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Woudl you need copper heatsinking for a 750W offline PSU?

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cupoftea

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Hi,
Supposing you had to do an offline SMPS (220VAC) for 24Vout at 750W.
The enclosure dimension should be no more than 275mm x 142mm by 40mm high.
You can have 2 cooling fans, each 12V, 0.18A, and obviously the associated cooling vents in the enclosure. The enclosure can be made of metal.
Ambient temperature withstand just like any usual Meanwell type offline PSU.

Would you use heatsinks made just of aluminium?, or would you need also to screw fairly large pieces of copper to the heatsinks, in order to provide sufficient cooling?....eg dont just screw the FETs/Diodes to the aluminium heatsink, but screw them to a decent chunk of copper, which itself then gets screwed to the aluminium heatsink? Also, have say a decent chunk of copper screwed to the top of the PFC FET diode heatsink, for that extra cooling..

So, would you need these extra copper heatsink pieces, or not?
 

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Depends on the efficiency. If the efficiency is assumed as %90, Power Dissipation over Transistor(s) will be at least 75W. So well ventilated heatsink will obviously be necessary.
 
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What do you exactly mean with extra cooling? Copper has higher thermal conductivity (384 versus 229 W/K*m), but using an extra piece of copper between power device and heatsink doesn't necessarily reduce the total thermal resistance. In case of a reasonably shaped heatsink it may even increase it.
 
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Thanks, we have been offered to buy a 750W PSU....and we took it apart, and noticed these copper pieces which have been screwed to the aluminium heatsinks, and wondered if this is common practice?
 

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What a slab of copper under a power device can do is reduce the thermal gradient immediately beneath and around the device, if the heatsink surface beneath the device is really too thin.
A properly dimensioned extruded heatsink will not suffer from that problem.

Its really a band aid solution, but it does work if the heatsink is for example, just a large flat plate of thin material.
 
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