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Self-oscillating switching amplifierSelf-oscillating switching amplifier description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070188222, Self-oscillating switching amplifier. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of priority to U.S. Provisional Application No. 60/572,548 filed May 18, 2004, and to U.S. Provisional Application No. 60/618,869, filed Oct. 13, 2004, and to U.S. patent application Ser. No. 11/132,810, filed May 18, 2005, all of which are fully incorporated herein by reference for all purposes. BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] The present invention relates generally to amplifiers; and specifically to switching amplifiers. [0004] 2. Description of Related Art [0005] Switching amplifiers typically generate a pulse-width modulated (PWM) waveform by comparing a triangular oscillator's waveform to the audio input signal. The linearity of the triangular waveform has a direct effect on the distortion of the audio output. Many other elements of a switching amplifier such as fluctuation of power supply voltages, switching delays, dead times, nonlinearities of most semiconductor components and real passive components, all contribute to relatively high distortion of switching amplifiers. [0006] Reduction in distortion is normally accomplished with negative feedback. In a switching amplifier there is normally a reconstruction filter to minimize EMI at the amplifier output. The low-pass nature of the reconstruction filter introduces a phase shift that quickly approaches and exceeds 180 degrees. That large phase shift makes the design of the feedback loop difficult due to the possibility of oscillation when the total phase loop shift exceeds 360 degrees, taking into account the inherent 180 degrees phase shift of the negative feedback path and the error amplifier's own phase shift which reaches 90 degrees at only tens of hertz. Very often the feedback signal is picked up from a switching node of the power switch, before the reconstruction filter, therefore not including the reconstruction filter, to circumvent the 180-degree phase shift of the reconstruction filter. A reconstruction filter typically uses an inductor and a capacitor. These devices have their own non-linearities especially when an iron or ferrite core is used in the inductor. Magnetic hysteresis is naturally non-linear, and can be a major cause of distortion of switching amplifiers. [0007] The need for stable feedback loop thus limits the bandwidth of typical switching amplifiers. Some switching amplifiers that process signals only in the digital domain don't even have any feedback and suffer from distortions. Limited bandwidth of conventional feedback for switching amplifiers thus limits their audio performance in terms of distortion. Even all linear amplifiers designed with global feedback by necessity have a dominant pole to assure loop stability. That dominant pole in turn limits the bandwidth of linear amplifiers, and more importantly-reduces their ability to attenuate distortion at high frequencies where human ears can really tell slight differences. Indeed studies have shown that high-order harmonic distortions are not pleasing to our ears whereas low order harmonics can be tolerated because they already exist in the audio program and because they are in fact harmonious with the fundamental. Tube amplifiers are often liked for their high second and third order harmonic distortions while their higher order harmonic distortions are often much smaller due to their class-A type of operation. [0008] Prior art of self-oscillating switching amplifiers such as U.S. Pat. No. 6,707,337 senses mainly switching nodes and integrates their voltages to generate ramps that eventually force comparators to change state. Again the oscillator loop does not include the reconstruction filter therefore their audio performance may be left to be desired. Furthermore it requires both the audio signal and its inverted signal to operate. The signal inverter itself is a source of phase shift and distortion. [0009] Thus there is a need for switching amplifier circuits capable of linear transfer function, high loop gain and bandwidth. BRIEF SUMMARY OF THE INVENTION [0010] The present invention provides a simple circuit for generating oscillation and for controlling the power switch of switching amplifiers in pulse width modulation (PWM) to achieve high audio performance. Oscillation is generated by forcing the loop phase shift to exceed 360 degrees while loop gain approaches unity. High audio performance is achieved by multiple feedbacks, with at least one of them covers the output reconstruction filter which may have its non-linearities. High frequency switching is accomplished via high speed circuits not only in the high gain section of the loop but also in the feedback circuit. Both high speed operational amplifiers and comparators are used to make switching frequency very high, in the 100-1000 Khz range and above, in spites of the low pass reconstruction filter used by necessity at the output section. [0011] Embodiments of the invention provide, for example, methods and apparatus for controlling a power switch by PWM to provide high audio performance via high frequency self-oscillation. Certain embodiments can also provide methods and apparatus for minimizing distortion of a self-oscillating switching amplifier via multiple feedback paths. Further, certain embodiments provide methods and apparatus for controlling a power switch such that distortion of a self-oscillating switching amplifier is minimized by dynamic change of mode of switching of the power switch. BRIEF DESCRIPTION OF THE DRAWINGS [0012] These and other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures, wherein: [0013] FIG. 1 is a schematic diagram illustrating a prior art switching amplifier. [0014] FIG. 2A-2B are schematic diagrams illustrating an example of a self-oscillating switching amplifier of the present invention. [0015] FIG. 3 is a schematic diagram illustrating additional feedback from a switching node of an example of a self-oscillating amplifier of the present invention. [0016] FIG. 4 illustrates a change from binary switching to ternary switching according to the amplitude of the audio input signal. [0017] FIG. 5 illustrates a schematic diagram for combining a pulsing supply voltage with a DC supply voltage. [0018] FIGS. 6A-6C illustrate embodiments for a differentiator used in an example of the present invention. [0019] FIG. 7 illustrates another schematic diagram for combining a transformer-supplied pulsing supply voltage with a DC supply voltage. [0020] FIGS. 8A-8B illustrate a bridged switching amplifier with a very direct path to maintain self-oscillation at high frequency. Continue reading about Self-oscillating switching amplifier... Full patent description for Self-oscillating switching amplifier Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Self-oscillating switching amplifier patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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