Arduino Forum

Using Arduino => General Electronics => Topic started by: luxy on May 04, 2013, 03:50 pm

Title: Burned FET
Post by: luxy on May 04, 2013, 03:50 pm
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..
Title: Re: Burned FET
Post by: runaway_pancake on May 04, 2013, 03:55 pm
Turning LED reflectors.
Is that with a motor?
Title: Re: Burned FET
Post by: DirtBiker on May 04, 2013, 04:02 pm
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.
Title: Re: Burned FET
Post by: luxy on May 04, 2013, 04:15 pm

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?
Title: Re: Burned FET
Post by: sparkylabs on May 04, 2013, 04:23 pm
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.
Title: Re: Burned FET
Post by: runaway_pancake on May 04, 2013, 04:28 pm


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.
Title: Re: Burned FET
Post by: luxy on May 04, 2013, 04:49 pm

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?
Title: Re: Burned FET
Post by: runaway_pancake on May 04, 2013, 04:52 pm
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
Title: Re: Burned FET
Post by: sparkylabs on May 04, 2013, 05:07 pm
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.
Title: Re: Burned FET
Post by: afremont on May 04, 2013, 05:21 pm
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.
Title: Re: Burned FET
Post by: sparkylabs on May 04, 2013, 05:31 pm
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
Title: Re: Burned FET
Post by: luxy on May 04, 2013, 10:45 pm
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.
Title: Re: Burned FET
Post by: strykeroz on May 04, 2013, 11:25 pm
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 (http://www.gammon.com.au/forum/?id=11637) (toward the bottom of that page) - not sure if that part is working for you as it currently is.

Geoff
Title: Re: Burned FET
Post by: runaway_pancake on May 04, 2013, 11:45 pm
Maybe it's like a lighthouse.
Title: Re: Burned FET
Post by: luxy on May 05, 2013, 08:46 am

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 :)
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!
Title: Re: Burned FET
Post by: runaway_pancake on May 05, 2013, 05:14 pm


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 :)
To be exact, i am using that kind of LED light - LED reflector, without any motor -->


There's a difference between "turning a reflector" and "turning on a reflector" (or "turning a reflector on"), even in your country - isn't there?
Title: Re: Burned FET
Post by: DirtBiker on May 05, 2013, 07:17 pm

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.



You should not have a problem with the IRL2505.  The dissipation would be around 100mw.
Title: Re: Burned FET
Post by: DirtBiker on May 05, 2013, 07:32 pm

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?


No, the problem was you were dissipating about 800mW with the IRL540.  Since it was in a sealed plastic box, this allowed it to get hot.  Too hot.  Once it get's hot enough the Rds goes up rapidly, increasing the dissipation and making it even hotter.   Thermal runaway ensues and the device burns up.  This can happen really quickly.
Title: Re: Burned FET
Post by: sonnyyu on May 05, 2013, 08:03 pm
There is huge argument about MOSFET thermal runaway:-

BJTs suffer from a property known as "thermal runaway." Thermal runaway happens because the conductivity of a BJT increases with temperature. Because transistors tend to heat up in proportion to current flowing through them this means that the conductivity and temperature of BJTs can increase exponentially. This can damage the BJT and makes designing circuits for BJTs more difficult. MOSFETs do not suffer from thermal runaway.

thermal runaway, ehow.com (http://www.ehow.com/info_8756672_advantages-mosfet-over-bjt.html)

Power MOSFETs typically increase their on-resistance with temperature. Under some circumstances, power dissipated in this resistance causes more heating of the junction, which further increases the junction temperature, in a positive feedback loop.

thermal runaway, wiki (http://en.wikipedia.org/wiki/Thermal_runaway)

Here we need an expert to show us a light.  

@OP, FET is normally referring as JFET. Your title "Burned FET" means "Burned Power JFET" to me and make me click it rightway. 8)




Title: Re: Burned FET
Post by: DirtBiker on May 05, 2013, 09:12 pm
@sonnyyu,

I agree.  It is difficult to cause a power MOSFET to go into thermal runaway, under normal conditions, that is.  But operating a TO220 MOSFET at near a watt where it is not a major source of resistance in the circuit and providing no cooling at all is one way this can be achieved.

Looking at the normalized Rds(on) vs. temperature curve for an IRL540 and taking it piece by piece shows us that at 850mw, the temperature of the 540 will want to rise by 52 degrees in free air at an ambient of 25 degrees.  At 77 degrees (25 + 52) the Rds will rise by a factor 30%, increasing the dissipation to 1.1w, causing the temperature to rise to 93, causing the dissipation to go to 1.4W, causing the temp to rise to 111, causing....The curve actually just gets steeper and steeper.

And all that assumes free air with a fixed ambient temperature of 25.  Sealed in a plastic box, this will happen much quicker as the ambient temperature will rise, being heated by the MOSFET.

The self-stabilizing effect becomes prominent when the MOSFET itself is a major contributor to the circuit's overall resistance.  Then a 1.5 factor increase in Rds will reduce the overall current dramatically and actually lower the dissipation of the device.

The fact that the load in this case is the major contributor to the circuits resistance makes the matter worse since the IRL540 going from .08 ohms to .12 ohms will have little effect on the drain-source current, but a large effect on the power dissipated by the device.

Actually, the load here is quite likely going to be a constant current load too.
Title: Re: Burned FET
Post by: sparkylabs on May 05, 2013, 09:47 pm
Thermal runaway for BJT's is a big problem when they are paralleled (hence the emitter resistors in high current power linear supplies) because in getting hotter one will prevail in low resistance (due to manufacturing tolerances) and start to take more current so getting hotter so taking more current and so on.

mosfets don't suffer in this way so are easier to parallel
Title: Re: Burned FET
Post by: luxy on May 06, 2013, 11:43 am
I calculated power dissipation too late, after everything was burned :D


There's a difference between "turning a reflector" and "turning on a reflector" (or "turning a reflector on"), even in your country - isn't there?


You're right, that's happen when you're translating literally ;)