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Old 26th October 2010, 04:11 PM   #301
EUVL is offline EUVL  Europe
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> http://www.semicon.toshiba.co.jp/eng...28g_chap03.pdf

Take a look at Figure 2.2 first.

Even with a heatsink temperature of 67C, I calculated that

for 32W per MOSFET + Keratherm 86/82 TO247 insulator, junction temperature will be just below 100C.
No reliability issue for me.
And there are still ways to reduce the peak sink temperature.

Average sink temperature will stay 57C.




Patrick

Last edited by EUVL; 26th October 2010 at 04:14 PM.
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Old 26th October 2010, 04:49 PM   #302
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thanh1973, You stated the Pd was approx. 135W. with what transistor? What Tj-case? What interface? I'm no expert but it came to mind that you could have a really sheety interface and Tj-c and there is no where near 135W being disapated in those sinks? It could all be in a 175C junction instead?
I'm not trying to criticise your work or efforts but I think we need that data too, in order to get a real working condition estimate?
However, the last big GB on these sinks kinda came up with the conclusion that 100W was a cool number
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Old 26th October 2010, 04:58 PM   #303
EUVL is offline EUVL  Europe
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> I'm no expert but it came to mind that you could have a really sheety interface and Tj-c and there is no where near 135W being dissipated in those sinks?

First I have a Ph.D. in mechanical engineering.

At steady state, ALL heat is dissipated through the heat sink, however "sheety" the interface is.
If you would believe me, and I know what I am talking about in mechanical things, then we have no transistor reliability problems even with 67C sink temperature. If you want to believe what people give out here as holy rules and not look at the science, then as I said, use a Hifi2000 case or something of similar size.

When I am eventually finished with the case design, the sink temperature will drop further.

There is nothing to criticise in thanh's experiment IMHO. The results agree with manufacturer's data 100%.
Please kindly take your time to read post #301, 302 and the reliability handbook from Toshiba.


Best regards,
Patrick

Last edited by EUVL; 26th October 2010 at 05:18 PM.
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Old 26th October 2010, 06:16 PM   #304
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It took me a while to reconcile today's calculations of 100C junction temperatures with what remembered calculating a while ago. I was looking at an AX with 4 outputs and derated the sink for lower delta-T perhaps too aggressively. I concur with Patrick that the junction temperatures should be around 100C and cause no problems.

Thinking further, since the extremities of the sink are relatively cool it stands to reason that the tips of the fins would be touchable, and this would be a warm but usable solution.

Funny that sometimes the gut reaction doesn't stand up to engineering scrutiny. Nothing says you can't overbuild if you wish...
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Old 26th October 2010, 06:21 PM   #305
EUVL is offline EUVL  Europe
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My famous AXJ100 Tower has been running at > 60C sink temperature for the last 4 years without the slightest problem.
There is still a thermal path from sink to junction which one can do something about.
There are good reasons why I do not use mica & goop but Keratherm for Class A.

And I don't sit on my amp when I listen to music, so what is the problem with a 67C sink, even if it were.

I shall not disclose any further technical details until the design is finished and the prototype is tested, if it continues like this.


Patrick

Last edited by EUVL; 26th October 2010 at 06:27 PM.
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Old 26th October 2010, 07:49 PM   #306
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Patrick, I hope I haven't offended you. My sincerest apologies if I have. I should have said my gut feelings sometimes don't stand engineering scrutiny - it was supposed to be a joke at my own expense.

I don't sit on my amp, either. The cat wants to now that the weather is turning cooler. Sorry if I am a bit of a burn safety nut. A bicycle accident has left me very slow to sense heat and susceptible to burns before I feel warm.

Last edited by BobEllis; 26th October 2010 at 07:54 PM.
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Old 26th October 2010, 08:43 PM   #307
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"Why don't you knock it off with them negative waves? Why don't you dig how beautiful it is out here? Why don't you say something righteous and hopeful for a change?" - Oddball, Kellys Heroes

Edit: If you have seen the movie, then you will know this is meant to be a light hearted joke. Relax everyone. Have a little faith in Euvl.

Last edited by Melon Head; 26th October 2010 at 09:07 PM.
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Old 26th October 2010, 10:15 PM   #308
flg is offline flg  United States
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Quote:
Originally Posted by EUVL View Post
First I have a Ph.D. in mechanical engineering.

At steady state, ALL heat is dissipated through the heat sink, however "sheety" the interface is.
None is disapatted to the surrounding atmosphere through the uninterfaced surface of the package? Or the leads?Hmmm?

Quote:
Originally Posted by EUVL View Post
If you would believe me, and I know what I am talking about in mechanical things, then we have no transistor reliability problems even with 67C sink temperature.
Well, You started this post with a attempt at quoting me, therby addressing my question to thanh but, I said nothing about a transistor reliability problem. I did have some problem with my post not actually being posted when I hit the button? Therfore I did not see your posts #s 301 and 302.

Quote:
Originally Posted by EUVL View Post
If you want to believe what people give out here as holy rules and not look at the science,
Wait a minute. Is not the reason thanh is doing this because the abitlty of controling all the variables is difficult and going by manufacturer's data is not always proven by actual application. I'm tempted to go back a few pages to find you Patrick saying something to that effect. Now however you come out with calculations that you seem to want to make a "holy rule" after it has been proven by bench verification?

Quote:
Originally Posted by EUVL View Post
...then as I said, use a Hifi2000 case or something of similar size.
This type of statement is getting old. Pardon my audacity but it is starting to sound like the Blowtourch thread.

Quote:
Originally Posted by EUVL View Post
When I am eventually finished with the case design, the sink temperature will drop further.
I have respected your desire to not be bothered by us pee-ons. Sorry, your post did quote me.

Quote:
Originally Posted by EUVL View Post
There is nothing to criticise in thanh's experiment IMHO.
First
Quote:
Originally Posted by EUVL View Post
First I have a Ph.D. in mechanical engineering.
IMHO? I could start over but...

>I'm not trying to criticise your work or efforts but I think we need that data too, in order to get a real working condition estimate? (Me)
I think it is strange you elude to the interface conditions but I cannot ask thanh the details of his work?

Quote:
Originally Posted by EUVL View Post
Please kindly take your time to read post #301, 302 and the reliability handbook from Toshiba.
Thank you, for a while I was having alot of trouble finding anything good in your post. As I said I've got some problem now and then where my posts are showing up hours after I type them??? I have no problem with your calculations, estimations or proven examples. My problem is they way you rudely addressed me answering my post directed at someone else. But you did not answer my question? Certainly not "kindly".
I guess I'll shut-up now. I'm probably getting myself in trouble saying what I said. I need to go out in the yard now
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Old 26th October 2010, 10:16 PM   #309
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Was going to post sometink about copper plates and heat spreaders but better not.
Hot here.
Al
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Old 27th October 2010, 01:58 AM   #310
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Hi flg
For the thermal pads I use Bergquist SP2000-0.015-00-122.
The mosfets used were IRFP240/9240.
Intially I determined there was not an issue with Tj-case so I didn't bother to record it.
I still need to do more tests here to try and spread the heat more evenly across the heatsink and bring down the temp around the mosfets, so I can record the temperatures of the mosfets in these followup tests.

Heat transfered from the mosfet casing directly to air is limited by the surface area of the mosfet, ambient temp and convection. Since the surface area of the mosfet is small relative to the heatsink surface area (as a guess 1:1000) the heat dissipated to the air directly from the mosfet is very small (you could probably say insignificant) compared to the heat dissipated via the heat sink to the air
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