Switching between 2 different power suplies - Page 1
Forum section: electronics / general

author: Razvan1203
author_rank: Newbie
timestamp: April 05, 2024, 09:14:22 am
content: Hello! I’m working at a circuit, which can use either 5V from USB or 9V from external battery. I implemented a jumper selector through which the user can choose where the voltage comes from.The problem is that I would have 2 different 5V signals: one from USB and one from a voltage regulator from the 9V signal. How can I design a circuit that automatically switches between those 2 whenever one is active and the other one is not.Remember that the 2 power supplies can never be activated at the same time.Thank you!

author: Ice-Tea
author_rank: Super Contributor
timestamp: April 05, 2024, 09:45:01 am
content: With two diodes ;-)

author: Doctorandus_P
author_rank: Super Contributor
timestamp: April 05, 2024, 06:17:23 pm
content: Two diodes would be the simplest option, but you will have the voltage drop over the diode's. You can use FET's as (nearly ideal) switches, but this needs some system to determine which fet to turn on. But it's a common problem, and there are dedicated IC's for this. From simple "high side switches" to more complex power manangement IC's.

author: Benta
author_rank: Super Contributor
timestamp: April 05, 2024, 09:07:09 pm
content: Why is this in the KiCAD forum?

author: mariush
author_rank: Super Contributor
timestamp: April 06, 2024, 09:27:57 am
content: If efficiency is not a big concern, two schottky diodes (one on each input) will do the job... the highest voltage will win.There are ideal diodes you could use to keep it as efficient as possible, but a lot of them only work up to 5.5v-6v... so if you want to use them you'd have to lower the voltage to 5v or less and then feed them to one of these ideal diodes.For example, LM66200 -https://www.ti.com/lit/ds/symlink/lm66200.pdf- is a dual ideal diode package, and it automatically puts on the output the higher voltage out of the two.It would be great to auto switch between USB input and internal battery for example, both voltages lower than the maximum 5.5v it can handleThe IC has an enable pin that's active low, so if it gets some voltage on the OE pin it would turn off the output so I suppose that could work for you in an indirect way (for example 9v in - buck regulator to 5v or 3.3v - connect output to OE pin - when you plug 9v in then the ideal diode disconnects and the two inputs on the ideal diode are disconnectedLink :https://www.digikey.com/en/products/detail/texas-instruments/LM66200DRLR/15856663There's also a single diode version LM66100 that has an EN pin ... you can use two of them and interconnect them to get a dual ideal diode packageLink :https://www.digikey.com/short/dhpbzfz9MAX40200 is another example of an ideal diode, which goes only up to 1A - 1.2A :https://www.digikey.com/short/890zf1jb(it's also available on LCSC cheaper)There's also power switch ICs which can select between two or more inputs.One good example, TPS22933 :https://www.digikey.com/en/products/detail/texas-instruments/TPS22933ARSET/3725886https://www.ti.com/lit/ds/symlink/tps22933.pdfHas three inputs (up to 12v) and auto selects highest of the three , and also has a built in LDO that can output 3.6v up to around 75mA that you could use a sort of stand-by power if you need to.Other examples ... TPS21**TPS2111APWR - 4.5A , up to 5.5vhttps://www.digikey.com/en/products/detail/texas-instruments/TPS2111APWR/652781TPS2115APWR and TPS2115ADRBR (different footprints, same ic)https://www.digikey.com/en/products/detail/texas-instruments/TPS2115APWR/652785- 1.25A, 5.5vhttps://www.digikey.com/en/products/detail/texas-instruments/TPS2115ADRBR/1843509- 2A , 5.5vTPS2121RUXR - 4.5A , 22vhttps://www.digikey.com/en/products/detail/texas-instruments/TPS2121RUXR/9859001https://www.digikey.com/en/products/detail/texas-instruments/TPS2121RUXR/9859001