how big of a heat sink should i get?

Question

im following plans off this site:

http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/step6/The-new-stuff-Constant-Current-Source-1/
(step 6)

and i am planning on using the same nfet as instructed as my electronics isnt up to par to deviate too much, i would like to run Q2 (Fairchild FQP50N06L) nfet with about 3 watts of power (2.8w to be exact). how big of a heat sink should i get for them to run safely? i was looking at some off ebay

http://www.ebay.com/itm/5pcs-21x15x10mm-IC-Aluminum-Heat-Sink-With-Needle-TO-220-Mosfet-Transistors-H1-/200800490404?pt=LH_DefaultDomain_0&hash=item2ec0a453a4

would those be enough for the nfets or should i look for bigger?

Answer 1

Here is a better place to get heatsinks than Ebay:



https://www.jameco.com/webapp/wcs/stores…

Answer 2

These are not specified, so just what they do is unknown. The rating of a heat sink is in watts per degree C. Thus you get the temperature rise for a given power.



A reasonable junction temperature (inside the transistor) may be 125°C (max is 175°C).

The thermal resistance of the device itself, junction to heat sink is 1.74°C/W, call it 2°C/W.

If the power dissipate by tghe transistor is 3W and the air temperature is 50°C and the junction is 125°C so the allowed temperature rise becomes 75°C. The thermal resistance junction to the air is 75°C/3W = 25°C per watt. Thus a heat sink like 23°C/W or less will do the job. These heat sinks are more than enough, just looking at them.



The links below show examples of heat sinks around 20°C/W. Consider these as minimum for 3W.



Power calculation:

Power_watts = (maximum supply voltage - Vf_all_leds - 0.7V) x current in amps

Answer 3

You need to know the thermal resistance of the heat sink and the safe operating temperature of the FET case. The temperature will be the power dissipated divided by the total thermal resistance, plus the ambient temperature. So if we allow the transistor to go to 75C that's about 50C above ambient, so the total thermal resistance has to be less than 50/3 degrees per watt. Rtjc for the FET is 1.24 so the heat sink has to have a thermal resistance less than about 15 deg/watt.

The thermal resistance of the heat sink is a lot more than the Rtjc and a small heat sink like this will probably be PCB mounted, so there is no need for grease or isolation mounting.

You need to watch the power supply voltage. Every extra volt will add 0.75A *1Volt = 0.75 Watts to the transistor heat. If your power supply is too high, consider adding LEDS until the transistor voltage is minimized.

Answer 4

That heat sink should ber fine. Be sure to use a washer and thermal compound.