0.5C/W from the heatsink graph, or the corrected C/W value for the temperature it will have ?
30W times 2 devices = 60W
60 x 0.5 = 30C above ambient
30C at 20C ambient makes 50C.
But temperature in La Réunion is usually a lot higher, I'd think.
If you did not correct the C/W value, it would be closer to 60C at 20C ambient
Thanks for your analysis. In réunion Island temperature where i live is about 28°c. The heatsinks Will be around 60°c. I don ´ t Know if it´s too hot or no. CAN you Help me please? It ´ s my first class A.
If i use fans for cooling the heatsinks, will it work?. Or must i change it?
This is mine.
http://www.audiophonics.fr/fr/kits-...-thermique-anodise-noir-200x80x40-p-7239.html
Fred.
The majority of heatsinks are not used to cool the output stages of class A amplifiers.
Most designers need a heatsink to handle the worst dissipation level, to protect the die of active devices reaching a temperature that will destroy them.
Suppose you have a TO-220 transistor that can handle 75W max, you want it to dissipate 25W at the most.
The die of the transistor should stay under 150C, say ambient air is at 25C, leaves 125C difference.
125 divided by 25 makes 5 C/W total thermal resistance from the die to the ambient air.
Of that number, the transistor package takes 1.67 C/W (125C divided by 75W)
Thermal resistance of a TO-220 package between the case and heatsink takes another 1 C/W
Which leaves 2.33 C/W for the heatsink.
2.33 C/W x 25W = 58.25C above ambient, makes a heatsink temperature of 83.25 C
The higher the temperature difference between heatsink and ambient, the more efficient a heatsink becomes.
If the C/W value of a heatsink would be measured at 50-55C by the manufacturer, it would be larger (=more expensive) than necessary for most applications.
So manufacturers measure heatsinks at a much higher temperature.
Bottomline of the long story : your heatsinks are not 0.45 C/W at the temperature you wish to use them. (désolé
)
You really need fans.
Most designers need a heatsink to handle the worst dissipation level, to protect the die of active devices reaching a temperature that will destroy them.
Suppose you have a TO-220 transistor that can handle 75W max, you want it to dissipate 25W at the most.
The die of the transistor should stay under 150C, say ambient air is at 25C, leaves 125C difference.
125 divided by 25 makes 5 C/W total thermal resistance from the die to the ambient air.
Of that number, the transistor package takes 1.67 C/W (125C divided by 75W)
Thermal resistance of a TO-220 package between the case and heatsink takes another 1 C/W
Which leaves 2.33 C/W for the heatsink.
2.33 C/W x 25W = 58.25C above ambient, makes a heatsink temperature of 83.25 C
The higher the temperature difference between heatsink and ambient, the more efficient a heatsink becomes.
If the C/W value of a heatsink would be measured at 50-55C by the manufacturer, it would be larger (=more expensive) than necessary for most applications.
So manufacturers measure heatsinks at a much higher temperature.
Bottomline of the long story : your heatsinks are not 0.45 C/W at the temperature you wish to use them. (désolé
)You really need fans.
Last edited:
Je vous en prie.
Fans are also the way to save your electrolytic capacitors.
Life span of caps is shortened if their operating temperature is higher.
As active devices, life of electrolytics is halved for every 10C higher temperature. Or, 20C lower equals living 4 times longer.
Means that if you hang electrolytic caps right above a 60C heatsink, you will have to exchange them every couple of years.
At 40C, a regular 85C electrolytic easily reaches 15 years use.
Fans are also the way to save your electrolytic capacitors.
Life span of caps is shortened if their operating temperature is higher.
As active devices, life of electrolytics is halved for every 10C higher temperature. Or, 20C lower equals living 4 times longer.
Means that if you hang electrolytic caps right above a 60C heatsink, you will have to exchange them every couple of years.
At 40C, a regular 85C electrolytic easily reaches 15 years use.
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