Author Topic: Power dissipation in product app notes  (Read 1616 times)

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Offline Stuartambient

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Power dissipation in product app notes
« on: July 22, 2016, 09:53:56 pm »
Bit confused here.  The data sheet is saying 4.17C/W thermal resistance (film to case).  In the application notes though it lets you select the model and how much power the app will generate.  I'm wondering why it only goes up to 17.4W in the table and not 30W as rated.  I must be missing something here.   :-//
 

Offline Kilrah

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Re: Power dissipation in product app notes
« Reply #1 on: July 22, 2016, 10:02:21 pm »
What are we talking about?
What does the * say?

How are we supposed to guess?
« Last Edit: July 22, 2016, 10:13:37 pm by Kilrah »
 

Offline Stuartambient

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Re: Power dissipation in product app notes
« Reply #2 on: July 22, 2016, 10:10:38 pm »
What are we talking about?
What does the
  • say?


How are we supposed to guess?

Sorry.  I'm speaking about some power resistors for current sensing.  In the application notes there is a quick guide to selecting a heat sink.  It allows you to find your chip and then the power dissipation of your app to get the C/W.  I'm wondering if the chip is rated for 30W why is the table only giving me options up to 17.4W.

They say this chips is fine for being attached to even a case panel so no big concern about the right size heat sink. Just confused and thinking maybe they left some numbers out of the application notes.

Sorry again for the confusion. 
 

Offline Kilrah

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Re: Power dissipation in product app notes
« Reply #3 on: July 22, 2016, 10:17:04 pm »
What does the * say?
Can't we have the entire datasheet? The reason is likely mentioned, or at least hints to it that we get no chance of seeing. We don't even know the title of the things you posted and what they're supposed to be.

It's probably that the heatsink selection takes the derating into account for continuous dissipation.
« Last Edit: July 22, 2016, 10:18:45 pm by Kilrah »
 

Offline Stuartambient

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Re: Power dissipation in product app notes
« Reply #4 on: July 22, 2016, 10:27:44 pm »
I attached the datasheet.  The above table is from the Application Notes.   Specifically it is the MP930.  I believe power rating and power dissipation are two different things so maybe that is what's throwing me. 
 

Offline suicidaleggroll

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Re: Power dissipation in product app notes
« Reply #5 on: July 22, 2016, 10:30:14 pm »
The "30 Watts" number is assuming a heatsink with 0 C/W, in other words "perfect", with a perfect interface, and with the ambient at 25 C.  That gives 4.17*30+25 = 150 C, which is pretty standard.

As soon as you stick a real heatsink on there, the maximum power comes down.  Keep Tamb at 25 C, but put a 2 C/W heatsink on with a 1 C/W interface, and you get a total film to ambient thermal resistance of 7.17 C/W, which at 17.4 W brings you right back up to the 150 C limit.

In a real application, you need to take your maximum ambient temp, your actual heatsink thermal resistance, and your actual thermal interface, to estimate the maximum power you can dissipate while staying below 150 C.

P = (150 - Tamb) / (4.17 + Hr + Ir)
Where Hr is the heatsink thermal resistance and Ir is the interface thermal resistance
« Last Edit: July 22, 2016, 10:34:32 pm by suicidaleggroll »
 

Offline Kilrah

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Re: Power dissipation in product app notes
« Reply #6 on: July 22, 2016, 10:32:55 pm »
Yep that's it.

Quote
* Derating Using Case Temperature (TC)
[...]
Quote
Up to 30 Watts at +25°C Case Temperature

You could dissipate 30W IF whatever cooling solution you had could maintain the case at 25°, which at ambient temp would mean an ideal heatsink with no thermal resistance that obviously doesn't exist, or an active cooling system that goes below ambient.

The app note considers a real world, normal heatsink with a 2°C/W thermal resistance.
 

Offline Stuartambient

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Re: Power dissipation in product app notes
« Reply #7 on: July 22, 2016, 11:07:54 pm »
Okay that is very helpful! Makes more sense.

I have some heat sinks and some fans, though I have no idea of their thermal resistance though.  Then again the notes say 

Quote
All Caddock TO-Style resistor packages have the resistor element electrically isolated from the heat sink mounting surface. Therefore, the chassis, metal panels or PC board metallization can effectively be utilized for heat sinking. The following are typical thermal resistances for commonly available heat sinking surfaces

It almost makes it sound easy to keep this thing cool.  Guess I'll need to experiment.  Temperature sensors would really be the only way to know what the case temp is though?
 


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