Abstract:
An AC signal producer comprises a controlling unit, a Class-D switch circuit, and a low-pass filter. The control unit receives a DC signal and produces a PWM control signal via checking reference tables. The Class-D switch circuit receives the PWM control signal and outputs a square-wave signal. The low-pass filter converts the square-wave signal into the AC signal. Thereby, disadvantages associated with the utilization of an oscillator and a transformer to convert the DC signal to an AC signal could be solved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an alternating current (AC) signal producer, and in particular to an AC signal producer utilizing a Class-D technique for converting a direct current (DC) signal to an AC signal and a method thereof. 
     2. Description of Related Art 
     A power converter in a power supply for converting a power source is classified into different types, such as AC to AC, AC to DC, DC to AC, and DC to DC. In general, the power converter for a general consumer electronic product or an electronic car product may convert an AC input from a power source to a DC voltage signal so that the general consumer electronic product or the electronic car product could be powered. If the electronic product is driven by an AC voltage signal, the power supply needs to further convert the DC voltage signal into the AC voltage signal. 
     Many AC power supplies use a transformer to step up or step down the voltage signal. One of the advantages of this method is that the noise is separated from the input end of the AC power supplies. Another advantage is the elimination of the noise of the output end that influences the power source at the input end. Moreover, the power provided by the transformer is more than the active component. 
     Please refer to  FIG. 1 , which shows a schematic diagram of the conventional power source converting the DC signal to the AC signal. When a DC system  10  converts the DC signal to the AC signal for an AC system  13 , the DC system  10  uses an oscillator circuit  11  to output AC signal. Next, a transformer  12  converts the AC signal to a voltage level required by the AC system  13 . Therefore, the DC system  10  requires the transformer  12  and the oscillator circuit  11  to output the AC signal of the required voltage level for the AC system  13 . 
     However, a structure using a transformer for converting the DC signal to the AC signal has some disadvantages, described in the below:
         1. The efficiency of power conversion is at about 50 to 60%.   2. The inductance of the transformer changes depending on the changes in the temperature, which may affect the oscillating frequency of the oscillator or even cause the oscillator not to oscillate.   3. The transformer is expensive, driving up the cost of the structure.   4. The value of the inductance of the transformer can&#39;t be controlled accurately. Therefore, the quality of the transformer is unstable.   5. The size of the transformer is large.       

     SUMMARY OF THE INVENTION 
     It is an object of the present invention to improve the structure for converting the DC signal to the AC signal. The present invention uses the Class-D technique for converting the DC signal to the AC signal. The Class-D technique is mostly used for amplifiers. An amplifier having the Class-D technique is highly efficient in terms of the conversion of signals. Because the Class-D technique is digitally controlled, the output waveform is not distorted by changes in the environmental temperature. 
     To achieve the above object, the present invention provides an AC signal producer comprising a control unit, a Class-D switch circuit, and a low-pass filter. The control unit has an AC waveform table. A DC signal is converted to an AC waveform according to the AC waveform table, and the AC waveform is further converted to a PWM control signal. The Class-D switch circuit receives the DC signal and the PWM control signal before outputting output a square-wave signal. The low-pass filter then converts the square-wave signal to an AC signal. 
     To achieve the above object, the present invention provides a method for operating an AC signal producer comprising the steps of receiving a DC signal; converting a DC signal to an AC waveform according to an AC waveform table, generating a PWM control signal according to the AC waveform, generating a square-wave signal via the PWM control signal, which is configured to control a Class-D switch circuit, and converting a square-wave signal to an AC signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be fully understood from the following detailed description and preferred embodiments with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic diagram of the conventional power source converting the AC signal to the DC signal; 
         FIG. 2  is a function block diagram of the AC signal producer according to the present invention; 
         FIG. 3  is a function block diagram of the control unit according to the present invention; 
         FIG. 4  is a schematic diagram of a Class-D switch circuit and a low-pass filter according to the present invention; 
         FIG. 5  is a flow chart illustrating a method for operating the AC signal producer according to the present invention; and 
         FIG. 6  is a schematic diagram of the application of the AC signal producer according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following detailed description is of the best presently contemplated modes of applying the invention. This description is not intended to be taken in a limiting sense, but is made merely for the purpose of illustrating general principles of embodiments of the invention. The scope of the invention is best defined by the appended claims. 
     Please refer to  FIG. 2 , which shows a function block diagram of an AC signal producer of the present invention. The AC signal producer  2  comprises a control unit  21 , a Class-D switch circuit  22 , and a low-pass filter  23 . The control unit  21  receives a DC signal. The DC signal is converted to a pulse-width modulation (PWM) control signal via a parameter table of the control unit  21 . The Class-D switch circuit  22  receives the PWM control signal and the DC signal, and outputs a square-wave signal. The low-pass filter  23  converts the square-wave signal to an AC signal. The AC signal is delivered to the general consumer electronic device or electronic car device requiring the AC signal. 
     Please refer to  FIG. 3 , which shows a function block diagram of the control unit  21  of the present invention. An AC waveform table  30  is stored in the control unit  21 . The AC waveform table  30  includes a sine wave angle vs. a radian table  31  and a binary code table  32 . The control unit  21  converts the DC signal to an AC waveform according to a voltage parameter and the angle and radian of the sine wave angle vs. the radian table  31 . Next, the control unit  21  converts the angle and radian of the AC waveform from the binary code table  32  to output the PWM control signal. In a second embodiment, the binary code table  32  can be any digital signal conversion table with different binary codes, and the number of bits of the binary codes is not limited. The function of the binary code table  32  is for converting the angle and radian of the AC waveform to the digital-based PWM control signal. 
     Please refer to  FIG. 4 , which shows a Class-D switch circuit  22  and a low-pass filter  23  of the present invention. The Class-D switch circuit  22  includes a p-channel metal-oxide semiconductor field-effect transistor (PMOS) switch M 1  and a n-channel metal-oxide semiconductor field-effect transistor (NMOS) switch M 2 . A source end of the PMOS switch M 1  receives the DC signal. A gate end of the PMOS switch M 1  receives the PWM control signal, and thus the PMOS switch M 1  could be turned on or off according to the PWM control signal. A drain end of the PWOS switch M 1  outputs the square-wave signal. A source end of the NMOS switch M 2  is connected to a ground. A gate end of the NMOS switch M 2  receives the PWM control signal, and thus the NMOS switch M 2  could be turned on or off according to the PWM control signal. A drain end of the NWOS switch M 2  is connected to the drain end of the PMOS switch M 2  and outputs the square-wave signal. The square-wave signal outputted from the Class-D switch circuit  22  is a disperse-dense square wave. After the square-wave signal is filtered by the low-pass filter  23 , the low-pass filter  23  outputs the AC signal, which corresponds to the AC waveform. 
     Please refer to  FIG. 5 , which shows a flow chart illustrating a method for operating the AC signal producer according to the present invention. First, the control unit  21  receives a DC signal (S 501 ). Next, the control unit  21  converts the DC signal to an AC waveform according to a voltage parameter and the angle and radian of the sine wave angle vs. the radian table  31  (S 503 ). Next, the control unit  21  converts the angle and radian of the AC waveform according to the binary code table  32  before outputting the PWM control signal (S 505 ). Next, the Class-D switch circuit  22  outputs the square-wave signal according to the PWM control signal (S 507 ). Finally, the low-pass filter  23  converts the square-wave signal to the AC signal corresponding to the AC waveform (S 509 ). 
     The present invention utilizes the Class-D technique for converting the DC signal to the AC signal despite the Class-D technique is usually utilized by amplifiers. The Class-D technique is highly efficient, with the power efficiency approaching 90%. 
     And since the Class-D technique employs digital-based control, causing no distortion on output waveforms as the conventional transformer for converting the DC signal to the AC signal may result in. Meanwhile, since the Class-D technique may be utilized in the integrated circuit the total cost for implementing the function of converting the DC signal to the AC signal may be reduced. 
     In general, a consumer electronic product has a power supply control unit for managing the power supply. As the AC signal producer of the present invention is readily integrated into the consumer electronic product, the size and the cost of manufacturing the consumer electronic product may reduce while the efficiency in converting the DC signal to the AC signal may increase. For example, a Vacuum fluorescent display (VFD) that is widely used in car audio plates generally requires an AC signal from the transformer and the oscillator in order to be powered. When the AC signal is provided by the transformer, the size of VFD may increase and the operation of the VFD may be affected by the temperature inside the vehicle. However, that problem may be minimized if the AC signal producer according to the present invention is utilized, as shown in  FIG. 6  in which the power required for the VFD  61  is prepared by the AC signal producer  2 . 
     The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.