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# need a little heatsink help

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#### strokedmaro

##### New Member
So I built a new project using two IRF3711 MOSFETs (data sheet attached)...My question is I'm not uber proficient at reading data sheets and want to know if, and in particular how you attach a heat sink to a T0-251AA package. They will dissipate between 5.5 and 8 watts depending on voltage.
Can someone explain the calculations required to figure out what the temperature would be? (12vdc to 13.8vdc x 25ohms)
Thanks!

#### Attachments

• irfr3711zpbf.pdf
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You use the thermal resistances of the various thermal interfaces from the transistor to the heat sink to calculate the transistor junction temperature, which is the critical temperature. (All thermal resistances are in series).

The first page of the data sheet lists the various thermal resistances. For 5.5 to 8 watts you will need a heat sink so the resistance from junction to case of 1.9C/W is used.

You attach the transistor to a heat sink that will keep the junction below the maximum junction temperature. The data sheet lists 175C as the maximum junction temperature, but for best reliability you should try to keep it below 125C.

Then you need to know the maximum ambient temperature the heat sink will see. The heat sinks need good ventilation so that they will see the outside air temperature. Mounting inside a closed box will significantly increase that temperature.

Then you calculate the required heat sink thermal resistance required. The junction temperature is Tj = 8W x (RΘjc + RΘch + RΘh) + Tamb, where RΘch = case to heat sink resistance and RΘh = heat sink resistance to air. RΘch can usually be neglected if you use thermal grease between the case and heat sink.

After calculating the heat sink resistance required, you select a heat sink with thermal resistance equal to or less than this value.

WOW. Tons of information...THANKS! couple of "tack" on questions.... Does this mean that MOSFETs are not suitable for applications that are supposed to be "splashproof/waterproof" because they require good airflow? Why is the drain connected to the "tab" of these MOSFETs? Doesn't that pretty much screw the possiblity of connecting 2 or 3 of them to a metal case for cooling? Thanks for the info!

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If the case needs to be waterproof then you will need some method to get the heat to the outside. You may be able to make the case of aluminum to act as a heatsink and mount the transistors to the case. That will make the case hot, of course.

If you want to connect more than one MOSFET to a single heatsink then you either need to use MOSFETs with isolated tabs, which some have, or use an insulating (mica) washer and non-conducting (nylon) screws to fasten them to the heatsink.

And, of course, if you attach more than one transistor to the heat sink then you have to add the power from all when calculating the heatsink temperature rise.

again, how do you attach a heat sink to a TO-251aa package?

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• TO-251aa.JPG
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Try using a clip similar to this one:

JimB

again, how do you attach a heat sink to a TO-251aa package?
I missed the package design. I glanced at the picture and thought it was a TO-220 type, which has a mounting tab with a hole. There likely are similar transistors to the one you selected in such a case.

5W to 8W in one of those tiny Mosfets will melt it.
Use a real Mosfet that has a case you can bolt to a real heatsink.

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