Thermal Design for LFPAK FETs

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 ?

 

"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: http://seventransistorlabs.com

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?
<http://www.fischerelektronik.de/en/latest-news/press-releases/releases/smt-heatsinks-for-lfpak/>

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 (though I haven't tried it yet), other than a dubious idea of pumping oil over 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 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.

Steve

 

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.

 

Aluminum-core PCB?

 

I have used these kind of heatsinks on a product:
http://uk.farnell.com/fischer-elektronik/fk-244-08-d-pak/heat-sink-for-d-pak-31-5-c-w/dp/4314001

 

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

 

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

Version 4
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I'd think that if you just want to characterize it you could run it in

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.

 

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