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Mosfet Question

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petesmc

New Member
Hi,

I can't really find very much basic information on mosfets and my books don't have much either so this may sound stupid. But whats the difference between connecting the load to +V and the drain, and 0V and the source?

It my mind, they should be exactly the same, but I don't know.

Thanks
 

Noggin

Member
Not 100% certain what you're asking, but there are basically two types of MOSFETs, N-fets and P-fets.

N-fets are good at pulling something to ground and are "turned on" by applying a high voltage to the gate. P-fets are good at pulling somthing to V and are turned on by applying a low voltage to the gate.

If you have a device and want it turned on when your control signal is high, use a P-fet. Tie the source to ground, the drain to the ground of your device, the device's power to a battery/power source, and the gate to the control signal.

If you have a device and want it turned on whe your control signal is low, use an N-fet. Tie the source to V+, the drain to the device's power line, gate to the control signal, and the device's ground to the system ground.

Keep in mind that depending on the current this could/will alter the device's ground when using the N-FET to supply a ground.

You can use an NFET to supply V+ and a PFET to supply ground (I don't like the way thats worded...) In VLSI we went over situations like that, and if I remember correctly, when using an N-FET to supply v+ you have a constant power drain (even when off) and if your gate is 5v, then the maximum voltage at the drain would be 5v - 0.7v (approximately).
 

Noggin

Member
Oh, but maybe your question was simpler :)

Source and drain on a non-depletion mode MOSFET are the same, the device is symmetrical. Can't comment on depletion mode, never studied those. You can turn a MOSFET around but you can't swap a P-FET for an N-FET or an N-FET for a P-FET (usually)
 

petesmc

New Member
Hi,

I think your second answer was what I'm looking for, but just to clarify, he's a very simple diagram. Is there a different between the following ?


+V -> LOAD -> N Mosfet -> 0V

and...

+V -> N Mosfet -> LOAD -> 0V

From your answer, I assume they are similar, however, using Multisim and Crocodile clips, I get a larger voltage drop across the mosfet using the second method, whereas I'm trying to minimize drop. I though they should be identical as you have describe. I will try it out on breadboard soon. Oh btw, which are 'normal' mosfets, depletion or non-depletion?

Thanks for you information post, it actually gave me a really good idea and saved me several op-amps in a project I'm working on.

Another few quick questions though, does the Rgs of a mosfet determine the voltage drop? And since I want a very low voltage drop, i should be looking for a very low R mosfet correct (large load)?

As well, is it okay for the voltage at the source to be large than at the drain? Just by around 5V? This is for a fairly high power mosfet, if that makes a difference.

Thanks

p.s. You visit SPF?
 

Optikon

New Member
petesmc said:
Hi,



Another few quick questions though, does the Rgs of a mosfet determine the voltage drop? And since I want a very low voltage drop, i should be looking for a very low R mosfet correct (large load)?


Thanks

p.s. You visit SPF?

Yes. You mean Rds right? Rds looks like a resistor. First figure out what your Rds really is in your circuit. Rds is strong function of Vgs. Fet datasheets will give one or two values of Rds at a given Vgs but they also usually provide curves. Find your operating point on the curve and multiply that Rds by your current and see if that makes sense for the voltage drop you are seeing.
 

petesmc

New Member
HI,

Thanks for the info, yes, it does work out right. Can anyone help with the above questions. Btw, in teh first reply, is the information correct? IN the 3rd and 4th paragraphs, shouldn't N be replaced with P and vice versa?

I just tried with a N Mosfet. I tried:

+V -> LOAD -> N Mosfet -> 0V

and...

+V -> N Mosfet -> LOAD -> 0V

...and there is definately a difference. The drop over the mosfet in the second example, is about 5V from a 12V source, whereas, using the other method, the drop is only 0.1V which is as expected for my load.

Thanks
-Peter
 

Sebi

Active Member
In the second case You can't totally opened the N-fet with same voltage.
The datasheet show a minimum gate-source voltage for Rdson. When the load connected between GND and source, no enough voltage for proper driving, because the source voltage increasing on load.
Sorry for bad english.....
 

Noggin

Member
If you would prefer the second option, replace the NFET with a PFET. You're seeing something related to what I posted:

N-fets are good at pulling something to ground and are "turned on" by applying a high voltage to the gate. P-fets are good at pulling somthing to V and are turned on by applying a low voltage to the gate.

Keep in mind though that if you replace the NFET with a PFET, you'll need a low voltage to turn it on, and a high voltage to turn it off.
 

Roff

Well-Known Member
Noggin said:
Source and drain on a non-depletion mode MOSFET are the same, the device is symmetrical.
This is true, but for most commercially available discrete MOSFETs, the substrate is internally tied to the source, so if you swap source and drain, the substrate diode will be forward biased, effectively shorting out the transistor.

As others have said, when you use an N channel as a high side switch, you have a source follower, which means the source will be at a lower voltage than the gate by the amount of the gate to source threshold. To overcome this, you have to drive the gate higher than the drain by several volts.
 

petesmc

New Member
Can someone explain to me, the correct usage of a pMosfet? Like where to place the load. Typical range of voltages to connect to drain, source and gate, and how to turn on/off the fet?

Thanks
 

digital_dave

New Member
petesmc said:
Can someone explain to me, the correct usage of a pMosfet? Like where to place the load. Typical range of voltages to connect to drain, source and gate, and how to turn on/off the fet?

Thanks

Check out International Rectifier's application note AN-940. It describes thier P-channel HEXFET power MOSFETs, but it should help.

David
 

Noggin

Member
Ron H said:
Noggin said:
Source and drain on a non-depletion mode MOSFET are the same, the device is symmetrical.
This is true, but for most commercially available discrete MOSFETs, the substrate is internally tied to the source, so if you swap source and drain, the substrate diode will be forward biased, effectively shorting out the transistor.

Thanks for pointing that out for him. Don't want to be responsible for him losing any smoke.
 
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