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Old 29th August 2012, 11:12 AM   #1741
Salas is offline Salas  Greece
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Quote:
Originally Posted by merlin el mago View Post
How much A can give IRF9520?
3A+ at 10Vin-out CCS by its safe operating area spec. This is 30W+ and I would avoid using that 40W part like that constantly, but at 2A to 2.5A it should be living longer given the sinking is good.
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Old 29th August 2012, 01:02 PM   #1742
AndrewT is offline AndrewT  Scotland
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Salas, you must de-rate the Pq to take account of case temperature (Tc).
Select a sink. Model the Ts & Tc for your operating conditions.
Use the Tc to find the de-rated SOAR.

That could easily be <<50% of the SOA @ 25C
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Old 29th August 2012, 01:08 PM   #1743
Salas is offline Salas  Greece
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Yes, that is why I generally said 2A TO 2.5A given some good sinking. Who will do it should calculate and evaluate closer.
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Old 29th August 2012, 01:18 PM   #1744
AndrewT is offline AndrewT  Scotland
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Salas,
that "guide" value for 2A to 2.5A for a 40W device is totally misleading.

eg.
Take 2A and 10Vds resulting in Pq=20W
Rth s-a 2C/W.
Ta = 40C
Ts ~ 40 + [2 * 20 * 1.2] ~ 88C
Rth c-s ~ 1C/W
Tc ~ 88 + 1 * 20 ~ 108C
De-rating factor = [150-108]/[150-25] = 42 / 125 = 0.336
The absolute max Pd for Tj =150C is 13.44W

For a lower continuous Tj I would recommend a continuous Pq << 13.44W, maybe 8W.
That leaves you continuous current @ ~800mA for 10Vds.

Can you see why your 2A to 2.5A is totally ridiculous?

Now increase the mains voltage by 6% and recalculate the worst case Vds voltage. Is it 12V or 13V or 14V?

What would be the short term Tj for this continuous situation?
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Old 29th August 2012, 01:26 PM   #1745
Salas is offline Salas  Greece
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Watch your language when referring to members. You are not the calcs policeman. I did not make any special calculations. Just said that 40W parts are usually safe at half their rating.
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Old 29th August 2012, 04:26 PM   #1746
AndrewT is offline AndrewT  Scotland
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And that is the statement that is ridiculous.

Now model again for 1.5A
If that is still too high then try again at 1A.
Eventually you will find a more sensible value than 2A to 2.5A.
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Old 29th August 2012, 06:16 PM   #1747
RCruz is offline RCruz  Switzerland
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Old 29th August 2012, 08:00 PM   #1748
Salas is offline Salas  Greece
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Quote:
Originally Posted by AndrewT View Post
And that is the statement that is ridiculous.

Now model again for 1.5A
If that is still too high then try again at 1A.
Eventually you will find a more sensible value than 2A to 2.5A.
You model a 2A biased Mosfet on a 2C/W tiny sink assumption and you want to pose the serious one. LOL. Sink it on a teaspoon is next.
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Old 29th August 2012, 08:40 PM   #1749
AndrewT is offline AndrewT  Scotland
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2C/W is hardly small.
I have used 5C/W and 7C/W and 10C/W in high current versions.
And I use 20C/W & 30C/W in ordinary DCB1.

And that 2C/W is only taking the one mosFET.
If all 4 mosFETs were fitted it would need to be bigger than 1C/W

Model your suggestion and let the Members see what you are proposing.
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Old 29th August 2012, 08:58 PM   #1750
mjock3 is offline mjock3  United States
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Boy Andrew, why do you have to be so contrary all the time?
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