Abstract:
A switching power conversion system and a method for start-up pop minimization in an audio amplifier assembly are disclosed. The switching power conversion system comprises a forward path including a compensator, a switching power stage and a demodulation filter. The system further comprises a DC-servo and a pre-charging circuit and a sequence control unit configured for providing a start-up sequence where the compensator and DC-servo are correctly biased and a bootstrap capacitor within the switching power stage is charged before the switching power stage is started. Hereby, it is e.g. possible to minimize the audible start-up pop in audio amplifier assemblies.

Description:
FIELD OF THE INVENTION 
       [0001]    This invention relates to a switching power conversion system such as DC-AC (Direct Current-Alternating Current), DC-DC or AC-AC conversion systems or any combination of the above mentioned. More specifically, the invention relates to startup pop elimination in an audio amplifier. 
       BACKGROUND 
       [0002]    The startup pop elimination system can be a central element of an audio power conversion system. 
         [0003]    Most audio power converters are based on a PWM (Pulse Width Modulation) modulator (digital modulator or analogue modulator) that converts a PCM (Pulse Code Modulated) signal received from a source such as a CD-player, or an analogue signal preceded by a D/A (Digital to Analogue) converter, to for instance pulse-width-modulated signals (digital or analogue PWM modulator). 
         [0004]    The output signal of the modulator is fed to a power stage where it is amplified. A typical power converter includes a switching power conversion stage, a filter and an analogue control system. 
         [0005]    At start-up of the audio power conversion system a general problem is the presence of an audible signal at the output of the system even though there is no input signal applied to the audio power conversion system. The signal at the output at start-up is called pop. 
         [0006]    A contributing source of the start-up pop can be transients when the control system is started from a saturated position. 
         [0007]    When the amplifier starts up the control system will find its correct bias value. It is therefore desired that the control system is correctly biased before startup. 
         [0008]    In WO 2008/072212 the close down pop is minimized by including a parallel power stage with a switch in serial at the output. This implementation is complex since one more power stage is needed. 
         [0009]    U.S. Pat. No. 6,538,590 describes a system using a serial resistor for ramping up. Not for a system with a control loop. 
         [0010]    US 2007/0139103 describes a system for quiet power up and power down of an audio amplifier, however it is only applicable in digital systems. 
         [0011]    There is therefore a need for an improved system and method for minimizing the start-up pop often present in audio power conversion systems. 
       SUMMARY OF THE INVENTION 
       [0012]    It is therefore an object of the present invention to provide a system for an audio amplifier assembly which alleviates all or at least some of the above-discussed drawbacks of the presently known systems. 
         [0013]    This object is achieved by means of a switching power conversion system for an audio amplifier assembly as defined in the appended claims. 
         [0014]    According to one aspect of the present invention, there is provided a switching power conversion system for startup pop minimization in an audio amplifier assembly, said system comprising: 
         [0015]    a forward path including a compensator, a switching power stage for amplifying an output signal from the compensator, and a demodulation filter for filtering an output signal from the switching power stage and providing an amplified output, said switching power stage including a bootstrap capacitor and a pre-charging circuit for charging the bootstrap capacitor; 
         [0016]    a DC-servo connected between the amplified output and an input of the compensator, thereby enabling reduction of offset voltages in the amplified output; 
         [0017]    a signal path connecting the output of the compensator to the DC-servo; and 
         [0018]    a sequence control unit configured for:
       ensuring correct biasing of the compensator and DC-servo at start-up;   charging said bootstrap capacitor by controlling said pre-charging circuit in said power stage; and   after said correct biasing is ensured and said bootstrap capacitor is charged, starting said switching power stage.       
 
         [0022]    The present invention is based on the realization that if the output of the compensator is connected to the input of the DC servo (DC-servo) before startup of the power stage, it makes it possible to get the control system out of saturation and correctly biased before startup of the power stage, thereby removing a contributing source to the start-up “pop”. The present invention thus provides for a simplified and a more cost-efficient alternative to previous known systems for minimizing the start-up “pop”. 
         [0023]    Further the inventors have realized that, at startup, the driver stage boot strap capacitor is normally charged by first having a low (sometimes referred to as negative) first pulse with a forced width. When the control system is biased correctly before start up, the first pulse after start-up can be either high (sometimes referred to as positive) or low, without generating an audible “pop”. This is also enabled by the fact that the invention also includes a pre-charge circuit of the boot strap capacitor in the driving stage which makes it possible to start up the power stage with a first high pulse or a first low pulse with small/short width. This is generally not possible in self oscillating systems. 
         [0024]    Further, in one exemplary embodiment said signal path further comprises a switch for connecting or disconnecting the DC servo from the output of the compensator, and wherein said sequence control unit is further configured for: 
         [0025]    closing said switch at start-up, thereby connecting the DC-servo to the output of the compensator and ensuring correct biasing of the compensator and the DC-servo at start-up; 
         [0026]    simultaneously opening said switch and starting said switching power stage. 
         [0027]    This is because that when the amplifier is running normally it is desired that the influence from the input from the compensator output is as little as possible. Thus, by adding a switch to the signal path the output from the compensator to the DC-servo is more or less completely attenuated. In the present context the term “switch” is to be understood as a device having a transfer function that can be either 0 dB (i.e. no attenuation through the device) and substantially −∞dB (i.e. a very high attenuation through the device). 
         [0028]    According to another aspect of the present invention there is provided a method for minimizing start-up pop in an audio amplifier assembly having a switching power conversion system comprising: 
         [0029]    a forward path including a compensator, a switching power stage for amplifying an output signal from the compensator, and a demodulation filter for filtering an output signal from the switching power stage and providing an amplified output, said switching power stage including a bootstrap capacitor and a pre-charging circuit for charging the bootstrap capacitor; 
         [0030]    a DC-servo connected between the amplified output and an input of the compensator, thereby enabling reduction of offset voltages in the amplified output; 
         [0031]    a signal path connecting the output of the compensator to the DC-servo, wherein said method comprises the steps of: 
         [0032]    ensuring correct biasing of the compensator and the DC-servo at start-up, thereby bringing the DC-servo out of saturation; 
         [0033]    charging said bootstrap capacitor; 
         [0000]    starting said switching power stage, after said correct biasing is ensured and said bootstrap capacitor is charged. 
         [0034]    With this aspect of the invention, similar advantages and preferred features are present as in the previously discussed aspect of the invention. 
         [0035]    The invention may advantageously be used for improved start-up in any audio amplifier assembly, in particular high precision DC-AC power conversion systems such as in high efficiency audio amplification. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0036]    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: 
           [0037]      FIG. 1 a    illustrates a block diagram representation of a switching power conversion system in accordance with an embodiment of the present invention. 
           [0038]      FIG. 1 b    illustrates a schematic drawing of a DC servo and a switch in accordance with another embodiment of the present invention. 
           [0039]      FIG. 2  illustrates a schematic drawing of a DC servo and switch for a DC servo which is connectable to a compensator in accordance with yet another embodiment of the present invention. 
           [0040]      FIG. 3 a    illustrates a block diagram representation of a bootstrap capacitor pre-charge circuit in accordance with yet another embodiment of the present invention. 
           [0041]      FIG. 3 b    illustrates a block diagram representation of a bootstrap capacitor pre-charge circuit in accordance with yet another embodiment of the present invention. 
           [0042]      FIG. 4 a    illustrates a pre-charge circuit of the positive side of the bootstrap capacitor of a driver in accordance with yet another embodiment of the present invention. 
           [0043]      FIG. 4 b    illustrates a sub-block of the pre-charge circuit in  FIG. 4 a    in accordance with yet another embodiment of the present invention. 
           [0044]      FIG. 5 a    illustrates a pre-charge circuit of the negative side of a driver in accordance with yet another embodiment of the present invention. 
           [0045]      FIG. 5 b    illustrates a sub-block of the pre-charge circuit in  FIG. 5 a    in accordance with yet another embodiment of the present invention. 
           [0046]      FIG. 6  illustrates a simulation showing voltage signals at different parts of a power conversion system in accordance with yet another embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0047]    In the following detailed description, currently 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. 
         [0048]      FIG. 1 a    shows a block diagram  1  of an embodiment of the invention which will be described in greater detail in the following. The system in  FIG. 1  comprises a switching power conversion system including a sequence controller  18  controlling the sequence of the upstart, a DC-servo  8  having input from the output  11  of the amplifier and from the output of a switch  6 , the switch  6  being connected to the output of the compensator  5 , feedback filters  12 ,  14 ,  16  filtering the feedback signals for the control loop, attenuator  3  and clipper  3  that attenuates and clips the input signal  10 , compensator  5  shapes the control loop of the amplifier  1 , comparator  7 , power stage  9  including pre-charge circuit (not shown in  FIG. 1 ) for charging the high side boot strap capacitor before startup and a demodulation filter  13 . 
         [0049]    The pop minimization can be done by the following sequence: 
         [0050]    Firstly, the input signal  10  should be as close to zero as possible, this can be done by a clipper  3  and/or an attenuator  3 . 
         [0051]    In step two, for having the amplifier control loop in balance at startup the compensator  5  output is connected to the DC servo  8  input by a switch  6 , this way the compensator  5  and the DC servo  8  can be biased correctly before startup of power stage  9 . 
         [0052]    In step three, the power stage  9  must be capable of starting up with either a negative first pulse or a positive first pulse. To make it possible to start up with a positive first pulse the high side boot strap capacitor must be charged. 
         [0053]    In step four, the switch  6  connecting the output of the compensator  5  to the input of the DC-servo  8  is opened at the same time as the amplifier power stage  9  is started up. 
         [0054]    In step five, the clipper  3  stops clipping and the attenuator  3  stops attenuating. 
         [0055]    Lastly, the amplifier (switching power conversion system)  1  runs normally. 
         [0056]    In general applications, when the power stage  9  is switching normally, the bootstrap capacitor is charged via a diode when the output pulse is low. Therefore, if there is no pre-charging of the bootstrap capacitor being performed, the first pulse needs to be negative. In a case where a self-oscillating control system is being used with a single ended power stage, the switching power conversion system will never start if the first pulse is positive. In other applications, which don&#39;t use self-oscillating control systems, the switching power conversion system will always eventually start because it is forced to switch and therefore a negative pulse will follow the (possibly initial) high pulse, and thereby enabling the charging of the bootstrap capacitor. 
         [0057]    However, if the power stage  9  is forced to start-up with a first negative pulse with a certain width, it will often lead to a signal at the output  11  of the amplifier, a “pop”. The width of the pulse depends on the capacitance of the bootstrap capacitor that needs to be charged during this negative period. A high capacitance value is desirable because the bootstrap voltage can then be held for a longer time when there is a long positive pulse, for example because of a clipped high positive signal at the output  11  of the amplifier. In these types of “forced systems” it can be said that the forced negative pulse will cause a considerable “pop”. While in a system according to the present invention the width of the pulse is not forced (due to the pre-charging of the bootstrap capacitor), meaning that the control system is not forced and the first pulse can be a high pulse or a short low pulse and thus the system will not present a “pop” at the output/load. 
         [0058]    In the above-described sequence, steps two and three can be switched or have an overlap. Moreover, the constriction block  3  can be a clipper or an attenuator or both. 
         [0059]    Further, regarding the charge of the boot strap capacitor in step three, it can be done by two current sources one putting current into the + (positive node) of the capacitor and one pulling current out of the − (negative node) of the capacitor, explained in more detail in reference to  FIG. 3   a.    
         [0060]    The charge can also be done by a resistor connected to the positive power supply and the + (positive node) of the capacitor and a resistor connected to the negative power supply and to the − (negative node) of the capacitor, explained in more detail in reference to  FIG. 3   b.    
         [0061]    Alternatively, the system may comprise an additional switch, so that when the amplifier has started up and is running normally, the DC servo  8  input signal is from the output  11  of the amplifier. There can be a switch in this path so the output signal  11  of the amplifier is not connected to the DC servo  8  in step three when the input of the DC servo  8  is connected to the compensator  5  output. This will ensure that potential noise at the output  11  of the amplifier does not disturb the biasing of the DC-servo  8  in the above-described step two of the sequence. 
         [0062]    The switch  6  from the output of the compensator  5  to the input of the DC-servo  8  can also be removed and the output of the compensator  5  then connected directly to the DC-servo  8 . When both the compensator  5  and the amplifier output  11  are directly connected to the DC-servo  8  without a switch  6  in the path between the compensator  5  output and the DC-servo  8  input there is gain and filter frequency considerations between the compensator  5  output and the amplifier output  11  influence on the DC-servo  8 . When the amplifier is running normally it is desired that the influence from the input from the compensator  5  output is as little as possible. In other words, when the amplifier is running normally, a DC offset between the output of the compensator  5  and the output  11  of the amplifier will only be suppressed by the often larger gain in the amplifier output  11  feedback to the DC-servo  8 , compared to the gain in the compensator  5  output connected to the DC-servo  8 . For example, in some systems the signal swing at the output of the amplifier is about 10 times larger than the output signal swing from the compensator output, so if both of these outputs are connected to the DC-servo with the same resistor values, the feedback from the amplifier output will have about 10 times more gain than the compensator output, which will be acceptable system in some applications. Thus, considerations are taken and depending on the desired application and specifications it is determined if the switch  6  between the compensator  5  and DC-servo  8  can be excluded. In  FIG. 1 b    a rough schematic drawing of a DC-servo and associated components is shown. The output  112  of the compensator and the input of the DC-servo  108  can be connected with a switch  106 . The switch can be either closed or opened. Thus, in the present context the term “switch” is to be understood as a device having a transfer function that can be either 0 dB (i.e. no attenuation through the device) and substantially −∞0 dB (i.e. a very high attenuation through the device). 
         [0063]    The resistor  101  connected between the output  111  of the amplifier and an input of the DC-servo  108  determines the gain of the DC-servo  108  when the amplifier is running normally. While the resistor  102  connected between the output  112  of the compensator  5  and the switch  106  determines the gain of the DC-servo when the control system is basing during the start-up sequence. Between the output of the DC-servo  108  and an input  110  of the compensator  5  there is a resistor  103  which, together with capacitor  107 , contributes to the gain of the DC-servo in both of the above-described situations. 
         [0064]    In  FIG. 2  a detailed schematic of a DC-servo system and a switch in an integrated circuit application is shown. Separate sections  201 - 209  of the schematic have been marked in order to clarify the circuit in the figure and a brief description of each section will be provided in the following. The DC-servo is provided with three switches in section  201 , each having a different resistor value associated with it when connected to the output of the compensator. The decode logic in section  207  is used to select which of the switches and consequently which of the resistor values to be used, depending on the application of the system. The switch and resistor(s) to be used may be selected by an I2C interface (sometimes called inter-integrated circuit, I 2 C). Section  202  contains the operational amplifier used in the DC-servo and the associated (external) capacitor  107 . Section  209  illustrates some ESD (electrostatic discharge) protection components together with a box used to indicate where the output  111  of the amplifier and a resistor ( 101  in  FIG. 1 b   ) may be connected. 
         [0065]    Further, a switch that is used to short-circuit the DC-servo capacitor, when the system is not in use, is illustrated in section  205 . The aforementioned switch in section  205  is controlled by an input signal  120 . Section  204  comprises two comparators in order to protect from DC at the output of the amplifier when the amplifier is disabled. During this measurement, performed by the comparators in section  204 , the switch in section  206  shorts the output of the DC-servo and the DC-servo OP-amp in section  202  is disabled. The switch in section  206  is controlled by an input signal  123 . The switch Moreover, the above-described configuration (resistor from amplifier output and capacitor at the DC-servo OP amp) creates a low pass filter (RC-filter) for the measurement of the DC level at the output of the amplifier. This is in order to prevent high frequency noise to be detected by the DC-protection in section  204 . Section  203  also contains two comparators in order to protect from DC at the output of the amplifier when the amplifier is running normally. The comparators in  203  are measuring if the DC-servo output is above a certain level. In case the DC-servo is saturated it cannot minimize the DC at the output of the amplifier and it is therefore an indication of DC voltage at the output of the amplifier. Both of the DC-protection circuits render in an output signal  121 , where an optional input/control signal  122  may be applied for masking the saturation when the DC-servo is initializing, i.e. during start-up. The control signals  120 ,  122 ,  123  may be provided from a control unit (not shown) as known in the art. 
         [0066]    Section  208  provides a voltage references for the comparators in sections  203  and  204 . 
         [0067]      FIG. 3 a    shows a block diagram of an exemplary embodiment of a bootstrap pre-charge circuit  301 . This embodiment utilizes current sources  306 ,  307  or current generators  306 ,  307  for charging the bootstrap capacitor  302 . The current generator  306  is used to provide a first current i 1  to the positive side  305  of the bootstrap capacitor  302 . A second current generator  307  is used to draw a second current i 2  from the negative side  304  of the bootstrap capacitor  302 . Preferably the magnitude of the second current i 2  is equal to the magnitude of the first current i 1 . This is because when the negative side  304 , is connected to a load, such as e.g. a speaker, (not shown) and for the pre-charge current (i 1 ) not to run through the load and cause the undesired “pop”, the current drawing block  307  is connected to the negative side  304 . 
         [0068]    In  FIG. 3 b    an alternative embodiment of the bootstrap capacitor pre-charge circuit  321  is depicted, in a block diagram representation. Here, the bootstrap capacitor  302  is charged via the resistor  308  which in turn is connected to the positive supply  310 . A connection to the negative supply rail  311  from the negative side  304  is made via a resistor  309 ; in order to draw current similarly to the setup discussed in relation to  FIG. 3 a   , i.e. to prevent pre-charge current from running through a load, such as e.g. a speaker, (not shown) which is connected to the negative side  304 . Preferably the current through  308  should be equaled by the current through  309  in order to ensure that no pre-charge current flows through the load. 
         [0069]    In  FIGS. 4 a -5 b    some more detailed examples of a pre-charge circuit and associated sub-blocks are illustrated in a detailed schematic configuration for integrated circuit applications. 
         [0070]      FIG. 4 a    shows a detailed schematic of a pre-charge circuit from the positive supply to the positive side of the bootstrap capacitor in an integrated circuit form. Similarly to the procedure in  FIG. 2 , the circuit has been divided into sections  401 ,  402  which will be further described. Section  401  contains a cascade transistor  403  for handling high voltages and a diode  404  for separation from the bootstrap voltage when the bootstrap capacitor is at a high voltage when the power stage is switching, and a resistor  405  for ESD protection. In section  402  the block for the current mirror for the positive side of the bootstrap capacitor can be seen. 
         [0071]      FIG. 4 b    shows a more detailed view of the current mirror block for the positive side of the bootstrap capacitor ( 402  in  FIG. 4 a   ). Section  411  comprises two transistors  413 ,  414  for receiving the input reference current in the current mirror, and section  412  contains the output transistors for supplying the precharge current from the current mirror. 
         [0072]      FIG. 5 a    shows a pre-charge circuit from the negative supply to the negative side of the bootstrap circuit. Section  501  in  FIG. 5 a   , similarly to section  401  in  FIG. 4 a   , contains a cascade transistor  504  that can handle high voltages and a resistor  503  for ESD protection. The circuit in section  502  includes a reference current for the current mirror for the positive side of the bootstrap capacitor and a current mirror for the negative side of the bootstrap capacitor. 
         [0073]      FIG. 5 b    illustrates the “inside” of the current mirror block  502  from  FIG. 5 a   . Section  511  contains transistors for receiving an input reference current in the current mirror, section  512  contains transistors for generating a reference current for the current mirror for the positive side of the bootstrap capacitor (see  FIGS. 4 a -4 b   ), and section  513  comprises transistors for the pre-charge of the negative side of the bootstrap capacitor. 
         [0074]      FIG. 6  illustrates a signal simulation of a system in accordance with an embodiment of the invention. The simulation serves to elucidate the inventive start-up sequence and the different signals are illustrated in voltage graphs which will be explained in the following. 
         [0075]    The top window  601  shows the signal from the SCU (Sequence Control Unit) or sequence controller, controlling when the switch for the DC-servo from the compensator is on in the start-up sequence. Window  602  illustrates the signal from the SCU controlling when the bootstrap capacitor is to be pre-charged. Next, one can see the output of the DC-servo in window  603  which indicates that the DC-servo is saturated in the beginning and afterwards biased to a correct bias value after the switch to the DC-servo is closed (window  601 ), i.e. connection is established between the compensator and DC-servo. 
         [0076]    Further, the signal (from the SCU) controlling the activation of the power stage is shown in window  604  and window  605  illustrates the voltage across the bootstrap capacitor in the power stage. Window  606  represents the output of the compensator, where it is indicated that saturation is present in the beginning and that it is biased to a correct bias value after the switch to the DC-servo is closed (window  601 ). 
         [0077]    Lastly, window  607  shows the output signal of the amplifier after it has been filtered through the demodulation filter (low pass filter) and an additional low pass filter for removing switching ripple so that the audio band signal is more visible. 
         [0078]    The invention has now been described with reference to specific embodiments. However, several variations of the switching power conversion system are feasible. For example, the system may be applied to several different applications, such as e.g. in two level or multi level modulation, single ended amplifiers, BTL (Bridge Tied Load) dual supply, etc. Further, the DC-servo used is not limited to first order DC-servos but a higher order DC-servo is equally applicable. 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.