I'm trying to make a high power UPS, I have a 13.6V PSU and an Inverter and a Lead acid battery.

Everything would be connected this way:

enter image description here

There's a problem with that circuit, if the battery is too discharged the initial charge current will be too high for it, so I need to limit the current while charging and not while discharging.

The simplest solution would be this:

enter image description here

But that solution isn't very efficient. 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 fets, so the end result should be this:

enter image description here

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?

By the way, the MOSFET I want to use is an IRF3205.

  • \$\begingroup\$ Power MOSFETs have a body diode between source and drain. \$\endgroup\$ Jul 30, 2018 at 0:31
  • \$\begingroup\$ Sorry but is that a problem? Putting the body diode in the schematic shows that the circuit should work: i.imgur.com/OuJzEl5.png The diode blocks current from the PSU into the battery. Basically I want the mosfet to be on when the PSU is off, and to be off when the PSU is on. Once the mosfet is off current will go thru and be limited by the 1 ohm resistor. \$\endgroup\$
    – Phoenix1
    Jul 30, 2018 at 0:41
  • \$\begingroup\$ Your N-FET is in reverse. \$\endgroup\$ Jul 30, 2018 at 3:38
  • \$\begingroup\$ Are you sure? i.imgur.com/lpTzd9t.png There are several images that explain how to use an Nfet for reverse polarity protection. I have it according to those. \$\endgroup\$
    – Phoenix1
    Jul 30, 2018 at 4:01
  • \$\begingroup\$ Anyway, I placed the Nfet the other way around and according to ltspice it is still conducting current from the supply to the battery. \$\endgroup\$
    – Phoenix1
    Jul 30, 2018 at 4:20

1 Answer 1


The reverse polarity circuit you're basing it on will protect against reverse voltage. It doesn't care which way the current is flowing.

Your FET is always on because the gate is always positive with respect to the source. You need to sense current separately and switch the FET depending on the direction of the current.

  • \$\begingroup\$ I see, thanks. I was thinking about turning the mosfet based on the supply voltage, like this: i.imgur.com/tDjopZj.png If the supply voltage exceeds (needs to be ajusted) 13V (that means the PSU is on) then the bjt turns on and drops the voltage to the gate of the mosfet, limiting the current to the batteries thru the 1 ohm resistor. Now the problem I see with that method is that the PSU would need to bring the batteries to 13V briefly for it to work, it may still not work. Could you please explain more deeply about current sensing in this case? \$\endgroup\$
    – Phoenix1
    Jul 30, 2018 at 15:55
  • \$\begingroup\$ That looks good. Check your BJT B-E voltage over the conditions to see how variable your threshold is. \$\endgroup\$ Jul 30, 2018 at 16:02

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