Dan Michaels email (remove spam text)
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?
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
The "reason" is --> the different symbols were not invented with
the newbie in mind. You get used to it.
hope this helps,
- Dan Michaels
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