Creating positive phase shift?

[Pabo]

I have tested putting a low pass filter in feedback loop of a self oscillating hysteresis stage with the result that one pole of the output filter is eliminated. I have though seen a fairly new patent claiming this technique but I would be surprised if this hasn´t been done about a thousand years ago in for example OPAMP circuits.

Has anyone seen such a circuit before which would make that claim invalid?

[phase_accurate]

I guess you are talking about the patent of a fellow Scandinavian, aren't you ?
Building a lead filter the way as K.N. did is per se not new at all. It is not even new to have a lowpass in the feedback path of a class-d amp (I did this 15 years ago as well !).
But it was new to do this around a switching amp's modulator in order to generate a phase-lead ! It is 1.) the total combination of all aspects that counts and 2.) what the patent office regards as novel.
It actually depends on what is exactly claimed in the patent. If it was just a lowpass in the feedback path of a switching amp without any closer definition what it is used for, then it could easily be killed by the circuit from my thesis (which would then be prior art).

K.N.'s trick could even be considered as novel although someone else patented something like a DC/DC converter using such a modulator ages ago, since the application of it to audio would be a different one. It always depends on what the examiners let you get away with .......

Regards

Charles

[Pabo]

Yes, unfortunately the claim is in combination with preceeding claims which in total gives a classd amplifier with global feedback.

Well, I can do without it. Using this technique would give me the possibility to apply about 6dB more of feedback but this isn´t the whole world is it. You still have to watch out for Nyqvist.

[phase_accurate]

Yes there are more possibilities to generate a phase-lead. K.N.'s version is not the only possible solution but I must enviously admit that it is quite refined and elegant.

The 6 dB of additional NFB you are talking about could give you increased load independancy so it might be well worth it.



Regards

Charles

[Pabo]

Another way of creating phase lead is of course to add a zero in front of the modulator but the disadvantage with this solution is that the switching ripple is increased which may cause high frequency instability for example during startup. This disadvantage is probably not present in K.N.s circuit, but I am not sure about it.

Transient simulations using K.N.s technique gives a very good behaviour at all frequencies and loads.

[classd4sure]

Hi,

IcePower patent? Or something newer? Care to share?

[Pabo]

classd4sure

ICEpower, Yes, it is one of Karsten Nielsens earliest patents. The title is something like Multi Enhanced Cascade Control, I think. Use it for DIY but be careful if designing professionally.

I have been designing classd using a current loop in the output stage but there are some problems with this technique. First of all it is very difficult to sense the current in a linear manner. I have used 2010 MELF resistors creating a shunt of 100mohms. With this shunt I measured the return current from the output filter including both the capacitor and load current. It is a very good technique when it comes to phase shift and high frequency behaviour. A square wave looks allmost ideal.

My problems are: The negative output is not ground but a triangular wave giving more noise on the output. I have not been able to make the loop as linear at high frequencies as when using voltage integrator technique before the filter.

One would think that the output impedance at high frequencies would be a problem but actually it can be pretty low. I have managed to come down to 0,5 ohms at 20kHz using about 18dB of feedback.

One idea that I have is to measure the current flowing into the capacitor/load but this would require a measurement circuit tracking the output signal +/-50V (kind of difficult). The circuit has to be able to measure a triwave at 700kHz in order to make it future proof.

Does anyone know of such a circuit?

[classd4sure]

Hi Pabo,

Thanks for sharing

You said "fairly new patent" which threw me, K.N.'s newest patent (2004) is the highly vague abomination straight off the mains with the modulator driving the voice coil directly with no output filter.

SO, you're efforts are very interesting.

Have you read the mueta patent? Maybe you can get some ideas from that. You could possibly also do something similar to what their current datasheet shows. That's all I can think of.

[Pabo]

classd4sure

Thanks for the tip

The Mueta patent is according to me a very complex way of going around the ICEpower patent and achieve a -1 slope in the output filter without using the output voltage. The amount of OPAMPs needed to create the current "set value" in the capacitor and to correct the output voltage is to high according to me. Otherwise it´s a good technique because it is a self oscillating current loop without the disadvantage of including the load current which causes higher output impedance.

[Kenshin]

if you elimating the pole of the output filter for bigger bandwidth,
that's of no good in audio device.

At large input at high frequencies , to drive the filtered output to full amplitude,the output amplitude of the main amplifier will be greater than the rail--this is impossible. so TIM distortion may occur.

but I have once tried an idea like this in a buck SMPS to eliminate the output LC filter's overshoot completely.

Originally posted by Pabo
I have tested putting a low pass filter in feedback loop of a self oscillating hysteresis stage with the result that one pole of the output filter is eliminated. I have though seen a fairly new patent claiming this technique but I would be surprised if this hasn´t been done about a thousand years ago in for example OPAMP circuits.

Has anyone seen such a circuit before which would make that claim invalid?