mosfet irf840 heating in simple LED driver

[onoffman]

Hi all ,
i picked this one circuit to drive

three smd 5050 led modules , current is set at 180mA

with formula Rs=0.7/I
Rs =3.9E


i made Rs with 15E and 5.6E parallel

PROBLEM : the MOSFET IRF840 heat al little nicely after 4 hours ,
i want to keep it cool w/o any heat sink

But IRF 840 can handle more is not it

 

[umesh49]

you can put your resistor in series with LED and ground the source. It should help you.
The topology you are using may not let the FET turned ON fully and having a finite resistance.

 

[Audioguru]

Heat= the current in the Mosfet times the voltage across the Mosfet. It the datasheet for the IRF840 it shows that without a heatsink it is very hot with a power dissipation of only 1W.
Then with your 180mA of current the voltage across it should not be more than about 1000mW/180mA= 5.6V. The LED voltage is determined by its color but you did not say its color.

EDIT: These are tiny little surface mount LEDs that are usually operated at only 20mA. Your 180mA will blow them up!

 

[D.A.(Tony)Stewart]

5050 LEDs can handle 30mA or 110mW at 25'C derated by -1.2 'C/mW

The voltage rises from 2.8 at low current up to 3.8V in some cases due to bulk ESR leaving too much voltage drop across the FET.

If these LED's are 3.3V typ at 20mA , you must choose a string of three (3) LEDs for a 12V supply with a 2Vmin~3Vmax drop across the transistor resistor pair.

If these are 3.3V parts a string of 3 yields 10V leaving 2V for the current sink.

If you are lucky, as I am , I get 3.0V power LEDs at a consistent voltage+/-0.1 so I can drive a string of 4 in series directly to 12.0V.

Remember to read the LED & MOSFET specs.

Use OHm's Law for VI drop and apply Ohms Law to Thermal resistance as well. where temperature rise is W * deg 'C/W ( thermal resistance)

 

[onoffman]

smd5050 has 3 leds inside..per module current is 60mA white ones ..3 means 180 no blowing may be yes suppy has to be trimmed i guess @Audioguru

- - - Updated - - -

my light is so versatile that it takes any voltage 4-15v due to ccs -constant current source

ccs with mosfet balances the dropping with 4ohm sens resistor so fully resistor is NOT
bearing full current the npn in getting on /off and controlling fet


tiny 5050 has three guys inside so per module 60ma is good as they are in parallel
series will only do work in 12v..my light takes the cell powerbank which makes it
easy to work after power cut

As audio guru said vcc has to go low ..but i want some trick so that i dont add resistive dropper

5050 LEDs can handle 30mA or 110mW at 25'C derated by -1.2 'C/mW

The voltage rises from 2.8 at low current up to 3.8V in some cases due to bulk ESR leaving too much voltage drop across the FET.

If these LED's are 3.3V typ at 20mA , you must choose a string of three (3) LEDs for a 12V supply with a 2Vmin~3Vmax drop across the transistor resistor pair.

If these are 3.3V parts a string of 3 yields 10V leaving 2V for the current sink.

If you are lucky, as I am , I get 3.0V power LEDs at a consistent voltage+/-0.1 so I can drive a string of 4 in series directly to 12.0V.

Remember to read the LED & MOSFET specs.

Use OHm's Law for VI drop and apply Ohms Law to Thermal resistance as well. where temperature rise is W * deg 'C/W ( thermal resistance)

 

[D.A.(Tony)Stewart]

If you want MOSFET to work cool with CC cct. choose voltage with less than 1 LED drop for CC MOSFET or resistor.

Using 3.5V for this small chip and using 12V that is 3 LEDs in series. NOT parallel.

12V on a 3.5V LED is a 8.5V drop which is a bad for heat and waste of power.

 

[onoffman]

If you want MOSFET to work cool with CC cct. choose voltage with less than 1 LED drop for CC MOSFET or resistor.

Using 3.5V for this small chip and using 12V that is 3 LEDs in series. NOT parallel.

12V on a 3.5V LED is a 8.5V drop which is a bad for heat and waste of power.

Excellent help yes true .. very much apreciated.
But my light has to do well in 5v supply how to achieve that.

probably keeping extra row of leds that should turn on if supply is >8V other wise act like a short for low voltages.
with tiny fet switch or any other way?

 

[Audioguru]

I found the datasheet for a Synergy SMD5050 LED that has three separate LEDs on it. Each LED has a forward voltage range from 3.0V to 3.4V so maybe they cannot be connected in parallel.
The datasheet is confusing because it does not say that the three LED are matched and can be connected in parallel or cannot be.

The datasheet is also confusing since it lists a maximum current of 20mA and 60mA. The datasheet does not say that the maximum current for one of the LEDs is 20mA and is 60mA for all three lighted at the same time.

I think the three LEDs should be connected in series and in series with something that limits their current to 20mA or less. Then their forward voltage will be 9.0V to 10.2V so each group of three LEDs needs to have its own current limiting circuit.

 

[onoffman]

Excellent help yes true .. very much apreciated.
But my light has to do well in 5v supply how to achieve that.

probably keeping extra row of leds that should turn on if supply is >8V other wise act like a short for low voltages.
with tiny fet switch or any other way?

use of this chip BP1360 30V/500mA Led driver may suit the need .waht say ?
costs .2$ but has PWM.

- - - Updated - - -

I found the datasheet for a Synergy SMD5050 LED that has three separate LEDs on it. Each LED has a forward voltage range from 3.0V to 3.4V so maybe they cannot be connected in parallel.
The datasheet is confusing because it does not say that the three LED are matched and can be connected in parallel or cannot be.

The datasheet is also confusing since it lists a maximum current of 20mA and 60mA. The datasheet does not say that the maximum current for one of the LEDs is 20mA and is 60mA for all three lighted at the same time.

I think the three LEDs should be connected in series and in series with something that limits their current to 20mA or less. Then their forward voltage will be 9.0V to 10.2V so each group of three LEDs needs to have its own current limiting circuit.

http://chioszrobots.files.wordpress.com/2014/03/ws2812.pdf
Some more place ihave seen that the SMD5050 gets a 20mA safecurrent for RGB types .
May be the white cal do 30mA .I can fry one to test that .

I will prefer to avoid IC as ordering is long process.

 

[Audioguru]

Your link shows a 5050 RGB chip with a smart chip driving it, all on one module. The smart chip is controlled with data sent to it. It provides 256 steps of brightness probably with pWM.

 

[onoffman]

Your link shows a 5050 RGB chip with a smart chip driving it, all on one module. The smart chip is controlled with data sent to it. It provides 256 steps of brightness probably with pWM.

link included for the current data ..

3leds in series works ! sad part is it is only usable at 9-12+

the parallel design gets it running from 5V powerbank .

Can we do Some trick like zerner at GATE to make the guy work on 12V , and remain cool ?

 

[D.A.(Tony)Stewart]

Can you be more clear what you want or can /cannot use?

If just driving PWM to RGB 20mA max, there are many solutions with parallel, serial in. You only need 3.5 to 5V depending on FET switch not 12V.

 

[onoffman]

wanted to make a versatile desklamp
wide voltage range , simple parts

my circuit works 5-12V with a little heat getting generated .
12V only version
============
After suggestions
i made circuit with three led in series .
this runs cool .but will not run off mobile power bank!

the guys running from power bank has to make another circuit with parallel LED
this kindof awkward.

The VCO circuit
============
The attached circuit is a VCO it gives as stepped down LED glow at 12V
mainly i designed to make it as low battery indicator .

@12V This is special it shows a fastblink(steady light for eye)
Battery voltage falls 8V it gives a blink (say 3Hz).

with a little trick we can make this maintain same fast blink even down at 5V (50hz)

So that the final circuit does all 5v -12V range with same light .
3 SMD5050 will be driven

 

[Audioguru]

Your simple multivibrator circuit is missing diodes that prevent the base-emitter junctions of the transistors from excessive reverse bias voltage that slowly destroys the transistors when the supply voltage is higher than about +6.8V without a load.