# MOSFET driver circuit, when is 5V too low?

Discussion in 'General Electronics Chat' started by Nicholas, Jan 21, 2016.

1. ### Nicholas Thread Starter AAC Fanatic!

Mar 24, 2005
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Hi guys

Yet another question. I hope you can answer this. The MOSFET is an IRL540.

I was given the good advice, on this forum, to insert a diode on the mosfet gat, to protect the microprocessor should the
MOSFET give in or something, and direct 30V back through the gate.

I have made one of my famous drawings(see below). When the diode and the 10K resistor is in place, my 5V goes to
4.48V. Will this ensure complete saturation of a logic level MOSFET?

On a less-important note; if I want to connect an LED + resistor from 5V to gnd (before the diode etc) then my current draw
goes from nothing, to 10-20mA...on a pin that has a maximum of 30mA. Will this not affect saturation of the MOSFET?

Nick

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Modern LED's will have a good brightness if the LED current is >1mA. So increase your led resistor value and Iled around 5mA or lower.
As for the MOSFET we need to know what is the load ?

Mar 24, 2005
121
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Thanks,

Nick

Feb 17, 2009
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5. ### Nicholas Thread Starter AAC Fanatic!

Mar 24, 2005
121
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I would think about 5 amps!

Oct 15, 2009
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Mar 24, 2005
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8. ### crutschow Expert

Mar 14, 2008
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Don't use "typical curves" for a design. They don't indicate worst case characteristics.
However the data sheet does shows that, for a Vgs of 4v, the ON resistance is a maximum of 0.11Ω with a drain current of 14A, so you are fine at 5A.

9. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The power dissipation in the MOSFET is around Ptot = Id^2*Rds(on) = 5A*5A *2*0.077Ω = 3.85 watts. So, the small heatsink will be needed.

10. ### crutschow Expert

Mar 14, 2008
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It's only half that value at 1.92W.
You multiplied by the "2" which is used in that formula as the "squared" indicator.
But even for that dissipation you should use a small heatsink.

11. ### Jony130 AAC Fanatic!

Feb 17, 2009
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This "2" in the formula I use because, as you know the Rds(on) resistance will rise with junction temperature and this "2" includes this effects (more or less).
http://www.vishay.com/docs/91300/91300.pdf (Fig 4)

12. ### crutschow Expert

Mar 14, 2008
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Okay. You just threw that fudge factor of 2 in there with no explanation so I didn't know.
But that's reasonably conservative since, according to the data sheet, the ON resistance doubling occurs at a junction temperature of about 140°C and you likely don't want to operate much above that junction temperature for good reliability.