While I was supposed to be concentrating on something entirely different (work) I had a revelation/really stupid idea, you be the judge.
I am having problems with sinking my FETS and after taking measurements with a thermocouple surface temp probe I did the calcs and decided the problem was not the heatsink itself but rather the case/sink interface.
MY IDEA:
Solder the case to a thin plate (1-3mm) of aluminium/copper then mount the copper plate complete with FETS to the heatsink. The case plate would effectively become part of the case with little to no difference in thermal resistance.
This would have 2 benefits:
1) The area over which the conduction between case and sink is much larger. This is of benefit because heat dissipation (W) is inversely proportional to surface area. Therefore a doubling of contact area of the case would double the heat dissipation ability of the case. This is why TO-3 cases are better than TO-220 cases.
2) Secondly the clamping force between plate and sink can be greater than the maximum clamping force of the mosfet and only limited by the method of fastening. Increasing the clamping force also decreases the thermal resistance. Note: mica/kapton insulating material and silicon grease is still required between the plate and the sink.
The only problem is will the heat of soldering destroy the FET? I have a couple of FETS which I am willing to sacrifice in the name of audio science and will experiment this weekend. I think I will get the plate up to soldering temperature in the oven and sink the FET as much as possiblem, then apply the solder and place the FET in a pool of solder. After the bond has taken i will cool the whole thing as quickly as possible.
Does anyone have any thoughts/suggestions?
Cheers
Dan
[Edited by ding on 06-07-2001 at 11:59 PM]
I am having problems with sinking my FETS and after taking measurements with a thermocouple surface temp probe I did the calcs and decided the problem was not the heatsink itself but rather the case/sink interface.
MY IDEA:
Solder the case to a thin plate (1-3mm) of aluminium/copper then mount the copper plate complete with FETS to the heatsink. The case plate would effectively become part of the case with little to no difference in thermal resistance.
This would have 2 benefits:
1) The area over which the conduction between case and sink is much larger. This is of benefit because heat dissipation (W) is inversely proportional to surface area. Therefore a doubling of contact area of the case would double the heat dissipation ability of the case. This is why TO-3 cases are better than TO-220 cases.
2) Secondly the clamping force between plate and sink can be greater than the maximum clamping force of the mosfet and only limited by the method of fastening. Increasing the clamping force also decreases the thermal resistance. Note: mica/kapton insulating material and silicon grease is still required between the plate and the sink.
The only problem is will the heat of soldering destroy the FET? I have a couple of FETS which I am willing to sacrifice in the name of audio science and will experiment this weekend. I think I will get the plate up to soldering temperature in the oven and sink the FET as much as possiblem, then apply the solder and place the FET in a pool of solder. After the bond has taken i will cool the whole thing as quickly as possible.
Does anyone have any thoughts/suggestions?
Cheers
Dan
[Edited by ding on 06-07-2001 at 11:59 PM]