Power MOSFET Circuit Help

What is the purpose of using a power MOSFET in the main voltage input of a
circuit? Why not just directly connect the main voltage input to a power
supply? Why go through the MOSFET?

I am trying to understand a circuit that goes directly from a 12V battery
to Drain of a P-channel power MOSFET. The Gate is connected to ground
through a 1MOhm resistor and connected to Source through a Zener diode. The
circuit is connected to the Source pin.

On Mon, 20 Jul 2009 11:49:15 -0500
"eeboarder" <jmeyer@emittechnologies.com> wrote:

> What is the purpose of using a power MOSFET in the main voltage input
> of a circuit? Why not just directly connect the main voltage input to
> a power supply? Why go through the MOSFET?
> 
> I am trying to understand a circuit that goes directly from a 12V
> battery to Drain of a P-channel power MOSFET. The Gate is connected
> to ground through a 1MOhm resistor and connected to Source through a
> Zener diode. The circuit is connected to the Source pin.

Accidentally reverse the battery.  Repeat your circuit analysis.
Illumination should follow.

-- 
Rob Gaddi, Highland Technology
Email address is currently out of order

eeboarder wrote:


> I am trying to understand a circuit that goes directly from a 12V battery
> to Drain of a P-channel power MOSFET. The Gate is connected to ground
> through a 1MOhm resistor and connected to Source through a Zener diode. The
> circuit is connected to the Source pin.

That is a protection against the battery reversal. Mosfet has less of 
the voltage drop compared to a diode.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

"Vladimir Vassilevsky" ...
> eeboarder wrote:
>> I am trying to understand a circuit that goes directly from a 12V battery
>> to Drain of a P-channel power MOSFET. The Gate is connected to ground
>> through a 1MOhm resistor and connected to Source through a Zener diode. 
>> The
>> circuit is connected to the Source pin.
>
> That is a protection against the battery reversal. Mosfet has less of the 
> voltage drop compared to a diode.

And when analyzing, remember the 'parasitic' drain-source diode inside the 
MOSFET - it is an essential part of the trick...

Arie 

>
>"Vladimir Vassilevsky" ...
>> eeboarder wrote:
>>> I am trying to understand a circuit that goes directly from a 12V
battery
>>> to Drain of a P-channel power MOSFET. The Gate is connected to ground
>>> through a 1MOhm resistor and connected to Source through a Zener
diode. 
>>> The
>>> circuit is connected to the Source pin.
>>
>> That is a protection against the battery reversal. Mosfet has less of
the 
>> voltage drop compared to a diode.
>
>And when analyzing, remember the 'parasitic' drain-source diode inside
the 
>MOSFET - it is an essential part of the trick...
>
>Arie 
>
>

Thanks again for the help!