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 29th August 2012, 01:02 PM #1741 AndrewT   R.I.P.   Join Date: Jul 2004 Location: Scottish Borders 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 @ 25°C __________________ regards Andrew T.
 29th August 2012, 01:08 PM #1742 Salas   diyAudio Chief Moderator     Join Date: Oct 2002 Location: Athens-Greece 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 #1743 AndrewT   R.I.P.   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 s-a 2C/W. Ta = 40°C Ts ~ 40 + [2 * 20 * 1.2] ~ 88°C Rth c-s ~ 1C/W Tc ~ 88 + 1 * 20 ~ 108°C De-rating factor = [150-108]/[150-25] = 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? __________________ regards Andrew T.
 29th August 2012, 01:26 PM #1744 Salas   diyAudio Chief Moderator     Join Date: Oct 2002 Location: Athens-Greece 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 #1745 AndrewT   R.I.P.   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. __________________ regards Andrew T.
 29th August 2012, 06:16 PM #1746 RCruz   nhinstruments diyAudio Member     Join Date: Sep 2006 Location: Wallis Beware my friend __________________ RC
Salas
diyAudio Chief Moderator

Join Date: Oct 2002
Location: Athens-Greece
Quote:
 Originally Posted by AndrewT 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.

 29th August 2012, 08:40 PM #1748 AndrewT   R.I.P.   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. __________________ regards Andrew T.
 29th August 2012, 08:58 PM #1749 mjock3   diyAudio Member   Join Date: Jan 2011 Location: Wisconsin Boy Andrew, why do you have to be so contrary all the time?
 29th August 2012, 09:08 PM #1750 wushuliu   diyAudio Member     Join Date: May 2008 Location: Pacific NW It's alright, Andrew seems to be safety first and 'an ounce of prevention'. Nothing wrong with that. Dirty job, but somebody has to do it! :P

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