Ok, I'm really trying to understand these darned MOSFET's here, so I managed
to find some good articles regarding them. I will summarize what I know to see
if it's true (for pMOS)

If the voltage drop from gate to source is greater than the threshold, the
MOSFET should turn on...

I took my IRF9530 (just a basic pmos with a Vgs threshold of -4V) to test this
out. Connecting source to +12 and gate to GND, the drain showed a drop of
+12V. So far, that seems right.. since Vgs = -12V, and the threshold = -4V.

BUT...

When I connected the gate to +5V, the drain still showed +12V. Why is this?
Vgs = 5 - 12 = -7V, which is still greater than the threshold, right?

Another thing is the signs here... I think that's what may be messin me up.
Are we talking about algabraically greater?? Because -7 isn't greater than -
4... nor is -12. So somewhere in my explanation (and understanding), I'm
mistaken. Can anyone clarify?


Quoting Tal <tal@ZAPTA.COM>:

> Any P channel will do. Drive it with an open collector and a pullup
> resistor and you will have a good Vgs drive voltage of 12V (your high
> side voltage). Make sure your mosfet can handle 12V Vgs (otherwise, add
> another resistor for voltage divider).
>
> Also, I am not sure what are the charactersistis of the fan you plan to
> use. If this is an inductive load, you may need add a reverse diode
> parallel to it. Anybody wants to comment on this ?
>
> BTW, N chan can also do high side but the drive is a little bit more
> complex (e.g. a charge pump to generate voltge above your VDD) so
> typically you want to avoid it.
>
> Tal
>
> > -----Original Message-----
> > From: pic microcontroller discussion list
> > [mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of Jai Dhar
> > Sent: Sunday, March 09, 2003 5:25 PM
> > To: PICLIST@MITVMA.MIT.EDU
> > Subject: Re: [EE]: MOSFET as a digital switch
> >
> >
> > Quoting Herbert Graf <mailinglist@FARCITE.NET>:
> >
> > > > So with a working circuit such as this:
> > > >
> > > >                          ------IRLZ24-------
> > > > +12V <-------| FAN |-----DRAIN  GATE  SOURCE---------- GND
> > > >                                   |
> > > >                                   |
> > > >                                   |
> > > >                                  Logic input
> > > >
> > > > would it be correct calling this a low-side switch?
> > >
> > >         That would be correct.
> > >
> > > > Is it
> > > > possible to convert
> > > > it to a high side, since I'm hoping to use the IRLZ24's
> > as a power
> > > > switch for my supply.. ie:
> > > >
> > > > +12V <------ (IRLZ24) ----- LOAD ---- GND
> > >
> > >         Although "possible" I would STRONG recommend
> > against it. The
> > > reason is in this config you are using the MOSFET in source
> > follower
> > > config. Why this is bad is power dissipation, the MOSFET
> > will drop a
> > > rather large amount of voltage and probably burn up (if you are
> > > running at "rated" current). This is due to the transistor
> > not being
> > > fully "on" (hard in the triode region).
> > >
> > > > I hope I haven't mixed up my high/low side terminology.
> > Would I need
> > > > a P- channel for this application?
> > >
> > >         Correct, you need a p-ch to do this properly. TTYL
> >
> > Can anyone recommend a good p-ch that can be driven by logic
> > level's? (do they even exist since they need a negative gate
> > voltage to turn on?).
> >
> > >
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