Power dissipation values in TIP142 (Darlington Pair)

[yasser_shoukry]

Hi all,

I need to know the difference between this two values in the TIP142 (Darlington Pair) Data Sheet:

-Continuous device dissipation at (or below) 25°C case temperature (Ptot) 125 W
-Continuous device dissipation at (or below) 25°C free air temperature (Ptot) 3.5 W

Thanks in advance

 

[alberr]

Re: Power Dissipation

Hi all,

I need to know the difference between this two values in the TIP142 (Darlington Pair) Data Sheet:

-Continuous device dissipation at (or below) 25°C case temperature (Ptot) 125 W

This is maximum power dissipation when case of device gets cooled with some heatsink with good thermal contact to obtain case temperature 25°C.

-Continuous device dissipation at (or below) 25°C free air temperature (Ptot) 3.5 W

Device without heatsink. Air convection is not so good way of cooling. Temperature of case on this occasion will be more than 25°C (temperature of air). Power must be reduced to prevent overheating.

 

[Kral]

Re: Power Dissipation

yasser_shoukry,
There are two related ratings: Thermal Resisance junction to case Rjc) , and Max operating junction temperature (Tjmax) that are used in selecting a heatsink. For example, if you need to dissipate 30 watts at an ambient (air) temperature of 50 Deg C, you would proceed as follows:
. Let Rhs = the required thermal resistance of the heatsink
For the TIP 142:
. Tjmax = 150 Deg C
. Rjc = 1 Deg C/watt
.
So the total Thermal Resistance, Junction to ambient must be
. (150-50)/30 = 3.33 DegC/W.
So Rhs = 3.33 - Rjc = 3.33 - 1 = 2.33 Deg C/Watt.
.
If Rjc is not given, you can use the slope of the Power vs (Case Temperature) curve.
Regards,
Kral