Why self oscillating class-D shall not be patentable.

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Explained without math:

There are two root trivial experiences:

1- Instability in fast linear amplifiers: anyone can find out that the output gets PWM modulated at a self oscillating frequency, as long as excess gain above 180 deg crossover frequency is moderate and propagation delay is low. But there are 2 problems: That thing is not designed to switch, and not designed for getting repeatable oscillation tuning.

2- Triangle wave PWM modulators: anyone can find out that the wave modulated against the triangle approaches a sinewave which is nearly equivalent to carrier residual as optimum frequency compensation is introduced. But there are 2 problems. Wave is no longer sampled at centered intervals as output approaches the rails. Amplitude of wave is reduced towards zero as output approaches the rails. More than optimum compensation is required.

And there is one trivial association: Merge experience 1 with experience 2.

3- The result is the elementary self oscillating amplifier, with natural variable frequency PWM.

Then comes the need for one trivial modification. The quality of this potentially EMI producing thing is directly in proportion with noise immunity.

4- Make it balanced. Make it buffered. Sense at best places. Make the op-amp inputs low impedance. Make the oscillation controlled at clipping.

5- Most patented stuff has been described.
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Eva, you know so much more about class D amplifiers than I. I would, however, try the argument that your first experience is that you have a flawed situation you like to avoid (self-oscillation) and it is not obvious, with the inconveniences you mention for a linear amplifier, that this "failure mode" could be cultivated into a high performance amplifier.

Patents on self-oscillating class D amplifiers are probably many. If you want to get into the market of such amplifiers, a lot of such patents are about to expire (after 20 years). I noticed a well known Philips patent that was first applied for in 2002. Thus, in 2022 the patent will expire.
Expiry of patents is more straight forward than finding reasons why they should not have been granted in the first place.
A patent is only granted to a novel idea that works.
Self oscillating PMW amplifiers are not novel and are not patentable save the first designer which as far as I am aware was Clive Sinclair in 1959 and the patent has now expired.

"................only granted to a novel idea that works". Rather, granted unless general laws of physics clearly indicate that it will not work. The benefit of the doubt...........
The Clive Sinclair argument seems very interesting.
As an owner of a number of patents, in the UK the novel idea must be demonstrated to work in practice, if I was unable to demonstrate my machines working, I was not permitted a patent but was allowed to use 'patent pending' until such time as it was perfected. This gave a little protection as far as the idea was concerned but still cost me in the order of £1000 in fees for each patent.

The US have their own rules and I am unable to comment on their system.
Today a lot of patents are only filed to blackmail money out of serious producers. You file a patent and block a product that is coming to the market. Instead of waiting for a court to judge and find the blocking patent as irrelevant, the producer pays to use a patent that is nonsense. So he can market his stock, that otherwise would be worthless.

The globalisation put´s producers in a situation where in a corrupt country like China, a state run company for example can close out foreign competition, using their patent court´s and state patent offices, while the own producer´s prosper on stolen patents.

So if you are astonished, why half of the worlds new patents come from a certain country, it is not because it´s engineers are so productive and well educated. Many patents are ridiculous, like patenting a shower head with an on/off switch.

There are organisations that do nothing else but recycle international, run out patents for this black mail scheme. Often these patents show that the issuer did not even understand what he patented, because he was unable to translate it right from the old document.

On the other side, in many Western and European country´s it is really hard to get something patented, as you have to prove it is a new idea and not just a solution any engineer would find if asked.
Some stuff, like electronics are fine for patenting any nonsense, as only small changes may look like something new, while in reality it is only a different combination of old stuff. The less common something is, like class D-amps, the easier it is to file all kind of irrelevant stuff, just for marketing reasons and protecting own profits.
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