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-   -   Self oscillating class d with BD modulation, it's possible! (http://www.diyaudio.com/forums/class-d/232904-self-oscillating-class-d-bd-modulation-its-possible.html)

Tekko 27th March 2013 11:45 AM

Self oscillating class d with BD modulation, it's possible!
 
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A few days ago i found a pdf depicting a self oscillating class d with BD modulation, something i have never before seen and dident even know wat possible.

I had to come up with the component values myself but this is the basic circuit.

And a link to the paper for those interested: Low cost implementation of filterless class D audio amplifier with constant switching frequency_ٶĿ

Osvaldo de Banfield 27th March 2013 01:02 PM

Once again. As ham radio, I ask not to use filterless class D audio, they generate and irradiates large EMI as that speaker cables becomes an antenna.

And self oscillating, the amplifier moves in frequency in the audio rate, so interferences strongly disturb AM reception.

Tekko 27th March 2013 03:08 PM

Read the paper, its constant switching frequency, its not changing. Also note in my sim that there IS a output filter used, i also do not like filterless.

Filterless is generally only used where the amp is within inches of the speaker.

I know you do not like self osc, i did so myself until i discovered the way to stabilize a self osc so that it does not vary in switching freq.

Osvaldo de Banfield 27th March 2013 04:04 PM

It appears that you don't read this paper. Clearly it is said that carrier frequency depend of the value of M, the modulation index. See figure 3 in your link.

gmarsh 27th March 2013 06:52 PM

I see a pair of 22uH inductors on the output. All is good.

If you're building this amp, I'd recommend changing the two 470n caps to a single 220n film cap (with twice the voltage rating) connected between the two outputs. Use a pair of 22n capacitors to ground. This reduces the amount of ripple current flowing through the circuit ground, which is good from an EMI perspective.

Feed the output through a common mode filter before you bring it out of the case.

gmarsh 27th March 2013 06:54 PM

Also, a self-oscillating amplifier will not be constant frequency. It'll be maximum frequency at zero output, and the frequency will drop as it gets close to the rails.

Unless you're doing some sort of trick where you feedforward the audio signal and dynamically adjust the control loop with it, but I don't see that here.

Tekko 27th March 2013 07:00 PM

Again read the paper, it clearly states the "variable hysteresis" to obtain a constant switching frequency from zero to near 100% modulation.

Workhorse 28th March 2013 06:19 AM

Tekko, if you take 2 UCD modules and connect a 2.2nF cap between outputs of both modules you get same class-BD mode but also with post filter feedback.

Tekko 28th March 2013 10:37 AM

No you do not! They need to be synchronized so that they run at not only the exact same switching frequency, but also in phase with each other. Only the audio signal is 180deg out of phase, thats how you get the BD mode.

Also the hypex ucd modules suffer the exact same switching frequency drop as a fixed hysteretic window class d, but also the UcD can be compensated for a constant switching freq by using complex zeros in the feedback path and what not.

Workhorse 28th March 2013 11:30 AM

Quote:

Originally Posted by Tekko (Post 3430983)
No you do not! They need to be synchronized so that they run at not only the exact same switching frequency, but also in phase with each other. Only the audio signal is 180deg out of phase, thats how you get the BD mode.


Wrong...!!!

With the sync cap between 2 half bridge UCDs spk outputs + to + of both, the switching frequency becomes same and the phase also becomes same, only one has to feed out of phase audio signals at input.


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