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Author Topic: does a irf520 mosfet need to be heatsinked?  (Read 1477 times)
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i want to run a 12 volt 2amp led strip tape off of it for dimming. i noticed it got a little hot after a few minutes.

Thank you
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Based on the datasheet (http://www.vishay.com/docs/91017/91017.pdf), the maximum case temperature at 2Amps (Fig 9, Page 5) looks to be just over 160C.

If you have a way to measure the case temperature, you can tell if you're driving the MOSFET too hard.
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If you are driving the mosfet from an Arduino pin (perhaps with a series resistor), the your main problem is that the IRF520 is not a logic level mosfet, instead it is designed for 10V gate drive.
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i want to run a 12 volt 2amp led strip tape off of it for dimming. i noticed it got a little hot after a few minutes.

Thank you

If it gets hot, it needs a heatsink. Regardless of what a spec sheet says, a good rule of thumb for any electronics is "if it's too hot to touch - it's too hot".

Some chips are spec'd to Tmax=150C - are you kidding me? it won't last long at that temperature.

Remember - too hot to touch == too hot. Period.
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i want to run a 12 volt 2amp led strip tape off of it for dimming. i noticed it got a little hot after a few minutes.

I always use the "rule of thumb":

If you put your thumb on the metal tab and it leaves a little white ring where the hole is, it needs a heatsink.
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ditto, heat sink it, for direct driving from an mcu (arduino or otherwise) you want logic level mosfets like IRL540, IRL530 or IRL520

remember the gate threshold voltage is the voltage that will start to make the mosfet conduct, not become fully conductive, even logic level parts will work with a gate voltage up to 10V
« Last Edit: April 01, 2013, 04:25:43 pm by sparkylabs » Logged


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You can simply calculate the power dissipation from the current and the Rds(on) figure, then plug in the degrees C per watt
value for the device package (including any heatsink) and calculate the rise above ambient.  Keep it low if you can.  For a
TO220 package without heatsink anything over 0.5W will be too much really.  You're running at 1W.

Using an IRF520 wasn't a good start, it has a huge(*) Rds(on) figure of 270mOhms, and is rated at 100V.  A 30V device with <10mOhm
Rds(on) would be much more sensible option for that load, absolutely no heatsinking needed and a voltage drop of ~0.02V rather
than 0.54V

(*) huge by modern standards, its probably a 30 year old design though.
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even an IRL540N has an Rsd on of 44mR and will take 100V
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While too hot to touch is too hot is an old adage nowerdays it is not true. Many modern chips run very hot. A heat sink will help but the heat sink can itself be too hot to touch.

However the OPs problem is almost certainly due to the fact that they are not turning it on fully.
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Modern chips do run hot some of them and I've reused mosfets that I've practically burnt my fingers on but really if you are going to use it for a period of time running like that a heatsink would not hurt.
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im using a POT on an analog pin and PWM from a digital pin on my arduino to drive the gate pin. its not hot enough to burn my hand but i didn't let it run for more than 5-10 minutes. i also noticed that voltage drop issue in the brightness of my led strip. sorry about the delayed response. i used an irf520 because the getting started with arduino book showed it for driving things such as small motors. the wiring is my arduino pin connected to pin 1 of the mosfet, led strip ground connected to pin 2, ground connected to pin 3, and i'm not sure why but according to the book, a diode connected across the main 12volt power to mosfet pin 2. What would be a good mosfet to use instead with less of a power drop? i do not know how to make much sense of mosfet data sheets
« Last Edit: April 09, 2013, 02:47:21 pm by wikitjuggla » Logged

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I know the frequency is low but the arduino output will not exactly be a low impedence signal source. The gate does have capacitance that take time to charge and during that time the switching over is a "slow" change in impendence rather than a snap one. Using a mosfet drive may be a good idea, this would also allow you to drive the gate at a higher voltage to help maximise switching speed and make sure the "on" resistance is as low as possible. Of if your worried just put a little heat sink in.

If you have an oscilloscope put put it across the load or the drain and source and see how long the transition from low to high and high to low take ("zoom" right in on the time scale). Nothing is ever truly instantaneous, only "for the practical purpose at hand"
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i used an irf520 because the getting started with arduino book showed it for driving things such as small motors. the wiring is my arduino pin connected to pin 1 of the mosfet, led strip ground connected to pin 2, ground connected to pin 3, and i'm not sure why but according to the book, a diode connected across the main 12volt power to mosfet pin 2.

That diode is needed when driving a motor, solenoid or other inductive load. It's not needed (but harmless) when driving LEDs.

What would be a good mosfet to use instead with less of a power drop? i do not know how to make much sense of mosfet data sheets

Here are a few suggestions:

IRLU8726PbF (very low Rds(on), but it's in a small package so hard to attach a heatsink)
IRLZ44
STP40NF10L
RFP30N06LE


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also there are in the same series as the 'L520 there are the IRL530 and IRL540, parts ending with "N" are better variants of the same.
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