Power Switching with mosfet

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


.

Relevant Pages

  • Re: Power Switching with mosfet
    ... Jon Slaughter wrote: ... to switch power to an IC. ... I can only bring the gate voltage about 1.7V above the V_DS. ... So whats wrong? ...
    (sci.electronics.design)
  • Re: Power Switching with mosfet
    ... to switch power to an IC. ... I can only bring the gate voltage about 1.7V above the V_DS. ... So whats wrong? ... It's all about how Vgs is developed for saturation. ...
    (sci.electronics.design)
  • Re: Power Switching with mosfet
    ... to switch power to an IC. ... I can only bring the gate voltage about 1.7V above the V_DS. ... It's all about how Vgs is developed for saturation. ... (Vgs < Vdd) ...
    (sci.electronics.design)
  • Re: 2n7000 spice models
    ... lots of MOSFET models have such weird structures. ... M2 turns on when the gate voltage is well above the drain voltage, ... The Wheatley and Ronan model looks something like this. ...
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  • Re: Modeling JFET IDSS in SPICE
    ... they are specified to run from 24 to 60 mA. (IDSS is the drain current ... with the gate grounded and the drain-source voltage in this case at ... Looking at the parameters available for JFETs in SPICE, ... JFET parameters vary over such a wide range that either ...
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