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29th August 2012, 11:12 AM  #1741 
diyAudio Chief Moderator

3A+ at 10Vinout 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.

29th August 2012, 01:02 PM  #1742 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

Salas, you must derate 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 derated SOAR. That could easily be <<50% of the SOA @ 25°C
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regards Andrew T. 
29th August 2012, 01:08 PM  #1743 
diyAudio Chief Moderator

Yes, that is why I generally said 2A TO 2.5A given some good sinking. Who will do it should calculate and evaluate closer.

29th August 2012, 01:18 PM  #1744 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

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 sa 2C/W. Ta = 40°C Ts ~ 40 + [2 * 20 * 1.2] ~ 88°C Rth cs ~ 1C/W Tc ~ 88 + 1 * 20 ~ 108°C Derating factor = [150108]/[15025] = 42 / 125 = 0.336 The absolute max Pd for Tj =150°C 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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regards Andrew T. 
29th August 2012, 01:26 PM  #1745 
diyAudio Chief Moderator

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.

29th August 2012, 04:26 PM  #1746 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

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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regards Andrew T. 
29th August 2012, 08:00 PM  #1748 
diyAudio Chief Moderator

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.

29th August 2012, 08:40 PM  #1749 
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders

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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regards Andrew T. 
29th August 2012, 08:58 PM  #1750 
diyAudio Member
Join Date: Jan 2011
Location: Wisconsin

Boy Andrew, why do you have to be so contrary all the time?

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