Polarity Protection circuit

Apr 21, 2004 6 Replies
C

I'm having problems with this annoyingly simple P-Chan FET circuit ....



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With the correct input voltage it seems to work fine, but when the input polarity is reversed the FET doesn't turn off for some reason.



With the input polarity _reversed_ the FET terminal voltages are:



Drain = -24V Source = -24V Gate = -12V



So the gate is at +12V wrt the Drain/Source. Since Vgs(th) for this FET is about -2 volts it really should be OFF.



At first I thought I was measuring leakage currents on the output so I loaded the output with a few Ohms and they got hot - so the FET really is conducting.



With power removed the FET switches off.



I replaced the FET with another the same and got the same result.



The body diode is the right way around.



All the zenners are for static protection. With reverse input polarity (and the FET conducting when it should be off) D3 is toast and has been removed until I can get the FET to turn off.



The potential divider on the gate is to reduce Vgs to 12V. Max Vgs for the part is +/- 20V.



Any ideas?



Colin


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J

Since the part's a P Channel FET, its internal Zener will have its anode connected to the drain and its cathode to the source, so whether or not it's on or off, current will flow through the Zener with the drain connected to +24V! Then, when you reverse the polarity of the input voltage With D3 in there, the drain will be connected to 0V through D3, the gate will go to about -12V, turning on the FET, connecting the cathode of D3 to -24V. Toast! When you connect a few ohms across the output with no D3 there, the source gets connected to 0V through the few ohms, the FET gets turned on and the few ohms get hot. Then, when you connect your voltmeter from source to ground, the source sees ground through your meter, the FET turns on and your meter says -24V. :-)
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J

See "PerfectDiodeForChargerIsolation.pdf" on the S.E.D/Schematics Page of my website.

...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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C

Yes that's ok. It has to be that way around or the diode will conduct when the polarity is reversed and the FET is meant to be off.

But thanks for this next bit......

Now I think I see whats happening... When the polarity is reversed and the output is open circuit the FET is off as intended...but when I pull the source to the 0V rail with a load the Gate at -12V will be below the source at 0V and it will turn on! oops.

Thanks for that. I should be able to work out a fix OK.

Colin

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C

Humm it looks like it might be easier with a relay! The problem with using a FET seems to be that most Low-ON-Restance types have a VGS max around 20V and I need +/- 24V.

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J

If you could use the FET's body diode in the forward direction, why can't you just use a single rectifier in series with the load? Or a Schottky if you need a low Vf?
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C

For info, I fixed the problem I was having by inserting a 1N4148 diode in series with R2.

Colin

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