Quick FET question

Basil

Hi All,

Just want to check something.

I am currently revising my altimeter board design.
Currently I am using a small N channel FET to interface to the prop, follwed by a higher current P channel.

This was done because I couldn't find a P channel which could drive ~10V @ 6-10A from a 3.3v output.

I have discovered this product. The fairchild FDW2508P, which is a P channel dual FET with Rds of ~20ma and gate threshold of -0.5V typical, -1.5V max

I noticed these values are very close to those on the N FET im current using (Fairchild FDV301) so Im thinking the new FET will interface directly to the prop?

If someone could shoot me down or confirm my Suspicions that would be great.

Alec

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-Alec

My our page

Phil Pilgrim (PhiPi)

If you need to switch significantly more than 3.3V on the high side, you still have to use an nMOSFET, NPN transistor, or other driver to control the pMOSFET's gate. This is because, even when the Prop output (if it were applied to the pMOSFET's gate) is high, it will still be enough lower than the pMOSFET's source voltage (10V in your case) to turn the transistor on. There just isn't a way to switch the transistor off without bringing the gate to within 0.5V or less of +10V.

Remember: the gate threshold is always spec'd relative to the source voltage — not to system ground, which the pMOSFET won't "know" anything about because it's not applied to any of its pins.

-Phil

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'Still some PropSTICK Kit bare PCBs left!

Post Edited (Phil Pilgrim (PhiPi)) : 8/28/2008 11:21:42 PM GMT

Basil

Ahhh ok then. Do the same rules apply to N FETs?

Thanks Phil

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-Alec

My our page

Phil Pilgrim (PhiPi)

Yes: the gate threshold is always spec'd relative to the source voltage.

-Phil

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'Still some PropSTICK Kit bare PCBs left!

Basil

Ok [noparse]:)[/noparse]

Thanks for the prompt answer

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-Alec

My our page