Maximum DC power for a Transistor before heatsink necesssary? - diyAudio
 Maximum DC power for a Transistor before heatsink necesssary?
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Join Date: Dec 2009
Maximum DC power for a Transistor before heatsink necesssary?

Hello,

I have a NPN transistor that has 60mA of emitter current and Vce = 3.18VDC for a power of 0.18 watts.

The only specifications I can find on the data sheet are:

PD Total Device Dissipation(TC=25°C) = 50 Watts

Derate above 25°C = 0.4W/C

RθJC Thermal Resistance, Junction to Case = 2.5 °C/W

It is a T-220 case.

I have attached a Power derating curve that is on the data sheet also.

For this example I feel that I am OK with out a heatsink because when I press my finger against the transistor it is just barely warm.

I just want to know how to do this for in the future.

How can I calculate at what power should I be using a heatsink?
Attached Files
 Power derating.pdf (71.4 KB, 9 views)

Last edited by skibum; 9th May 2012 at 03:20 PM.

 9th May 2012, 06:25 PM #2 diyAudio Member     Join Date: Nov 2007 Location: Near Dallas Texas USA You need to know the thermal resistance junction to ambient. For a TO-220 it's probably 50 to 60 degrees C per Watt. If it's not on the manufacturers' spec sheet, look for other parts made by the same manufacturer, or a similar part by another company. Then just figure the junction temperature based on power dissipation.
 9th May 2012, 06:47 PM #3 diyAudio Member   Join Date: Mar 2007 Location: Oxfordshire A rule of thumb I use is 1.5 watts for TO220 . 1 watt for TO126 and 2 W for TO3P . Seems to work if there is ventilation . The printed circuit can dissipate heat if plenty of copper is connected with the transistor . I ran some BD 135 in pairs at 1.13 Watts for days with no problems . Another rule of thumb . If you can not hold on to them for 5 seconds they are too hot . Small clip on heat sinks cost very little money .
 10th May 2012, 01:07 PM #4 diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders I have pulled up an ONsemi MJE15032 because I knew it was rated at 50W when Tc=25°C The datasheet shows 2.0W when Ta=25°C Sometimes the manufacturer shows two plots on the P vs Tc/Ta graph. ONsemi show both. Again some manufacturers show Rth j-c and Rth j-a for the two operating conditions of infinite heatsink or no heatsink. All these forms can be used to determine the same de-rated SOAR. eg. 50W @ Tc=25°C gives Rth j-c = {150-Tc} / P = 125 / 50 = 2.5C/W for the infinite heatsink. 2.0W @ Ta=25°C gives Rth j-a = {150-Ta} / P = 125 / 2 = 62.5C/W for the no heatsink. You have 180mW and no sink. Tj = {Rth j-a * P} + Tambient = {62.5 *0.18} + 40°C = 51°C. 51<<100 so OK. You can re-arrange the formula to give Pmax = {Tjlimit - Tambient} / Rth j-a. If you set your Tjlimit to 100°C and Tambient = 50°C then, Pmax = {100-50} / 62.5 = 0.8W compared to absolute maximum of 2W when Ta= 25°C __________________ regards Andrew T. Sent from my desktop computer using a keyboard Last edited by AndrewT; 10th May 2012 at 01:10 PM.
 10th May 2012, 01:36 PM #5 diyAudio Member   Join Date: Mar 2007 Location: Oxfordshire I remember doing all those calculations for a design once and getting nowhere . I didn't realize that the heat transfer to the heat sink is not ideal and doesn't happen in real life . I say when real power comes in double your estimate . We did this at work recently . The guys I work with are no idiots . The design was to be air cooled . Now we have a small fan as it didn't quite work out . This cost the project months because the boss didn't want a fan . It is the quietest fan we could find and comes on when needed . Some customers in Asia turn of the air con . It blows our devices up if 45 C in the room .
 10th May 2012, 02:07 PM #6 diyAudio Member   Join Date: Dec 2009 Thanks Andrew I will still do the finger test but it's nice to have an equation.
 10th May 2012, 02:11 PM #7 diyAudio Member   Join Date: Mar 2007 Location: Oxfordshire I know if we get into trouble we have Andrew . I have many at work and I am lazy . I dig a hole and they get me out . I am the boss you know . It was me who didn't want fans . Boss of projects that is . Not money boss thank goodness . Last edited by nigel pearson; 10th May 2012 at 02:37 PM.

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