Normally open, normally closed, or normally nothing. A MOSFET question.

Thread Starter

ballsystemlord

Joined Nov 19, 2018
39
Hello,
My problem is a simple one:
I purchased a set of IRFZ44N MOSFETs and I noticed that they are normally nothing. They are open and you have to apply the opposite polarity voltage to close them (turn them off) and the opposite again to open them. The datasheet did not specify either way what there normal state is.
Do you guys have any suggestions how I can hunt down some normally closed MOSFETs?
Is there some pertinent detail that I'm missing?
Thanks!
 

wayneh

Joined Sep 9, 2010
17,153
A key part of using a MOSFET properly is to take control over the gate voltage. As soon as you do that, whatever the gate charge may have been becomes irrelevant.

Your question is like asking where to buy capacitors without any charge on them. Why would you care?
 

ebeowulf17

Joined Aug 12, 2014
3,282
Following up on Wayne's comment, you almost always see a resistor tying the gate pin to the source pin (pull down resistor in this case, pull up resistor in some other configurations.) This resistor guarantees that the the gate pin is always at a known voltage and that the MOSFET is inactive, unless you're deliberately driving the gate with some other signal. The gate should never be left "floating."

upload_2018-11-19_21-28-26.gif
 
Datasheet: https://www.infineon.com/dgdl/irfz44n.pdf?fileId=5546d462533600a40153563b3575220b

You have to look INTO the datasheet. The graphs provide useful info,

Vgs(leakage) - You have to provide a path for this current. Hence the resistor to ground in many circuits.

Vgs(threshold) - Thats when the thing STARTS to conduct. LOGIC FETS like rto turn fully on at normal logic levels.

Rds(ON) = pretty much the minimum on resistance you can get. Note that it's specified at
VGS= 10V, ID= 25A. Then note in Fig 3, you can see Id start to level off. This basically tells yu that you need >10V (Vgs) to use this as a switch.
No more than 20 V. +-20 V can be applied to the gate.
 

shortbus

Joined Sep 30, 2009
8,797
I purchased a set of IRFZ44N MOSFETs and I noticed that they are normally nothing. They are open and you have to apply the opposite polarity voltage to close them (turn them off) and the opposite again to open them.
Sounds like your only using the Drain and Source of the mosfet. They conduct one way and not the other because of a built in diode. Like the others have said you need to use the gate, not just on that particular mosfet number, but all mosfets.

Also be aware that the mounting tab needs to be insulated from anything else in the circuit, the drain pin and the mounting tab are one and the same. Without insulation it could be a direct short.
 

crutschow

Joined Mar 14, 2008
27,701
They are open and you have to apply the opposite polarity voltage to close them (turn them off) and the opposite again to open them. The datasheet did not specify either way what there normal state is.
Do you guys have any suggestions how I can hunt down some normally closed MOSFETs?
I would interpret "close" as conducting (ON), not off.
Normally closed MOSFETS (ON with 0V gate to source voltage or depletion-mode) are uncommon.
Most MOSFETS are OFF with Vgs = 0V. A Vgs voltage must be applied (enhancement-mode) to turn them ON (positive for N-MOSFETs and negative for P-MOSFETs).

The data sheet will tell you the Vgs voltages required to turn on the MOSFET.
 
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