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Class D Switching Power Amplifiers and Power D/A conversion

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Old 24th August 2011, 09:14 PM   #1
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Join Date: Jun 2011
Default Heat

Is it normal for a class D amp to run a lil warm when idle and cool down when its being used? Seems a lil weird. The heat isn't bad or anything, just an observation of use, it always a lil warm when sitting idle but cools down a lot when its being run pretty hard. Whats going on to make it react opposite like that? Its realy strange.
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Old 25th August 2011, 04:48 PM   #2
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Join Date: Dec 2003
Location: Munich
Seems like your design uses an output inductor with inductance and short dead time.
In this case, during idle the inductor current does not force the switching node to slope from rail to rail during dead time and the switch which is being turned ON operates in hard switching. Means you have hard switching of both MosFets.
At higher load, the load current will force the switching node to slope from rail to rail during dead time in one direction (depending on the polarity of the LF output current). Hard switching takes place only in one or the other MosFet (depending on the polarity of the LF output current) , means less switching losses.
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Old 25th August 2011, 08:29 PM   #3
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Interesting.. it doesn't get realy hot at all, just a lil warm to to touch, but the heat goes away with use and this isn't a home brew amp, its one built into a set of speakers. Was just wondering about the opposite heat response and what causes it to react like that since I never seen a heat response like that in any other type of amp. Class d is all new to me and the SQ on these things is spectacular! Warm, thick, lively sound from ss is incredible. I have been reading up on this new technology and its very interestng. So basically are you saying the output inductor is a lil too small for the system and causing the hard switching of both mosfets or the chip? I don't know if this system uses a chip or descreet mosfets as I have never taken it apart and don't intend to, it works great, sounds awsome and the heat is not bad at all, I just noticed how it reacts to load and was curious. Its rated for 20w/ch so probably a chip amp. I am planning on building a class D amp around 200 or 400 w/ch to drive large speakers and use as a keyboard amp after I study the tech a lil better.
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Old 26th August 2011, 07:52 PM   #4
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Join Date: Dec 2003
Location: Munich
The opposite way. If the inductor value is high, you will get less inductive ripple current and more hard switching during idle.
But there is no reason to say that such a design would be wrong.
Short dead time is fine for low distortion and higher L values are helpfull to achieve low voltage ripple at the speaker without excessive output caps.
It is not a must to operate in soft switching during idle.
As long as idle losses and EMI remain acceptable - I tend to say: Don't worry.
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