Abstract:
An AC power feedback control device for a vacuum fluorescent display is provided. In the AC power feedback control device, the Class-D drivers are driven by the PWM controller so as to generate a sine wave voltage. After being filtered by the LPFs, the sine wave voltage is ready for filaments. The output voltage outputted by the filaments is detected by simple feedback elements so as to control and modulate the duty cycle of the PWM controller.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an AC power feedback control device, and more particularly to an AC power feedback control device for a vacuum fluorescent display (VFD). 
     BACKGROUND OF THE INVENTION 
     Vacuum fluorescent display (VFD) is commonly utilized in the application of a small display to provide a better contrast of brightness. VFD has the advantages of better brightness, wider visual angle, larger operating temperature range and lower production cost. 
     In VFD, the driving method for the filaments depends on the type of the filaments. Generally, there are two kinds of the driving methods for the filaments. One is the AC power driving method and the other is the dc power driving method. 
     For generating an AC power to drive the filaments of a VFD operated in a dc power supply, an AC sine wave signal is generated by using a transformer based on the LC oscillation effect resulted from an inductor and a capacitor. The frequency can be adjusted by changing the inductance of the inductor and the capacitance of the capacitor, the amplitude can be adjusted by changing the turn ratio of the coils of the transformer, and the phase can be adjusted by changing the central tap of the transformer and incorporating a Zener diode. 
     Although the circuit topology of the above traditional AC power control device is simple, it lacks of a feedback control loop. When the input voltage is changed, the output voltage will be influenced undesirably. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an AC power feedback control device for a vacuum fluorescent display. In the AC power feedback control device, the output voltage outputted by the filaments is detected and stabilized by simple feedback circuits controlling and modulating the duty cycle of the AC sine wave signal inputted to the filaments. 
     According to the foregoing object of the present invention, an AC power feedback control device is provided. The AC power feedback control device includes a controller generating a control signal; a filtering apparatus filtering the control signal to generate a sine wave signal with a first peak; a filament driven by the sine wave signal; a first peak detector detecting a value of the first peak of the sine wave signal; and a first comparator and a second comparator comparing the value of the first peak with a first and a second predetermined voltage values respectively so as to modulate the control signal. 
     Preferably, the filtering apparatus comprises at least a class-D driver and at least a low-pass filter. 
     Preferably, the filament is for lighting a vacuum fluorescent display. 
     Preferably, the first peak is a maximum peak of the sine wave signal. 
     Preferably, the AC power feedback control device further includes a second peak detector detecting a value of a second peak of the sine wave signal; and a third comparator and a fourth comparator comparing the value of the second peak with a third and a fourth predetermined voltage values respectively so as to modulate the control signal. 
     Preferably, the second peak is a minimum peak of the sine wave signal. 
     The foregoing and other features and advantages of the present invention will be more clearly understood through the following descriptions with reference to the drawings, wherein: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an AC power feedback control device according to the present invention; 
         FIG. 2  is a circuit diagram showing the class-D driver of  FIG. 1  according to the present invention; 
         FIG. 3  is a circuit diagram showing the first peak detector of  FIG. 1  according to the present invention; 
         FIG. 4  is a circuit diagram showing the second peak detector of  FIG. 1  according to the present invention; and 
         FIGS. 5(   a )-( i ) are illustrative diagrams showing the relationships of the sine wave signal and four limits of the peaks. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for the purposes of illustration and description only; it is not intended to be exhaustive or to be limited to the precise form disclosed. 
     Please refer to  FIG. 1 , which is a block diagram showing an AC power feedback control device according to the present invention. In  FIG. 1 , the AC power feedback control device  1  includes a controller  10 , two class-D drivers  11  and  11 ′, two low-pass filters  12  and  12 ′, a filament  13 , a first peak detector  14 , a first and a second comparators  15  and  16 , a second peak detector  17 , and a third and a fourth comparators  18  and  19 . 
     A control signal generated from the controller  10  is inputted to the two class-D drivers  11  and  11 ′. Two high-frequency driving signals are then generated from the class-D drivers  11  and  11 ′. The low-pass filters  12  and  12 ′ filters off the high-frequency signals. The Two sine wave signals are tehn generated to drive the filament  13 . For stabilizing the output voltage, the first peak detector  14  and the second peak detector  17  are incorporated here to detect the peaks of the sine waves which are feedbacked. That is to say, the first peak detector  14  is used to detect a value of the maximum peak of each the sine waves which are feedbacked and the second peak detector  17  is used to detect a value of the minimum peak of each the sine waves which are feedbacked. 
     In the feedback circuit constituted by the element blocks  14 ˜ 19 , one of the input terminals of the first comparator  15  is to receive an upper limit V 1  for the maximum peak of the sine wave which is feedbacked, one of the input terminals of the second comparator  16  is to receive an lower limit V 2  for the maximum peak of the sine wave which is feedbacked, one of the input terminals of the third comparator  18  is to receive an upper limit V 3  for the minimum peak of the sine wave which is feedbacked, and one of the input terminals of the fourth comparator  19  is to receive an lower limit V 4  for the minimum peak of the sine wave which is feedbacked. The value of the maximum peak of the sine wave detected by the first peak detector  14  is compared with the upper limit V 1  and the lower limit V 2  and the value of the minimum peak of the sine wave detected by the second peak detector  17  is compared with the upper limit V 3  and the lower limit V 4 , four feedback signals are then generated. Sampling values of the sine waves can be adjusted based on the four feedback signals to change the duty cycle of the controller  10 , so as to stabilize the output voltage. 
     The preferable schemes of all the element blocks of the AC power feedback control device are described as follows. 
     The controller  10  of the AC power feedback control device  1  can be a pulse-width modulation (PWM) controller or a pulse-frequency modulation (PFM) controller. The filament  13  is one for lighting a vacuum fluorescent display (VFD). 
     Please refer to  FIG. 2 , which is a circuit diagram showing the class-D driver of  FIG. 1  according to the present invention. The class-D driver  11  or  11 ′ is a half-bridge converter constituted by a non-overlap controller, a PMOS transistor P 1  and a NMOS transistor N 1 . The input terminal IN of the class-D driver  11  receives a control signal from the controller  10  and the output terminal OUT of the class-D driver  11  generates and transmits a high-frequency signal to the low-pass filter  12 . 
     Please refer to  FIG. 3 , which is a circuit diagram showing the first peak detector of  FIG. 1  according to the present invention. The first peak detector  14  is constituted by a diode D 1 , a capacitor C 1  and a resistor R 1 . The input terminal IN of the first peak detector  14  receives a sine wave signal and the output terminal OUT of the first peak detector  14  transmits the peaks of the detected sine wave signal to the first comparator  15  and the second comparator  16  for being compared. 
     Please refer to  FIG. 4 , which is a circuit diagram showing the second peak detector of  FIG. 1  according to the present invention. The second peak detector  17  is constituted by a diode D 2 , a capacitor C 2  and a resistor R 2 . The input terminal IN of the second peak detector  17  receives a sine wave signal and the output terminal OUT of the second peak detector  17  transmits the peaks of the detected sine wave signal to the third comparator  18  and the fourth comparator  19  for being compared. 
     The corresponding adjusting methods of all combinations of the four feedback signal are described as follows. The comparing outcomes of the first, second, third and fourth comparators are expressed in a four-bit number combination as “XXXX”, wherein “1” represents a-peak smaller than the limit and “0” represents a peak bigger than the limit. 
     (a) the feedback signal is “0000” 
     The “0000” feedback signal means that the maximum peak is bigger than the upper limit V 1  and the lower limit V 2  and the minimum peak is bigger than the upper limit V 3  and the lower limit V 4 . Accordingly, the sine wave signal is upward shifted. To adjust the sine wave signal, the offset must be decreased, as shown in  FIG. 5(   a ). 
     (b) the feedback signal is “0010” 
     The “0010” feedback signal means that the maximum peak is bigger than the upper limit V 1  and the lower limit V 2  and the minimum peak is smaller than the upper limit V 3  but bigger than the lower limit V 4 . Accordingly, the sine wave signal is upward shifted or expanded. To adjust the sine wave signal, the offset must be decreased or the amplitude must be shrunk, as shown in  FIG. 5(   b ). 
     (c) the feedback signal is “0011” 
     The “0011” feedback signal means that the maximum peak is bigger than the upper limit V 1  and the lower limit V 2  and the minimum peak is smaller than the upper limit V 3  and the lower limit V 4 . Accordingly, the sine wave signal is expanded. To adjust the sine wave signal, the amplitude must be shrunk, as shown in  FIG. 5(   c ). 
     (d) the feedback signal is “1000” 
     The “1000” feedback signal means that the maximum peak is smaller than the upper limit V 1  but bigger than the lower limit V 2  and the minimum peak is bigger than the upper limit V 3  and the lower limit V 4 . Accordingly, the sine wave signal is upward shifted or shrunk. To adjust the sine wave signal, the offset must be decreased or the amplitude must be expanded, as shown in  FIG. 5(   d ). 
     (e) the feedback signal is “1010” 
     The “1010” feedback signal means that the maximum peak is smaller than the upper limit V 1  but bigger than the lower limit V 2  and the minimum peak is smaller than the upper limit V 3  but bigger than the lower limit V 4 . Accordingly, the sine wave signal is in the predetermined range. There is no need to adjust the sine wave signal, as shown in  FIG. 5(   e ). 
     (f) the feedback signal is “1011” 
     The “1011” feedback signal means that the maximum peak is smaller than the upper limit V 1  but bigger than the lower limit V 2  and the minimum peak is smaller than the upper limit V 3  and the lower limit V 4 . Accordingly, the sine wave signal is downward shifted or expanded. To adjust the sine wave signal, the offset must be increased or the amplitude must be shrunk, as shown in  FIG. 5(   f ). 
     (g) the feedback signal is “1100” 
     The “1100” feedback signal means that the maximum peak is smaller than the upper limit V 1  and the lower limit V 2  and the minimum peak is bigger than the upper limit V 3  and the lower limit V 4 . Accordingly, the sine wave signal is shrunk. To adjust the sine wave signal, the amplitude must be expanded, as shown in  FIG. 5(   g ). 
     (h) the feedback signal is “1110” 
     The “1110” feedback signal means that the maximum peak is smaller than the upper limit V 1  and the lower limit V 2  and the minimum peak is smaller than the upper limit V 3  but bigger than the lower limit V 4 . Accordingly, the sine wave signal is downward shifted or shrunk. To adjust the sine wave signal, the offset must be increased or the amplitude must be expanded, as shown in  FIG. 5(   h ). 
     (i) the feedback signal is “1111” 
     The “1111” feedback signal means that the maximum peak is smaller than the upper limit V 1  and the lower limit V 2  and the minimum peak is smaller than the upper limit V 3  and the lower limit V 4 . Accordingly, the sine wave signal is downward shifted. To adjust the sine wave signal, the offset must be increased, as shown in  FIG. 5(   i ). 
     In the above situations (a)˜(i), the controller  10  is a PWM controller. Therefore, the expand or shrink operation is implemented by increasing or decreasing the duty cycle. For those skilled in the art, it is achievable to infer that the expand or shrink operation can be implemented by increasing or decreasing the feaquency of the control signal when the controller  10  is a PFM controller. 
     Under the situation of the sine wave signals being bigger than ground level and the voltage value of the minimum peak being around the ground level, the second peak detector  17 , the third comparator  18  and the fourth comparator  19  can be omitted. The maximum peak will be detected only through the first peak detector  14 , the first comparator  15  and the second comparator  16 . 
     In conclusion, the feedback control function of the AC power feedback control device provided in the present invention is achieved with four comparators. The four comparators generates four feedback signals to be processed to stabilize the output voltage. The AC power feedback control device provided in the present invention has at least the following advantages: 
     (1) the feedback circuit is simple and easily achievable and has low cost; 
     (2) the easy setting procedures for the amplitude and phase of the waveform is suitable for panels with VFD; 
     (3) the control device can be applied in the situation of an unstable power supply, e.g vehicles; and 
     (4) the control device can correct the waveform if the load varies. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.