MOSFET not conducting even when supplied with specified gate to source voltage

Thread Starter

vaibh_p

Joined Jan 11, 2020
7
Hi , I am learning how to use MOSFET as a switch and than doing pratical on NI Circuit Design simulator.
I have use one N-Channel MOSFET IRFZ34 and one P-Channel MOSFET IRF9530 and the circuit is shown in below to figures.

However I had noticed one strange thing that in datasheet of N_Channel IRFZ34 , the "Gate Source Threshold Voltage" is specified as "2.0V to 4.0V" ,
but when I give 3.3V at "Gate" of N-channel MOSFET IRFZ34, it didn't conduct as shown in figure below and when I had applied 4V , it conducts.

Am I doing some mistake in connection or in understanding the datasheet? or any other problem because as per datasheet "Vgs= 2 to 4V".qw.png4v.png3.3v.png
 

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ericgibbs

Joined Jan 29, 2010
9,124
hi v,
Welcome to AAC
the "Gate Source Threshold Voltage" is specified as "2.0V to 4.0V" ,

Means it can be as low as 2v or as high as 4v, there is a wide spread in MOSFET Threshold voltage.

E
 

Thread Starter

vaibh_p

Joined Jan 11, 2020
7
hi v,
Welcome to AAC
the "Gate Source Threshold Voltage" is specified as "2.0V to 4.0V" ,

Means it can be as low as 2v or as high as 4v, there is a wide spread in MOSFET Threshold voltage.

E
hi ericgibbs,
yes you are right, but that's my question was about. Why didn't the N-Channel MOSFET conduct when i had given sufficient threshold voltgae of 3.3V when the range is(2 to 4V) ?
 

jpanhalt

Joined Jan 18, 2008
8,351
Also note that "threshold" voltage is the Vgs at which the mosfet just begins to conduct. It is not fully on. You need to go to the datasheet and look at drain-source current versus gate voltage (Vgs).

When you say "it didn't conduct," what voltage (Vds) did you test and what current , if any did you observe? That is, for example, were you measuring milliamps or just seeing whether an LED lit.
 

ericgibbs

Joined Jan 29, 2010
9,124
hi,
You are misreading the specification, it means that type of MOSFET could have a threshold of 2V or 4V, or any value between 2v and 4v,,,,not 2v to 4v
They do vary for Vgth due to manufacturing variations.
E
 

Alec_t

Joined Sep 17, 2013
10,616
The datasheet is telling you that, allowing for manufacturing tolerances, you need at least 4V Vgs to be sure that the FET turns on enough to pass a pitiful 250uA, but that you may strike lucky and get away with only 2V Vgs to get that trickle of current.
 

Thread Starter

vaibh_p

Joined Jan 11, 2020
7
The datasheet is telling you that, allowing for manufacturing tolerances, you need at least 4V Vgs to be sure that the FET turns on enough to pass a pitiful 250uA, but that you may strike lucky and get away with only 2V Vgs to get that trickle of current.
Hey Hi
Alec_t
I have attached file , among them in first capture from datasheet , it is specified that Min=2.0V and Max=4.0V , so i thought with 3.3V I can draw average current. Clear me if i am still wrong somewhere.
 

Thread Starter

vaibh_p

Joined Jan 11, 2020
7
hi,
You are misreading the specification, it means that type of MOSFET could have a threshold of 2V or 4V, or any value between 2v and 4v,,,,not 2v to 4v
They do vary for Vgth due to manufacturing variations.
E
No , it is given as for Vgs --> Min=2.0V and Max=4.0V , so I assumed that for 3.3V I can draw sufficient current or enough voltage to bring the MOSFET into conduction. Please clear me out if i am still wrong in some point. Thanks
 

crutschow

Joined Mar 14, 2008
23,814
No , it is given as for Vgs --> Min=2.0V and Max=4.0V , so I assumed that for 3.3V I can draw sufficient current or enough voltage to bring the MOSFET into conduction. Please clear me out if i am still wrong in some point. Thanks
As has been stated, Vgs(ths) can be anywhere from 2V to 4V (it's a manufacturing variation from unit to unit), so the actual value depends upon the particular unit you have.
Thus if you happen to have a unit with a Vgs(ths) of greater than 3.3V you will get no current at 3.3V.
Normally such a MOSFET (not a logic-level type) is specified at a Vgs of 10V to be fully on (look in the data sheet where the on-resistance is specified).
Make sense?
 

Thread Starter

vaibh_p

Joined Jan 11, 2020
7
Also note that "threshold" voltage is the Vgs at which the mosfet just begins to conduct. It is not fully on. You need to go to the datasheet and look at drain-source current versus gate voltage (Vgs).

When you say "it didn't conduct," what voltage (Vds) did you test and what current , if any did you observe? That is, for example, were you measuring milliamps or just seeing whether an LED lit.
Thanks , yes you are correct , in the datasheet , in the graph of Id vs Vgs , the threshold voltage starts from 4V to 10V.
But please help me to understand following points-

- If Vgs is from 4V to 10V , than in datasheet why it is mentioned as Min=2.0V , Typical=Didn't mention anything and Max=4.0V in Normal Specification tab.

- But in graph of Id vs Vgs, Vgs starts from 4V, that means it starts conducting from Vgs=4V, than why it is mentioned as Minimium Vgs=2.0V

Can you please guide me why such two different values are mentioned.
 

jpanhalt

Joined Jan 18, 2008
8,351
That is what everyone has pointed out to you. Start conducting means it will pass very little current. In your simulation, you need to look at the device model and see what that is.
 

Thread Starter

vaibh_p

Joined Jan 11, 2020
7
Thanks to all of you guyies , now I got the actual what is Vgs and tried with 3-4 different MOSFET and got the desired result....Thanks a lot....

and one thing , I am on several other forums and joined AAC today only and now its my best forum ever. I am expecting that my query will be resolved within a day but actually with you guyies it just solved within few hours only , amazing forum , just loved it....happy to be a part of such a great forum....Thank You
 

BobTPH

Joined Jun 5, 2013
2,138
I think of it as the voltage at which the MOSFET is no longer off.

Given a threshold of 2V to 4V, I would use the 2V as the voltage I must stay below to keep it off, and give it a margin of error so maybe 1.5V max.

Bob
 

Thread Starter

vaibh_p

Joined Jan 11, 2020
7
I think of it as the voltage at which the MOSFET is no longer off.

Given a threshold of 2V to 4V, I would use the 2V as the voltage I must stay below to keep it off, and give it a margin of error so maybe 1.5V max.

Bob
yes , initially I also thought same, but doing pratical I had found that it works on 4V though datasheet has given range 0f 2v-4v
 
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