Heatsinking surface mount high power MOSFETS

** What do you think the problem is ?

.... Phil

The usual problem ... phsssst ... *POOF* :-)

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"Joerg the Jerkoff "

** Piss off - FUCKWIT !

Solder it to a water-cooled copper block. And derate it 3:1.

John

PCBs aren't great heatsinks. One can bolt a thru hole TO-220 or similar package directly to a real heatsink. I fail to see how the surface mount variant can realize the same power specs under all operating conditions.

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What are high power MOSFETs in DPAK and D2PAK packages good for when one can get the TO-220 equivalent? Why pay for capability that you will never realize? I can see these packages being useful in low power applications where the pad on the PCB is a sufficient heatsink and the board consists of mainly SMT components.

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"Phil Allison"

PCBs aren't great heatsinks.

** Who says you have to use the PCB ??

One can bolt a thru hole TO-220 or similar package directly to a real heatsink.

** One can CLAMP any flat object onto a flat heatsink.

One way would be to cut a rectangular hole in the PCB under the device and use a piece of flat copper to couple the heat through to a large aluminium heatsink underneath. All mating surfaces smeared with heatsink compound of course.

Clamping is left to your imagination.

I fail to see how the surface mount variant can realize the same power specs under all operating conditions.

** Same power specs as what ??

BTW

Do not mangle posts by inverting the order of the names.

.... Phil

Maximum power is 300W at Tc of 25C, right?

So, find some way to hold the case temperature to 25C (or less) at the maximum allowable ambient temperature, while dissipating 300W, or derate it below 300W for a higher case temperature.

Or use a MOSFET with better thermal design such as this one:

..which will (according to the data sheet) handle 300W with a case temperature of >80°C-- which might even be practical with a metal-core PCB and a fat heatsink and limited maximum Ta.

Best regards, Spehro Pefhany

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Because, you aren't paying for the power dissipation. Power ratings are lies anyway, and the manufacturers admit it. Either IR or Fairchild obtains the measurement by submerging the device in a nucleated boiling liquid, like stirred freon, so it will it boil at the slightest increase in temperature, carrying away a tremendous amount of power. This is BS for three reasons:

1., the case (plastic) generally isn't cooled; 2., the leads are infinite heatsinks; 3., the heatsink tab itself is never so well thermally coupled.

What you *are* paying for is a really low Rds(on). You can't ever use the thing at more than 1/3 - 1/4 times rated current, but by doing so, you get awesome efficiency.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

Look at through-hole devices. They are characterized at 25°C surface temperature as well. You would need an infinite heatsink for reaching this value at room temperature. So any engineer knows how to derate the datasheet value to his particular setup. The same is true for max. current ratings. Those figures are just for reference. Usually ST gives you also another value in the datasheet with practically obtainable values, you only refer to a package, which device do you have in mind? ciao Ban

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If they spec a thetaJA, how is this a lie?

You can use ceramic or aluminium PCBs. Aluminium PCBs are easely available since these are often used to mount LEDs on. You'll still need to do the math to stay within the temperature limits of the die. The power rating is usually a value which can only be achieved if the mounting base is kept at 25 degrees Celcius (or less).

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Here is the solution that may be used for LOW volume. You solder the D2Pak to the custom designed copper block ( used $16 Wal-Mart hot plate for prototypes). The MOSFET is dropped "face down" in the rectangular hole so that copper block rests (makes contact) on the large PCB pad (flat primary transformer winding in my case). Gate and source leads are soldered to the pads on the edge of the hole. Same screw (I use 6-32) is used to clamp the copper block between heat sink and transformer winding. I had a meeting with Infineon FAI trying to find out how they do it. I assumed that they should have solution. How wrong I was!!

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BTW, I needed to dissipate ~15-20 W or so.

You can never really get 300W away from the package. If you do your layout with no parts on the back below the D2PAK you can arrange to press the back of the PCB against an aluminum heat sink. PCB has about 10 times the thermal conductivity of air. Most of the heat will leave the device into the PCB. Adding some vias will help but can make trouble because they can connect things to the heat sink.

If you make your power supply very efficient, you will find that the losses in the MOSFET are kept low by using one that has a maximum current well above the current it actually carries. You want the breakdown voltage to be only a few times the maximum the voltage.

LN2 will do it. If you make a little "steam engine" you can use the heat of the MOSFET to boil the N2 to run a generator to power up the system.

thetaJC is the lie based on ~300W, which is what the OP asked about.

Tim

--
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Website: http://webpages.charter.net/dawill/tmoranwms

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Case study using Digikey pricing. Manufacturer's parts are STP80PF55 (TO-220) and STP80PF55T4 (D2PAK). The surface mount part is $4.14 and the thru hole is $2.54, volume pricing doesn't change the disparity. Specs are identical except for package.

Even if they were priced the same, the only reason I would design for D2PAK is if it is a switching application and current levels are way below the 80A max since even with the very respectable 18 mOhm on resistance, the device would have to dissipate around 28 W of heat at

40 A and 100% DC. However, that would only be after checking pricing and specs for a lower power device commensurate with the actual power levels involved.

Choosing D2PAK over TO-220 would be absurd for a high power application. The awesome efficiency you mention also applies to the TO-220 case. Also, heatsinking the TO-220 device would be a fairly standard exercise.

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