Author Topic: Ruggeduino Output Rails Protection Doesn't Make Sense To Me  (Read 2637 times)

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Offline yrrapt

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Ruggeduino Output Rails Protection Doesn't Make Sense To Me
« on: September 23, 2013, 10:03:21 pm »
Hi,

I've been looking at the Ruggeduino board (http://ruggedcircuits.com/html/ruggeduino.html) for some inspiration for circuit protection in one of my projects.  What interests me in particular is the output protection to stop applied voltages raising the rails and blowing other components connected to those rails.

They show the circuit they use but as far as I can understand it the P-type MOSFET is connected backwards and supplies power to IO through the diode across the drain and source regardless of gate voltage.  When the IO voltage is higher than the regulator voltage the diode blocks any current and the comparator turns the MOSFET off.  But then surely just using a Schottky would have sufficed without the extra circuitry.



Can someone tell me what I'm missing here?

Cheers,
Tom
 

Offline Short Circuit

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Re: Ruggeduino Output Rails Protection Doesn't Make Sense To Me
« Reply #1 on: September 23, 2013, 10:20:30 pm »
Guess they wanted to elimate the voltage drop that comes with just a diode.

btw Too bad their mosfet will be destroyed with any 5VIO transient/spike above 20-25V due to the limited Vgs(max) of the mosfet...  So much for rugged :palm:
 

Offline dannyf

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Re: Ruggeduino Output Rails Protection Doesn't Make Sense To Me
« Reply #2 on: September 23, 2013, 11:36:35 pm »
Quote
the P-type MOSFET is connected backwards

They are trying to protect against a high transient on the +5VIO line (assuming that it is within Vgs(th) ratings of the mosfet).

Quote
So much for rugged

Being rugged doesn't mean it is unfallable.
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Online sleemanj

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Re: Ruggeduino Output Rails Protection Doesn't Make Sense To Me
« Reply #3 on: September 24, 2013, 12:42:50 am »


They show the circuit they use but as far as I can understand it the P-type MOSFET is connected backwards and supplies power to IO through the diode


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Offline dannyf

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Re: Ruggeduino Output Rails Protection Doesn't Make Sense To Me
« Reply #4 on: September 24, 2013, 12:56:45 am »
Quote
Watch

That came from an old IRF application note. N-ch mosfets can be utilized as well.
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Offline yrrapt

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Re: Ruggeduino Output Rails Protection Doesn't Make Sense To Me
« Reply #5 on: September 24, 2013, 07:48:36 am »
I was under the impression that the P-channel wouldn't conduct current in this direction.  Obviously a bit of a fundamental understanding failure on my part, back to the books!

Thanks for the help guys.
Tom
 

Offline Harvs

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Re: Ruggeduino Output Rails Protection Doesn't Make Sense To Me
« Reply #6 on: September 24, 2013, 09:07:49 am »
Quote
I was under the impression that the P-channel wouldn't conduct current in this direction.

Common misconception.  A mosfet will have a low impedance in both directions with a Vgs into saturation.  That's what makes them good as synchronous rectifiers.
 


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