Is 4.2W too much for this heatsink?

[Fusion916]

Attached is a picture of a KSE340 with 4.2W idle power. Too much? The heatsink gets very hot after about 5-10 minutes.

 

[ticknpop]

Is there voltage on the heat sink?
If no voltage on the sink , then test can you keep your hand on the heat sink for 10 seconds without pulling it away? If you can keep it there 10 seconds your OK for temperature

 

[Fusion916]

Is there voltage on the heat sink?
If no voltage on the sink , then test can you keep your hand on the heat sink for 10 seconds without pulling it away? If you can keep it there 10 seconds your OK for temperature

No voltage on the heatsink.

I can keep my hand on there for about 5 or so seconds but its very very hot.

 

[Andrew Eckhardt]

Not good. That style of sink always seemed like an oddity to me. Such high thermal mass and total area but such a wimpy connection to it from the device. With continuous dissipation the mass only helps for a few minutes, but the 1/16" material behind the transistor is going to get significantly hotter than the rest of the sink. Also, if you're going to push a 340, you might want to start with a non-encapsulated version and think about mounting it directly to an isolated sink without an insulating pad. Dangerous, but thermally max performance option. That sink would be okay at that power if you used a transistor with exposed thermal pad and a heavy flat bar of aluminum between it. (grease, of course)

 

[gentlevoice]

Hi,

Just a thought: It looks as if you haven't got many tracks on the bottom side of your pcb so might it be an option to drill some holes in the pcb to improve convection along the heatsink fins? I guess it would improve the cooling capacity of the heat sink although don't know how much.

Greetings,

Jesper

 

[sajti]

It should be OK, for 4.2W, if You can keep Your hand on it for 5sec it can be maximum 50-55C, which is OK.
I agree with gentlevoice to make some holes on the PCB. It can decrease the temperature even with 10degree

Sajti

 

[sofaspud]

It does look to be sufficient for 4.2W. There also looks to be room on the rear of the sink to fit a TO-3 style heatsink, or long vertical sink similar to the one below, using the single bolt to clamp everything together. Don't forget the thermal grease.

 

[Fusion916]

Kind of mitigated the power issue with a few diode drops on the collector of the hot device. Still gets warm but I can keep my finger on it indefinitely now.