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I noticed that there are more N-channel mostfets than P-channel mosfets on the market.

Then I realised that probably in many applications, the high powered device runs on a higher voltage and therefore a P-channel mosfet cannot be used.

An N-channel mostfet connects the ground lines of the high-power device to ground. However this is not "common" ground. Its ground + the potential of the mosfet.

I actually have a lot of components (all 5V) to be controlled by one MOSFET switch including digital logic. I need this to isolate out that part of the board when my board is only being powered by PC USB. (ie: 500mA limited, and this part of the board draws 1.5A and should only work when on USB charger)

I was thinking of putting in a P-channel mosfet into my design so that my microcontroller is controlling the highrail of my high-powered device (a light, a motor, etc), rather than the ground rail. It feels like that would make sense because then one maintains a solid ground plane on the entire board. Perhaps this is not as important as I was lead to beleive in school?

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  • \$\begingroup\$ Could you ask for the differences of P-Ch and N-Ch power switching in your question? :) \$\endgroup\$ May 14 '13 at 12:29
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    \$\begingroup\$ A solid ground plane is not the best idea if you are dealing with small analogue circuits and digital circuits and/or power circuits. You would tend to have ground planes separated and linked at one point to prevent circulating currents from one type of circuit affecting others. \$\endgroup\$
    – Andy aka
    May 14 '13 at 12:35
  • \$\begingroup\$ @abdullahkahraman - Thats kinda a vague question. N-ch switching can be used for connecting devices to the low rail, P-ch can be used for connecting to the highrail. \$\endgroup\$
    – SpiRail
    May 14 '13 at 12:51
  • \$\begingroup\$ @Andyaka: Thanks Andy, that seems to make good sense. Interesting. Then why is there a such a push to put common ground everywhere? \$\endgroup\$
    – SpiRail
    May 14 '13 at 12:54
  • \$\begingroup\$ @SpiRail I was asking you to add that wishing the possible answerer explain the details on the grounding differences. Btw, grounding is actually a big topic and cannot be generalised like that. \$\endgroup\$ May 14 '13 at 12:59
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I think we'd need to see the exact situation you're thinking about to give you a real answer, but in general,

  1. getting the grounding scheme right can be VERY important, and

  2. Sometimes getting the grounding scheme right can mean separating your grounds, or even isolating part of your circuit such that one part does not share a common ground with the rest of your circuit.

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