Re: MOSFET for Reverse Polarity Protection

On 26 Apr 2005 16:41:03 -0700, Winfield Hill
<hill_a@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

>Jim Thompson wrote...
>>
>> Jim Thompson wrote:
>>
>>> Winfield Hill wrote:
>>>
>> [snip]
>>>>>
>>>>> 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.
>>>>
>>>> Yes, for a battery as a load. But ordinary loads only need one FET.
>>>
>>> Oooops! Guess you are right... no loads that can source... one FET,
>>> body diode pointed toward load side.
>>
>> 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.
>
> 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.

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

[MY EMPHASIS BY CAPITALIZATION]

From: "Andrew" <ajhuk@xxxxxxxxxxx>
Newsgroups: sci.electronics.design
Subject: Using zener diode to protect test circuit?
Date: Sat, 23 Apr 2005 15:56:05 +0100
Message-ID: <WZ-dnXBz4sqM__ffRVnyjg@xxxxxxxxx>

"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
http://makeashorterlink.com/?K388412FA) 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.
.

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