Amplifier topologies that operate in a partially on state, such as Class A and AB, act like resistors and produce heat, thereby wasting energy. Thus, Class D amplifiers are substantially more efficient than non-switching linear amplifiers. Higher efficiency and less waste heat allows the Class D amplifier to utilize a smaller power supply and to be offered in a more compact package than a comparable linear amplifier.
Unfortunately, existing Class D amplifier designs suffer several disadvantages, including disadvantages related to modulation, isolation, feedback, and board layout. Both H-bridge switch states result in a differential voltage across the outputs leading to current flow through the load. These two-state Class D amplifiers typically compare a reference triangle waveform to an audio error waveform audio feedback using a single comparator. The output of the comparator is a single PWM signal with the same frequency as the reference triangle waveform.
Thus, a differential voltage is always present at the output causing power to be lost via the loudspeaker or low pass filter even in the absence of an audio input to the amplifier.
Existing Class D amplifiers typically require large power transformers to accommodate a relatively inefficient output stage and to meet government regulations requiring high voltage isolation between AC mains and all user-accessible inputs and outputs.
This isolation is typically achieved by incorporating one or more power transformers between the AC mains and the input and output stages. Unfortunately, such power transformers are large and expensive. Even in applications where the outputs are not user-accessible, no effort is typically made to isolate the input stage from the output stage.
Where input-to-output isolation is attempted, small-signal audio transformers are typically used. Unfortunately, these transformers suffer from limited frequency response, making implementation difficult. Existing high-power Class-D amplifier designs incorporate a control or feedback loop to minimize distortion. Conventional control theory requires filtering, attenuating, and summing the output signal with the input signal.
This typically involves a feedback loop comprising a differential RC low pass filter, followed by an attenuating differential amplifier, and then a summing amplifier to combine the feedback signal with the input signal. For high power applications where common-mode voltages can exceed 70 Vdc, precision matching of feedback resistors is a critical concern. The RC low pass filter is required to attenuate the PWM switching energy and to pass the audio signal to the differential amplifier.
This can result in decreased efficiency as power is lost in the RC low pass filter even in the absence of an audio input signal. Unfortunately, precision matching and increased power handling requirements for the RC low pass filter resistors result in increased cost and size. Existing Class-D amplifier designs incorporate pairs of multi-pole differential LC low pass filters to filter the ever-present differential switching output voltage.
Typical multi-pole differential LC filter designs dissipate a majority of attenuated energy in the first LC low pass filter pair. No advantage is gained from common-mode filtering because the output of the H-bridge continues to be a differential voltage.
As a result, high power designs are required to incorporate expensive high power inductors that can dissipate the switching energy even when no audio input signal is present. Existing Class D amplifiers typically exhibit high harmonic distortion above 1 kHz as a result of pulse transient damping issues and poor triangle waveform damping generation. Excessive pulse undershoot and overshoot result from high inductance board layouts and power supplies. Some existing designs attempt to reduce pulse overshoot and undershoot on H-bridge outputs by incorporating large, expensive RC snubbers.
Additionally, pulse transient damping issues also lead to increased EMI emissions that increase the cost of shielding the amplifier. Triangle waveform generation has always been a source of distortion in Class D amplifier designs.
Triangle waves are typically generated using RC oscillators made of operational amplifiers or logic gates. These Class D amplifier designs suffer from high frequency noise superimposed on the triangle waveform; in turn, the high frequency noise results in increased harmonic distortion.
Thus, existing Class D amplifiers typically exhibit undesirable harmonic distortion much greater than 0. Due to the above-identified and other problems and disadvantages in the art, a need exists for an improved Class-D audio switching amplifier. The present invention overcomes the above-identified as well as other problems and disadvantages in the art of Class D and linear audio amplifiers by providing a Class D switching amplifier operable to provide increased efficiency, increased reliability, and reduced distortion through use of four state modulation, input-to-output driver and feedback signal isolation, dual topology output filtration, and a low inductance board layout.
Though not limited thereto, the amplifier is particularly ideal for applications without user-accessible outputs, such as powered loudspeakers, wherein isolation of input-to-output drive and feedback signals allows for the elimination of large expensive power transformers required by the prior art.
Furthermore, though not limited thereto, the amplifier is particularly ideal for high-power applications involving, for example, 50W or more. The preferred Class D switching amplifier broadly comprises an input stage; a triangle stage; a gate drive stage; an output stage; a filter stage; and a feedback stage.
The error signals represent the combined audio input and error for both positive and negative swings. The triangle stage is operable to derive a low noise triangle waveform, TRIANGLE, having reduced high frequency noise that might otherwise lead to excessive distortion. The optoisolators preferably allow isolated pulse transmission with minimal delay and pulse width distortion. As mentioned, the present invention introduces a unique four state modulation scheme that advantageously increases efficiency and allows for common-mode filtering to reduce loss during no-audio conditions.
Using the four state modulation scheme of the present invention, in the absence of an audio input signal the H-bridge outputs are common-mode in phase and no current is delivered to the load. In the presence of an audio input signal, the H-bridge outputs differentially drive current through the load at double the frequency of the triangle waveform. The input stage is isolated from the output stage using optoisolators. Alternatively, small signal transformers may be used in place of the optoisolators; however, the optoisolators, being more cost and space effective, are preferred.
Many available optoisolators provide fast data transmission while minimizing pulse distortion effects. Another benefit of floating the output stage is reduction of typical Class D chassis referenced DC voltage present at the amplifier output.
Additionally, the present invention improves upon prior art Class D feedback topology by isolating feedback signals and referencing the RC low pass filter to the input stage ground. This improvement eliminates potentially damaging differential and common-mode voltages present in the feedback circuit. As a result, precision resistor matching is no longer required, and less power is lost in the RC low pass filter.
Thus, isolating the feedback signals substantially reduces costs and increases efficiency and design reliability. The filter stage includes an LC low pass output filter operable to attenuate the high frequency switching, pass the amplified audio signal, reduce radiated emissions, and smooth the output current. In prior art Class D amplifiers, differential LC low pass filter designs are used with 3 dB cutoffs at no less than 25 kHz. Regardless of whether audio is present at the output or not, the filter is absorbing energy at the switching frequency.
With prior art modulation schemes no advantage was gained from common-mode filtering because the output of the H-bridge was a differential voltage waveform. As a result, high power designs were required to incorporate expensive high current, low resistance inductors in the LC low pass filters that could absorb the switching energy with or without an audio signal present. The modulation scheme of the present invention results in a common-mode voltage in the absence of audio that allows for use of a combination common-mode and differential LC low pass filter constructed with inexpensive series surface-mounted inductors.
The first two-pole LC low pass filter combination is arranged in a common-mode topology; the second two-pole LC low pass filter combination is arranged in a differential topology. Through use of a four pole combined common mode and differential LC output filter, inductor current is reduced in the absence of an audio signal.
By incorporating a differential second two-pole combination, the filter maintains beneficial rejection of high frequency differential signal components. The present invention utilizes a unique low inductance board layout and modularization that advantageously lowers pulse overshoot and undershoot, leading to reduced distortion, reduced radiated emissions, and increased efficiency. The low inductance design allows for elimination of expensive RC snubbers common in prior art Class D amplifiers.
With the improved board layout, harmonic distortion has been reduced to less than 0. Furthermore, the unique board layout reduces overall size and allows for small lightweight construction. A preferred embodiment of the present invention is described in detail below with reference to the attached drawing figures, wherein:. Referring to FIG. The amplifier 20 is operable to provide increased efficiency, increased reliability, and reduced distortion through use of four state modulation, input-to-output drive and feedback signal isolation, dual topology output filtration, and a low inductance board layout.
Though not limited thereto, the amplifier 20 is particularly ideal for applications without user-accessible outputs, such as powered loudspeakers, wherein isolation of input-to-output drive and feedback signals allows for the elimination of large expensive power transformers required by the prior art. The preferred Class D switching amplifier 20 broadly comprises an input stage 22 ; a triangle stage 24 ; a gate drive stage 26 ; an output stage 28 ; a filter stage 30 ; and a feedback stage Thus is the ERROR signal derived, representing the combined audio input and error for both positive and negative swings.
It will be appreciated by those with ordinary skill in the electrical arts that alternative circuit topologies may be devised substantially equivalent in function to those described herein and shown in the figures. Referring also to FIG. The triangle stage 24 is operable to derive a low noise triangle waveform, TRIANGLE, and broadly comprises an ultra-low noise voltage regulator 46 ; a ramp capacitor 48 ; and a low ESR capacitor 50 ; and a switched current source IC The ultra-low noise voltage regulator 46 reduces high frequency noise that might otherwise lead to excessive distortion, and provides the ramp capacitor 48 with a clean supply rail with minimized high frequency transients.
The low ESR capacitor 50 provides a low impedance path to ground for high frequency transients. The switched current source IC 52 provides a switched current capacitive charging circuit; a suitable switched current source IC 52 is available from Unitrode in the model UC By incorporating such a switched current source IC, a majority of the components required for modulation signal derivation are self-contained, which results in reduced cost and minimizes board real estate usage.
The optoisolators 58 , 60 preferably allow isolated pulse transmission with minimal delay and pulse-width distortion. It will be appreciated that the gate drive stage 26 , regardless of which circuit topology is used, includes a first internal branch and a second internal branch. Descriptions There is no relevant information available for this part yet. Programmable controllers 2. The world's leading manufacturers of consumer and business technologies look to SMA for the products, expertise, and worldwide support they need to make their visions a reality.
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