Abstract:
The output voltage of the output of a power supply for providing power to an audio amplification and transduction system having a plurality of channels with each channel transmitting an audio signal includes monitoring the amplitude of the audio signals and providing a highest amplitude signal representative of the contemporary highest amplitude of the audio. AC electrical power has a variable frequency controlled by the highest amplitude signal and is converted to DC voltage having the output voltage related to the variable frequency. This DC voltage is delivered to the channels.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
         [0001]    Not applicable to this invention.  
         STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
         [0002]    This invention is not related to any federally sponsored research or development.  
         TECHNICAL FIELD  
         [0003]    This invention relates to tracking power supplies, and more particularly to frequency modulated power supplies for multiple track audio applications.  
         BACKGROUND OF THE INVENTION  
         [0004]    It is an important object of the invention to provide an improved power supply for multiple track audio applications.  
         BRIEF SUMMARY OF THE INVENTION  
         [0005]    According to the invention, a power supply for converting line AC electrical power to varying voltage DC electrical power to power audio amplification and transduction circuitry, the audio amplification and transduction circuit having a plurality of input channels, each channel including an audio signal having an amplitude, includes a connector, for connecting the power supply to a source of line AC power. The power supply further includes a first converter device, electrically coupled to the connector, for converting the line AC power to DC power and a switching circuit, electrically coupled to the first converter device, for converting the DC power from the first converter device to AC power. The switching circuit is controllable so that the AC power from the switching device has a variable frequency. A power converter circuit, electrically coupled to the first converter device, has an input voltage and an output voltage, wherein a ratio of the input voltage to the output voltage is responsive to the variable frequency. A tracking device tracks the amplitudes of the plurality of channels as a function of time. The power supply further includes a device for determining the highest absolute value of the amplitudes of the plurality of input channels as a function of time and a frequency modulator for controlling the switching circuit to control the ratio of input voltage to output voltage so that the power supply provides sufficient power to amplify the audio signal in the track corresponding to the highest absolute value of the amplitudes.  
           [0006]    In another aspect of the invention, a method for controlling the output voltage of the output of a power supply for providing power to an audio amplification and transduction system having a plurality of channels, each of the channels transmitting an audio signal having an amplitude, includes the steps of monitoring the amplitudes of the audio signals in the plurality of channels as a function of time; determining, as a function of time, the maximum of the amplitudes; obtaining AC line electrical power; converting the electrical power to DC electrical power; converting the DC electrical power to AC electrical power having a variable frequency, wherein the variable frequency is responsive to the maximum of the amplitudes; converting the AC electrical power to AC electrical power having a variable voltage in a manner such that the reduced voltage is responsive to the variable frequency; and converting the variable voltage AC power to DC electrical power having a corresponding variable voltage.  
           [0007]    Other features, objects, and advantages will become apparent from the following detailed description, which refers to the following drawing in which: 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0008]    [0008]FIG. 1 is a block diagram of a multichannel audio device incorporating the invention;  
         [0009]    [0009]FIG. 2 is a more detailed block diagram of the multichannel audio device of FIG. 1;  
         [0010]    [0010]FIG. 3 is a plot of the voltage conversion ratio as a function of the normalized switching frequency for a resonant circuit according to the invention; and  
         [0011]    [0011]FIG. 4 is a plot illustrating the tracking of the audio signals. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]    With reference now to the drawing and more particularly to FIG. 1, there is shown a block diagram of a power supply according to the invention. Terminals  10 ,  11  of connector  12  connect to AC line source (not shown). Connector  12  is in turn electrically coupled to first AC/DC converter  14 . First AC/DC converter  14  is electrically coupled to controllable switches  16  by power lines  13   a,    13   b.  Controllable switches  16  are coupled to resonant circuit  18  by power lines  15   a,    15   b.  Resonant circuit  18  is electrically coupled to primary winding  19  of transformer  20  by power lines  17   a,    17   b.  Secondary windings  21   a  and  21   b  of transformer  20  are coupled to second AC/DC converter  22  by power lines  20   a,    20   b.  Second AC/DC converter  22  is coupled to rails  24 ,  26 . Rails  24 ,  26  may be connected to audio amplification and transduction circuitry  28 . Audio signal processor  30  is connected by a signal line  31  to a source of audio signals. If the audio signals are digitally encoded, signal line  31  is typically a single line. If the audio signals are analog, signal line  31  typically includes a separate line for each audio channel. Power supply control (PSC) module  33  of audio signal processor  30  is coupled by signal lines to error amplification device  36 ; error amplification device is also coupled to rail  26  as shown or to rail  24 . Error amplification device  36  is further coupled by signal lines to compensation device  38  and to frequency modulator  40 . Frequency modulator  40  is coupled to controllable switches by signal line  42 .  
         [0013]    In operation, connector  12  plus into an AC outlet to provide line voltage to AC/DC converter  14 . AC/DC converter converts the line AC voltage to a DC voltage so that the voltage across power lines  13   a,    13   b  is DC at the peak line voltage or at twice the peak line voltage if AC/DC converter include a voltage doubler rectifier. Controllable switches  16  switch at a high frequency so that the voltage across power lines  15   a,    15   b  has a square or trapezoidal waveform at a frequency of &gt;20 kHz. Electrical power is then conducted through resonant circuit  18  to transformer  20  so that voltage across power lines  20   a  and  20   b  has a square or trapezoidal waveform at substantially the same frequency as the frequency across power lines  17   a,    17   b  at a voltage V o . Second AC/DC converter  22  converts the square or trapezoidal waveform to two unidirectional voltages so that the voltage at output terminal  24  is +V o  and the voltage at output terminal  26  is −V o .  
         [0014]    PSC module  33  determines the highest absolute value of the amplitudes of the signals in the audio input channels at each point in time. Error amplification device  36  compares the signals from PSC module  33  with the voltage from rail  26  to develop an error signal, amplifies the error signal and transmits the amplified error signal to compensation device  38 . Compensation device  38  adjusts the gain and phase of the amplified error signal, and transmits the adjusted error signal to frequency modulator  40 . Frequency modulator  40  determines an appropriate switching frequency for controllable switches  16 , and transmits that frequency over signal line  42  to controllable switches  16 .  
         [0015]    The resonant circuit  18  between the controllable switches  16  and the transformer shapes the current to a quasi-sinusoidal waveform. A current having a quasi-sinusoidal waveform has very low harmonic content. A power supply according to the invention is advantageous because the high electromagnetic interference (EMI) emissions that are associated with conventional power supplies using pulse modulation or pulse width modulation are reduced. The power supply can be conveniently placed in an audio system chassis near radio receiving circuitry with relatively little EMI shielding.  
         [0016]    Referring now to FIG. 2, there is shown the power supply of FIG. 1, showing the circuitry that performs the functions of some of the blocks of FIG. 1 and also shows some elements that are useful in a practical embodiment of the invention. In FIG. 2, like reference numerals refer to like elements in the several views of the drawings. The grouping of the elements of FIG. 2 is according to the logical arrangement of the elements as shown in FIG. 1. The elements of FIG. 1 do not necessarily represent distinct physical elements and the elements of FIG. 2 may be physically grouped differently than they are grouped in FIGS. 1 and 2. EMI filter  46  is not shown in FIG. 1. Its purpose is to prevent EMI emissions generated by the current switches from flowing to the line source. First AC/DC converter  14  may include a bridge rectifier  48  and two capacitors  50  and  52 . First AC/DC converter  14  may include a full bridge rectifier with a capacitor, having an output voltage of at peak line voltage. Alternatively, first AC/DC converter may include a voltage doubler, having a voltage output of twice the line peak voltage. Controllable switches may include two semiconductor switches  54 ,  56  which alternately turn on and off at a high frequency (&gt;20 kHz, in one implementation between 100 kHz and 200 kHz) with a duty ratio of slightly less than 50% to invert the DC input voltage to high frequency square wave voltage. The duty ratio of slightly less than 50% allows a small dead time during which both switches are turned off. The dead time avoids “shoot through” of the switches and allows zero-voltage switching as will be explained later. Resonant circuit  18  has two inductors  62 ,  64 , and a capacitor  66 . Capacitor  66  and inductor  62  are arranged in a conventional series resonant manner. The parameters of inductor  62 , and capacitor  66  are selected such that the resonant frequency of the inductor  62  and the capacitor  66  is lower than the range of switching frequencies. Capacitors  58 ,  60  of controllable switches  16 , connected in parallel with the switches, slow down the voltage transition across the two switches, changing the square wave waveform to a trapezoidal waveform, further reducing the EMI emissions that result from the switching. The dead time for the two switches  54 ,  56  should be chosen so that it allows the transition of the voltage across the two switches (from high to zero or from zero to high) to be completed within the dead time. With an adequate dead time, the two switches are turned on and off at zero voltage, which reduces switching losses. A dead time of approximately 300 ns is adequate for a switching frequency of 100 kHz to 200 kHz. Transformer  20  may be a conventional step-down transformer with center tap windings at its secondary. In an exemplary embodiment, transformer  20  is a step-down transformer that has a leakage inductance for inductor  62  and a magnetizing inductance for inductor  64 . Its two secondary windings  21   a  and  21   b  are tightly coupled. Second AC/DC converter  22  may include a bridge rectifier  68  and two capacitors  70 ,  72 . Audio amplification and transduction circuitry  28  may include rails  24 ,  26 , audio power amplifiers  78 ,  80 , and electroacoustical transducers, represented by load resistors  82 ,  84 . Audio signal processor  30  and PSC module  33  may be conventional digital and analog devices, or, in a specific embodiment, may be digital signal processors running appropriate software on digital audio signals. In the case of digital audio signals, the multiple channels may be transmitted over a single signal line  31 . Error amplification device  36 , compensation device  38 , opto-isolator  39 , frequency modulator  40 , and half-bridge driver  41  all include conventional circuit elements arranged in a conventional manner. In one embodiment, frequency modulator  40  and half-bridge driver  41  are implemented on a single monolithic circuit L6598 available commercially from ST Microelectronics of Carrollton, Tex. (website us.st.com).  
         [0017]    A characteristic of a power supply according to the invention is that the ratio of the voltage at the output lines (the voltage at line  24  and the voltage at line  26 ) to the voltage at the input lines (across lines  15   a  and  15   b ) is determined by the relationship of the switching frequency at the input lines  24  and  26  to the resonant frequency of capacitor  66  and inductor  62 .  
         [0018]    Referring to FIG. 3, there is shown a plot of voltage conversion ratio versus normalized switching frequency for a power supply according to FIGS. 1 and 2. Each of the curves represents a different load resistance seen at line  24  or at line  26 . The normalized switching frequency is expressed as multiples of the resonant frequency, which is determined by the parameters of capacitor  66  and inductor  62  of resonant circuit  18 . A typical resonant frequency is 100 kHz. So, for example with load L 3 , and a switching frequency of 1.5 times the resonant frequency, the voltage conversion ratio is about 0.45, so that with a line voltage of 120 volts rms, a peak voltage of 170 volts, and a turns ratio of primary windings  19  to secondary windings  21   a  or  21   b  of five, the voltage Vo is about (170×0.45)/5 or about 15.3 volts.  
         [0019]    Referring to FIG. 4, there is shown an example of the function of PSC module  33 . Curves  86 ,  88 ,  90  represent the audio signals in channels  1 ,  2 , and  3 , respectively, of a three channel audio system. PSC module determines the highest absolute value of the amplitudes of the three signals. To represent the absolute values, the rectified values of each of the curves are shown in dashed lines. The highest absolute value is shown as heavy line  92 . The feedback loop including error amplification device  36 , compensation device  38 , and frequency modulator  40  act to maintain a voltage at rail  24  and at rail  26  that matches curve  92 . In another implementation, the PSC module  33  builds in a small offset DV, for example 3 v, to provide headroom for the amplifier, resulting in offset line  94 . In this implementation, the feedback loop including error amplification device  36 , compensation device  38 , and frequency modulator  40  act to maintain a voltage that matches curve  94 .  
         [0020]    It is evident that those skilled in the art may now make numerous modifications of and departures from the specific apparatus and techniques disclosed herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques disclosed herein.