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I'm making a breakout board for an ESP32-S2-MINI-2 module. This module has USB OTG capabilities and a UART interface, which is usually exposed with a USB port via a USB-UART converter.

I would like to expose both of these interfaces via a USB port each, but this poses the question on how to merge two 5V rails and protect both in regard to ESD and reverse polarity.

My idea was to merge the two 5V rails right after the USB ports, then use a reverse polarity device (P-FET or diode), and an ESD IC on each port.

However, if I look at the schematic for the ESP32-S2-devkitc, they have applied a reverse polarity diode and an ESD diode on each of the 5V rails before they get merged, which to me seems like a waste of components. The same goes for ESD on the same line.

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Shouldn't one ESD diode and one reverse polarity diode be enough for both ports? Even if both ports are being simultaneously, one component per net should be enough.

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No you need two diodes because you can't connect 5V of USB connectors together.

For simple reason, if you do connect 5V from both connectors together, and one port is supplied by $5 mains adapter, and you plug it into a $1000 computer which gets fried. In any case, back-feeding power out from an input connector is simply wrong and forbidden by any logic.

And that's why each port has separate ESD protection. You want to eliminate ESD before it destroys the respective diode used to combine the 5V together or gets passed through the diode to next chips.

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  • \$\begingroup\$ So the point I was missing was that one port could be at 5V exactly and another at 4.95V, and thus creating a flow of current from the 5V to the 4.95V one? \$\endgroup\$
    – StefanoN
    Commented Nov 23, 2022 at 22:53
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    \$\begingroup\$ No, the other port could be a turned off PC and the other turned on PC and one PC tries to power the other by sending current wrong way through USB port. For the same reason, you don't have mains cords that plug into two outlets or cables with two cigarette lighter plugs to connect two cars together. \$\endgroup\$
    – Justme
    Commented Nov 23, 2022 at 23:01

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