Diode ORing Circuit - Page 1
Forum section: electronics / beginners

author: Thane of Cawdor
author_rank: Regular Contributor
timestamp: January 16, 2022, 05:35:38 pm
content: Hi everyone,I have an application where I have two voltage rails which I want to automatically switch to a supply rail for downstream regulation. The first supply is 12V and is capable of delivering up to 5A. The second supply is a USB 5V rail. The downstream regulator can take both voltage levels.Can this method of simple diode ORing work as automatic voltage selection?I'm looking to minimise component count and potential damage to either supply through reverse current flow. It's also difficult to find a low-component count power MUX controller IC which can operate up to the 12V 5A level. The main downsides I can think of for this method is the potential for reverse leakage (given Schottky diodes) and high forward voltage drop at 5A. Any other suggestions?Thanks

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 16, 2022, 05:39:11 pm
content: It is ok if 5V USB is fully capably to deliver enough current for your load.A load will see 12 VDC if primary power supply is ok. And it will see 5VDC from USB when there isn't primary power supply.

author: SiliconWizard
author_rank: Super Contributor
timestamp: January 16, 2022, 06:03:04 pm
content: The reverse leakage will be in the order of a couple µA if you select appropriate Schottky diodes. Up to you to figure out if this is too much. At 5A, Vfd will be in the order of 0.5V-0.6V for Schottky diodes, and 1V+ for typical silicon diodes. Unless maybe you used batteries, the reverse leakage current should not be a big problem.So the real problem here will be the power dissipation in the diodes. At 5A, that will be somewhere between 2.5W and 5W+ depending on the diodes you'll use. That's a lot of power.An alternative, more efficient solution would be to use PMOS transistors instead.You can have a look at this:https://www.ti.com/lit/pdf/sboa383(there are integrated controllers doing just that, if you want to minimize parts.)

author: ledtester
author_rank: Super Contributor
timestamp: January 16, 2022, 06:52:01 pm
content: A term you can search for is "power or-ing" or "power mux".Here's a good overview/introduction to the topic:https://www.eetimes.com/fundamentals-of-power-system-oring/and a stackexchange discussion:https://electronics.stackexchange.com/questions/96398/or-ing-power-supplies-diode-or-mosfetand there are devices you can get to do this, e.g.:https://www.mouser.com/c/?type=Power%20MUX

author: ghost1325
author_rank: Contributor
timestamp: March 17, 2022, 04:46:52 am
content: FYI[ Specified attachment is not available ]

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 17, 2022, 06:33:29 am
content: Everything in that picture is wrong! Maybe FYD (For Your Disinformation)

author: Zero999
author_rank: Super Contributor
timestamp: March 17, 2022, 08:37:50 pm
content: Use a diode for the 12V power supply.If the 12V power turns on/off quickly, the 5V can be switched with a MOSFET with little loss. The is a region when the MOSFET will be off and the body diode conducts, but that's a non-issue if the 12V supply is switched quickly.