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.)

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?


What's the standard way to solve this problem?

  • 1
    \$\begingroup\$ What's wrong with usual diodes? \$\endgroup\$
    – user76844
    Dec 14 '17 at 9:45
  • 3
    \$\begingroup\$ Can I buy an ideal diode? What a weird question, how should we know if you can buy something. Do you know if I can buy bananas today? What you mean (and should ask) is: Should I use this component or... \$\endgroup\$ Dec 14 '17 at 9:48
  • \$\begingroup\$ Gregory, mostly that it wastes power and gets very hot. It also reduces my 5V supply to 4.5V which isn't ideal. Bimpelrekkie, you're absolutely correct. The title was mean to be a little tongue-in-cheek as I always thought that ideal diodes didn't exist! (But the datasheet seems to suggest that they do.) \$\endgroup\$ Dec 14 '17 at 9:58
  • \$\begingroup\$ YOu mean one of these linear.com/product/LTC4358 \$\endgroup\$
    – Trevor_G
    Dec 14 '17 at 12:26
  • 1
    \$\begingroup\$ At the very least, use Schottky diodes! \$\endgroup\$ Dec 14 '17 at 12:28

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 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?

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.


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