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Remove heat from a DPAK how to??

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rakeshm55:
Hi,
i plan to use SUD50P06-15 P- Channel Mosfet for one of my project. It is a DPAK  package.

Heat Sink (Chassis) used in my system will be connected to ground. How remove heat out of Pchannel Mosfet??

 Its thermal PAD is connected to drain. So connecting to board ground not possible.

Since DPAK is SMD what is the standard practice  for DPAK devices.

Gyro:
The standard practice is to solder it down to a PCB with a decent area of copper to act as a heatsink. If it's being used for switching (cleanly) then the low Rds makes that sufficient normally.

rakeshm55:
It is used as a highside switch with 6A of continuous current. Expect a dissipation of over 1W.

Considering junction to Ambient 65C/W . A temperature rise of 65C above ambient. Onboard ambient can go upto 75C So junction temperature of 140C is anticipated.

I guess it is too close to max thermal limit.

max_torque:
What i do is to use a multilayer pcb, solder the drain to the isolated top layer pour, and via that with lots of vias, to Layer 3, where the same pour is repeated.  Then on the bottom layer and layer 2 i put a chassis ground referenced copper pour, again via'd together and make all those pours overlap, the amount they overlap and the total area depends on the total heat flux. 

That way the heat comes out of the dpak into top, spreads via the vias ( :-DD ) into layer 3, and then the heat can  move sideways through the (relatively) thin pcb material, which has a poor, but not terrible thermal impedance into the layers that are thermally coupled to the chassis.  You do have to watch out if the device is switching fast, as those overlapping layers ARE capacitively coupled!

Alternatively, you sink the heat into the top layer of the pcb only, and find space for a solder / bolt on heat sink, that is large enough to reject the heat into the air inside the enclosure, and then that heat gets rejected to the enclosure walls.  The problem with this approach is static air is a terrible conductor, and hence for any significant power level you might need a fan blown heat sink

Gyro:
Yes, you do need to be careful with SMD FETs, many of the parameters are fictitious (stated at 25'C Tj or pulse).

Although Junction to Ambient is 65'C/W, Junction to Case is <1'C/W. It's not unreasonable to be able to dissipate 1W on a PCB. Common practice these days is to pick a lower Rds-on part rather than resort to a heatshink if things get tight.

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