Electronics > Beginners
mosfet; p-channel - basic question
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tester43:
MOSFET: IPP120P04P4L-03
Datasheet: https://www.infineon.com/dgdl/Infineon-IPP_B_I120P04P4L_03-DS-v01_01-EN.pdf?fileId=db3a30432f69f146012f783b3b5a2e3f

Maximum Vgs = +-16v

Voltage I will be controlling: 24V peak

Does it mean that I can't just tie the Gate to GND because it will create 24V Vgs which is more than 16V and will break the device?
Zero999:
Yes, you'll kill the device, if the MOSFET is in the +24V supply and the gate is connected to 0V. If speed isn't an issue, connect a 10V zener between the gate and drain and connect that to 0V, via a resistor. If this is being run off a microcontroller, then the zener can be avoided: use BJT amplifier in common base configuration.


V2 = Vinput
VGS = (Vinput-VBE)*R2/R1
VBE = 0.6V

Vinput = 5V
VGS = (5-0.6)*1000/470 = 9.36V.

Why not switch the 0V side? It's much easier and can be done with an N-channel MOSFET.
tester43:
Hi again :)
Thanks for this solution.
Regarding "why not N-Channel" part: i'm still thinking which way to go with my soldering station power.
I have all the info I need: AC Zero crossing interrupt signal, opto coupler to drive the gate etc. The only thing that is bugging me is the idea of doing all measurement with AC voltage present at the connector. I know that it will be in zero (or close to it) but this concept is somehow.... counter-intuitive :)
I like this idea of having P-Channel oriented FET. With it I dont have any AC on my load when I need to do the thermocouple reading.
Load is T12/T15 hakko tip:
http://dangerousprototypes.com/forum/download/file.php?id=12167&mode=view

http://dangerousprototypes.com/forum/download/file.php?id=11012&mode=view

StillTrying:
If the 24V is raw rectified DC, one minor problem you'll have with these simple methods of switching the P-Channel is that it won't switch on and off at the zero crossing point, but every time the 24V rises and falls through 2V - 4V. So its near-zero dissipation will be a bit higher than near-zero.
tester43:
hi, yes - it's because of threshold voltage - if I remember correctly.... Would need to go to -10V at 0 AC.
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