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I am using the IRF540N. Its gate to source threshold voltage is 2v - 4V (VGS = VDS, ID = 250µA.)

Does "gate to source threshold" mean I need to apply voltage above 4V to the gate?

How is Id = 250uA related to the threshold voltage?

There is one more drain current in datasheet given a 33A. In what situation do we need to use this?

JRE
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  • As you noticed the gate threshold is specified for minimal current flow. You'll need quite a bit more to pass useful currents through a power FET. Hopefully the data sheet has a graph of gate voltage versus current, or gate voltage vs. channel losses or resistance or similar. – Chris Stratton Apr 30 '19 at 04:16
  • I've modified your question somewhat - it was very difficult to understand. Please read through it and let me know if it says what you intended. – JRE Apr 30 '19 at 11:00

1 Answers1

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Gate to source threshold means, i need to apply voltage above 4V to gate?

Yes.
The gate threshold voltage is the voltage range (the spread in production devices) in supplied devices that will result in 250uA of current flowing from drain to source. Since this is not useful in most applications you need to apply more than 4V to ensure that all devices you might buy will turn on.

How much more voltage (V(GS)) you need depends on the current flow required in your application.
In the specification of the IRF540 you would probably want the lowest possible RDS(on) to give the lowest V(DS) and lowest power loss in the device. For example if you were using the device as a switch.

enter image description here

In the IRDF540 datasheet it is clear that you would use V(GS) of about 10V to ensure you could get 15-27A.
Again from the datasheet you could view the Typical Characteristics graph and deduce the typical I(DS) and V(DS) you might expect for a given V(GS):

enter image description here

Notice in this graph, if your load current was only 10A, then you could use around 6V V(GS) and typically expect this to work. But it's up to you to understand the V(GS) voltage range of your supplied devices. That is why in many designs the manufacturer may buy devices with a more defined VGS(th) to get better performance at a lower V(GS).

NOTE: In the datasheet I showed here, the IRF540 is not capable of 33A continuous, so you must be reading data supplied by some other device manufacturer.

Jack Creasey
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