Heat sink sizing for power amps
 User Name Stay logged in? Password
 Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read Search

 Solid State Talk all about solid state amplification.

 Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
 4th February 2011, 07:28 PM #1 prairiemystic   diyAudio Member   Join Date: Jul 2004 Location: Edmonton, AB Canada Heat sink sizing for power amps I'm trying to ballpark heatsink requirements for Class B power amplifier (output transistors). I started with the calculations here: Elliot Sound Amplifier Efficiency Theoretically, a rail-to-rail amp with +/20V rails, puts 25W RMS into an 8ohm load. This is Iout=1.768A RMS with Voutput transistor=one rail leading to... the output stage dissipating 10.36W There is some argument over using RMS power (0.707), average power (0.636) or music power (?) in these calcs. An amp used in a dance club at full bore would dissipate less than max. sinewave on a bench. Extending the same calculation to a Leach low-TIM amp: With a 80VCT transformer, rails at +/-58VDC and assuming 5 volts/rail loss in the driver/output stages/emitter resistors... =about +/-53VDC output swing into 8 ohms is 175W, with the output transistors dissipating around 96W. Leach used Wakefield 423K thermal resistance of 0.94 °C/W or a 47°C rise above ambient at 50W in convection. That's about 110°C/230°F on the bench at full power sinewave. But to what to use for calcs in music?
mnemneth
diyAudio Member

Join Date: Feb 2006
Location: Dundee
Quote:
 Originally Posted by prairiemystic Theoretically, a rail-to-rail amp with +/20V rails, puts 25W RMS into an 8ohm load. This is Iout=1.768A RMS with Voutput transistor=one rail leading to... the output stage dissipating 10.36W There is some argument over using RMS power (0.707), average power (0.636) or music power (?) in these calcs. An amp used in a dance club at full bore would dissipate less than max. sinewave on a bench. Extending the same calculation to a Leach low-TIM amp:
No argument at all.

RMS power is a myth and "Music power" is a gimmick put forward by amplifier makers to make their product seem better than it is and should be regarded with the utter contempt is deserves.

Average power rules. Simples.

What you would need to find is the maximum power your amp could produce in the most arduous of circumstances and I would say that would be a square wave at half your rail to rail voltage into your minimum load value.

This would pre-suppose output into a resistive load, of course.

Sandy

 4th February 2011, 09:24 PM #3 flg   diyAudio Member     Join Date: Jul 2005 Location: North East A square wave, if I remember right, would be 1.33% greater rms voltage than a sine wave of equal peak voltage. Did I get that right? __________________ "It was the perfect high end audio product: Exotic, inefficient, expensive, unavailable, and toxic." N.P.
 4th February 2011, 11:45 PM #4 prairiemystic   diyAudio Member   Join Date: Jul 2004 Location: Edmonton, AB Canada I looked at National Semi's chip-amp app note AN-1192.pdf and they calculate Pdiss max=(Vcc tot^2)/(2pi^2)*RL. I have no idea where the pi squared came from. Using their eqn with my previous Leach amp calc, I get 71W instead of the 96W for Pdiss. Also, they mention that using a real "audio" application (avg. music power) doesn't work well, as unregulated power supplies have 15-35% higher no-load voltage "causing the overall maximum power dissipation to be higher than expected", and the two cancel out.
 5th February 2011, 12:00 AM #5 nigelwright7557   diyAudio Member     Join Date: Apr 2008 Location: Carlisle, England In class AB the worst case of power into the heatsink comes at 2/3 power and that is 40% of total power is dissipated in the heatsink and 60% into the load. Class D heatsinks are usually much smaller for the same power. __________________ PCBCAD51 pcb design software. 2018 version out now with lower prices >> http://www.murtonpikesystems.co.uk

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Site     Site Announcements     Forum Problems Amplifiers     Solid State     Pass Labs     Tubes / Valves     Chip Amps     Class D     Power Supplies     Headphone Systems Source & Line     Analogue Source     Analog Line Level     Digital Source     Digital Line Level     PC Based Loudspeakers     Multi-Way     Full Range     Subwoofers     Planars & Exotics Live Sound     PA Systems     Instruments and Amps Design & Build     Parts     Equipment & Tools     Construction Tips     Software Tools General Interest     Car Audio     diyAudio.com Articles     Music     Everything Else Member Areas     Introductions     The Lounge     Clubs & Events     In Memoriam The Moving Image Commercial Sector     Swap Meet     Group Buys     The diyAudio Store     Vendor Forums         Vendor's Bazaar         Sonic Craft         Apex Jr         Audio Sector         Acoustic Fun         Chipamp         DIY HiFi Supply         Elekit         Elektor         Mains Cables R Us         Parts Connexion         Planet 10 hifi         Quanghao Audio Design         Siliconray Online Electronics Store         Tubelab     Manufacturers         AKSA         Audio Poutine         Musicaltech         Holton Precision Audio         CSS         exaDevices         Feastrex         GedLee         Head 'n' HiFi - Walter         Heatsink USA         miniDSP         SITO Audio         Twin Audio         Twisted Pear         Wild Burro Audio

 Similar Threads Thread Thread Starter Forum Replies Last Post portreathbeach Chip Amps 5 28th February 2010 11:56 AM cfcubed Pass Labs 5 23rd July 2008 04:59 PM ECM Car Audio 25 6th September 2006 04:07 AM webercarbmann Swap Meet 13 21st January 2004 06:38 PM rosss Pass Labs 11 23rd February 2003 05:23 AM

 New To Site? Need Help?

All times are GMT. The time now is 06:44 AM.