USB 5V and 5V AC/DC Power Module protection needed?

Question

Im designing a small board that with have an input 220VAC and with an AC/DC module will generate the 5V for my 5V components and then with a regulator I generate 3.3V for my 3V3 components.

There will be also and a usb to uart interface for development purposes.

The way im going to use this board is firstly with the usb power supply I will power all the components. Does the AC/DC module need a protection for reverse current on its output?

And secondly while the board is working via the ac/dc module, I may need to plug the usb to programm the board. Is it safe or I need some kind of protection in that case too?

UPDATE Question:

Thank you for your advice. I think I will implement an ideal diode OR-ing but with a quick simulation it seems that it doesn't block reverse current in my case. Simulation Here I think this circuit is for reverse polarity protection? I need reverse current protection?

Answer 1

Does the AC/DC module need a protection for reverse current on its output?

I don't think so. Considering the electrical and mechanical specs, I'm pretty sure this is a flyback regulator. The output has a series diode already, so there's no need to place a diode to block reverse current.

And secondly while the board is working via the ac/dc module, I may need to plug the usb to programm the board. Is it safe or I need some kind of protection in that case too?

No. As I stated above, the flyback regulator has a series diode at its output so there's no need to place another one. D6 allows the circuit to be powered from USB as well, and if you don't want this functionality then you can remove it.

One thing here: USB specification dictates the output voltage to be within 4.75 .. 5.25V range. And the output accuracy of your module is 2% which means the output voltage can be as low as 4.9V. So, if the USB bus voltage is spec max (5.25V) and the module's output voltage is spec-low (4.9V) then D6 will quite possibly be forward biased, so the circuit will be supplied partially by USB as well. This unlikely causes a problem, but if you want the circuit to be supplied from module only then you should consider this.

Answer 2

There is not much to add to @Rohat's answer in general.

The datasheet of the converter does not explicitly mention reverse current protection at all. So it may or may not be protected against this situation already - we can not know for sure. From the fact that it is not mentioned we can not conclude it is not needed. The reverse powering case might simply have been forgotten by the folks...

I know some almost identical converters from other manufacturers whose datasheet explicitly mention that an additional protection diode is recommended in case of reverse loading (powered by USB only in your case).

Therefore my recommendation would be to add an ideal diode OR-ing to combine both power inputs.

That not only decouples them and let's the stronger take over, it also reduced power loss in the Schottky diode. Of course this may be a bit over-engineered - but if you can implement it, why not :)