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2000\12\09@095614 by

Page 939 of AoE shows use of a mosfet to control power to a circuit.  Checking my Forrest Mims Getting Started, it is using a
P-channel mosfet.  The load is between the drain and ground.  The source is connected to +5V.  The text indicates the p-channel is
used because of the positive supply.

Then in Switching Power Supply Design by Abraham Pressman, 2nd ed., I found similar circuits (p. 356) except he recommends an
N-channel for positive supply and vice versa.  He also places the load between the power supply and drain (for positive supplies).

Are these consistent?  Does it matter?

Just a beef, but the symbols on p. 355 of SPSD appear intended to confuse newbies!  Neither match my Mims book.  There are slightly
different symbols (extra line and dot).  Result is that for an N-mosfet sometimes the arrow points in and sometimes out.  (Must be a
reason?)

DS

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David Scott wrote:
>Page 939 of AoE shows use of a mosfet to control power to a circuit.
Checking my Forrest Mims Getting Started, it is using a
>P-channel mosfet.  The load is between the drain and ground.  The source is
connected to +5V.  The text indicates the p-channel is
>used because of the positive supply.
>
>Then in Switching Power Supply Design by Abraham Pressman, 2nd ed., I found
similar circuits (p. 356) except he recommends an
>N-channel for positive supply and vice versa.  He also places the load
between the power supply and drain (for positive supplies).
>
>Are these consistent?  Does it matter?
>

Hi David,

I don't have these books, but it all sounds ok. You can use either
N-channel or P-channel in a +Vcc system due to the concept of "duality",
meaning that the arrangements with respect to each other are basically
equal and opposite.

Typically, you would use a P-channel with source tied straight to
+Vcc and load from drain to gnd. Here the MOSFET is off with gate=HI,
and on with gate=LO.

For N-channel, the arrangement is the "dual" or opposite of the P-channel.
MOSFET tied with source=gnd, load between drain & +Vcc, and you pull
the gate=HI to turn on, gate=LO to turn off. So you can see the duality.

You could also use the N-channel as a "source-follower", similar to a
standard BJT emitter follower, by tying drain=+Vcc and putting the
load between source and gnd, HOWEVER --> in this case, due to the
2-3V theshold voltage required between gate and source, you probably
cannot pull the gate high enough to maintain full Vcc across the load.
==================

>Just a beef, but the symbols on p. 355 of SPSD appear intended to confuse
newbies!  Neither match my Mims book.  There are slightly
>different symbols (extra line and dot).  Result is that for an N-mosfet
sometimes the arrow points in and sometimes out.  (Must be a
>reason?)

The "reason" is --> the different symbols were not invented with
the newbie in mind. You get used to it.

hope this helps,
- Dan Michaels
Oricom Technologies
http://www.oricomtech.com
=========================

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At 08:50 AM 12/9/2000 -0600, you wrote:
>Page 939 of AoE shows use of a mosfet to control power to a circuit.
Checking my Forrest Mims Getting Started, it is using a
>P-channel mosfet.  The load is between the drain and ground.  The source
is connected to +5V.  The text indicates the p-channel is
>used because of the positive supply.

Positive supply with grounded load = "High side drive". P-channel mosfets
are more expensive and have poorer performance than N-channel ones.
Unfortunately in a high side circuit, a drive voltage higher than the
positive supply is needed in order to use an N-channel mosfet, so simple
circuits usually have P-channels.

>
>Then in Switching Power Supply Design by Abraham Pressman, 2nd ed., I
found similar circuits (p. 356) except he recommends an
>N-channel for positive supply and vice versa.  He also places the load
between the power supply and drain (for positive supplies).
>
>Are these consistent?  Does it matter?

It allows you to use N-channel mosfets, and that's good. If your load
the negative connection. In a switching power supply, you can quite often
choose, so choose N-channel where you can put the mosfet at the negative
side and the load at the positive side.

>Just a beef, but the symbols on p. 355 of SPSD appear intended to confuse
newbies!  Neither match my Mims book.  There are slightly
>different symbols (extra line and dot).  Result is that for an N-mosfet
sometimes the arrow points in and sometimes out.  (Must be a
>reason?)

Sorry I don't have that book. Maybe they only had symbols for junction fets
in their drawing program?

Cheers,

Bob Blick

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Hi David,

There are really two issues here. First of all, the choice of N versus P
channel. The two circuits you describe are two different kinds of circuit.
Both are called "common source" configurations (because the source is
connected to a "common" or supply node, either GND or V+), but one is of
the type known as a "high-side" configuration and the other a "low-side"
configuration. This just refers to the fact that in one case, the
transistor is on the high-potential side of the load (in the PMOS case) and
in the other, it is on the low-potential (closer to GND) side (NMOS case).

Both configurations work equally well, and which you use depends on whether
it is acceptable to switch V+ or GND. For example, if you are controlling
something which has multiple independent ground connections, or has
multiple things connected internally to ground and you are just controlling
one of those via the V+ line, then you must use a high-side circuit (the
PMOS version) because you don't have control over the GND line, and you
can't stick a switch(or FET) in it.

For even more concrete examples, imagine that you have a device which
contains a motor and some other electronics. The electronics accepts a +5V
supply and the motor takes a 12V supply, but they share a common ground (so
you only have 3 wires). You want to control the motor via PWM, but it is
unacceptable to switch the power to the electronics on and off, it must be
stable. So, you can't put a switch between the device and ground, you must
put it between +12V and the 12V input line.

The opposite situation could occur, where you have a common supply but
different grounds. In that case, you want to use an NMOS FET to switch the
ground line.

The second issue is the one of symbols. There are many different symbols
for mosfets. I have seen at least 3 different conventions, and yes, some of
them have an arrow direction opposite to that of the others. Probably the
best thing to do is to become familiar with one convention, and then when
you are unsure, look it up, or try to figure it out from the circuit (i.e.,
think about whether an NMOS fet would work in that situation, etc.)  At
least for the 3 conventions I have seen, there are other distinctive
differences (other than just a reversed arrow), so it isn't completely
ambiguous.

Sean

At 08:50 AM 12/9/00 -0600, you wrote:
{Quote hidden}

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>Page 939 of AoE shows use of a mosfet to control power to a circuit.
Checking my Forrest Mims Getting Started, it is using a
>P-channel mosfet.  The load is between the drain and ground.  The source is
connected to +5V.  The text indicates the p-channel is
>used because of the positive supply.
>Then in Switching Power Supply Design by Abraham Pressman, 2nd ed., I found
similar circuits (p. 356) except he recommends an
>N-channel for positive supply and vice versa.  He also places the load
between the power supply and drain (for positive supplies).

>Are these consistent?  Does it matter?
>
>Just a beef, but the symbols on p. 355 of SPSD appear intended to confuse
newbies!  Neither match my Mims book.  There are slightly
>different symbols (extra line and dot).  Result is that for an N-mosfet
sometimes the arrow points in and sometimes out.  (Must be a
>reason?)

Yes.
Yes.
Say what? :-)

The P channel FET is equivalent to a PNP transistor.
The Drain is most negative in use and source most positive.*
(* - there are useful modes where drain may be positive relative to source
but they are less standard - ignore for now).
Turn on by taking the gate negative relative to the source.
By using a P channel MOSFET as you described it is placed in the positive
"battery / Vcc" lead and the load connects from the drain to ground. This
allows the load to be ground references and is a major reason for P channel
FETs to be used in this application.

An N channel FET is pretty much the mirror image of a P channel FET. Read
the above and swap the terms positive and negative and Vcc & ground.
I haven't tried but this should work :-).
Basically the source is grounded and the drain connects to the 'cold" side
of the load and the gate is taken high relative to source to turn it on.
The disadvantage is that the load floats relative to ground when off.
N channel FETs are usually higher performance per dollar than P channel
FETS.

AFAIK the symbol for a standard N channel MOSFET should always have the
arrow pointing "in" unlike the somewhat equivalent NPN transistor which has
its arrow pointing "out".

regards,

Russell McMahon

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