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Class D Switching Power Amplifiers and Power D/A conversion

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Old 10th May 2005, 08:43 AM   #11
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Quote:
Originally posted by JohnW
Is this a full range ribbon - or just Mid - Hi?

If full range (Bass), have you considered that you’re going to have to use a full bridge (2x RDSon losses), with a Half Bridge I can't image how you would prevent PSU pumping with such a Low Load Resistance....

Regulated PSU that can Sink as well as Source maybe, but then you might as well use a full bridge.

John
Even for the mid/high I would seriously look at full bridge.
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Old 10th May 2005, 10:49 AM   #12
JohnW is offline JohnW  Hong Kong
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Hey wait a minute – I have almost 10 Farads of bulk reservoir capacitance here

Joking aside – maybe those 2 Farads “Bake bean Cans” used for car audio would be perfect here!!!

John
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Old 10th May 2005, 11:05 AM   #13
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Obviously the stored energy is no different from what you need for a 25W amp at 8 ohms, only that electrolytic's energy density becomes worse at lower voltages (the oxide becomes a smaller fraction of the volume).
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Old 10th May 2005, 11:50 AM   #14
JohnW is offline JohnW  Hong Kong
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I was thinking along the lines that the only thing that limits how much the PSU rails increasing with a Half bridge design (during pumping) is the amount of energy that can be stored into the bulk caps – so the larger the caps the more energy that can be stored – reducing the increase in rail voltage (with non sinking PSU).

As the PSU rails would be much lower for a ribbon design, the 12V rating of these cheap Car audio “Bake Bean Cans” Mega Farad Caps. would allow them to be used in this application – however by choice I would still go for a full bridge design….

John
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Old 10th May 2005, 06:54 PM   #15
bcarso is offline bcarso  United States
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Bruno said: "Obviously the stored energy is no different from what you need for a 25W amp at 8 ohms, only that electrolytic's energy density becomes worse at lower voltages (the oxide becomes a smaller fraction of the volume)."

I wonder if the "ultracapacitor" parts are approaching sufficiently low ESR now to be adequate for this application, if assisted by low-ESR smaller C's in parallel? Their typical voltage limitation for single caps of 2.5V would be less of a problem here. They are certainly appealingly compact.
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Old 10th May 2005, 07:57 PM   #16
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Quote:
Originally posted by bcarso
I wonder if the "ultracapacitor" parts are approaching sufficiently low ESR now to be adequate for this application, if assisted by low-ESR smaller C's in parallel? Their typical voltage limitation for single caps of 2.5V would be less of a problem here. They are certainly appealingly compact.
Let me run though a few numbers while I think about what to answer.

A typical 25W amplifier will have something like 1.5J worth of energy storage per rail, sufficient for rectification and freedom from significant pumping. Point is, for an amp running off +/-2.5V (delivering 25A) this means 480000uF per side (albeit at only 2.5V).

For standard electrolytics, 480000uF/2.5V would be quite a bit bigger than 4800uF/25V, for technological reasons.

The smallest Ultracap (www.epcos.com) is 4F, 2.3V. Sounds like enough. Unfortunately the ESR is specified as 380mOhm. We're not going to draw 25A from these.

Of course, you'd need to use a synchronous buck converter anyway to make +/-2.5V at this kind of current. The caps can go on the input side of this converter so you get smaller ones.
You'd have to use two cpu style buck regulators to do that, one with the control chip somewhat unusually arranged for the negative voltage.

The alternative proposal is to use a full bridge. OK, that's twice the ON losses but you are allowed to run it at up to at least 5V (the FETs are 7V anyway). The extra losses are gladly incurred because otherwise you'd lose extra power in the +/-2.5V supply. You can directly run this amp off a computer SMPS and no large caps are needed.

The latter argument is so persuasive that even if you don't expect pumping (mid or tweeter ribbon), a full bridge is still the obvious choice.
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Old 10th May 2005, 08:08 PM   #17
bcarso is offline bcarso  United States
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380mohm---arrggh. I had thought I had seen some substantially lower numbers when considering these for bus energy storage but if that's as good as they get they are certainly precluded!

Full bridge it is.
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Old 12th May 2005, 03:20 AM   #18
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The ribbon will be used between 80 and 4,000 Hz. Over this frequency range, the ribbon measures a flat 5 uH in inductance and 0.1 ohms in resistance. There is a 12 degree phase shift.

At this low resistance, only 5V - 7V on the outputs are required. This should allow use of very low Ron Mosfets.
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Old 12th May 2005, 09:45 AM   #19
Pucco is offline Pucco  Sweden
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VICOR has some new "DC transformers", VTM and BCM modules that should be perfect for a low voltage high current application like this. 48V in and 1.5V , 4V , 6V , 8V or 12V out at 200W to 300W. All bulk capacitance at the input, 5mohm impedance, 95% efficiency.
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Old 18th May 2005, 03:46 AM   #20
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Default High current

Hey all, I have been reading this topic and am interested in how efficient this amplifier will be. Although very low Rds on MOSFETs are avaliable, their Rds on does increase with the current. So, while having a quick look at things, it may be hard to get the amplifier to be any more than 25% efficient if the Rds on at 25A will be much more than 0.1Ohms.
If this is not a problem, then sweet.



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