reverse current protection

[danv]

I’m using reverse current protection as in figure d but without the capacitor, it works well.

**broken link removed**

but I am concern if the input voltage is say 60V then the Vgs is -60 and the datasheet Vgs=+-20V
I’m using a p-mosfet (PMOS_BSS84) and the resistor of 1 M to GND.
Is it OK or I need a protection for the gate source voltage to not exceed 20volts.

 

[smanches]

Well, if I'm not mistaken, I could be wrong. I think all those mosfets are hooked up backwards. Isn't the source supposed to attach to the appropriate charge carriers (Bat- for nfets, and Bat+ for pfets?)

 

[schmitt trigger]

You DO REQUIRE to protect the Mosfet's gate from overvoltage.

The easiest approach is with a zener diode.
The diode's ratings should be 2 to 4 volts greater than the the Vgs required for maximum channel enhancement, which is the gate voltage for the rated RDSon.

So, if your Fet is rated 0.1Ω @ 10 Vgs, get yourself a 12 volt zener.

 

[smanches]

I'm actually confused about this. Doesn't the body diode conduct in reverse conditions, so a mosfet can only be used one way.

How does this work for polarity protection?

PS: I know why they were hooked up backwards now. duh.

 

[smanches]

I think I understand this now. They are using the body diode in the mosfet, not the mosfet itself. The gate is tied to the source to keep the mosfet turned off. And since the mosfet is hooked up in reverse, you are actually using the body diode as your reverse polarity protection.

So this assumes the mosfet body diode is better than a standard rectifier?

You would not need any gate protection for this, since the gate is tied to the source, it will always be safe from transients. Zener diodes are only to protect against transients, not a full time over-voltage condition.

 

[Roff]

I think I understand this now. They are using the body diode in the mosfet, not the mosfet itself. The gate is tied to the source to keep the mosfet turned off. And since the mosfet is hooked up in reverse, you are actually using the body diode as your reverse polarity protection.

So this assumes the mosfet body diode is better than a standard rectifier?

You would not need any gate protection for this, since the gate is tied to the source, it will always be safe from transients. Zener diodes are only to protect against transients, not a full time over-voltage condition.

No. Keep in mind that drain and source are interchangeable, except for the body diode. In these circuits, if the power supply is connected correctly, the gate voltage will turn the channel on, effectively shorting out the body diode. If the supply is reversed, the body diode is back-biased, and the gate voltage turns the channel off.