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
allows. This might not work if your load was in a vehicle, and already had
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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