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A Nmosfet needs to have a higher voltage to the gate compared to the source so it is usually used as a lower side switch to provide ground to the load, the source will be connected to the ground and the drain will be the output.
It can be also used as a high side switch, in that case the drain is connected to the positive supply and the source to the load but in that case you will have to use a method like bootstrapping or an isolated power supply to actually drive the gate with a voltage about 5-10v higher than the drain voltage.
The problem is that when the mosfet conducts then the source voltage will be almost as high as the drain voltage so you need to provide the gate with a bias higher than that.
The Nmosfet has lower Rds-on (on resistance), is cheaper and also has lower gate capacitance, that is why is is preferred for higher efficiency (like for example switching PSU).
A Pmosfet is the exact opposite of a Nmosfet, it works with a voltage in the gate lower then the one connected to the source, it is usually connected as a high side switch, source to positive supply and drain as output.
You probably intend to drive the mosfet from the mcu with 3 or 5v, if what you want is a switch then a Nmosfet would be the obvious solution connected at the ground side of the load.
You also have to select a mosfet that has logic level gate threshold voltage (turns on at a low gate voltage).
As Alex, explained about the gate voltage that needs to be higher than the drain. You can also get Logic level gate mosfets, which will trigger directly from a microcontroller output at 5v. Check out this link Metal Oxide Semiconductor MOSFET Tutorial
In my circuit mosfet will take triggering pulses from micro controller(planing to use PIC10) and the drain is connected to the negative supply n source is connected to load(string of series connected LEDs) .so i have to use N-type logic level mosfet for my application..right???
Yes a Nmosfet will work fine in your project, just select a logic level mosfet that can work correctly with your mcu output voltage, for example IRL3803.
If you are going to use fast PWM then you will also need a proper high current driver like
IRL3803 was an example of a mosfet with low gate threshold voltage (1v) and very low Rds-on (6m ohm), you can use any mosfet depending on your needs.
IRL3803 can be used for lower current too or you can use a cheaper models if you want.
You haven't specified the output current for your application, what is the output current for the string of series connected LEDs?
It also depends on the availability of specific mosfets in your country , the case package you prefer etc.
IRF540 for example is widely available and cheap, can work fine with a few amperes load, case it TO220
Selecting a n-type or a p-type is primarily based on your application.
If cost is a constrain for your design, then p-type mosfets is not a good option, because they are a bit more expensive than n-type because of the manufacturing costs.
To select a mosfet, you have to know how much current will be going through the fet to switch on/off the load. Therefor you look on the datasheet for Rds(on). It is good to choose one with a low Rds(on), because it will determine the power the device consumes under the current you are applying to it.
n-type mosfets switches with a high voltage on the gate, and a p-type switches with a low voltage on the gate.
apart from voltage, current rating we need to look into the other paramers like
1) rise time and fall time if is in the high frequency application.
2) rDS (Drain-Source ) resistance, to find the power dissipation and provide adequate heat sink to it.
3) P-channel or N-channel. Its upto you, depends on the circuit configuration.
4) Always provide resistance between Drain to Source. Its for the protection purpose.
5) If you are using MOSFET with in-built anti-parallel diode, make sure the max diode anode current is geater than worst case current.
for my application an ordinary MOSFET which acts as a switch (for 3A) will be enough.Now i am confusing about that depletion or enhancement type will be suitable for me.Can you please suggest low current model
Do you want a different case from the TO220 of IFR540, if not then why don't you use it?
The majority of the mosfet you will find are enhancement-mode and that is what you need too, you can select any mosfet you like that can work with your current and has a gate threshold low enough to work with your driver voltage level.
It will probably be different in different parts of the world, that is why I have suggested a cheap mosfet that is widely available and cheap.
Also IRF540 can be found anywhare and can work with low or high current.
IRF510 has a lower current rating but the Rds-on resistance is ten times worse and the price is the same.
If the size is the problem?
Maybe someone from your country can help or you can check in any online shop i your area.