Thermal Design for LFPAK FETs

attractive (low source inductance in particular appeals to me) if I knew how to heatsink them to a significant portion of their rated dissipation (typically ~50W). NXP's LFPAK thermal design document discusses under-1-watt applications only. Has anyone any clever ideas on how to get the heat out ?

You clamp or better yet solder it to a lot of copper. There's no way you can get to 50 watts using PCB copper.

What part number did you have in mind?

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"Steve K"

Some of the NXP (and Renesas) MOSFETs in the LFPAK (SOT669) package look quite attractive (low source inductance in particular appeals to me) if I knew how to heatsink them to a significant portion of their rated dissipation (typically ~50W). NXP's LFPAK thermal design document discusses under-1-watt applications only. Has anyone any clever ideas on how to get the heat out ?

** The LFPAK is about the same size as the chip used in many TO3P/ TO264 power fets and bjts.

So, you could clamp or solder the LFPAK to a copper header and then bolt or clamp that onto a large aluminium heatsink.

IOW create your own large flat pak.

But why bother ?

.... Phil

Some manufacturers offer copper core PCBs (the core of which can be soldered to). Don't ask about the price. :)

Tim

-- Deep Friar: a very philosophical monk. Website:

On 12/5/2012 5:23 PM, Steve K wrote: >Some of the NXP (and Renesas) MOSFETs in the LFPAK (SOT669) package >look quite attractive (low source inductance in particular appeals to >me) if I knew how to heatsink them to a significant portion of their >rated dissipation (typically ~50W). NXP's LFPAK thermal design >document discusses under-1-watt applications only. Has anyone any >clever ideas on how to get the heat out ?>

Not using bonding wires certainly gets the inductance down. But any discussion of how to mount the part requires knowing the ambient temperature. Since you are switching the FET, what is the duty cycle? Does the "on" period exceed a millisecond? [Generally using PWM in temperature analysis doesn't work if the on time is too long. For instance, 1 day on and 10 days off isn't 10% duty cycle in the eyes of a semiconductor.]

A lot of cheesy consumer grade products spec the maximum ambient at 40 degrees C because they figure if it is used in the presence of a person, said person will get out of Dodge when the ambient exceeds 40. This is really common for devices with displays.

BTW, there are plenty of places in the world where the ambient exceeds

40 degrees C!

Heatsink?

Cheers

if you look at similar solderable heatsinks for D2PAK they are around

25K/W so they are not much help with 50W

-Lasse

Thanks for all the comments...a couple suggested soldering to a copper tab and bolting that to a heat sink. That's pretty much what I came up with (t hough I haven't tried it yet), other than a dubious idea of pumping oil ove r the PCB.

Why bother ? Potentially tens of MHz operation (~50% duty cycle) at a few percent of the component cost of RF power FETs (if the heatsinking kluge ca n be done cheaply). I was looking mostly at the lowest Qg versions of the NXP PSMN series. I had looked at the Fischer heatsinks and they only seem good for a few watts.

Steve

b and bolting that to a heat sink.  That's pretty much what I came up wit h (though I haven't tried it yet), other than a dubious idea of pumping oil over the PCB.

few percent of the component cost of RF power FETs (if the heatsinking klug e can be done cheaply).  I was looking mostly at the lowest Qg versions o f the NXP PSMN series.  I had looked at the Fischer heatsinks and they on ly seem good for a few watts.

isn't that something like where directfets shines? they have the "right" side up so you can put a heatsink on top Maybe mount them on back side sandwiched between pcb and heatsink?

another trick I've seen is to using the kelvin connection on a sense fet as reference for the gate drive

-Lasse

Par for the course when doing datasheets for devices that expect pads of copper to be heat sinks is to just solder the part to a big piece of PCB and characterize the theta JA. You dremel wide breaks in the copper to change the size of the heat sink.

To get the junction temperature on a chip is easy since you can characterize a parasitic diode. [Force a small constant current into the pin. Make sure the part is not generating any heat itself, that is, don't operate it. Then sweep temperature and measure the diode voltage.] For a discrete FET, this could be harder or maybe impossible since there is no free diode to play with when you make the chip generate heat.

There may be a way to put two parts on the PCB with one generating heat and the other to measure junction temperature, but I'd have to mediate on if that is kosher.

bolting that to a heat sink. That's pretty much what I came up with (though I haven't tried it yet), other than a dubious idea of pumping oil over the PCB.

percent of the component cost of RF power FETs (if the heatsinking kluge can be done cheaply). I was looking mostly at the lowest Qg versions of the NXP PSMN series. I had looked at the Fischer heatsinks and they only seem good for a few watts.

Aluminum-core PCB?

I have used these kind of heatsinks on a product:

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Failure does not prove something is impossible, failure simply 
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nico@nctdevpuntnl (punt=.) 
--------------------------------------------------------------

bolting that to a heat sink. That's pretty much what I came up with (though I haven't tried it yet), other than a dubious idea of pumping oil over the PCB.

percent of the component cost of RF power FETs (if the heatsinking kluge can be done cheaply). I was looking mostly at the lowest Qg versions of the NXP PSMN series. I had looked at the Fischer heatsinks and they only seem good for a few watts.

$$$$$$!!!!!!!

I'd think that if you just want to characterize it you could run it in reverse using the body diode as both thermometer and to generate heat

toggle between a high current to generate heat and a small current to measure temperature

-Lasse

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I'm a little unsure about the toggling. I've done this with very slow bench meters to measure the diode, but you have added time into the equation.

I'm not ruling this out, but you would probably want to test the scheme on a device that does have a spare diode. That way you would have a benchmark on the scheme's accuracy.

attractive (low source inductance in particular appeals to me) if I knew how to heatsink them to a significant portion of their rated dissipation (typically ~50W). NXP's LFPAK thermal design document discusses under-1-watt applications only. Has anyone any clever ideas on how to get the heat out ?

If you mount them on a largish copper area, you can couple this to the package wall or other radiators either directly or through thermally conductive pads/insulators. The large copper area sor of defeats the intention of thhe smaller package though, doesn't it?

There's not much use heatsinking them inside a container, unless there's somewhere for the heat to go, but everything will reduce thermal impedance.

The Aavid 5731/5733/5734/7109 types, at 4-16 degC/W are similar to the Fischerelektronik parts.

A 1 in square thermal pad can easily beat the lower end of the above numbers, even at a considerable thickness.

RL