VGS vs VGS(th) Mosfet confusion

Using Arduino General Electronics
#1

Hello,

I am trying to wrap my head around mosfets and there is something I just dont understand.

I am looking at this mosfet: http://aosmd.com/res/data_sheets/AO4484.pdf

It has a VGS of +/- 20V and a VGS(th) of 1.7-3v.

I understand that VGS(th) needs a max of 3v to open up the gate.

  1. What is the VGS rating of 20v for? What if I exceed 3v on the gate?
  2. What is the difference between VGS and VGS(th)?
  3. Will PWM duty cycle on an Arduino reflect the voltage on the gate? What is the voltage range on the pwm pins on an Arduino?
  4. Can I used this as a logic level mosfet? I want to drive 4 amps at 20v from an arduino nano.

Thank you so much for helping me clarify. Ive been researching for a few hours and im stuck. :stuck_out_tongue:

#2

Look at the RDS(ON) line.

RDS(ON)
Static Drain-Source On-Resistance
VGS = 10V, ID = 10A
TJ=125°C
8.2
10
mΩ
12.5
16
VGS = 4.5V, ID = 8A <———<<<<
10
12.5

For those cold winters night:
https://www.embeddedrelated.com/showarticle/809.php

#3

mmimadi:
I understand that VGS(th) needs a max of 3v to open up the gate.

Wrong. That would be "minimum of ~2.2volt (typical) to start opening the gate".

"Threshold" is the point where the fet just starts/stops conducting.
The gate/source voltage where the fet only can conduct 250uA.
It needs at least two to three times that voltage to fully conduct.

Gate/source voltage should be kept below +20volt (or -20volt), to not damage the fet.
5volt (4.6volt on USB supply) from the Nano should be ok.
Leo..

#4

You also need to check what SECTIONS of the datasheet the parameters are stated in.

Vgs is in "Absolute Maximum Ratings" i.e. exceed these and you will probably kill the device.

Vgs(th) is in "Static Parameters" i.e. working values.

Steve

#5

Wawa:
Wrong. That would be "minimum of ~2.2volt (typical) to start opening the gate".

"Threshold" is the point where the fet just starts/stops conducting.
The gate/source voltage where the fet only can conduct 250uA.
It needs at least two to three times that voltage to fully conduct.

Gate/source voltage should be kept below +20volt (or -20volt), to not damage the fet.
5volt (4.6volt on USB supply) from the Nano should be ok.
Leo..

Thank you, may I ask how you calculated that and came up with that figure?

#6

If you look at figure 5 of the device spec you can see the effect of various gate voltages on the FETs ability to conduct current (i.e. Rds on). 3 volts is the transition area when a small change in Vgs can make a large change in conductivity.

The figure is "typical" but from it you can probably get away with Vgs = 4V and for higher current capability you would need 6 to 8 volts.

#7

No calculation involved really.

Given that you have the FET in a circuit with source to ground and drain to some supply voltage with a load resistor, as you apply increasing voltage to the gate, it will turn on just sufficiently to allow 250 μA to pass (that is, drain current) when you apply a voltage in the range VGS(th) of somewhere between 1.7 and 3 V (usually around 2.2 V) depending on the particular device.

But 250 μA is not generally a useful amount of current if you want it to switch a load. The specification is that if you need it to switch 8 A effectively with a minimum resistance then at a gate voltage (VGS) of 4.5 V, the resistance RDS(ON) will be no more than 12.5 mΩ (milliOhm) which at 8 A would be a voltage drop of 100 mV.

You are warned not to put more than 20 V either negative or positive between the gate and source - VGS.

Leo points out that there is a diode in series with the USB port on the Nano, so if you are powering it from the USB at 5.0 V, then the “5V” pin and the ATmega sees only about 4.6 V. OK, that was two calculations - the diode drop from the USB and the voltage drop across the FET at 8 A. :grinning:

#8

mmimadi:
Hello,

I am trying to wrap my head around mosfets and there is something I just dont understand.

I am looking at this mosfet: http://aosmd.com/res/data_sheets/AO4484.pdf

It has a VGS of +/- 20V and a VGS(th) of 1.7-3v.

I understand that VGS(th) needs a max of 3v to open up the gate.

Wrong. The threshold voltage is where the device switches fully off. In other words below the threshold there is no channel at all (the gate voltage induces a channel between source and drain via electric field).

However to be fully conducting the gate voltage needs to be much higher, so that the channel is fully formed and low resistance. Typically this is at 3 times the threshold voltage.

  1. What is the VGS rating of 20v for? What if I exceed 3v on the gate?

Absolutel maximum rating, exceed it and the device may instantly be destroyed

  1. What is the difference between VGS and VGS(th)?

Vgs simply means the voltage between gate and source, Vgs(thr) is the threshold gate-source voltage

  1. Will PWM duty cycle on an Arduino reflect the voltage on the gate? What is the voltage range on the pwm pins on an Arduino?

No, a PWM signal is a logic signal, its either high or low, nowhere inbetween (well not for more than about a nanosecond during transitions).

  1. Can I used this as a logic level mosfet? I want to drive 4 amps at 20v from an arduino nano.

Definitely not from that Vthr rating. However it does have a Rds(on) rating for Vgs=4.5V, so it definitely is a logic-level FET. I suspect the Vthr value is nonsense, a misprint (they do happen in datasheets!). Its probably more like 1.5V in reality.

Thank you so much for helping me clarify. Ive been researching for a few hours and im stuck. :stuck_out_tongue:

Anyway, ignore the threshold voltage, its not useful. The Rds(on) ratings quote a Vgs value - those are the one's to use for switching. Typically a logic level MOSFET has a 4.5V Vgs entry for Rds(on), and only a 10V one if not logic-level.

#9