Correct placement of drain and source for a p channel enhancement mosfet

Thread Starter

samyang

Joined Jul 24, 2022
35
Hello. I am confused by where the drain and the source are located in a P channel enhancement mosfet. Is the source on the top or the bottom? I have attached two photos for reference. Thanks.

2. If I apply zero volts to the gate of a p channel mosfet, and 5 volts to the top (assuming that the top is the source), because there is a negative voltage difference between the gate the top (source), the mosfet will turn on correct? Even though it says that there must be a negative voltage applied to the gate to turn on a p channel enhancement mosfet, using two positive ones in this scenario would work as well? Thanks.

I have attached data sheet for mosfet below.
 

Attachments

Last edited:

LowQCab

Joined Nov 6, 2012
2,329
Depending upon the Specifications,
5-Volts may not be enough Voltage to turn it on completely,
which quite often results in over-heating, and possibly complete failure of the FET.

"" using two positive ones in this scenario would work as well? ""
I don't understand this sentence, two positive what ? in what scenario ???

For the FET to Turn "ON" completely,
the Gate-Voltage must be made "lower" ( negative ) with respect to the Source-Pin.
For the "average", cheap P-FET, that Voltage needs to be around ~8 to ~10-Volts to
completely "turn-ON" the FET.
Below this Voltage range,
there will very likely be an unacceptably high Resistance value between the Source and Drain Pins.
.
.
.
 

Thread Starter

samyang

Joined Jul 24, 2022
35
Depending upon the Specifications,
5-Volts may not be enough Voltage to turn it on completely,
which quite often results in over-heating, and possibly complete failure of the FET.

"" using two positive ones in this scenario would work as well? ""
I don't understand this sentence, two positive what ? in what scenario ???

For the FET to Turn "ON" completely,
the Gate-Voltage must be made "lower" ( negative ) with respect to the Source-Pin.
For the "average", cheap P-FET, that Voltage needs to be around ~8 to ~10-Volts to
completely "turn-ON" the FET.
Below this Voltage range,
there will very likely be an unacceptably high Resistance value between the Source and Drain Pins.
.
.
.
Thank you for your response. I have attached the datasheet of the mosfet I plan to use above. Looking at the graph in the data sheet, I beleive it says that because I will have -5V, the mosfet will turn on and allow 0.5 amps of current to pass through. Also, for a p channel enhancement mosfet, is the source and drain location the ones in the first photo? Thanks.
 

Irving

Joined Jan 30, 2016
2,979
2. If I apply zero volts to the gate of a p channel mosfet, and 5 volts to the top (assuming that the top is the source), because there is a negative voltage difference between the gate the top (source), the mosfet will turn on correct? Even though it says that there must be a negative voltage applied to the gate to turn on a p channel enhancement mosfet, using two positive ones in this scenario would work as well? Thanks.
Correct. Maybe this will help:

1659300493232.png
 

dl324

Joined Mar 30, 2015
14,902
I have attached the datasheet of the mosfet I plan to use above.
It's considered impolite to modify a post in a manner that makes a member's post seem clueless.

Please use correct spelling. MOSFET is capitalized because it's an acronym.
 

Irving

Joined Jan 30, 2016
2,979
Yes... on a P-channel the Source must always be more +ve than the Drain (except in very specific circumstances). These illustrate the most common arrangement, with the load on the Drain side.

If you explain in detail what you want to acheive we can advise more.
 

Thread Starter

samyang

Joined Jul 24, 2022
35
Yes... on a P-channel the Source must always be more +ve than the Drain (except in very specific circumstances). These illustrate the most common arrangement, with the load on the Drain side.

If you explain in detail what you want to acheive we can advise more.
Thank you for your response. I have attached a photo of the schematic I am referring to. So on the very left the signal is either 3.3 or 0V. I want to turn on the LED when applied 0V before the Resistor on the left. Thus, I have chosen a pull down resistor and a p channel MOSFET. The 5v goes into the source of the MOSFET, so when the transistor within the optocoupler is off, there is 0V applied to the gate and 5V applied to the source of the p channel enhancement MOSFET, which should turn on the mosfet according to the datasheet in my original post.
 

Attachments

Papabravo

Joined Feb 24, 2006
18,978
This simulation shows what happens to the currents and voltage in a PMOSFET as the gate goes from being positive with respect to the source to being negative with respect to the source.

1659302865276.png
 

Irving

Joined Jan 30, 2016
2,979
Thank you for your response. I have attached a photo of the schematic I am referring to. So on the very left the signal is either 3.3 or 0V. I want to turn on the LED when applied 0V before the Resistor on the left. Thus, I have chosen a pull down resistor and a p channel MOSFET. The 5v goes into the source of the MOSFET, so when the transistor within the optocoupler is off, there is 0V applied to the gate and 5V applied to the source of the p channel enhancement MOSFET, which should turn on the mosfet according to the datasheet in my original post.
Your diagram makes it difficult to understand the orientation of the MOSFET. But the function you want works as this simulation shows; when the OptoLED is 'on' ie V(lite)=3.3v, the output LED is off ie I(D1) = 0mA (Ignore R2, that's there to keep the simulator happy!)

1659362429178.png

Is there a specific reason for using a P-channel MOSFET? You can achieve the same result with an N-channel MOSFET just by inverting the whole arrangement...

1659363109940.png

Actually, you don't need the MOSFET at all, though I don't recommend this for other reasons, but this works just as well (for small LEDs < 15mA anyway)

1659364062083.png
 
Top