Question: I'm trying to switch over from USB power to external power if it's present. (External power is 6V to 13V). The current is 2A.

To do this I'm using diodes 1 and 3. See image below. ("OR" configuration.)

![](https://istherecake.com/IMG_20171214_092901.jpg)

I'm then using diode 2 as reverse voltage protection in case the external power is connected backwards. (To avoid destroying the computer by raising the 5V USB power line to 13V.)

Is there a better way to do this? The amount of voltage and heat wasted by the 3 diodes is huge (at 2A).

Can I perhaps use 3 ideal diodes? Something like this?

<http://datasheet.octopart.com/FR014H5JZ-ON-Semiconductor-datasheet-85537694.pdf>

What's the standard way to solve this problem?

Answer: 
That's right, your reverse protection chip uses an internal MOSFET to achieve lossless switching.

I seem to recall seeing a "drop-in replacement for a diode" kind of FET-based device, but I can't seem to find it now.

MOSFET's with dedicated secondary-side controllers are used for synchronous rectification in higher-end modern power supplies (and in low-volt VRM's on modern PC motherboards).

Speaking of a "diode OR" in a redundant power supply scenario, I've found another [relevant link](http://www.linear.com/product/LT4351) at LT's website...
For D2 and D3, you should consider Schottky diodes. These have a significantly lower voltage drop than standard diodes, maybe 400 mV instead of 1 V at typical load current, so you'll lose a lot less power.

For D1, if the drop of a Schottky diode is still too high, the part you've linked to looks like a valid option - I have no experience with this type of IC though. You may also find these answers helpful:

[Switch between 5V power supplies?](https://electronics.stackexchange.com/questions/401/switch-between-5v-power-supplies)

[Switch between battery and USB power](https://electronics.stackexchange.com/questions/21570/switch-between-battery-and-usb-power)

Where does your 'USB' supply come from though? You're not guaranteed to be able to draw more than **100 mA** from an actual USB port unless your device negotiates with the host for more.