Re: Power Switching with mosfet

On Mon, 22 Oct 2007 19:22:53 -0700, D from BC
<myrealaddress@xxxxxxxxx> wrote:

On Tue, 23 Oct 2007 01:23:06 GMT, "Jon Slaughter"
<Jon_Slaughter@xxxxxxxxxxx> wrote:

I only have an n-channel enhancement mosfet(2N7002) and I'm trying to use it
to switch power to an IC.

I can only bring the gate voltage about 1.7V above the V_DS. I'm not
switching the entire voltage to the IC.

Some question I have, What is the difference between a high side mosfet
driver and a low side? A low side brings the drain within a few mV above
ground but the high side brings the source within only several hundred mV
below the drain. Shouldn't it be symmetric?

i.e., whats the difference between

V
|
Mosfet
|
Load
|
G

V
|
Load
|
Mosfet
|
G

I'm calling the first one a high side and the second a low side, I think
thats the correct terminology. Both mosfets are the same as is the load.

The low side works fine but the high side doesn't. It seems that in either
cause it should be the same. V_DS is the same in both cases? Obviously this
isn't true but I can't seem to understand why?

The way I'm looking at it is basically as two resistors in series so the
voltage drop across each should be exactly the same.

So whats wrong?

Thanks,
Jon


It's all about how Vgs is developed for saturation.

High Side:
(Vgs >Vdd )

Low Side:
(Vgs < Vdd )


D from BC

Ooops ...let me fix that..

High Side:
(Vg > Vdd )

Low Side:
(Vg < Vdd)


D from BC
.

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