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Transistor vgs,rds and vgthreshold confusion

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skmdmasud

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Hi.. :)
i am trying to learn basic about reading datasheet of transistors namely FETs. What i dont understand is for lets say IRF540

1. Vgs is rated max 20V but in the Gate Threshold V it says Min 2V and Max 4V but in graph below
vgs.JPG

the drain to source current starts from 4.5Vgs, so whats the threshold all about? Should i apply 20V to make it behave like a closed switch or 4V??

2. In some MOSFET the Rds(on) is 23mΩ and in some it is 0.05 or 0.005Ω. Shouldn't the Rds be very small because its the resistance between the drain to source when operating like a closed switch.

Thanks and Regards
 
Good questions.

Gate threshold is the voltage that the FET just starts to turn on usually measured at only 250 micro amps.
The curve shows the more useful part of the curve where the FET is on but may not have the lowest Rds on yet.
Often there is a spec with the voltage gate to source of 10 volts or so. Above this Rds does not get much lower. This number will be lower in what are called logic level FETs. The 20 volt number is a maximum rating above which damage may occur and should be avoided.
And yes Rds on is small, but different FETs will have different values as it will vary depending on what it is designed to do. For example a high speed one may have a higher Rds on than a slower one.
 
The graph for a Mosfet is for a "typical" device. But there is a fairly wide range of voltages and currents even if they have the same part number then you can get a more sensitive one or a less sensitive one within the wide range.

You cannot ask for a more sensitive Mosfet of a certain part number, you get whatever they have.
 
ok great. can i say from the graph that at 5v it will handle around 30A.
No, because the graph shows a "typical" device. You cannot order a typical one, you get whatever they have which might be much worse.
It is guaranteed to conduct at least 16A when its gate-source voltage is 10V.
 
If you need one that turns on with 5 volts look for one that is called logic level. They often have an L after the part number.
 
Hi,

Yes i agree this is a good question.

There is some mystery to this when you first look at it because they seem to be quoting this and that and it seems to contradict in many places.

The relation of the gate voltage to the output current and resistance is not an exact specification. It varies between devices of the same part number and is affected by temperature. That means you could never design a circuit that takes an exact input voltage and passes a given current and it stays like that forever after. What you can do however is make sure your drive is beyond anything that could ever be needed, and that helps to ensure that you get the output you need.

For most MOSFET's the input range is from 0 to 20 volts. The 'turn on' voltage is somewhere around 4.5 volts, and that's where a typical MOSFET starts to turn on. It does not mean that every MOSFET you buy and hook up and test will turn on at 4.5 volts however, because it is just a characteristic specification not an absolute specification. It's characteristic because there are other types of lower voltage MOSFET's that will start to turn on at 1.5 volts, and this type is usually referred to as a 'logic' level MOSFET. That type will be fully turned on at 5v input or even less, while the 'normal' MOSFET might require 12v to be fully turned on.

The temperature has a lot to do with it too. If you want to make sure the MOSFET is fully turned on you have to feed it a high enough gate voltage to ENSURE that any specification of the input is met. If it starts to turn on between 3v and 5v, then that just means that the resistance at the output STARTS to decrease, so you have to feed it a much higher input to get it to turn on fully. 12v will usually turn it on fully but check the data sheet. You should be able to find a spec that tells you that the resistance will low at some input level, and going over that level means you should be getting the full turn on.

The logic level MOSFET's are used in lower voltage circuits where the drive signals can not be as high. These come in very low Rds types too.
 
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