Patent Publication Number: US-10326415-B2

Title: Self-oscillating amplifier system

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claim the benefit of and priority to European Patent Application No. EP17176373.3 filed Jun. 16, 2018. The entire disclosure of the above application is incorporated herein by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to the field of DC-DC and DC-AC power conversion, with particular relevance for high definition switching audio power amplification. 
     BACKGROUND 
     Switching Class D audio amplifiers have found increasing use in the industry in recent years, due to the improvements in output stage switching devices and equally in modulation and feedback control methods. The classical switching power amplifier system consists of a pulse modulator, for converting an analog or digital source into a pulse-modulated signal, which is amplified by a switching power stage. A passive demodulation filter reproduces the power modulated power signal. 
     Most switching class D amplifiers are based on variants of Pulse Width Modulation (PWM). Some of the challenges in switching amplifier design relates to the shortcomings of PWM, one particular is problems associated with carrier generation since it limits system bandwidth and complicates design. Additionally, a stable and robust control system design is difficult to implement. 
     As a remedy to this, self-oscillating amplifiers were introduced, these are commonly classified into two categories: hysteresis-controlled oscillation and phase-shift controlled oscillation. An example of the latter in the form of a non-hysteresis controlled oscillation modulator (COM) is disclosed in WO 98/19391, by the present applicant, ICEpower A/S, incorporated herein by reference. Other oscillating modulator methods and systems have been disclosed in prior art as WO 2004/47286 by the present applicant, ICEpower A/S. These methods are characterized by self-oscillation being determined by feedback after the output filter, i.e. having the output filter as an integral, determining part on self-oscillation conditions. Such architectures will in the following be referenced to as global loop controlled oscillation modulators (GCOM). The global loop controlled oscillation modulator based switching amplifier systems disclosed in prior art have a particular advantage in terms of wide closed loop gain bandwidth enclosing the output filter, such that filter distortion and output impedance is minimized. Moreover, the (resulting/achieved) open loop gain of GCOM is defined as the difference between the open loop gain of the audio operational frequency which is generated and the open loop gain at the actual switching frequency. Therefore, it is desirable to have as much open loop gain as possible in the operational range and as much open loop attenuation as possible outside of the operational range. 
     Thus, there is still a need for improvement in the art, particularly there is a need for switching power conversion systems with improved closed loop bandwidth and reduced phase shift across the audio band. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a self-oscillating amplifier system which alleviates all or at least some of the above-discussed drawbacks of the presently known systems. 
     This object is achieved by means of a self-oscillating amplifier system as defined in the appended claims. 
     According to an aspect of the present invention, there is provided a self-oscillating amplifier system having an input for receiving an input signal and an output. The self-oscillating amplifier system comprises a forward path from the input to the output. The forward path comprises a pulse modulator configured to modulate the input signal and form a pulse modulated signal, a switching power amplification stage configured to receive the pulse modulated signal, amplify the pulse modulated signal, and form a switching output signal, a demodulation filter configured to demodulated the switching output signal and form a demodulated signal at the output, and a first forward branch preceding the pulse modulator, the first forward branch comprising a differentiating forward filter for controlling a switching frequency. Furthermore, the self-oscillating amplifier system comprises a feedback path from the output of the system to the input of the system, the feedback path comprising a phase lead network for compensating at least a portion of a phase lag of the demodulation filter. The forward path further includes a second forward branch connected in parallel with the first forward branch, said second forward branch comprising an integrating forward filter. 
     The present invention is at least partly based on the realization that it may be beneficial to separate a part of the forward path into two parallel paths/branches; a first, differentiating path for controlling the switching together with the feedback path, and a second, integrating path for controlling the behavior of the amplifier system within the operational frequency range. 
     The advantages arise from the fact that the integrating contribution of the forward path may have a 180 degrees phase shift close to the operating (audio) frequency range, and hence tend to cause sub-harmonic oscillations, i.e. undesired self-oscillation far below the switching frequency. By placing this integrating contribution in a separate path, more options are available for counteracting such undesired self-oscillations. For example, the integrating forward filter can be restricted or switched off without shutting down the self-oscillation of the amplifier system. 
     Through the invention a new and improved self-oscillating amplifier system, suitable for use in high fidelity audio applications, is presented. Embodiments of the inventive amplifier system exhibit an increased open loop gain and a flatter frequency response across the operational frequency band of the amplifier system (e.g. audio band, approx. 20 Hz-20 kHz) as compared to known systems. Flat frequency response is to be understood as that the frequency response curves (magnitude and phase response), in a graphical representation of the frequency response, e.g. Bode plot, are substantially flat or constant across the operational frequency band. In more practical terms, a flat frequency response is to be construed as that a system more accurately reproduces an input through the output without enhancements in a particular area (frequency range). Stated differently, a flat response means that all frequencies are treated with the same amount of gain (within a certain bandwidth). 
     The integrating forward filter may in accordance with an embodiment of the present invention be an active filter (i.e. comprising active components) and/or a higher order filter, such as a second order filter. By having a higher order integrating filter it is possible to maximize the open loop gain within the operational frequency range and attenuation outside this operational frequency range, leading to better system performance. 
     The demodulation filter is preferably a higher order passive filter, e.g. a second order LC filter. 
     Further, in accordance with an embodiment of the present invention, the integrating forward filter is configured to form an integrated signal, and the second forward branch further comprises an amplitude limiting circuitry configured to limit the amplitude of the integrated signal. An amplitude limiting circuitry is in the present context to be understood as a circuit solution which is operable to maintain a signal amplitude at a predefined node within certain static or dynamic limits. 
     This amplitude limitation of the integrated signal ensures that sub-harmonic oscillation caused by the integrating filter is avoided. Stated differently, the amplitude limitation of the integrated signal keeps the amplifier system from oscillating at a too low frequency. 
     Such amplitude limitation is particularly beneficial in situations where the amplifier system is subjected to abnormal operating conditions, such as e.g. very large voltage variations on the input, during start-up of the amplifier, or after a state of current limitation. Under such conditions, the integrating filter may generate very large signal excursions at the output of the integrator and thereby cause sub-harmonic oscillation, i.e. create a scenario in which the amplifier system locks onto a frequency close to the resonant frequency of the demodulation filter. By limiting the contribution of the integrated signal this contribution is prevented from dominating the forward path, and such “low” frequency sub-harmonic oscillation is effectively avoided. 
     An integrating filter may for example be understood as an electronic integration circuit which has an output voltage (i.e. output signal/integrated signal) that is proportional to the input voltage integrated over time. The integrating forward filter is included in the second forward branch which may or may not comprise additional components (e.g. amplitude limiting circuitry, inverters, etc.). 
     Further, in accordance with another embodiment of the present invention, the amplitude limiting circuitry may be configured to limit the amplitude of the integrated signal to be within a predefined amplitude range, such as, e.g. a voltage range of ±1 V, ±1.5 V, or ±2 V, etc. Stated differently, the amplitude limiting circuitry controls the output signal of the integrating forward filter (i.e. the integrated signal) to not exceed this predefined range. Alternatively, one may set a predefined current range, within which the integrated signal is kept. 
     Even further, in accordance with another embodiment of the present invention, the amplitude limiting circuitry is connected to the input of the system. The amplitude limiting circuitry is then further configured to determine a difference between the integrated signal and the input signal of the system, compare the difference to a predefined threshold value, and act on an input of the integrating forward filter in order to limit the amplitude of the integrated signal when said difference exceeds the predefined threshold value. In other words, the amplitude limiting circuitry is configured to subtract the input signal from the integrated signal and to limit the amplitude of the remaining/resulting signal from that operation. The amplitude limiting circuitry accordingly indirectly controls the output of the integrating forward filter by directly acting or controlling its input signal. 
     By having the amplitude of the integrated signal compared to a reference signal, namely the input signal of the system, a robust and effective means for amplitude limitation can be achieved. An advantage compared to setting a fixed limiting range for the integrated signal is that the risk of over-modulating in the amplifier system is reduced, thus providing for a more robust amplifier system. This is because the output signal from the integrating branch is somewhat proportional to the input signal (during normal operation) so removing the input signal from the integrated signal and limiting the remaining signal means it is actually the compensation signal that is limited, not the usable signal. Thereby, tighter limiting thresholds can be set. 
     Moreover, in another embodiment of the present invention, the integrating filter comprises an active integrating circuit having an operational amplifier. The amplitude limiting circuitry may then be configured to limit the integrating signal by comparing the integrated signal with a reference signal (e.g. the input signal of the system) and generate a control signal at the inverting input of the operational amplifier. Whereby a simple and robust realization of the inventive system can be achieved. 
     Yet further, in accordance with yet another embodiment of the present invention, the phase lead network of the feedback filter is a first phase lead network, and wherein said differentiating forward filter comprises a second phase lead network for compensating at least a portion of the phase lag of the demodulation filter. In conventional non-hysteresis self-oscillating amplifiers, the phase lead network (may also be referred to as a lead compensator) is provided in the feedback loop of the amplifier. However, by separating the leads, one in the feedback path, and one in the forward path, it is possible to more precisely shape the closed loop frequency response and compensate for the poles of the demodulation filter. More specifically, it is now possible to control the switching frequency with less negative impact on the bandwidth of the amplifier system. A lead-zero in the feedback path will result in a −1 slope of the closed loop response. A lead-zero in the forward path is designed to partly compensate for this as the loop gain will be increased at high frequencies. It is possible to select these two lead-zeroes to achieve a wider closed loop response at the same time as controlling the switching frequency. 
     Yet further, in accordance with another embodiment of the present invention, the self-oscillating amplifier system further comprises a low pass filter preceding the forward path outside the feedback loop. In other words, the low pass filter is connected between the system input and a summing box into which the (now filtered) input signal and feedback signal come together. This further shapes the loop and provides for a flatter frequency response within the operational frequency band of the amplifier system. 
     These and other features of the present invention will in the following be further clarified with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For exemplifying purposes, the invention will be described in closer detail in the following with reference to embodiments thereof illustrated in the attached drawings, wherein: 
         FIG. 1  is a schematic block diagram representation of a self-oscillating amplifier system in accordance with an embodiment of the present invention; 
         FIG. 2  is a graph illustrating a comparison of the closed loop amplitude and phase characteristics between a self-oscillating amplifier system according to an embodiment of the present invention and of a conventional prior art self-oscillating amplifier system; 
         FIG. 3  is a schematic block diagram representation of a self-oscillating amplifier system in accordance with another embodiment of the present invention; 
         FIG. 4  is a schematic block diagram representation of a self-oscillating amplifier system in accordance with yet another embodiment of the present invention; 
         FIG. 5  is a schematic block diagram representation of a self-oscillating amplifier system in accordance with yet another embodiment of the present invention; 
         FIG. 6  is a schematic circuit representation of the self-oscillating amplifier system in accordance with an embodiment of the present invention; and 
         FIG. 7  is a schematic circuit representation of the self-oscillating amplifier system in accordance with another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, preferred embodiments of the present invention will be described. However, it is to be understood that features of the different embodiments are exchangeable between the embodiments and may be combined in different ways, unless anything else is specifically indicated. Even though in the following description, numerous specific details are set forth to provide a more thorough understanding of the present invention, it will be apparent to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well known constructions or functions are not described in detail, so as not to obscure the present invention. 
       FIG. 1  illustrates a block diagram representation of a self-oscillating amplifier system  1  according to an embodiment of the invention. The amplifier system  1  has an input  2  for receiving an input signal (V in ) and an output  3 . A load in the form of a speaker is connected to the output  3  of the amplifier system  1 . The amplifier system has a forward path  4 , extending from the input  2  of the amplifier system  1  to the output  3  of the amplifier system  1 . The forward path  4  has at least two parallel forward branches  31 ,  32  preceding the pulse modulator  5  which in turn is configured to modulate the (filtered) input signal in order to generate or form a pulse modulated signal (e.g. PWM signal). The first forward branch  31  includes a differentiating forward filter  8 , while the second forward branch  32  includes an integrating forward filter  11 . Thus, the first forward branch  31  may to some extent be construed as a “differentiating forward branch/path” while the second forward branch  32  may be construed as an “integrating forward branch/path”. The pulse modulator  5  may for example be realized as a comparator stage. The pulse modulated signal is fed to a switching power stage  6  which is configured to receive the pulse modulated signal, amplify the pulse modulated signal, and form a switching output signal. The pulse modulator  5  and switching power stage  6  may however be construed as a single power switch and comparator stage, as readily understood by the skilled artisan. 
     Further, the forward path has a demodulation filter  7 , preferably realized as a passive higher order filter, such as a second order LC filter, configured to receive the switching output signal and to demodulate the switching output signal in order to generate or form a demodulated signal at the output  3  of the amplifier system  1 . The amplifier system  1  further has a feedback path  9  comprising a phase lead network  10  which is utilized for compensating at least a portion of a phase lag introduced by the demodulation filter  7 . The feedback path  9  extends from the system output  3  to the system input  2 , where the feedback signal is combined with the system input signal (may also be referred to as a reference signal) in a summing box/junction  15 . The summing box  15  is preferably realized as an error amplifier which subtracts the feedback signal from the input signal and amplifies the difference, as known in the art. The feedback path  9  (may also be referred to as a global feedback) can be voltage controlled, as described in WO 98/19391 or current controlled as described in WO 02/25357, both documents hereby incorporated by reference. The feedback path may further include an optional feedback filter  12  in order to suppress errors in the switching power stage  6  and the demodulation filter  7 . However, the feedback filter  12  may also be reduced to unity gain. 
     In this embodiment illustrated in  FIG. 1 , the parallel forward filters  8 ,  11  consists of a first forward filter  8 , which is a differentiating forward filter  8  used for controlling the switching frequency of the amplifier system, and a second forward filter  11  which is an integrating forward filter  11 . The differentiating forward filter  8  may be such that implementation is possible without operational amplifiers. However, other realizations where the differentiating forward filter  8  comprises a compensation block with a more complex transfer function, are feasible. The integrating forward filter  11  may for example be an active filter comprising active components. Also, the integrating forward filter  11  may be a higher order filter, such as a second order filter. By providing a higher order integrating forward filter one can achieve higher gain within the operational frequency band and larger attenuation outside the operational frequency band, as compared to first order integrating forward filters. 
     Advantageously, the first forward branch  31  further comprises a second phase lead network (see e.g. ref.  14  in  FIG. 5 or 6 ) which together with the first phase lead network  10  compensates at least a portion of the phase lag of the demodulation filter  7 . Stated differently, the two phase lead networks are used to control the switching frequency of the amplifier system by introducing a lead-zero in both the feedback path  9  and in the forward path  4  in order to compensate for the poles of the demodulation filter  7 . Conventionally, only the feedback path  9  would comprise a phase lead network, however, by splitting the phase lead network into two, the overall performance of the amplifier system  1  is improved. More specifically, it provides for improved closed loop characteristics with reduced phase turn and relatively constant gain across the operational frequency band (e.g. audio band), i.e. larger small signal bandwidth. 
     Further, by arranging the two forward filters  8 ,  11  in parallel it is possible to shut down or stop the integrating forward filter  11  during abnormal operating conditions of the amplifier system  1  (e.g. large voltage variations at the input  2  or a state after current limit) without stopping the self-oscillation of the whole amplifier system  1 . Stated differently, one can reduce the negative impacts of these abnormal operating conditions by stopping the integrating forward filter  11 , and still have the amplifier system  1  running, a feature which is not possible with conventional switching amplifier systems. The trade-off is that the amplifier system  1  may be subjected to more non-linearity while the integrating forward filter  11  is stopped. Moreover, the integrating forward filter  11  provides means for the error amplifier (represented by the summing box  15 ) to more accurately perform its job. In more detail, the feedback factor of a system is its ability to correct for errors. It is defined as the open loop gain minus the closed loop gain at a certain frequency. If for example the open loop gain at 1 kHz is 60 dB and the closed loop gain is set to 20 dB the feedback factor is 40 dB. Thereby the system&#39;s ability to correct for errors when generating 1 kHz is 40 dB. Higher order harmonics in the error will be corrected with the feedback factor at that certain frequency and in many systems the open loop gain decreases at higher frequencies in order to maintain phase margin meaning the ability to correct errors is decreased the higher the frequency. Thus, by increasing the open loop gain, the feedback factor is increased and error correcting ability of the system is improved. 
       FIG. 2  shows a schematic plot of the closed frequency response loop frequency response for a self-oscillating amplifier system  20   a ,  20   b  according to an embodiment of the invention (broken lines) and for a self-oscillating amplifier system according to the prior art  21   a ,  21   b  (solid lines). The schematic plot is based on a simulation of an amplifier system comprising a pair of parallel forward filters (differentiating and integrating) and a phase lead network in the same branch as the differentiating forward filter. The advantages of the inventive system are immediately apparent as the skilled person readily appreciates the flat, i.e. relatively constant, gain characteristics  20   a  across the whole operational frequency band (the audio band in this illustration) of the inventive system, while a conventional system has a slightly sloped gain  21   a  at the upper range of the operational frequency band. The −3 dB point is substantially higher in frequency for the inventive system, approx. 242 kHz vs approx. 60 kHz. 
     Moreover, the phase characteristics are improved since the phase shift at 20 kHz is approx. 5 degrees for the inventive system, while it is approx. 16 degrees for the conventional prior art system. Thus, the inventive self-oscillating amplifier system exhibits larger small signal bandwidth and therefore allows for more accurate reproduction of the input signal, as compared to conventional prior art systems. 
     Further, as illustrated in  FIG. 3 , the second forward branch  32  may further include an amplitude limiting circuitry  13   a  which is configured to limit an output of an of the integrating forward filter  11 . The integrating forward filter  11  is accordingly configured to generate an output in the form of an integrated signal, which signal&#39;s amplitude is then kept within certain limits by means of the amplitude limiting circuitry  13   a.    
     However, the amplitude limiting circuitry  13   a ,  13   b  may be realized in various different ways. As for example, illustrated in  FIG. 4 , instead of limiting the integrated signal formed by the integrating forward filter  11  within predefined limits (e.g. by means of transistors and fixed biasing levels), the amplitude limiting circuitry  13   b  may be connected to the input  2  of the system  1  in order to retrieve a reference signal and compare it to the integrated signal and from this comparison control the amplitude range of the integrated signal. This allows for more dynamic amplitude control, and is advantageous in that the risk of over-modulation in the subsequent modulating step is reduced. 
     The first forward branch  31  is designed to only have −180 degrees of phase shift (which is the required condition for oscillation) at a frequency far above the operational frequency range. The criteria for obtaining self-oscillating conditions are discussed in for example WO 2004/47286 and EP1994638, incorporated herein by reference. The second forward branch  32  however, will have more than −180 degrees close to the operational frequency range meaning that it can cause sub-harmonic oscillation. The control signal to the modulator  5  is the sum of the two paths  31 ,  32 . However, sub-harmonic oscillation in the system requires that the excursion of the integrated signal is much larger than normal. Accordingly, by limiting the integrated signal generated in the integrating forward filter  11 , one will also prevent it from locking the system  1  onto a too low switching frequency. Stated differently, by limiting the output of the integrator  11 , the differentiating path (i.e. first forward branch) will be in control. 
       FIG. 5  illustrates another block diagram representation of a self-oscillating amplifier system  1  according to another embodiment of the present invention. More specifically, the system  1  comprises a low pass filter  16  at the input of the amplifier system  1 , i.e. preceding the forward path  4 , outside the feedback loop  9 . This is advantageous in order to further flatten the closed loop frequency response and to compensate for any potential peaking in the closed loop frequency response introduced by the forward filter(s)  8 ,  11 . 
       FIG. 6  illustrates a schematic circuit realization of a self-oscillating amplifier system  1  having an integrating forward filter  11  with an amplitude limiting circuitry  13   a  configured to limit the output of the active integrating forward filter  11  to be within a predefined amplitude range. Various blocks (same reference numerals are used for the corresponding blocks in the appended figures) and their corresponding functions have been discussed in the foregoing, and will for the sake of brevity not be repeated, however, this illustration in  FIG. 6  serves to show how an example of how a circuit implementation may be realized. 
     The predefined amplitude range can for example be controlled by setting appropriate biasing levels at the gates/bases of the two transistors  51   a ,  51   b . The biases may for example be set to be ±3.0V, ±2.0V, ±1.5V, ±1.25V or ±1.0V. Stated differently, the transistors  51   a ,  51   b  are configured to control the magnitude (i.e. absolute value) of the output signal of the integrating branch so that it is less than or equal to 3.0V, 2.0V, 1.5V, 1.25V, 1.0V. In operation, an input signal (e.g. audio signal) is received at the input  2  and fed to an error amplifier  15  which is configured to amplify the difference between the output signal (provided via the feedback loop  9 ) and the input signal to produce an error signal which is fed via the filters of the two parallel forward branches  31 ,  32  to a modulating stage  5  (e.g. a comparator or zero cross detector). The modulated signal is subsequently amplified in the switching power stage  6  and then demodulated before reaching the output  3 . 
     The output of the integrating forward filter  11  is further connected to an inverter for inverting the integrating signal before being fed to the modulating stage  5 . This is however readily understood by the skilled artisan to be a specific implementation, and the circuit may be realized by other equivalent components or arrangements as known in the art. Further, the switching power stage  6  is here represented by a pair of schematic switches being connected by a positive and negative supply rail. The output  3  of the amplifier system  1  is connected to a load, here in the form of a resistor. 
       FIG. 7  illustrates another schematic circuit realization of a self-oscillating amplifier system  1  having a different realization of an amplitude limiting circuitry  13   b  for limiting the output of the integrating forward filter  11  of the second forward branch  32 . More specifically, the amplitude limiting circuitry  13   b  is here connected to the input  2  of the system  1  and configured to compare (subtract) the input signal fed to the system input  2  with the output (integrated signal) of the integrating filter  11  and generate a control signal at the negative (inverting) input of the operational amplifier in the integrating filter  11 , and thereby control and limit the amplitude of the integrating signal. However, the illustrated embodiment of  FIG. 6  is just an example of how the “comparing” amplitude limiting circuitry  13   b  may be realized. The skilled person readily understands that there are other possible circuit configurations which would provide an equivalent “comparing” function. 
     The invention has now been described with reference to specific embodiments. However, several variations of the self-oscillating amplifier system are feasible. For example, the (global) feedback loop may be current controlled or voltage controlled, the differentiating (first) forward filter may comprise a cascade of active and/or passive filters, the amplifier system may comprise additional local feedback loops, etc. Further, the pulse modulator may be configured to modulate the signal in numerous ways and with interchangeable components providing the same functionality. Such and other obvious modifications must be considered to be within the scope of the present invention, as it is defined by the appended claims. It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting to the claim. The word “comprising” does not exclude the presence of other elements or steps than those listed in the claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements.