Hi. I want to protect a high power circuit (~400W) from putting in the batteries the wrong way, and also I'd like to stop it sparking when the batteries are first inserted (the circuit includes a large decoupling cap). I've figured that if I use a FET I can't get away with using just 1 because the body diode can't provide soft start AND reverse protection. So I need to use 2 back-to-back, but this is okay since the FET's channel is symmetrical. Is this the simplest way to do it or is there a better alternative? For simplicity I've assume the gates can withstand full voltage in the below

 

Just put a Diode in series with the battery supply, or a Diode in parallel across the circuit reversed, and a fuse in series with the battery, that way the fuse will blow as it conducts through the diode.

 

But that wouldn't provide soft-start

 

Why does it need soft start, explain?

 

Why does it need soft start, explain?

To avoid stress on the battery and the capacitor. Also the spark upon inserting the batteries is disconcerting and unprofessional

 

Then use the mosfet soft start method you posted, with the bottom mosfet, and put a diode reversed across the supply rails, with a fuse in series with the battery, that will prevent reverse polarity.

 

Below is the LTspice simulation of your circuit.
The 100kΩ resistor was incorrectly connected.
I also had to add a diode (D1) to keep the gate-source voltage from momentarily going positive when a negative input is applied, which caused a spike of negative voltage across the load.

As can be seen the input (Green trace) is passed to the load (Red trace) when positive but is blocked when negative.

The peak start-up current through the load capacitor C1 (yellow trace) is about 7.5A.
This can be reduced by using a larger C2 capacitor.

 

Then use the mosfet soft start method you posted, with the bottom mosfet, and put a diode reversed across the supply rails, with a fuse in series with the battery, that will prevent reverse polarity.

The power diode and fuse plus fuse-holder is likely more expensive than an additional MOSFET, and would require replacement of the fuse for an accidental reverse voltage application, so you would also need a spare supply of fuses.

 

This is great, thanks @crutschow. I see the diode is necessary since C2 will present a low impedance when reverse polarity is initially applied. D1 will charge C2 v quickly and effectively peg M2's gate to within ~1V of M2's drain.

I do like @Dodgydave 's reverse biased diode suggestion. Assuming the device was to be fuse protected anyway, this offers lossless reverse protection at the cost of 1 diode which could probably even be a 4148 provided it will outlive the fuse (edit: I'd need something more substantial than a 4148 which will only survive 2A for 1us. perhaps a 1N4001)

 

This is great, thanks @crutschow. I see the diode is necessary since C2 will present a low impedance when reverse polarity is initially applied. D1 will charge C2 v quickly and effectively peg M2's gate to within ~1V of M2's drain.

I do like @Dodgydave 's reverse biased diode suggestion. Assuming the device was to be fuse protected anyway, this offers lossless reverse protection at the cost of 1 diode which could probably even be a 4148 provided it will outlive the fuse (edit: I'd need something more substantial than a 4148 which will only survive 2A for 1us. perhaps a 1N4001)

If you want to go with the reverse diode use a MUR1560G.