Author Topic: Why use an N channel MOSFET to connect GND  (Read 1398 times)

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Offline ilium007Topic starter

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Why use an N channel MOSFET to connect GND
« on: March 10, 2019, 01:22:48 am »
Hi - I came across this Freescale MC33926 breakout from Pololu and found this in the schematic. The numerous GND pins on the SMD package are connected to GND via an N channel MOSFET triggered by Vin. I am wondering why these pins aren't just tied direct to a GND plane on the PCB ?

I am starting out with electronics design and wondered why the designer may have chosen to go this way ?

 

Offline tsman

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Re: Why use an N channel MOSFET to connect GND
« Reply #1 on: March 10, 2019, 01:26:55 am »
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The carrier board has a reverse-protection MOSFET for added protection to the motor driver chip.
 
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Offline retrolefty

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Re: Why use an N channel MOSFET to connect GND
« Reply #2 on: March 10, 2019, 02:47:22 am »




I suspect it's to properly sequence powering up to prevent glitches on the output drivers.
 

Offline bson

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Re: Why use an N channel MOSFET to connect GND
« Reply #3 on: March 10, 2019, 03:03:56 am »
Note that the source pin is on the top and drain on the bottom, and it connects GND to Earth/Chassis.
 

Offline Mr.B

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Re: Why use an N channel MOSFET to connect GND
« Reply #4 on: March 10, 2019, 03:05:11 am »
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The carrier board has a reverse-protection MOSFET for added protection to the motor driver chip.

This...^
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Offline georges80

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Re: Why use an N channel MOSFET to connect GND
« Reply #5 on: March 10, 2019, 03:10:15 am »
http://www.ti.com/lit/an/slva139/slva139.pdf

Been using these schemes for years now for reverse polarity protection. Minimal voltage drop compared to using a diode (even a schottky).

The zener is needed if the gate voltage would otherwise be too high (depends on the supply rails).

cheers,
george.
 

Online NiHaoMike

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Re: Why use an N channel MOSFET to connect GND
« Reply #6 on: March 10, 2019, 03:14:52 am »
The part that trips most beginners up is that MOSFETs can conduct in both directions when turned on. And in fact, will do so better than a diode - look up "synchronous rectification" as well as the poorly named "ideal diode".

Here's a video explaining different options of reverse polarity protection:
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Offline ilium007Topic starter

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Re: Why use an N channel MOSFET to connect GND
« Reply #7 on: March 10, 2019, 04:59:26 am »
Thanks for all the replies. I have another weeks worth of reading to do !
 


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