what is the highest temperature for the BJT's to operate safelly

sparkle · 22nd August 2006, 02:10 PM

I have MJ15003's and they operate arround 32W and the heatsink is bad for them (not good enough).....What is the maximum temperature on which those transistor can operate not to become damaged during time......??!!
Has anyone tried to operate them (long term) on 70 degrees Celsius)?! The amp is JLH class A, single ended......

pinkmouse · 22nd August 2006, 02:22 PM

The principle is simple, the hotter the device, the shorter the life. If your heatsinks are running at 70 degrees, I feel that that is too hot. Get bigger heatsinks, or a fan.

BobEllis · 22nd August 2006, 02:32 PM

From the data sheet:

Rjc = .7 C/W
Junction temp max = 200C

So add ~1.2 C/W for the isolator if using Mica/grease and your junction is 1.9C/W above the heat sink temperature. Your Junctions are ~61 C above heat sink temperature or 131C.

Nelson Pass recommends 100C max for reliability using IRF devices with a max junction temp of 150C. Scaling that to your 200C max, you are probably OK, but as PM said cooler is better. Not to mention the danger of burning yourself or your guests on the 70C heatsinks.

sparkle · 22nd August 2006, 02:40 PM

no,no, no - heatsink is not 70 Celsius - but the top cover of the MJ15003 - the "hat" that covers the chip inside - the "L" profile where i have attached the transistors is arround 60 Celsius .....

this seems to me to be at the edge but still o.k. - agreed ??

thnx for the answers

BobEllis · 22nd August 2006, 02:50 PM

so, if your case temperature is 70 C, then your junction is .7xPd greater, or ~92C. Shouldn't be a problem at all. not even on the edge

poobah · 22nd August 2006, 02:51 PM

You're OK... just barely. Like Pinkmouse, Bob, & Nelson... cooler would be better. U.S. military says 110 C on all junctions... max. Lifetime goes in half for every 11 C increase in operating temp.

sparkle · 22nd August 2006, 02:58 PM

o.k. thnx guys......
barelly allive

JensRasmussen · 22nd August 2006, 05:06 PM

Quote:

Originally posted by poobah
You're OK... just barely. Like Pinkmouse, Bob, & Nelson... cooler would be better. U.S. military says 110 C on all junctions... max. Lifetime goes in half for every 11 C increase in operating temp.

Hi,

Do you have any links to any thermal derating documentation rearding lifetime in semiconductors?

\Jens

AndrewT · 22nd August 2006, 05:13 PM

Hi Jens,
that's a good question.
Now you have me wondering if all the previous estimates of lifetimes that I have seen, have been "old wives' tales" or based on factual evidence.

Nelson Pass · 22nd August 2006, 05:28 PM

Keep in mind that temperature has a dramatic influence on the
second breakdown of bipolars, so the 70 deg figure is very
safe at lower voltages, but when you get above 50V or so,
it gets more dicey.

22nd August 2006, 02:10 PM	#1
sparkle diyAudio Member Join Date: Oct 2003 Location: middle of nowhere...	what is the highest temperature for the BJT's to operate safelly I have MJ15003's and they operate arround 32W and the heatsink is bad for them (not good enough).....What is the maximum temperature on which those transistor can operate not to become damaged during time......??!! Has anyone tried to operate them (long term) on 70 degrees Celsius)?! The amp is JLH class A, single ended......

22nd August 2006, 02:22 PM	#2
pinkmouse diyAudio Moderator Join Date: Apr 2002 Location: Chatham, England	The principle is simple, the hotter the device, the shorter the life. If your heatsinks are running at 70 degrees, I feel that that is too hot. Get bigger heatsinks, or a fan. __________________ Al I conceive of nothing, in religion, science or philosophy, that is more than the proper thing to wear, for a while. Charles Fort

22nd August 2006, 05:13 PM	#9
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi Jens, that's a good question. Now you have me wondering if all the previous estimates of lifetimes that I have seen, have been "old wives' tales" or based on factual evidence. __________________ regards Andrew T.

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22nd August 2006, 02:32 PM	#3
BobEllis diyAudio Member Join Date: Sep 2003 Location: Upstate NY	From the data sheet: Rjc = .7 C/W Junction temp max = 200C So add ~1.2 C/W for the isolator if using Mica/grease and your junction is 1.9C/W above the heat sink temperature. Your Junctions are ~61 C above heat sink temperature or 131C. Nelson Pass recommends 100C max for reliability using IRF devices with a max junction temp of 150C. Scaling that to your 200C max, you are probably OK, but as PM said cooler is better. Not to mention the danger of burning yourself or your guests on the 70C heatsinks.

22nd August 2006, 02:40 PM	#4
sparkle diyAudio Member Join Date: Oct 2003 Location: middle of nowhere...	no,no, no - heatsink is not 70 Celsius - but the top cover of the MJ15003 - the "hat" that covers the chip inside - the "L" profile where i have attached the transistors is arround 60 Celsius ..... this seems to me to be at the edge but still o.k. - agreed ?? thnx for the answers

22nd August 2006, 02:50 PM	#5
BobEllis diyAudio Member Join Date: Sep 2003 Location: Upstate NY	so, if your case temperature is 70 C, then your junction is .7xPd greater, or ~92C. Shouldn't be a problem at all. not even on the edge

22nd August 2006, 02:51 PM	#6
poobah diyAudio Member Join Date: Nov 2005	You're OK... just barely. Like Pinkmouse, Bob, & Nelson... cooler would be better. U.S. military says 110 C on all junctions... max. Lifetime goes in half for every 11 C increase in operating temp.

22nd August 2006, 02:58 PM	#7
sparkle diyAudio Member Join Date: Oct 2003 Location: middle of nowhere...	o.k. thnx guys...... barelly allive

22nd August 2006, 05:28 PM	#10
Nelson Pass The one and only Join Date: Mar 2001	Keep in mind that temperature has a dramatic influence on the second breakdown of bipolars, so the 70 deg figure is very safe at lower voltages, but when you get above 50V or so, it gets more dicey.

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