MOSFET for Reverse Polarity Protection

I have not used it myself.

Common power MOSFETs are large semiconductor area devices and have rather high capacitance, and are therefore fairly rugged devices, not unlike a diode. But they do have an Achilles heel: their low gate-voltage limit, however you can easily add gate protection:

.. -|>|- .. IN ------- D S ----+---- + out .. pmos G | .. | \ | Active reverse-voltage protection .. +--|>|--' .. 10k | 15V zener insures Vgs < 20V .. GND ----/\/\--'

If you're ambitious, you can add an electronic-fuse shutoff feature.

--
 Thanks,
    - Win

That's interesting (and well stated). Do you have any links for us?

--
 Thanks,
    - Win

Because of the body diodes it takes TWO fets back-to-back to ensure reverse protection.

In my LiIon charger chip it was used to control both directions... turn off charging path at full charge... and STOP discharging at minimum allowed cell voltage.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
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I love to cook with wine.      Sometimes I even put it in the food.

Yes, for a battery as a load. But ordinary loads only need one FET.

--
 Thanks,
    - Win

Oooops! Guess you are right... no loads that can source... one FET, body diode pointed toward load side.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I a, presently developing a 6 volt battery powered product and added this sort of thing to it.

I used a SOT-23 20 volt several amp logic N-channel mosfet connected backwards in the negative battery line with the gate tied to the positive line (through the power switch), and a 6.8 volt zener gate to source. Works a charm. The load current is about a quarter amp and the voltage drop is a lot less than .1 volt. My low voltage cutoff (by other means) is at 4.2 volts where there is still plenty of gate voltage.

I don't have the part number with me, but it is something like this:

Steve I believe it is pretty well standard practice to use an N-channel MOSFET in the return lead of military power supplies (28V input). Drain to supply negative, source to the negative of the PSU and the gate driven by a protected derivative of the positive supply.

Kind regards

Graham Holloway WPS/Accuphon Electronics (Tel/Fax 0(044)1233 662599)

[snip]

Turns out... 'tain't quite true...

One device will give reverse-polarity protection, but it takes TWO devices to give BOTH reverse-polarity protection AND over-voltage protection.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Yes, certainly, but that's a rarely-required capability. For example, this thread's O.P. didn't ask for that feature. But it's good to keep in mind nonetheless.

--
 Thanks,
    - Win

The original-original poster (OOP for short :) was...

[MY EMPHASIS BY CAPITALIZATION]

From: "Andrew" Newsgroups: sci.electronics.design Subject: Using zener diode to protect test circuit? Date: Sat, 23 Apr 2005 15:56:05 +0100 Message-ID:

"I am designing a test instrument that will be powered from the circuit under test. Consequently there is a high probability of the INCORRECT VOLTAGE OR POLARITY being supplied to my circuit. I cannot protect against reverse polarity by using a series diode as this will drop the supply voltage too much.

An idea that I have seen (in the Microchip ICD1

is to place a 5.1V zener diode across the power input. This will short the supply if applied with the wrong polarity and attempt to clamp it to 5.1V IF TOO HIGH A VOLTAGE IS APPLIED. In either case the diode will draw a potentially large current since a series resistor is not practical as that would drop the voltage unacceptably under normal circumstances. However, this would probably work if the test power supply were current limited."

Which is why I posted a full solution to the query.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

OK, I stand corrected. He'll need two FETs and your circuit.

--
 Thanks,
    - Win

No problem. Two threads just got stirred together.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Win, I just had a remembrance that I thought you'd get a kick out of...

I did a charge pump design that has the PMOS bodies (N-well) tied together and left to float.

Clocking the pump devices causes the N-well to charge up (it's just a big capacitor) to the highest potential in the pump chain... so you don't have to worry about back-flow... it's automatically biased to prevent it.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Nice, that reminds me of some of Widlar's tricks.

--
 Thanks,
    - Win

(It's in silicon as of a few weeks ago, and working :-)

Widlar was devious ;-)

Years ago I did reverse engineering for ICE (Integrated Circuit Engineering).

One of the circuits I analysed was the "protected power transistor" (actually a spin on the LM317 regulator).

I think I posted it here (or a.b.s.e) at one point or another in time, but can't locate it right now... Hey! I'm home alone and on my third glass of wine ;-)

In there I found merged PNP-NPN combinations made in one well and a resistor buried under an isolation diffusion.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Don't have any idea (I wouldn't), but I know that I first saw the circuit in one of National's Bob Pease shows. Bob seemed to approve of the circuit, which means it's probably a reasonably good circuit.

He also mentioned that it is patented, although I think he thought up the circuit prior to doing the patent search.

[snip]

--Mac

What form does that typically take?

--
 Thanks,
    - Win

The latter sounds like a carefully-engineered circuit.

--
 Thanks,
    - Win

Sorry, no links. Aquired knowledge while working for various power supply companies in the UK. BTW, it's also common to include surge protection circuitry with the reverse-polarity circuitry as well.

Regards

Graham Holloway