Abstract:
A digital audio signal amplifier and a digital audio signal amplifying method provide safety and power efficiency by integrating a switching mode power supply and a digital audio amplifier into an apparatus. The digital audio amplifier includes a pulse modulation unit generate a pulse modulated audio signal by pulse-modulating an input audio signal, a switching unit to switch a DC voltage based on the pulse modulated audio signal, an insulation transformer to transform the output of the switching unit and to output the result of the transforming, and a low-pass filter to obtain an audio signal corresponding to the input audio signal by low-pass filtering the output of the insulation transformer, and to output the output audio signal. By integrating a switching mode power supply and a digital audio amplifier into an apparatus, the digital audio amplifier satisfies the insulation requirement and at the same time increases power efficiency.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit and priority under 35 U.S.C. §§119 and 365 from Korean Patent Application No. 10-2005-0100402, filed on Oct. 24, 2005, in the Korean Intellectual Property Office, and International Application No. PCT/KR2006/004341, filed on Oct. 24, 2006, the disclosures of which are incorporated herein in their entirety by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present general inventive concept relates to a digital audio amplifier, and more particularly, to a digital audio signal amplifier in which both safety and power efficiency are achieved by integrating a switching mode power supply and a digital audio amplifier into one body, and a digital audio signal amplifying method thereof. 
     2. Description of the Related Art 
     In general, a digital audio system includes a direct current (DC) voltage supply unit generating a DC voltage from a commercial alternating current (AC) voltage; and a digital audio amplifier pulse-modulating the DC voltage using an audio signal, low-pass filtering the pulse-modulated DC voltage, and obtaining an output audio signal corresponding to the input audio signal. 
     Here, for the safety of users, the DC voltage supply unit is required to have a function of insulating the commercial AC voltage from the digital audio amplifier. Accordingly, a switching mode power supply (SMPS) having an insulation transformer is generally used as the DC voltage supply unit. Meanwhile, the digital audio amplifier is generally a D class digital audio amplifier to increase the output power. 
       FIG. 1  is a block diagram illustrating a conventional digital audio system  100 . The audio system  100  of  FIG. 1  includes an SMPS  110  and a D class audio amplifier  130 . 
     The SMPS  110  converts an input commercial AC voltage into a DC voltage using a first rectifying and smoothing unit  112 , switches the DC voltage in a switching unit  116  operating at a rate corresponding to a pulse width modulation (PWM) signal having a frequency of tens to hundreds of kHz or higher generated in a PWM modulation unit  114 , transforms the switched DC voltage with an insulation transformer  118  having an appropriate winding ratio, rectifies the output of the insulation transformer  118  through a second rectifying and smoothing unit  120 , and outputs a DC voltage. Also, though it is not shown, an error signal is fed back to the PWM modulation unit  114  to adjust a frequency of a pulse modulated signal and a duty ratio, so that a constant DC voltage can be output irrespective of changes in the input voltage or output current. 
     The insulation transformer  118  insulates a primary coil side, i.e., the commercial AC voltage source, from a secondary coil side, i.e., the digital audio amplifier  130 . Also, an error signal detected in the secondary side is fed back to the PWM modulation unit  114  of the primary side through a photo coupler (not shown) to provide electrical isolation. 
     Meanwhile, the D class digital audio amplifier  130  switches the DC voltage provided by the SMPS  110  at a D class output end  134  by a PWM audio signal generated from the PWM modulation unit  132 , low-pass filters the switched DC voltage through a low-pass filter  136 , and generates an output audio signal corresponding to an original audio signal. Meanwhile, though it is not shown, in order to stabilize the audio output a negative feedback signal is generated from the output of the insulation transformer  118  to control the pulse modulation of the PWM modulation unit  114 . 
       FIG. 2  illustrates waveform diagrams illustrating an operation of the D class digital audio amplifier  130  of  FIG. 1 . Referring to  FIG. 2 , a PWM audio signal is obtained by modulating a 180 kHz or higher triangular wave carrier with an input audio signal of 20 kHz or lower. The switched DC voltage is obtained by switching the DC voltage with the PWM audio signal. Also, the output audio signal is obtained by low-pass filtering the switched DC voltage. As illustrated in  FIG. 2 , the input audio signal is similar to the output audio signal. 
     However, the conventional digital audio system as illustrated in  FIG. 1  has a problem in that the power efficiency is low and the manufacturing cost is high. 
     For example, assuming that the power efficiency of the SMPS  110  is P 1  and the power efficiency of the digital audio amplifier  130  is P 2 , the power efficiency Pt of a conventional digital audio system  100  is P 1 ×P 2 . Here, since each of P 1  and P 2  has a value less than 1, and Pt becomes less than P 1  and P 2 , respectively, the conventional digital audio system  100  has very low power efficiency. 
     Meanwhile, in the SMPS  110  and the digital audio amplifier  130  of  FIG. 1 , the PWM modulation units  114  and  132  perform the same PWM modulation, the rectifying and smoothing units  112  and  120  perform the same rectifications, and the switching unit  116  and the D class output end  134  perform the same switching operation. That is, it can be seen that the units performing the same operation are disposed redundantly, accordingly manufacturing costs are increased. 
     SUMMARY OF THE INVENTION 
     The present general inventive concept provides a digital audio amplifier capable of increasing power efficiency and reducing manufacturing costs by eliminating redundant parts in an SMPS and a digital audio amplifier. 
     The present general inventive concept also provides a digital audio signal amplifying method capable of increasing power efficiency and reducing manufacturing costs. 
     Additional aspects and utilities of the present general inventive concept will be set forth in part in the description which follows and, in part, will be obvious from the description, or may be learned by practice of the general inventive concept. 
     The foregoing and/or other aspects and utilities of the present general inventive concept may be achieved by providing a digital audio amplifier including a pulse modulation unit to generate a pulse modulated audio signal by pulse-modulating an input audio signal, a switching unit to switch a direct current (DC) voltage based on the pulse modulated audio signal, an insulation transformer to transform the output of the switching unit and to output the result of the transforming, and a low-pass filter to obtain an audio signal corresponding to the input audio signal by low-pass filtering the output of the insulation transformer, and to output the audio signal. 
     The switching unit and the low-pass filter may be disposed on the primary side and on the secondary side, respectively, of the insulation transformer, and the digital audio amplifier may further include a photo coupler photo-coupling the pulse modulated audio signal and providing the result to the switching unit. The pulse modulation unit may pulse-width modulate the input audio signal. 
     The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a digital audio amplifying method including generating a pulse modulated audio signal by pulse-modulating an input audio signal, switching a DC voltage based on the pulse modulated audio signal and generating a switched DC voltage, transforming the switched DC voltage through an insulation transformer, and low-pass filtering the output of the insulation transformer and outputting an output audio signal corresponding to the input audio signal. 
     The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a digital audio amplifier including a pulse modulation unit to modulate an input audio signal to generate a pulse modulated audio signal, a switching unit to switch a voltage according to the pulse modulated audio signal, an insulation transformer to receive the switched voltage and to transform the switched voltage, and a low-pass filter to filter the transformed voltage to output an audio signal corresponding to the input audio signal. 
     The insulation transformer may be connected between the switching unit and the low-pass filter. 
     The voltage may be a DC voltage, and the switched voltage may be an AC voltage. 
     The amplifier may further include a photo-coupler connected between the switching unit and the pulse modulation unit. 
     The amplifier may further include a photo-coupler connected to the pulse modulation unit to receive the audio signal therethrough. 
     The amplifier may further include a feed-back unit to transmit a feedback signal from the insulation transformer to the pulse modulation unit. 
     The amplifier may further include a DC voltage source connected to the switching unit to generate a DC voltage as the voltage, and the DC voltage source may not have at least one of another pulse modulation unit to generate a PWM signal, another switching unit to switch another voltage to generate an AC voltage, and another insulation transformer to transform the AC voltage to generate the DC voltage as the voltage. 
     The insulation transformer may include a primary coil side connected to the switching unit and a secondary coil side connected to the low-pass filter such that the transformed voltage is insulated from the switched voltage. 
     The amplifier may further include a DC voltage source connected to the switching unit to generate a DC voltage as the voltage, and the DC voltage source may receive an AC voltage and include a bridge diode circuit to rectify and smooth the AC voltage to generate the voltage. 
     The foregoing and/or other aspects and utilities of the present general inventive concept may also be achieved by providing a digital audio amplifier including a pulse modulation unit to modulate an input audio signal to generate a pulse modulated audio signal, a switching unit to switch a voltage according to the pulse modulated audio signal, a low-pass filter to filter the switched voltage to output an audio signal corresponding to the input audio signal, and an insulating transformer connected between the switching unit and the low pass filter to provide an insulation between the switched voltage and the audio signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other features and advantages of the present general inventive concept will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a block diagram illustrating a conventional digital audio system; 
         FIG. 2  illustrates waveform diagrams illustrating an operation of a D class digital audio amplifier of the conventional digital audio system of  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating a digital audio amplifier according to an embodiment of the present general inventive concept; 
         FIG. 4  is a circuit diagram illustrating a digital audio amplifier according to an embodiment of the present general inventive concept; 
         FIG. 5  illustrates an example illustrating a full-bridge type digital audio amplifier according to an embodiment of the present general inventive concept; 
         FIG. 6  illustrates an operation of a Schmitt trigger of the digital audio amplifier of  FIG. 5 ; 
         FIG. 7  is a circuit diagram illustrating a digital audio amplifier according to an embodiment of the present general inventive concept; 
         FIG. 8  is a circuit diagram illustrating a digital audio amplifier according to another embodiment of the present general inventive concept; and 
         FIG. 9  is a circuit diagram illustrating a digital audio amplifier according to still another embodiment of the present general inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the embodiments of the present general inventive concept, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in order to explain the present general inventive concept by referring to the figures. 
       FIG. 3  is a block diagram illustrating a digital audio amplifier according to an embodiment of the present general inventive concept. The digital audio amplifier  300  of  FIG. 3  includes a rectifying and smoothing unit  302 , a pulse modulation unit  304 , a switching unit  306 , an insulation transformer  308 , and a low-pass filter  310 . As used herein, the term “insulation transformer” has the same meaning as the term “isolation transformer.” 
     The rectifying and smoothing unit  302  converts a commercial AC voltage into a DC voltage. The pulse modulation unit  304  pulse-modulates an input audio signal and provides the obtained pulse modulated audio signal to the switching unit  306 . The switching unit  306  switches the DC voltage provided by the rectifying and smoothing unit  302  using the pulse modulated audio signal and generates a switched DC voltage. The insulation transformer  308  includes a primary coil side connected to the switching unit  306  and a secondary coil side connected to the LPF  310  to transform and output the switched DC voltage output from the switching unit  306 . The low-pass filter  310  low-pass filters the output of the insulation transformer  308  and outputs an output audio signal corresponding to the input audio signal. The output audio signal is provided to a speaker to reproduce sound corresponding to the output audio signal. 
     Here, the commercial AC voltage is insulated from the low-pass filter  310  by the insulation transformer  308 . 
     Referring to  FIG. 3 , compared to the conventional digital audio system  100  of  FIG. 1 , the digital audio system of  FIG. 3  does not have the problem of redundancy of the rectifying and smoothing units, the pulse modulation units, and the switching units. Accordingly, power efficiency and manufacturing costs are improved. 
       FIG. 4  is a circuit diagram illustrating the digital audio amplifier  300  according to an embodiment of the present general inventive concept. According to the embodiment shown in  FIG. 4 , the pulse modulator  304  is disposed on the secondary coil side of the insulation transformer  308 , and the PWM audio signal that is the output of the pulse modulator  304  is provided to the switching unit  306  disposed on the primary coil side of the insulation transformer  308 , through a photo coupler  410 . 
     Referring to  FIG. 4 , the digital audio amplifier  300  includes a fuse  402 , a filter  404  to prevent electromagnetic interference (EMI), a capacitor unit  406  having capacitors to filter noise between signal lines or between signal lines and the chassis, a negative feedback signal generator  408 , the photo coupler  410  having a photo-diode and a photo-transistor to photo-couple the pulse modulated audio signal between the pulse modulation unit  304  and the switching unit  306 , and a gate driver  412  to amplify the output of the photo coupler  410  to a level which is enough to drive switching transistors  306   a  and  306   b  of the switching unit  306 . 
     The rectifying and smoothing unit  302  includes a bridge rectifier  302   a  and a smoothing capacitor  302   b . The switching unit  306  includes switching transistors  306   a  and  306   b  to perform an operation complementary to each other. The switching unit  306  of  FIG. 4  performs a switching operation corresponding to a half-bridge type D class digital audio amplifier. However, it should be noted that the present general inventive concept is not limited thereto. The present general inventive concept can be constructed to perform a switching operation corresponding to a full-bridge type D class digital audio amplifier. 
       FIG. 5  illustrates an example of a full-bridge type digital audio amplifier according to an embodiment of the present general inventive concept. The full-bridge type digital audio amplifier may include a rectifying and smoothing circuit to receive an input audio signal and a triangular wave carrier to generate a switching signal to be transmitted to a switching unit having a first pair U of transistors Q 1  and Q 2  and a second pair V of transistors Q 3  and Q 4 . An output power (or voltage) Vuv of the switching unit is applied to a load such that an audio signal is reproduced. One or more power sources Ed/ 2  are connected to the transistors Q 1 , Q 2 , Q 3 , and Q 4  to supply a power to the switching unit, and a signal Ooc is connected to the power sources Ed/ 2  to control the power sources Ed/ 2 . The switching unit of  FIG. 5  may be used as the switching unit  306  of  FIG. 4   
     Referring back to  FIG. 4 , the insulation transformer  308  includes a ferrite core in order to reduce EMI and enhance efficiency. 
     The pulse modulator  304  includes an integration circuit  304   a  and a Schmitt trigger  304   b . The integration circuit  304   a  integrates an audio signal and a negative feedback signal of the negative feedback generator  408  according to an output of the insulation transformer  308  and a reference, and the output of the Schmitt trigger  304   b  is oscillated by the output of the integration circuit  304   a . A PWM audio signal is obtained through the integration circuit  304   a  and the Schmitt trigger  304   b.    
       FIG. 6  illustrates an operation of the Schmitt trigger  304   b  of  FIG. 5 . The Schmitt trigger  304   b  has two threshold voltages. For example, if an input voltage (Vin) becomes greater than a first threshold, the output voltage (Vout) is transited to a logic level 1. Once the output voltage (Vout) becomes the logic level 1, the logic level 1 is maintained while the input voltage is between the first threshold voltage and a second threshold voltage. If the input voltage (Vin) becomes less than the second threshold voltage, the output voltage (Vout) returns to a logic level 0. 
     Accordingly, the Schmitt trigger  304   a  may output a pulse width modulated signal corresponding to a change in the output of the integration circuit  304   a  as the PWM audio signal. 
     Referring back to  FIG. 4 , the PWM audio signal output from the pulse modulation unit  304  is provided to the gate driver  412  through the photo coupler  410 . The gate driver  412  amplifies the output of the photo coupler  410  to a level which is enough to drive the switching transistors  306   a  and  306   b  of the switching unit  306 . 
     The DC voltage is switched, i.e., pulse width modulated, by a switching operation of the switching unit  306 . The output of the switching unit  306  is transformed through the insulation transformer  308 , and low-pass filtered through the low-pass filter  310  and output as an output audio signal. 
     The negative feedback signal generator  408  is used to provide a stable output audio signal regardless of a change in the input voltage and the output current. The negative feedback signal generator  408  generates a negative feedback signal corresponding to the magnitude of the output audio signal, from the output of the insulation transformer  308 . The negative feedback signal generator  408  can be implemented simply, for example, by a resistor. 
     The negative feedback signal is provided to the integration circuit  304   a . The integration circuit  304   a  integrates the audio signal and the negative feedback signal, that is, amplifies the synthesized signal of the audio signal and the negative feedback signal. 
     When the level of the output audio signal is changed to be higher than a normal level due to a change in the input voltage and output current when the output audio is in a stable state, that is, a state in which the negative feedback signal has a constant value, the negative feedback signal decreases, and as a result, the duty ratio of the PWM audio signal decreases. Accordingly, the duty ratio of the switched DC voltage decreases and therefore the level of the output signal falls. 
     When the level of the output audio signal is changed to be lower than a normal level due to a change in the input voltage and output current, the negative feedback signal increases, and as a result, the duty ratio of the PWM audio signal increases. Accordingly, the duty ratio of the switched DC voltage increases and therefore the level of the output signal rises. 
       FIG. 7  is a circuit diagram illustrating a digital audio amplifier according to an embodiment of the present general inventive concept. In the present embodiment of  FIG. 7 , a pulse modulator  304  is disposed on the secondary coil side of an insulation transformer  308 , and the PWM audio signal output from the pulse modulator  304  is provided to a switching unit  306  disposed on the primary coil side of the insulation transformer  308  through a photo coupler  410  similar to the embodiment of  FIG. 4 . However, the embodiment of  FIG. 7  is different from that of  FIG. 4  in that a comparator  304   c  is employed instead of the Schmitt trigger  304   b.    
     The comparator  304   c  compares the output of an integration circuit  304   a  and a triangular wave carrier and outputs a PWM audio signal. The operations of the comparator  304   c  and the digital audio amplifier  300  can be clearly understood with reference to the waveforms shown in  FIG. 2 . 
       FIG. 8  is a circuit diagram illustrating a digital audio amplifier  300  according to another embodiment of the present general inventive concept. Compared to the embodiment of  FIG. 4 , the embodiment of  FIG. 8  include a pulse modulator  304  disposed on the primary coil side of an insulation transformer  308  such that an input audio signal and a negative feedback signal are provided to the pulse modulator  304  through photo couplers  802  and  804 , respectively. By doing so, insulation of the audio signal and the negative feedback signal is achieved. 
       FIG. 9  is a circuit diagram illustrating a digital audio amplifier  300  according to still another embodiment of the present general inventive concept. In the embodiment of  FIG. 9 , a pulse modulator  304  is disposed on the primary coil side of an insulation transformer  308 , and an input audio signal and a negative feedback signal are provided to the pulse modulator  304  through photo couplers  802  and  804 , respectively, similar to the embodiment of  FIG. 8 . 
     However, the embodiment of  FIG. 9  is different from that of  FIG. 8  in that a comparator  304   c  is employed instead of the Schmitt trigger  304   b.    
     If the digital audio amplifier according to the present general inventive concept is employed, the PWM modulation units and switching units which are usually used in both the SMPS and the D class digital audio amplifier can be simplified such that the number of parts and components, manufacturing cost, and installation space can be reduced. 
     In addition, since the circuit becomes simplified, reliability is increased. 
     Also, if the digital audio amplifier according to the present general inventive concept is used, the SMPS can be omitted, which improves power efficiency reduces the space required for heat radiation. 
     According to experiments performed by the applicant, when the SMPS and the D class digital audio amplifier were used, the power consumption was 130 watts. However, according to the present invention, the power consumption was 111 watts, that is, power consumption was reduced by 19 watts. As a result, by lowering the current rates of a variety of filters preventing EMI, a bridge rectifier to rectify an AC voltage, and a smoothing condenser, a digital audio amplifier can be manufactured at a lower cost. 
     According to the digital audio amplifier of the present general inventive concept, by integrating a switching mode power supply and a digital audio amplifier into a single unit, the digital audio amplifier satisfies the insulation requirement and at the same time increases power efficiency. 
     Although a few embodiments of the present general inventive concept have been shown and described, it will be appreciated by those skilled in the art that changes may be made in these embodiments without departing from the principles and spirit of the general inventive concept, the scope of which is defined in the appended claims and their equivalents.