MOSFET Power and ground Switching - Page 1
Forum section: electronics / projects

author: TommyGunn
author_rank: Contributor
timestamp: March 01, 2013, 05:33:26 am
content: I'm trying see if i can get rid of some jumpers in one of my designs.I have 3 different USB power sources on a single device Lets call them A B and C. If I have just A or B or C plugged in i would like to be able to power my device from the one that is plugged in.The thing is that it gets a bit trickier when I have 2 plugged in. A and B, or B and C, or A and C. here i run the risk of getting ground loops. so I currently have jumpers that select only one power source. Is there a way to do this ground and power switching with mosfets?

author: dr_p
author_rank: Frequent Contributor
timestamp: March 01, 2013, 09:44:29 am
content: I'm not sure, but how about diode-OR the 3 power inputs? Each positive and ground with it's separate diode.

author: TommyGunn
author_rank: Contributor
timestamp: March 01, 2013, 03:31:17 pm
content: That's the first thing that came to mind but I can't have too much of a voltage drop. As for a Diode-OR on the ground, im not sure if it would work. I think ill bread board it today.I would ideally like a solution without the v-drop.

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 01, 2013, 04:31:06 pm
content: They make OR-ing controller ICs that will switch the MOSFETs. This will address the problem of the voltage drop. You might search through Digikey's product index (Product Index > Integrated Circuits (ICs) > PMIC - OR Controllers, Ideal Diodes) and see if you can find something to fit the bill.Switching the 0V terminals of the sources with a controller IC might be a trick. If you think about it, since the IC is an active component, you need a 0V return for the controller IC to work (it can't power-up to switch the sources otherwise.) So you'll likely have to rely on the body diode of N-channel MOSFETs to provide a return path. Both the high-side P-Channel MOSFETs and low-side N-Channel MOSFETs would be 'backwards' from how they would normally be connected so that the body diodes can conduct to allow the controller to power-up. Once powered-up the controller can switch on the appropriate power source and turn the others off.The problem with this is that the 0V terminals of the sources won't be able to wander more than about -0.7V from 'circuit ground.'

author: fcb
author_rank: Super Contributor
timestamp: March 01, 2013, 07:35:03 pm
content: I guess the gold-standard method would be little relays, with some sort of opto-coupled 'lockout' circuit driving each relay coil - all a bit complex, but you could make it isolate the unused USB ports. Depends if your making a $50 thing or $500 thing.

author: dr_p
author_rank: Frequent Contributor
timestamp: March 01, 2013, 09:19:14 pm
content: Quote from: TommyGunn on March 01, 2013, 03:31:17 pm2 times 0.15V (Schottky diodes) isn't a lot IMHO.BTW, why do you need the grounds isolated too?

author: TerminalJack505
author_rank: Super Contributor
timestamp: March 01, 2013, 11:47:38 pm
content: Another thought is to use isolated 5v to 5v DC-DC converters on each of the sources. The converter outputs could then share a common ground. You can then use an OR-ing controller IC and MOSFETs to switch just the high-sides of the converted voltage.