down the wrong path here?, or is the fet option workable?
elevator4:
The p-fet should work. What sort of isolation are you looking for?
There are also ICs for the purpose, eg TI TPS2111
-E
With the FET, the 2 power supplies are effectively connected together - ie, there is no isolation. To do this "properly" (limiting how much you draw from each supply), the easiest solution is to use an IC designed for that purpose.
The TPS2111 looks like a good choice, and fairly cheap solution. It has internally 2 FETs which is what you need so that only one supply or the other is connected.
If you can stand the voltage drop, diodes are a good way to handle this as well, which is what we do on our boards, as it switches from USB or external power or power from an FTDI cable. But in our case we are using the nominal 5V to power a 3.3V low drop out regulator.
I’m not looking to switch between two power rails, for example, when I plug into a USB port and I require, lets say 600mA, I want to use the 500mA from the USB port the board is plugged into and supply the additional 100mA from a second source. Ideally the second source would be isolated such that it would not be ‘back’ powered by the USB port.
But the problem is that I have very limited board space and it needs to be cheap, A diode was my initial thought, but the voltage drop from it and the fact that the second supply may not be the same exact voltage ruled it out as an option.
What happens when the second supply is not there and you exceed 500 mA (In many cases a PC or hub will disable the 5V)?
Without separating the loads, it is hard to take 100 mA from supply A and 500 from supply B. Is the high load momentary (like starting up a motor)? In which case much bigger capacitor than normal can take up the slack. I would worry about when a USB decides to shut you down for over current it might not come back for quite some time, and your secondary supply will have to pick up it all.
I have picked up supplies meant for USB battery charging that can supply 2.1A, and they are very small, less than 1 cubic inch.
To answer your question, I think we need some more information about what the loads are.
The power source will be run from a USB port on a PC or TV, so from the port I am limited to 500mA, which should be fine for 90% of all cases, but there is a change that the user could plug in an additional USB device into my dongle to power a web cam (for example) so under that case the 500mA may not be enough and there would need to be some suplemental power from another source, for the moment I went with a fet, it doesn’t give much (if any isolation)…
Why not power your device from the upstream USB port and wire the downstream port’s power (where the web cam would plug in), to the external supply? Anything plugged into the downstream port requires the external power supply to be plugged in.