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Author Topic: How to calculate Transistor Power in Watts  (Read 13310 times)
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thetrueman
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« on: May 19, 2008, 06:49:48 06:49 »

Hi all,

I am looking at the datasheet of 2SD1047 transistor which says the transistor is 12A 140V 100W. I want to know how Power is calculated?

If I use this transistor at 12V 10A load how it affects the wattage?

Shahzad
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zuisti
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« Reply #1 on: May 19, 2008, 10:34:43 10:34 »

I am looking at the datasheet of 2SD1047 transistor which says the transistor is 12A 140V 100W. I want to know how Power is calculated?
If I use this transistor at 12V 10A load how it affects the wattage?

Hi thetrueman,
> 12A 140V 100W datas mean this:
max. 12 A collector current, max 140v collector-emitter voltage but only max 100w dissipation power (I * U).

On your example (transistor at 12V 10A load) if the transistor is well opened (it has enough basic current), then it's working like a shortcut. In the reality a small (saturation) voltage remains on it. In this case we can be calculate with 2V (see the datasheet diagrams). So the dissipation ('wattage') will:
2v * 10A = 20W

Hope ths helps.
zuisti
 
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thetrueman
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« Reply #2 on: May 19, 2008, 11:14:49 11:14 »

Thank you, Now I understand basic things but under wchich terminology I'll find 2V (as you calculated) in the datasheet to calculate the total power.
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zuisti
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« Reply #3 on: May 19, 2008, 11:37:39 11:37 »

Thank you, Now I understand basic things but under wchich terminology I'll find 2V (as you calculated) in the datasheet to calculate the total power.

In the datasheet (for ex. http://www.farnell.com/datasheets/82792.pdf), the terminology is:
Collector to Emitter Saturation Voltage (see the 12th diagram, Vce(sat) - Ic).
The 2v at 10 A is a typical value only (the worst case value may be higher!).

The current gain of this transistor (at 10A) is only 20, so use min 0.5A basic current to open it !!
zuisti
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sputnik
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« Reply #4 on: May 20, 2008, 05:04:11 05:04 »

I think he wants to know how they come up with only 100w

I know it sounds confusing how they would only get 100w power.

At 140V 12A thats 1680w!!!  Right???   Wrong!!!!!!!!!!

What they mean is, is can handle 140v, but only at .7A as this equals the 100W.  OR it can handle up to 12A at only 8.33v. Or any combination of the two as long as it doesnt exceed 100W of power.


This power rating is a PACKAGE rating. Meaning the package is only capable of dissipating 100W of power before melting down.  Either the plastic, internal connections, solder or something else to do with the package construction will fail at the heat 100w will generate.  There arent many power transistors that will handle much more power than that by themselves. Either parallel mosfets for higher power or use a large fan cooled heat sink.
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thetrueman
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« Reply #5 on: May 20, 2008, 07:58:05 07:58 »

I think he wants to know how they come up with only 100w

I know it sounds confusing how they would only get 100w power.

At 140V 12A thats 1680w!!!  Right???   Wrong!!!!!!!!!!

What they mean is, is can handle 140v, but only at .7A as this equals the 100W.  OR it can handle up to 12A at only 8.33v. Or any combination of the two as long as it doesnt exceed 100W of power.

Yes sputnik, I want to know what you mentioned. But now there are two opinions so I'm little bit confused. Can anybody clarify that which one from above two opinions are more to adopt for my application? Thanks.
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