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Author Topic: Burned FET  (Read 1050 times)
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Hello,

i made circuit for turning LED reflectors with FET (IRL540). I connected 4 LED reflectors with 40W of common power consumption. Power supply is a little bit more than 12V. First circuit worked without problems for a while but after about an hour FET was totaly burned.

Now i am wondering  why (which information in datasheet i missed).

Here's circuit diagram..


* scheme.png (92.22 KB, 979x660 - viewed 28 times.)
« Last Edit: May 04, 2013, 08:52:53 am by luxy » Logged

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Turning LED reflectors.
Is that with a motor?
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That's almost a watt across the MOSFET.  A TO220 package can usually handle a watt or two with lots of cool air flow.  Was the MOSFET covered up or kept from free airflow?  You might try a little heat sink on it to help it out and make sure it has sufficient airflow.
« Last Edit: May 04, 2013, 09:04:02 am by DirtBiker » Logged

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Turning LED reflectors.
Is that with a motor?

If not with FET, how else?

Now i putted another FET (IRL2505) in the same circuit and after more than hour it's almost completely cold. Without heatsink on it, everything is in completely sealed plastic box.

I saw that maximal Gate-to-Source voltage for IRL2505 is 16V while for IRL540 is 10V. Since i am powering the whole circuit with a little bit more than 12V, this could be problem?
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the IRL2505 has a full on resistance of 0.008 ohm's, the IRL540 is 0.044+ ohm's so there will be more voltage drop. Also full on condition will not happen unless you use 10+V on the gate.
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Turning LED reflectors.
Is that with a motor?

If not with FET, how else?

I asked if it's done with a motor.
Anyway.
Do you have a flyback diode placed across that motor?

> > > And I would add a 10K resistor from IC1_pin15 to Gnd.
« Last Edit: May 04, 2013, 09:30:23 am by Runaway Pancake » Logged

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i made circuit for turning LED reflectors with FET (IRL540).

I think i don't need flyback diode if i am turning LED reflector. Or am i wrong?
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If it's a motor then you need the diode.
I'm not going to argue.

About "how else" -- http://en.wikipedia.org/wiki/Mirror_galvanometer
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All inductive loads need a back e.m.f. diode across the load end of. motors in particular as not only do you have the back e.m.f. from power stored in the inductor you also have the momentum of the motor that keeps going for a bit supplying power to the back e.m.f.
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Your Gate to Source voltage will be only 5V since that is as high as the output pin of the micro goes, no need to worry about that.  Now, whether 5V is enough is another question.  I would think that 150R resistor is slowing down the gate charging making the heat problem worse.
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if it is an on/off cycle that is not happening many times a second then the resistor is not a problem, if it is something like pwm then yes you need that gate to swing as fast as possible (drivers become a good idea). I never really get this mania over resistors, there are alteady internal resistors in the MCU's
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Switching speed is not a problem because i am turning fet on or off only one time per day. I changed it to IRL2505 with lower Rds and now it's ok. I know that i need diode for inductive loads but in my case i am driving only led reflectors.

Thank you for quick response and helpful comments.
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Hi,

By "turning LED reflectorsr" do you mean you're only turning the LED light on/off or are you rotating the reflector with a motor?  I think there's some confusion here on that definition.

Also, as a side note since you're using FTDI for programming I'd normally put an 0.1uF ceramic cap between the programming header and the reset pin on the ATmega - as per Nick Gammon's explanation (toward the bottom of that page) - not sure if that part is working for you as it currently is.

Geoff
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Maybe it's like a lighthouse.
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By "turning LED reflectorsr" do you mean you're only turning the LED light on/off or are you rotating the reflector with a motor?

With LED reflector i mean only LED light. In our country we say reflector meaning light in case for mounting and rotary light for the one with motor smiley
To be exact, i am using that kind of LED light - LED reflector, without any motor --> http://www.ledlampcn.com/3-led-floodlight/1-1d.jpg

Quote
Also, as a side note since you're using FTDI for programming I'd normally put an 0.1uF ceramic cap between the programming header and the reset pin on the ATmega

It works ok without ceramic cap but i will put them in.. Thanks for advice!
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