FET Static Power Dissipation - diyAudio
 FET Static Power Dissipation
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 14th April 2013, 05:37 PM #1 BRSHiFi   Previously known as kingden   Join Date: Aug 2008 Location: Evanston, IL FET Static Power Dissipation For a FET I am using in an output stage, the Gate to Source voltage needs to be about 4 volts for proper biasing. I set the bias to something a little above 4. The FET is conducting about 1 amp of current at idle. The power supply is +/-40 volts. In class AB, there is 0 volts at the output at idle, so 40 drain to source. Does this mean it will dissipate 40 watts at idle, or do I use the RDSon value to compute this (Power Dissppated = I^2 * RDSon)?
 14th April 2013, 06:02 PM #2 CBS240   diyAudio Member     Join Date: Aug 2005 Location: K-town V X I = P 40V X 1A = 40W __________________ All the trouble I've ever been in started out as fun......
 14th April 2013, 06:05 PM #3 BRSHiFi   Previously known as kingden   Join Date: Aug 2008 Location: Evanston, IL Normally do Class AB fet stages dissipate this much static power at idle? If so, heat-sinking is the biggest must in the world.
 14th April 2013, 06:18 PM #4 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders +-40Vdc supply equals 80Vdc rail to rail. 1A passing as output bias from rail to rail (also equals zero output current) amounts to bias dissipation = 80W __________________ regards Andrew T. Sent from my desktop computer using a keyboard
 14th April 2013, 06:26 PM #5 BRSHiFi   Previously known as kingden   Join Date: Aug 2008 Location: Evanston, IL I am just doing half the system for clarification sake. Yes, the entire system would be 80 watts. My question is, am I biasing too much into class A or is this normal?
 14th April 2013, 06:58 PM #6 BRSHiFi   Previously known as kingden   Join Date: Aug 2008 Location: Evanston, IL The FETs are IRFP240 for the nmos and IRFP9240 for the pmos.
 15th April 2013, 10:54 AM #7 AndrewT   diyAudio Member   Join Date: Jul 2004 Location: Scottish Borders In Borbely's opinion you are biasing too much for FET outputs. He suggests at least 500mA irrespective of how many output pairs are adopted. __________________ regards Andrew T. Sent from my desktop computer using a keyboard

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