Mosfet Doubts

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I had worked on some mosfets last year. Cannot remember the exact model no.

Now i planning to use Mosfets in some LED driver circuit. The Mosfet im planing to use is the IRFZ44N. It states that the Vgs(threshold) is 4.0V.

Does this mean in can directly interface it with my AVR logic outputs. Or do i require a mosfet driver. I remember last year i had made a driver wherin the gate voltage was above 8volts for a valid logic high.

Regards

Rodney

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You need to read the data sheet carefully. You might find that the MOSFET won't pass enough current for your application with 4V on the gate, and you need a lot more.

Leon

Leon Heller G1HSM

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Thats what i thought. But they have specified the MAX Vgs(threshold) as 4.0 volts

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It will pass 250 uA at 4V and 100A at 9V!

Leon

Leon Heller G1HSM

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The datasheet information states, that the threshold-voltage is maximal 4V. That spech does not
limit the Gate voltage you apply. The gate voltage is
limited by +/- 20V.

You have to look at the "output characteristic"
diagram to see how much current you get
at which gate voltage.

The curve with smalles Vgs relates to Vgs=4.5V,
and at that pooint te MOSFET already can deliver
some amps.

Nevertheless I prefer "Logic Level Mosvets"
in applications where 5 V logic directly
drives a gate.

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Can i know some common logic level mosfets that i could use for my designs.

Here in India, we have to see which Mosfets are available and then design our circuit, Especially for custom jobs where the quantities are very small.

Regrds

Rodney

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I often take a IRLR2705, but that is because it
is easy to get for me.

Look what your supplier can deliver.
Often the Logic-Level types
have an "L" in their part-number, but check the datasheet !

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Depending on the application, it might not be enough just to have enough voltage on your driver. If you are switching high powers you often need a lot of charge moved on to the gate before the internal resistance drops. I did that mistake a few years ago where I had 24 volt to the gate, and that should have given an internal resistance of just a few milliOhms. But since I switched too fast and didn't have enough charge moved onto the gate, I got about 1 Ohm. The MOSFET made a very nice smoke machine. Adding an appropriate gate driver did the trick and it worked ever after that.

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I use the IRLR3303, source grounded (T0-252 surface mount) for switching 6 amperes @ 12 volts, with the gate directly connect to the an AVR output (pullups on). I use a 10K from gate to source for turnoff.

spec: w4.5 Vgs Rdson = 0.045 ohms.

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0.045 ohm, 6 A. 0.045*6²=1.62W. That is going to get real hot unless well cooled.

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Yep, you are correct Agwanll. A properly designed layout/heat sink is required. What is important here however, is that the FET I referenced is of a logic gate design, and in a common source circuit, an AVR output is more than adequate to turn it on (more or less fully).

My point is that a FET driver is not necessarily needed as there are multiple logic level FETs available.

Additional information is required from the OP to determine an actual, suitable FET, to design a properly working circuit.

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The gate of a MOSFET is basically a capacitor; big MOSFETs generally have big gates, so to change the voltage on the gate quickly a large current is needed to minimize the time the FET spends in its linear region which dissipates power.

Now, if you rarely change the state of the FET, say a to control a relay or indicator lamp, it's not a big problem that the switching is not that fast and the current an AVR output pin can deliver is sufficient.

But if you need to switch a load quickly, like in a switching power supply at several hundred KHz, you want to maximize switching speed and that means big currents, up to several amps. Only for a very short time though, and MOSFET drivers can do this well. A push-pull driver made from a PNP and NPN transistor works well too and is cheaper then a dedicated MOSFET driver.

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What jayjay1974 said...

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Absolutely, switching losses need to be considered. But the OP asked...

Quote:
Does this mean in can directly interface it with my AVR logic outputs. Or do i require a mosfet driver.

so without any further information, I say no he does not need a MOSFET driver.

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you could use one of the ON Semi NUD3112 FET switches and use the power supply rail for the power source. Those devices will run off an I/O pin. These are little 4 pin SMDs which are already set up to take a digital input on the FET gate and turn them on or off. They are low cost and I have designed them into a product and I drive them directly from an ATMega128 I/O pin.