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Class D Switching Power Amplifiers and Power D/A conversion

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Old 2nd August 2004, 05:34 PM   #11
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Default Circuit Comments

Sorry but I erred in my analysis of this circuit. It is a P channel FET connected with the source to the supply and the drain to the load. I'm so used to seeing it the other way round with an N-Channel FET.

The citcuit should push the FET into saturation which should limit the heating. However, there will still be some heating from charging the 2.2uFd cap. However, the modulation scheme is very primitive and there is no supply voltage rejection.

This means that even if the signal is higher in frequency, there will be a major supply ripple component to it. Unless the supply is very well regulated.

Something like this could work as say a siren driver but I seriously doubt that there would be any fidelity to it.

By the way, the self oscillating idea is OK. The best sounding class D amps use a self oscillating setup.

However, one problem you have is that you are using the BE junction of transistors to act as comparitors in your switching. These have fairly soft "knees" which means that there is a linear region between on and off. As well, the point they turn on or off varies with temperature.

Then there are similar problems with the diodes in series with the transistor. These will limit the precision where you can switch. That is why the good class-D amplifiers use a precision comparitor like the LM319 for this purpose. The accuracy of the modulator and its stability will determine your sound quality. Even the LM311 ir LM339 is not good enough.

To get good Class D you need to switch between high and low at each point as fast as possible, with as little time spent in the in between levels as possible.
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Dan Fraser
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Old 2nd August 2004, 11:13 PM   #12
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Default Re: Circuit Comments

I had originally designed this part of the circuit using a BJT because of cost, but it seemed to switch to slow and required much more current to drive. This tended to disrupt the audio just slightly. The FETs seem to work much better


Yes a P-channel hexFET, at least I think it is hex type. I had to go to some greif to get one, it cost me $10 as compared to $1 for the N-channel. The N-channel is for the negative side.) The signal from the FET does not actually drive the speaker though. It drives the collector of the output transistor----a dependent current source, which is a class AB biased BJT EF power amp, full range. The collector is switched from 35V to 50V during the short time that audio signal exceeds 20V in order to achieve a larger signal out, without breaking the other output device down and It can only handle 90V, but is linear to 7A. Therefore I am not looking for acuracy from this "puse" circuit other than the switching time. The occilations I was experiencing were high enough freq. to shoot right through the hi-current (lower Vce breakdown) transistors and appeared on the peak of my audio signal.

The speaker that I am designing this circuit for is a 4 way speaker box, with internal passive filters. I have noticed that this load seems to have a higher impeadence for higer freq. than for lower ones and needs to have more voltage in order to achieve the power needed. I like highs. The AC load line changes with dynamic range. Extending the Vce effectively extends the linear operation of the AC load line that exists with higher frequencies, requiring more voltage and less current as opposed to lower freq. and while one output has Vc =50V(for less than 20% of time) the other Vc=-35V, and vise-versa. The max voltage possible on 'off' transistor is only 85V.

I don't want to parralell output BJT devices.

As for supply rails ripple voltage I expect that this will happen, even with 15,000uF filter caps, but since this circuit is a regulator at 50V, It should not affect the circuit.
I hope to do some more testing tomorrow.

Chris
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Old 4th August 2004, 01:04 AM   #13
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After further experimenting, I think I am going to go a different route with the switching FET regulator. An inductor did take some of the occilations out, but only the higher frequencies, there is still some occilations left.

I am going to try using the FET in a source follower configuration instead of a feedback loop configuration. By referencing the gate to 55V zener reference voltage, there should be about 50V on the source, it doesn't have to be perfect as it may change slightly with varying current. Without the feedback loop it shouldn't occilate. The circuit is less complex anyway.
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Old 5th August 2004, 12:56 AM   #14
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sweeeeeeet

in scraping the feedback regulator loop for a simpler source follower, I have eliminated the occilations, but to drive the capacitance of the FET up to 20KHz I found that I need to have a lower impeadence for the gate drive maybe a smaller power transistor. This helped with the phase change of the output with relation to the input. That is one major criteria for this circuit.
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