Class D Crest factor?
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 Class D Switching Power Amplifiers and Power D/A conversion

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 14th January 2012, 08:45 AM #1 eem2am Banned   Join Date: Jun 2011 Class D Crest factor? Hello, Can you confirm, that it is physically impossible for a Class D amplifier to be able to draw more than peak_power/2 continuously?
 14th January 2012, 10:05 AM #2 Cristi Banned   Join Date: Dec 2008 Location: Hong Kong or Europe You ask who? :-D negative. it is possible, as long as you drive the amp with max. amplitude of the input signal. in fact the amp will draw the output power / eff. i do not comment weather or not can withstand this for long term, this depends on the particular design. imagine that you use a class D amp to drive whatever load needs a continuous wave, sine or other shape, a particular example is a DC-AC inverter, who
ontoaba
diyAudio Member

Join Date: Nov 2009
Location: Kudus, Malang, Dieng
Quote:
 Originally Posted by Cristi You ask who? :-D negative. it is possible,.... who

Most of them has over current protection at its supply. So high current that higher than the current protection treshold just work until capacitor discharged and protection work in.

iand
diyAudio Member

Join Date: Feb 2007
Location: London
Quote:
 Originally Posted by eem2am Hello, Can you confirm, that it is physically impossible for a Class D amplifier to be able to draw more than peak_power/2 continuously?
On an unclipped sinewave, this is obviously the case by definition for any amplifier.

On a squarewave, peak and average powers (and voltages, and currents) are the same.

Cristi
Banned

Join Date: Dec 2008
Location: Hong Kong or Europe
Quote:
 Originally Posted by ontoaba ...WHO
I was writing the reply while the forum went to sleep for the daily maintenance. when i pushed submit, just part of the message was posted. can be deleted. anyway i forgot my idea.

 14th January 2012, 06:55 PM #6 eem2am Banned   Join Date: Jun 2011 -ok, i see your meaning, though i used to work on SMPS for flat screen TV. I did loads of thermal tests on the flyback which supplied the Class D audio amplifiers. I distinctly remeber that at peak audio power, the flyback failed the thermal test by a long way. However, no managers were concerned. -Because all they were bothered about was that the flyback could continuously supply peak audio power divided by three....which it could. Given this fact, on these TV's which are sold the world over, how can we be saying that Class D amplifiers can supply more than peak power divided by two.?
Saturnus
diyAudio Member

Join Date: Jun 2005
Location: Copenhagen, Denmark
Quote:
 Originally Posted by eem2am -Because all they were bothered about was that the flyback could continuously supply peak audio power divided by three....which it could.
That's a very conservatory specification. If I were in the finance department of this company I would demand why these engineering managers agreed to a too high specification. Continuous output is typically -9/-10dB or 8/10 times lower than the peak output, so 3 times lower is a very conservatory specification and it probably should have been even lower at peak power divided by 4 or even 6, to give at least 50% headroom. 200%+ headroom seem exaggerated.

Naturally that could be tempered by the opposite cost of needed large capacitors but it was something I would definitely address if I was in your finance department.

Last edited by Saturnus; 14th January 2012 at 07:09 PM.

 14th January 2012, 07:50 PM #8 eem2am Banned   Join Date: Jun 2011 OK thanks, in my post (2 back) i accidentally said the Class D was supplying.....of course, i meant the smps (the smps that supplies the Class D amplifier.) or rather , that the class D amplifier "draws" power Anyway, Heatsinked fets heat up in about 2 seconds, so i wouldnt want to de-rate that much. Last edited by eem2am; 14th January 2012 at 07:53 PM.
Saturnus
diyAudio Member

Join Date: Jun 2005
Location: Copenhagen, Denmark
Quote:
 Originally Posted by eem2am Anyway, Heatsinked fets heat up in about 2 seconds, so i wouldnt want to de-rate that much.
They're only at peak output for a few microseconds or picoseconds at a time though, or roughly 10% of the time as noted above so why should it be a concern?

 17th January 2012, 01:02 AM #10 Eva   diyAudio Member     Join Date: Oct 2003 Location: Near the sea Detecting a too hot power supply is far easier than detecting a too hot voice coil. Replacing a shorted diode or transistor is easier than replacing a couple of TV speaker drivers with custom sizes and specs. __________________ I use to feel like the small child in The Emperor's New Clothes tale

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