Transistor temperature question - Page 1
Forum section: electronics / projects

author: rcbuck
author_rank: Frequent Contributor
timestamp: March 20, 2022, 05:11:37 pm
content: I have long wondered about transistor dissipation numbers given in data sheets.I have a feedback RF amplifier stage that uses a 2N3904. The data sheet says total dissipation is 625 mW which is to be derated at 5 mW for each degree above 25. After running for 30 minutes the transistor case measures 40 degrees C. That means 625 - 75 = 550 mW maximum dissipation at 40 degrees C.The transistor is drawing 20.8 mA with a Vce drop of 9.49 volts. That equals 197.4 mW dissipation. The transistor is hot to the touch but not too hot to prevent continuous holding on to it. Do I need to be concerned about it being hot? The circuit is in open air and will run a little warmer when it is put in the enclosure (which is vented).Maybe I am being overly cautious but I just don't like parts that run hot. Is the 2N3904 actually designed to run for years at this temperature with no negative effect?

author: TimFox
author_rank: Super Contributor
timestamp: March 20, 2022, 05:19:36 pm
content: The 625 mW at 25oC, derated by 5 mW/K, is forambienttemperature (air outside the case).The other rating, 1.5 W at 25oC, derated by 12 mW/K, is forcasetemperature.Your 40oC case temperature implies a power rating of 1.2 W, so your 200 mW dissipation is conservative.

author: Cerebus
author_rank: Super Contributor
timestamp: March 20, 2022, 09:45:54 pm
content: What you need to worry about is thejunction temperature.You know yourpower dissipation(197.4 mW) and you know yourcase temperatureat steady state (40ºC).The Fairchild data sheet for the 2N3904 in a plastic TO-92 case gives ajunction to case thermal resistanceof 83.3 ºC/W. So, your 197.4 mW will lead to a temperature difference from junction to case of 16.4 ºC, adding that to the case temperature gives you a junction temperature of 56ºC in round figures. According to the datasheet the rated "Operating and Storage Junction Temperature Range" is -55 to 150ºC, which leaves your junction temperature a very comfortable 94ºC below its maximum.

author: TimFox
author_rank: Super Contributor
timestamp: March 20, 2022, 09:55:36 pm
content: The ONSEMI datasheet for the 2N3904 I quoted is a bit easier to understand for this non-heatsinkable transistor.It only quotes power dissipation as a function of case temperature or ambient temperature.https://www.onsemi.com/pdf/datasheet/2n3903-d.pdf

author: rcbuck
author_rank: Frequent Contributor
timestamp: March 20, 2022, 11:01:35 pm
content: Quote from: Cerebus on March 20, 2022, 09:45:54 pmThanks for the detailed answer. I was somewhat confused about junction temperature vs case temperature. I did not look at the Fairchild data sheet.Quote from: TimFox on March 20, 2022, 09:55:36 pmTim, the numbers I quoted were from the Onsemi data sheet. They seemed to be simple enough but your reply gives more detail about the actual device rating. Actually you can put a heat sink on the TO-92 case. I have a dozen or so of these on hand that I bought about 25 years ago. You could probably epoxy a small copper sheet deformed with a drill bit to fit the back side of the TO-92 and accomplish the same thing.