Maximum DC power for a Transistor before heatsink necesssary? - diyAudio
Go Back   Home > Forums > Amplifiers > Power Supplies

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 9th May 2012, 03:12 PM   #1
skibum is offline skibum  France
diyAudio Member
 
Join Date: Dec 2009
Default 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=25C) = 50 Watts

Derate above 25C = 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
File Type: pdf Power derating.pdf (71.4 KB, 9 views)

Last edited by skibum; 9th May 2012 at 03:20 PM.
  Reply With Quote
Old 9th May 2012, 06:25 PM   #2
diyAudio Member
 
Loudthud's Avatar
 
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.
  Reply With Quote
Old 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 .
  Reply With Quote
Old 10th May 2012, 01:07 PM   #4
AndrewT is online now AndrewT  Scotland
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=25C
The datasheet shows 2.0W when Ta=25C
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=25C gives Rth j-c = {150-Tc} / P = 125 / 50 = 2.5C/W for the infinite heatsink.
2.0W @ Ta=25C 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} + 40C = 51C. 51<<100 so OK.

You can re-arrange the formula to give Pmax = {Tjlimit - Tambient} / Rth j-a.
If you set your Tjlimit to 100C and Tambient = 50C then,
Pmax = {100-50} / 62.5 = 0.8W compared to absolute maximum of 2W when Ta= 25C
__________________
regards Andrew T.

Last edited by AndrewT; 10th May 2012 at 01:10 PM.
  Reply With Quote
Old 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 .
  Reply With Quote
Old 10th May 2012, 02:07 PM   #6
skibum is offline skibum  France
diyAudio Member
 
Join Date: Dec 2009
Thanks Andrew

I will still do the finger test but it's nice to have an equation.
  Reply With Quote
Old 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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Power tube maximum current draw from a power supply razorrick1293 Tubes / Valves 4 4th November 2011 10:48 PM
Mica in Heatsink TO3 power transistor or no ? aldovan Solid State 2 6th June 2008 03:39 AM
Heatsink for two STK4050V in maximum power ElectroPower Chip Amps 2 24th July 2007 02:27 PM
For 35A rectifiers, what is maximum without heatsink geminni Solid State 10 18th March 2007 12:54 PM
isolating power transistor from heatsink Wagener Solid State 10 23rd May 2004 09:57 PM


New To Site? Need Help?

All times are GMT. The time now is 03:45 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2