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Old 29th January 2012, 08:51 PM   #1
effebi is offline effebi  Italy
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Default Can a D class give DC output?

Hi all,
I am new to D class stuff, so forgive me if the question is stupid.
I have an application where I need to drive a speaker-similar load (1 Ohm ) with a sine wave of about 5-10 KHz constant frequency. The problem is that the mean value of the sine get shifted up or down so there is a good DC content in output. Peak to peak values never exceed 50 V either side.
Actually we are using straight class B amplifiers (something like 10 TO3 pairs). Would it be possible to do it more efficiently with a class D?
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Old 29th January 2012, 08:55 PM   #2
stoc005 is offline stoc005  United States
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A class D amplifier is bipolar SMPS with a wide bandwidth control loop.
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Old 29th January 2012, 09:38 PM   #3
gmarsh is offline gmarsh  Canada
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No problem for a class D amp to do that, and save a bit of power.

Got any specs for the application, like a maximum THD level of the sine wave?
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Old 29th January 2012, 09:45 PM   #4
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Quote:
Originally Posted by stoc005 View Post
A class D amplifier is bipolar SMPS with a wide bandwidth control loop.
...means basically, yes. But take care with supply rail pumping.
Solution a) Go for bridged configuration or a full bridge design.
Solution b) If you need the load between amp out and GND then use an unbridged amp, but power the classD amp from an SMPS with synchronous rectifiers.

P.S.
Note that class D amps do have an superimposed HF ripple on the output.
Usually in the range between 300mVrms...1Vrms. Typical frequencies of such ripple are ranging between 150kHz...500kHz.
If this is Ok for your load, then a high current class D amp should work to power it.
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Old 29th January 2012, 09:58 PM   #5
benb is offline benb  United States
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Yes.
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Old 29th January 2012, 10:08 PM   #6
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When the mean gets shifted up and down, that means it is not DC, just AC with a low frequency.
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Old 30th January 2012, 07:30 AM   #7
effebi is offline effebi  Italy
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Thanks to all for the kind answers. The thing starts to lok feasible.Then:
Quote:
Originally Posted by vacuphile View Post
When the mean gets shifted up and down, that means it is not DC, just AC with a low frequency.
Yeah, *very* low, in the sense that it can stay shifted (i.e. +10 V) for 1/2 hour. Prcatically a true RMS meter always reads a lot of DC on the output.
BTW, for me HF content is not a big deal, and THD = 1% is OK. As I said, we are using pure class B stuff, with some ugly crossover distortion.
Then, full bridge could be a problem, since the load is usually referred to ground (really the amplifier has a current feedback sensing a 0.1 ohm shunt to the ground) What do you mean with a sync. rectifier? That the clock of the SMPS should be in sync with the amplifier (and maybe pi out of phase)?
And ever seen a D class connected in current feedback?
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Old 30th January 2012, 08:07 AM   #8
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If the SMPSU uses synchronous rectification (using MOSFETs) instead of diodes for the output voltage supply, then it may be able to absorb in an almost lossless manner the back-EMF caused by 'bus pumping' in the half-bridge class-D amplifier.
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Old 30th January 2012, 08:14 AM   #9
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I guess by 'current feedback' you mean that the output of the amplifier is a regulated current into the load, rather than a regulated voltage across the load. The answer is yes, this is quite possible. I have made a class-D amplifier using a self-oscillating hysteretic output stage with the sensing signal for the hysteretic PWM modulator being the voltage developed across the current-sense resistor in series with the load.
If you don't need a very high frequency for the PWM oscillation, you could use a full-bridge output where the required current-sensed feedback is produced by a Hall-effect current sensor, such as the Allegro ACS712 series.
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Old 30th January 2012, 05:44 PM   #10
Pafi is offline Pafi  Hungary
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Output of a bridged amp (and current sense resistor) can be grounded also, just the power supply have to be floated.

Hall-effect current sensors are very noisy (even the expensive LEM ones too), plus open loop ones like ACS712 are very inaccurate (and slow). Resistor is much better!
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