Using mosfet as efficient diode (Making an UPS) - Page 1
Forum section: electronics / projects

author: Samueru
author_rank: Newbie
timestamp: July 30, 2018, 02:56:06 am
content: Hi, I'm trying to make a high power UPS, I have a 13.XV PSU (variable) and an Inverter and a Lead acid battery.My first idea was to connect everything this way:However, There's a problem with that circuit, if the battery is too discharged the initial charge current will be too high, so I need to limit the current while charging and not while discharging.The simplest solution would be this:That way when charging current flows thru the 1 ohm high power resistor, and while discharging it goes thru the Schottky diode instead, however finding very high current (at least 60A Schottky diodes is impossible to me). And also the design isn't very efficient this way.So now the next thing that comes to my mind is using a mosfet as diode, like the reverse polarity protection circuits. I only have N channel mosfets, so the end result should be this:However according to LTspice that circuit doesn't work. As long as it is the default Nmos it does work, but if I try to replace it with with any Nmos in ltspice's catalog (Mosfets with a VGS above 20V), the mosfet still conducts when it shouldn't. Why?The mosfet I want to use is a IRF3205.I also don't know how much initial charge current the batteries could pull if I lower the supply voltage to 13V, they should still charge, just much slower. If it is low enough that might be the solution.

author: Damianos
author_rank: Frequent Contributor
timestamp: July 30, 2018, 03:22:00 am
content: Welcome to the forum!Your first idea is what is needed (adding some protection for over discharge and short circuits).Consult the datasheet of the battery and application notes from the battery manufacturer. Somewhere it maybe noted that, when using it in float - stand by mode, there is not a need for limiting the charging current.

author: Samueru
author_rank: Newbie
timestamp: July 30, 2018, 03:48:17 am
content: Quote from: Damianos on July 30, 2018, 03:22:00 amThere's already an overdischarge protection in the inverter. Though know that I think about it I need to check how much current the PSU pulls of the batteries when its off.Quote from: Damianos on July 30, 2018, 03:22:00 amIt's gonna be 3 automotive batteries in parallel.Unfortunately there's no info about max charge current in their website:http://www.titan.com.ve/productos_v_livianos.phpIt does spec a charging voltage between 13V and 14.1V. Since they're automotive batteries I guess that they can take very high charge currents from a car's alternator. However I may still need to limit the current because the power supply is only capable of 60A, and about 20A will be constantly be used by the inverter.I know I'm not using the right type of battery for this, it should be a deep cycle battery, however here car batteries are about 10 times cheaper than car batteries in the US (which are already cheaper than deep cycle batteries). I'll always try to prevent them from going below 11.8V though.

author: Samueru
author_rank: Newbie
timestamp: July 30, 2018, 05:22:13 am
content: Alright I think I have it:If I do that, the mosfet will be on as long as long as the supply voltage is under 13V (needs to be adjusted), and if the voltage exceeds 13V then the bjt turns on and the gate voltage to the mosfet drops, turning it off.However, the PSU would still need to be able to bring the batteries to 13V briefly for this to work. I'm not sure if it could work in real life.