P channel MOSFET Current Flow

[Nishitha M C]

Dear Sir,
I am using FDS4935 P channel MOSFET for my application. I have a doubt, when the gate voltage is 0v and source will be 5v, then our P channel MOSFET will be ON. I learned it When Vgs < Vth MOSFET will be ON then what will be the drain current? when i connect the LED at the drain, whether full 5A will flow as per datasheet.

 

[Nishitha M C]

And also what abt the Vds and Vd?

 

[WHONOES]

The LED will only draw as much current as it needs. If it is rated at 20mA then that is what it will demand regardless of what the FET is rated at. Bear in mind that the LED will need a series resistor to limit the current into it. As an example, if your supply is 5V and the LED forward voltage is 2V and its current demand is 20mA, then the resistor needs to be 3 / 0.02 = 150Ω.
Drain source voltage needs to be enough to accommodate the power supply voltage with a bit in hand. eg, for a 20V supply, a 30V Fet would be adequate though there is nothing to stop you using one with a higher Vds.

 

[Nishitha M C]

Ok sir.... Another question is I am using source voltage is 5v and gate voltage is 0v means what about the voltage will appear in the drain because i am connecting the LED at the drain pin. LED required some forward voltage to be active. If my source is 5v means drain voltage is also 5v?

 

[WHONOES]

If you power supply voltage is 5V then the voltage that will appear on you Fet drain minus the forward voltage of the LED. If you include a current limiting resistor in the LED as you should if it does not have one then, the drain voltage will be almost zero.

 

[BobK]

Unless you are approaching the current limit of the MOSFET, the voltage at the drain can be considered to be 5V when the MOSFET is fully turned on. You cannot just connect an LED between 5V and ground, it will likely be destroyed, so, as already stated, you must have a resistor in series with the LED.

Bob

 

[WHONOES]

Surely the drain voltage will be less than the supply voltage when the Fet is fully enhanced?

 

[BobK]

Yes. The datasheet for that part indicates a 28 mOhm resistance at -4.5V Vgs. So, with a 20mA LED powered through the drain, the voltage at the drain will be:

5 - 0.028 * 0.02 = 4.9994V

And at 1A it will be 4.972V

That is why I said it can be considered to be 5V.

Bob

 

[WHONOES]

Must confess, I was thinking in terms of an N channel device. That's what you get for only scanning a post. You are correct for a P device the voltage would be within 7mV of the 5V rail. The datasheet I found quoted Rds as 35mV for 4.5V Vgs.