Logic Level MOSFETs

[cyberfreak]

Hi

I need to confirm the theory about Logic Level MOSFETs. How does it work? 0V to switch it off and 5V to switch it on, type of thing? Can someone please help me?

[audioguru]

Hi Cyberfreak,
A logic-level Mosfet does not abruptly switch off with a gate voltage of 0V, nor switch on at 5V. Just do a Google search and look at a datasheet.

I have used a little 2N7000 Mosfet, and its datasheet is here:
http://www.fairchildsemi.com/ds/2N/2N7000.pdf
The datasheet says that its gate threshold voltage is typically 2.1V, and it is turned on a little bit, drawing 1mA. With a 4.5V gate voltage, it draws 75mA and has a Rds of typically 1.8 ohms.

Like all Mosfets, it works better with a higher gate voltage. The 1st curves in the datasheet also shown below, show that with a gate voltage higher than 4.5V it can conduct much more current. At 10V, it is excellent.

Because its resistance changes gradually with gate voltage, if you want it to switch quickly to stay cool, you must switch its gate voltage quickly. That is difficult due to its high gate capacitance which must be charged and discharged by your gate driver.

[cyberfreak]

Hi AudioGuru

You helped me shine some light on the matter. The MOSFET that I am looking at using, is the FDS6961A.

http://www.fairchildsemi.com/ds/FD/FDS6961A.pdf

Looking through the datasheet and comparing it with the 2N7000, you used, I found that Vgs=4.5V will draw Id=2.8A, which is more enough. I only need 1.8A, but I have 12V to my disposal. As you said the higher Vgs the lower Rds(on); less powerloss, etc. BUT, what if I want it to be switched OFF completely? Can I apply a 0V?

I'm using a NOR gate CD4001 to drive the MOSFET.

[Kevin Weddle]

All you have to do is look at a digital circuit to tell that the logic level is a nuisance to the circuit. So what they do is use the logic level the best way they can. The result is high gain. The phase splitter handles the high gain while the input transistor acts like two diodes to handle the large input level.

[audioguru]

Hi Cyberfreak,
Your Mosfets are tiny little things so you must be careful with power dissipation. Use your CD4001 driver with the 12V supply to drive the Mosfets to their best conduction. If your 1.8A load is continuous, each Mosfet will dissipate 292mW worst case (max. Rds). If both Mosfets in the package are conducting 1.8A at the same time, the power dissipation will be well within the package's thermal limit at room temperature.
What is your application?
What is the max frequency of switching?

Of course you can apply 0V to the gate, that is how they are normally switched off. The datasheet shows it conducting only 1uA or less.