When a heatsink is required on transistor ?

[andro]

TIP41 NPN
in datasheet i see following

Continuous device dissipation at max 25ºC case temperature (see note 1) Ptot 65 W
Continuous device dissipation at max 25ºC free air temperature (see note 2) Ptot 2 W

so what is the difference between p = 65 W and P = 2 W ?
and which one should i use to calculate the heatsink required for that transistor ?
and which one gives me indication on the rising junction temperature ?
i am confused about power dissipation and using heat sink
would you please give me an example on how using the last two terms in designing simple DC circuit

Best regards

 

[vimalkhanna]

The 65 W device dissipation is what is transferred to load as the collector output voltage to ground x load currents .
There is a min Vce drop reqd for maintaining the device in active mode before saturation .
This Vce x Ic = 2Watts dissipated as heat .
The heatsink design needs to radiate this in free air .Kindly select a proper fin size /area /alum. content /etc

 

[srizbf]

"..and which one should i use to calculate the heatsink required for that transistor ?"

dont worry about which one in the starting of heatsink design.

we decide the heatsink based on circuit power.

you must calculate the power dissipation across the transistor in stedy state.

for that calculated power disiipation , then refer the data sheet.
find the Rth of the transistor in question .
then you calculate the area reqd for the heatsink.

 

[keith1200rs]

The 65 W device dissipation is what is transferred to load as the collector output voltage to ground x load currents .
There is a min Vce drop reqd for maintaining the device in active mode before saturation .
This Vce x Ic = 2Watts dissipated as heat .
The heatsink design needs to radiate this in free air .Kindly select a proper fin size /area /alum. content /etc

That is NOT correct. 65W is what the TRANSISTOR can dissipate if you hold the case at 25C. 2W is what the transistor can dissipate if you have the transistor in free air at 25C - the transistor case wil be quite hot.

Keith

 

[Gozaki]

Look here : **broken link removed**

 

[andro]

@ keith1200rs
i am still confused yet

"65W is what the TRANSISTOR can dissipate if you hold the case at 25C"
case at 25C ,so it should be at 25C despite of ambient temperature ??? i mean the ambient maybe at different temperature high or low
so i understand from that : the transistor can dissipate thermal power at two rates 65 W and 2 W and that is the max heat transfer at two temperature rates
so the transistor can be working at case Temperature of 25C at max 65 W
and at free air at temperature of 25C at max 2 W
so i am right or wrong in what i understood ???
thanks a lot

 

[keith1200rs]

The 2W and 65W both come from the same limitation - the 150C junction temperature. The junction to case thermal resistance is around 1.92C/W so 65W will make it rise by 125C so if the case is held at 25C the junction will be at 150C - the maximum. That would be an "ideal" situation - in practice it would be very difficult to achieve. The other extreme is no heatsink: the thermal resistance of the junction to ambient is around 62.5C/W so 2W would make the junction rise by 125C above ambient so 150C again - the maximum.

I haven't read the link posted by Gozaki but it looks like it covers heatsink calculations in depth. To give a simple example, if you had a 10C/W heatsink and wanted to work at 50C then you need to add the 1.92C/W to the heatsink to give 11.92C/W and then from (150-50) = 100C - the maximum temperature rise, you arrive at 8.4W that you could dissipate (100/11.92) = 8.4W.

Keith.

 

[bigdogguru]

Here are a few appnotes concerning heatsinks and their properties, thermal conductivity, convection, etc:

Heatsink Characteristics

HEATSINK CALCULATION AND EXAMPLES

Thermal Management Using Heat Sinks

Hope they are of some value to this discussion.

BigDog