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 transforms the square-wave signal into the AC signal. Thereby, the defect of using an oscillator and a transformer to perform the DC to an AC function in the DC system is solved by the present invention.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an AC signal producer, and in particular to a device that uses a Class-D technique for transferring a DC signal to an AC signal and a method thereof. 
         [0003]    2. Description of Related Art 
         [0004]    A power supply for transferring a power source is classified into different types of servers, such as alternating current to alternating current, alternating current to direct current, direct current to alternating current, and direct current to direct current. In general, the power source for a general consumer product or an electronic car product needs to be transferred via the power supply. For example, the power source of a general consumer product or an electronic car product is driven via the DC levels or the alternating current to the direct current. If the electronic product is driven by the alternating current, the power source needs to transfer direct current to alternating current. 
         [0005]    Many AC power supplies use a transformer to step up or step down the voltage. One of the advantages of this method is that the noise is separated from the input end of the AC power. Another advantage is the elimination of the noise of the output end that influences the current of the input end. Moreover, the power provided by the transformer is more than the active component. 
         [0006]    Please refer to  FIG. 1 , which shows a schematic diagram of the power source transferring direct current to alternating current. A DC system  10  transfers the direct current to alternating current for an AC system  13 . The DC system  10  uses an oscillator circuit  11  to output an AC voltage. Next, a transformer  12  transfers the AC voltage to a voltage for an AC system  13 . Therefore, the DC system can use the transformer  12  and the oscillator circuit  11  to output alternating current for the AC system. 
         [0007]    However, a structure using a transformer for transferring direct current to alternating current has some disadvantage, as are described below:
       1. The efficiency of transferring power source, at about 50˜60%, is low.   2. The inductance of the transformer changes depending on the temperature.   3. The transformer is expensive, as is the cost of the structure, too.   4. The value of the inductance of the transformer can&#39;t be controlled accurately. Therefore, the quality of the structure is unstable.   5. The volume of the transformer is large.       
 
       SUMMARY OF THE INVENTION 
       [0013]    It is an object of the present invention to improve the structure for transferring direct current to alternating current. The present invention uses the Class-D technique for transferring direct current to alternating current. The Class-D technique is mostly used for amplifiers. An amplifier having the Class-D technique is highly efficient. Because the Class-D technique is digitally controlled, the output waveform is not distorted by changes in the environmental temperature. Therefore, the Class-D technique can be incorporated in the integrated circuit to reduce costs. 
         [0014]    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 transferred to an AC waveform according to the AC waveform table, and the AC waveform is transferred to a PWM control signal. The Class-D switch circuit receives the DC signal and the PWM control signal to output a square-wave signal. The low-pass filter then transfers the square-wave signal to an AC signal. 
         [0015]    To achieve the above object, the present invention provides a method of an AC signal producer comprising the steps of receiving a DC signal; transferring 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 to control a Class-D switch circuit; and transferring: a square-wave signal to an AC signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention can be fully understood from the following detailed description and preferred embodiments with reference to the accompanying drawings, in which: 
           [0017]      FIG. 1  is a schematic diagram of the power source transferring alternating current to a circuit current according to the prior art; 
           [0018]      FIG. 2  is a function block diagram of the AC signal producer according to present invention; 
           [0019]      FIG. 3  is a function block diagram of the control unit according to the present invention; 
           [0020]      FIG. 4  is a schematic diagram of the circuit of the Class-D switch circuit and the low-pass filter according to the present invention; 
           [0021]      FIG. 5  is a flow chart of the method of the AC signal producer according to the present invention; and 
           [0022]      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 
       [0023]    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. 
         [0024]    Please refer to  FIG. 2 , which shows the 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 (Director Current signal). The DC signal is transferred to a PWM control signal (Pulse-Width Modulated 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  transfers the square-wave signal to an AC signal (Alternating Current signal). The AC signal applies power to an AC power working device. 
         [0025]    Please refer to  FIG. 3 , which shows the function block diagram of the control unit  21  of the present invention. An AC waveform table  30  is set 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  transfers the DC signal to an AC waveform according to a voltage parameter setting and the angle and radian of the sine wave angle vs. the radian table  31 . Next, the control unit  21  transfers 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 other digital signal transferring 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 transferring the angle and radian of the AC waveform to the digital control signal. 
         [0026]    Please refer to  FIG. 4 , which shows the Class-D switch circuit  22  and the low-pass filter  23  of the present invention. The Class-D switch circuit  22  includes a PMOS switch M 1  and a NMOS switch M 2 . The source end of the PMOS switch M 1  receives the DC signal. The gate end of the PMOS switch M 1  receives the PWM control signal, and the PMOS switch M 1  turns on or off according to the PMOS control signal. The drain end of the PWOS switch M 1  outputs the square-wave signal. The source end of the NMOS switch M 2  is connected to the ground. The gate end of the NMOS switch M 2  receives the PWM control signal, and the NMOS switch M 2  turns on or off according to the PMOS control signal. The drain end of the NWOS switch M 2  is connected to the drain end of the PMOS 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. 
         [0027]    Please refer to  FIG. 5 , which shows the steps of the method of the AC signal producer. First, the control unit  21  receives a DC signal (S 501 ). Next, the control unit  21  transfers the DC signal to an AC waveform according to voltage parameter setting and the angle and radian of the sine wave angle vs. the radian table  31  (S 503 ). Next, the control unit  21  transfers the angle and radian of the AC waveform from the binary code table  32  to output 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  transfers the square-wave signal to the AC signal corresponding to the AC waveform (S 509 ). 
         [0028]    The present invention utilizes the Class-D technique for transferring the DC signal to the AC signal. The Class-D technique is used for amplifiers. The Class-D technique is highly efficient, approaching 90%. 
         [0029]    The wave form will be distorted via the influence of the environmental temperature when using the prior art with the transformer to transfer the DC signal to the AC signal. However, the Class-D technique that uses the digital control will prevent that from occurring. Moreover, manufacturing costs are reduced via the design of the Class-D technique being incorporated into an Integrated Circuit (IC). 
         [0030]    In general, a consumer electronic product has a power supply control unit for supplying power. The AC signal producer of the present invention is integrated into a system. Therefore, the space and cost of the system will be reduced, and the efficiency of the system of transferring the DC signal to the AC signal is improved. For example, a Vacuum fluorescent display (VFD) is wildly used in car audio plates. The AC power supplies for Vacuum fluorescent displays are assembled by a transformer and an oscillator which generates the AC signal to the VFD. However, the VFD will have the problems that the surface of the circuit is larger and the temperature of the car is higher by using the transformer. Please refer to  FIG. 5 , which shows the application of the present invention. The AC power supply for the Vacuum fluorescent display (VFD)  61  is generated by the AC signal producer  2 . Therefore, the problem of the prior art that occurred due to the transformer and the oscillator will be eliminated. 
         [0031]    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.