Shop OBEX P1 Docs P2 Docs Learn Events
Heatsinking — Parallax Forums

Heatsinking

LightfootLightfoot Posts: 228
edited 2006-11-25 09:40 in General Discussion
I have a setup where a TO-220 power·mosfet transistor can carry 3 amps at·12 volts, up to 40 watts (36 to be exact).· What kind of heatsink should I use?· The FET is mounted on a PC board.

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔

Comments

  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-11-24 15:05
    Well, we covered·this when you were Three of Them.

    You've, doubtless, seen the sort that have the fins on them -- won't be enough (no way.)· What you need is a considerable piece of metal.· The fin-type might help, if placed between the device and the metal/mass.

    You could try and just use the black-anodized piece screwed on and see how that goes, or nothing at all; it's not as though lives are at stake.· [noparse][[/noparse]Right?]· What's the worse thing that could happen (buying another transistor)?· For that matter, it could get hotter than heck and, for your intents and purposes,·"work."
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-11-24 15:48
    Just try to hold on to a 40 watt light bulb and you will get an idea of how much heat is involved.

    If you want it to be cool to the touch, quite large.

    The specification sheet should tell you want the thermal absolute limit is.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "If you want more fiber, eat the package.· Not enough?· Eat the manual."········
    ···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan
  • stamptrolstamptrol Posts: 1,731
    edited 2006-11-24 15:58
    Gents,

    Assuming the "power mosfet" is rated at 3 amps and 12 volts, there is no conceivable way for you to get the device to dissipate 40 watts within those ratings.
    I suspect the 36 watts is being described as the load that the particular mosfet can control.

    A power mosfet when "on" typically has a D-S voltage drop less than one volt, at full load of 3 amps, that is 3 watts dissipation. Similarly, if the "on" voltage really were 12 volts, you're really not dealling with a proper power mosfet.

    Cheers

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Tom Sisk

    http://www.siskconsult.com
    ·
  • LightfootLightfoot Posts: 228
    edited 2006-11-24 20:03
    What should I look for in the datasheet to determine the type of thermal management needed?·

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-11-24 20:05
    It's hard to know exactly what a lot of people are fixing to do oftimes.· My advise tend to worst case outcomes.· However, losing a transistor, where I'm sitting, isn't the end of the world.·

    I think that what anyone would/should take away from reading my previous post would be: give it a shot and find out what happens.· You can't reach for the gold ring if both your hands are shielding your stones.

    Post Edit:· If it "works", then fine.· If it doesn't "work", then Stop.

    Man -- Doc, it hurts when I do this.

    Doc -- Then don't do that.


    Post Edited (PJ Allen) : 11/24/2006 8:09:06 PM GMT
  • stamptrolstamptrol Posts: 1,731
    edited 2006-11-24 20:43
    Lightfoot,

    Look in the specs for Rdson, that is the resistance when conducting. Knowing that, take your load current, square it , and multiply by Rdson. That is the power you're dissipating. I think you'll find it to be very small. You may even find in the spec a note as to power dissipation capability in the un-heatsinked mode.

    Also, take PJ Allen's advice........Try it. The old techs used to say if the MOSFET is not hot enough to melt solder, its probably OK. Well, realistically, you should be able to hold your finger on it when operating (something around 110 to 130 deg F)

    Good luck.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Tom Sisk

    http://www.siskconsult.com
    ·
  • LightfootLightfoot Posts: 228
    edited 2006-11-25 00:10
    PJ Allen is right, I connected a 30 ohm resistor on the drain of the fet.· The load is driven from a 3 amp current transformer and fet did not get hot.· Oh boy that 5 watt wire wound resistor was smokin'!

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
  • PJAllenPJAllen Banned Posts: 5,065
    edited 2006-11-25 00:42
    Encouraging, I guess.· But, what's the supply?· If it's 12V and you have a 30-ohm resistor (load) then your draw is only 400mA, but it is 5W.

    [noparse][[/noparse]Just asking because you originally mentioned 12V, 3A.]
  • LightfootLightfoot Posts: 228
    edited 2006-11-25 00:52
    Yeah, just seeing if it would self-destruct at a low current.· Now to try a 4 ohmer.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2006-11-25 09:40
    When you say you want to sink 40 watts, just consider that I usually use a 30 watt soldering iron.
    Generally there is a watt rating in the fact sheet. If you go above that, there is just too much heat and destruction.

    While you may calculate that you want 40 watts, the device may sink only 5 or 10 watts. You have to put more FETs in parallel.· Often the voltage and amp ratings are maximums for duty cycles of far less than 100%.· So, when you add the two together.... smoke appears.

    Look at the wattage rating.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    "If you want more fiber, eat the package.· Not enough?· Eat the manual."········
    ···················· Tropical regards,····· G. Herzog [noparse][[/noparse]·黃鶴 ]·in Taiwan

    Post Edited (Kramer) : 11/25/2006 9:44:11 AM GMT
Sign In or Register to comment.