Parallel Surface mounted FETs or single PTH FETs in SMPS?

[treez]

Hello,
We are doing a 650W Buckboost converter using the LT8705 IC.
(Vin = 18-32V, Vout=28-34V, Fsw=160KHz, Continuous current mode.)

We have a choice between using one TO220 IPP037N06L3G NFET in each FET place, or using three paralleled BSC026N04LS NFETs for each FET placement.

Which do you think will give the lower FET junction temperatures?

BSC026N04LS datasheet (TDSON8 package):
http://www.infineon.com/dgdl/BSC026...b02d7&fileId=db3a3043410d6ee40141127f49e92b5d

(rdson=2.6mR , Qg=16nC, internal diode reverse recovery time =24ns )

IPP037N06L3G datasheet (TO220 package):
http://www.infineon.com/dgdl/IPP037...b02d7&fileId=db3a30431b3e89eb011b4561341f7d38

(rdson=3mR , Qg=59nC, internal diode reverse recovery time =44ns )

LT8705 Datasheet:
http://cds.linear.com/docs/en/datasheet/8705fb.pdf

The FET datasheets give the K/W , but for environmental conditions which don’t apply to us... With the TO220 package, we would screw the metal drain tab to a metal heatsink via a thin, electrically insulating thermal pad.

With the TDSON8 (surface) packages, we would mount them on a 2oz copper PCB, which is 1.92mm thick, we would have thermal vias going through this 1.92mm PCB to a thin , electrically insulating, thermally conducting pad, and then to a heatsink. (thermal vias from the chip's underside heat pad to a bottom copper pad) The problem here is that we obviously cannot screw the surface mount packages to the heatsink…well..we could screw the PCB to the heatsink, so as to “pressure” the TDSON8 FETs to the heatsink, but that might stress the PCB.

So , in summary, which gives the lower FET junction temperature?, one TO220 or three paralleled TDSON8 packages in each FET placement position?

 

[mtwieg]

The problem here is that we obviously cannot screw the surface mount packages to the heatsink…well..we could screw the PCB to the heatsink, so as to “pressure” the TDSON8 FETs to the heatsink, but that might stress the PCB.

This is the normal way to cool surface mount power devices. Cooling through the PCB generally doesn't work well, unless you use tons of vias-in-pad which are filled with metal. You may want to look at directfet packaged devices, they are excellent if you are looking for high density, and there are lots of app notes on how to mount heatsinks on them.

 

[jpanhalt]

They are spec'd with the same thermal area on the PCB:

Why not just solder the SMD device to a copper heat sink on the PCB?

BTW, 2 oz copper is not "1.92mm thick." My sources show it more like 2.8 mils (less than 0.1 mm).

John

 

[treez]

The PCB is 1.92mm thick, as you suggest, 20z copper is 70um thick