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Old 9th December 2014, 05:43 AM   #1
akis is offline akis  United Kingdom
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Join Date: Jul 2008
Default Output stage efficiency with varying rails voltage

I am powering a class AB output stage from batteries, and the rail voltages will vary from +/-21V to +/-25V.

The output must remain full amplitude and constant (as the supply decreases due to batteries getting slowly depleted) and the efficiency must be maximised at all times.

In order to maximise efficiency the signal amplitude must be as close to the rails as the output stage will allow.

But with varying rails from 21V to 25V, I have designed for the 21V, which means that when the batteries are fresh, 25V, then we have a lot of wasted power.

Ideas for improving the situation:

I was thinking of placing a DC-DC converter LM2596 at the batteries end, to provide a constant DC, say 20V, so then I can design for a fixed 20V. The DC-DC converter would be 80%-90% efficient and will not get too hot. not as much as the output stage would get if it were fed with 25V but used only 20V.

PS: Sorry meant to say LM2679

Another idea that has been suggested is to use a class D output stage - except I have no knowledge of this and the immediate question is would it be efficient at full volume which is how my device works?

Last edited by akis; 9th December 2014 at 06:05 AM.
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Old 9th December 2014, 11:33 AM   #2
brig001 is offline brig001  United Kingdom
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Join Date: Jan 2009
Class D is the only way if you want something efficient and this explains it better than I can: Class-D Amplifiers

Assuming you are playing music rather than a continuous waveform, your efficiency will be low. The article says less than 50%, but I would say less than 20% as music typically has more than 10dB dynamic range, so your average music signal will be at less than 10% power on the first graph on the page.

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