# Logic and questions about FET's!

Discussion in 'General Electronics Chat' started by electronice123, Feb 22, 2009.

1. ### electronice123 Thread Starter Senior Member

Oct 10, 2008
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0
I have been reading about FET's a bit and have a few questions:
I'm working with an IRFP450,

Do FET's need any gate current to work?

Is the threshold voltage the minimum gate voltage that will allow maximum current to flow?

I'm trying to build a circuit but I don't know how to figure what resistance I need between a NAND gate and the gate of the IRFP450 FET.

The Voltage is 10V applied to a 4093, the output of the 4093 will be used to drive the IRFP450's gate. On the 4093 datasheet the high level output current at 10V typ is 2.25mA. Do I need to add a resistor or is 2.25ma nothing to worry about?

If so how do I figure the resiatance required? Ohms law???

Last edited: Feb 22, 2009
2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Often you can get away with no resistor at all between the drive source and the gate. However, if your MOSFET is driving an inductive load you may what to place some resistance, say 100 ohms or so, just to protect the output of the driver from any spikes that could couple back from the drain to the gate.

hgmjr

Oct 10, 2008
304
0
OK thanks!

4. ### leftyretro Active Member

Nov 25, 2008
394
2
In steady state the gate is insulated from the source and drain leads and therefore requires no DC current, just voltage. However to transition from on to off or off to on there is a large capacitance associated with the gate that has to charge or discharge. If the signal driving the gate does not have a low enough impedance then it can effect how fast the MOSFET can switch and slow switching is associated with high heat dissipation if the device is switching larger currents. In general if driving large loads one wants as much current drive capacity as possible. There are many specific MOSFET gate driver chips that utilize low impedance totem pole outputs to help with this requirement.

Lefty

5. ### Audioguru New Member

Dec 20, 2007
9,411
896
It is a Mosfet, not a JFet.
The DC input current is zero. But it gas a high gate capacitance that takes a lot of current for it to switch on or off quickly.

Look at the datasheet. When the gate voltage is at the threshold then the minimum drain current is only 0.25mA which is nowhere near max. the datasheet says the gate voltage must be 10V for it to turn on completely.

The CD4093 has a dead short output current of about 20mA to begin charging the high gate capacitance of the Mosfet. It will take a fairly long time to fully charge so the Mosfet will turn on and off fairly slowly.
A low value resistor (4.7 ohms to 100 ohms) is connected in series with the gate at the gate pin to stop the Mosfet from oscillating at a VHF frequency.