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Class D Switching Power Amplifiers and Power D/A conversion

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Old 24th February 2005, 12:49 PM   #21
subwo1 is offline subwo1  United States
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You're welcome, lumanauw. I believe the core that looks like the dumbbell is called a drum core.

The output filter capacitor often falls in the range of 0.47 to 1 uf. In your scope screen pictures, what is the s/division setting? In other words, what approximately is the period or frequency of oscillation?
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Old 24th February 2005, 01:18 PM   #22
Alme is offline Alme  Ukraine
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Hi lumanauw,

the filter inductance in full-range amplifiers I know is from 9uH to 20uH, for subwoofer it is higher and may be over 100uH. In calculation I used the fact that load impedance and inductor together make low-pass filter and its cutoff frequency has to be high enough above the highest signal frequency. I don't know exactly that stuff about LC-filter inside feedback loop, I just learn these topologies now, but I suppose that filter will put additional phase shift to your feedback that has to be compensated somehow. Then I calculated filter capacitor from cutoff frequency of LC-filter (this has to be in between highest signal and carrier frequencies) and also so called characteristic impedance (RO), RO=sqrt(L/C)=has to be close to your load impedance - that is condition of "good balance between L and C" as you say.
Jaka helped to define your cores, now you can see AL and other parameters in catalog. For the future, to reveal unknown core, you make 10 turns on it, measure L, and to get AL divide that value by 100 (or make 20 turns, measure and divide value by 400, and so forth).
Even if it's self oscillating, there is some 'central' frequency, at least you better know the order of it (80kHz, or 160kHz, etc.) I found your core in catalog and supposing your switching at 200kHz, got 20 turns winding for 0,1tesla voltage operation (this value I think will keep the core just warm) and this corresponds to 30uH inductance - too high for 2Ohm full-range operation - cutoff at 10kHz. You can try to decrease number of turns but check core's temperature at idling switching.
Inductor size also depends on the highest current you want to flow thru it. With 20 turns this core will allow 17A of maximum current at 0,2T current operation, probably slightly low value. Anyway this is 300W RMS at 2 Ohm. For 1Ohm you better find a core with lower mu (because glueing 2 such cores together won't solve this problem). Or maybe I'm mistaken and your feedback will successfully solve core non-linearity distortions at higher flux (they claim maximum of 0,75T for MPP and 1,05T for Kool Mu).
Hope I've been helpful.

Regards,
Alex.
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Old 24th February 2005, 02:15 PM   #23
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Hi,

not much to add to the excellent advice already given, besides that even at 50A peak (with 20turns) MPP core wil still retain 50% of initial inductance while KoolMu will stay at 45%. One of advantages (or disadvantages) of powdered cores is gradual saturation. This means more headroom for mistakes, but also nonlinear inductance vs bias current. So stay with those cores, with gapped ferrite you would need something even bigger in volume. Plus fringing flux from large airgaps has nasty habit of cooking nearby windings. I doubt you will find drum (or bobbin as it is sometimes called) core sufficiently large for your application.

Second, I think you will find it easier to wind several parallel strands of thinner wire than one thick.

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Jaka Racman
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Old 24th February 2005, 05:35 PM   #24
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Second, I think you will find it easier to wind several parallel strands of thinner wire than one thick.
Not just that, It will also show less losses due to skin-effect !

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Charles
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Old 25th February 2005, 03:17 AM   #25
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Hi, Subwo1,

Sorry, I dont take note at that time. I will measure it. The signal is 20hz sinewave, but I dont know what frequency the pulses are. The uH at that time is 200uH (with saturating condition)

This is the "Drum" core that I bought yesterday. There are 2 more cores, one is light green and one is yellow. Don't know what they are.
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Old 25th February 2005, 03:22 AM   #26
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I've used this small+many wires in my SMPS. Like this one, I use 60pcs of 0.3mm wire. (but the insulation is not as good as big wires, you will have to wrap them again with asbestos tube in the ends, otherwise they will scratch each other)

Is this "Skin Effect" attention is a must in output ClassD LC filter too? I usually see factories (Crown BCA and other classD amps) only use 1 big wire, not "litz-wire-kind" bunch.
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Old 25th February 2005, 06:37 AM   #27
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Hi,

I do not think it is necessary to use litz wire for Class D filter. Usually you have only one layer winding, so it cools down efficiently and besides that high frequency AC ripple current of the switching stage will have relatively low amplitude compared to the full output current. I suggested some (3 or 4) parallell (not twisted) strands because they would cover toroid more evenly and would be easier to wind.

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Old 25th February 2005, 08:26 AM   #28
subwo1 is offline subwo1  United States
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Quote:
Originally posted by lumanauw
I've used this small+many wires in my SMPS. Like this one, I use 60pcs of 0.3mm wire. (but the insulation is not as good as big wires, you will have to wrap them again with asbestos tube in the ends, otherwise they will scratch each other)
Hi lumanauw,
Hopefully it is really fiberglass tube instead. I think it would be OK to use heat shrink tubing in this case. I just wrap the ends of enameled wire with electrical tape. But, I actually would go with Jaka Racman's suggestion and use something like 4 parallel windings separately wrapped around a core. Because litz wire has such thin insulation, I try to avoid it. Even though these are relatively low voltages present at your amplifier, in some other applications the voltage can arc between windings and short them together.
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Old 25th February 2005, 10:08 AM   #29
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The "low uH with many turns" cores works. All of them works, none giving the "ringing" like before
Funny thing is, the uH value seems not important, from x1 to x5 like gives no difference?

Since all of them works, how to determine the "best"?

I also have another question. The top gate pulse voltage is like this :
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Old 25th February 2005, 10:15 AM   #30
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But the lower gate voltage pulse looks like this :

They are different shape, is this OK / normal?

I also experimented with gate resistor value. With 470ohm, when there is no signal the pulses are gone. But with 680ohm, when there is no signal, the pulses are still there.

Which is the right one for self-oscilating classD? It seems with the pulses ON all the time, the current draw seems less (I haven't measure exactly yet)

Hi, Subwo1,

The frequency of the selfoscilating is 29.800hz. I make this for subwoofer, when I finished (and understand all), I dare to make the fullrange self-oscilating.

Hi, Alme,

What is (sqrt)(L/C)? Sorry, I'm not good with English. Is this squareroot of (L/C)?
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