Patent Publication Number: US-8115420-B2

Title: Filament power supply circuit for vacuum fluorescent display

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a driving circuit for a vacuum fluorescent display and, more particularly, to a driving circuit for supplying power to the filament of a vacuum fluorescent display. 
     A vacuum fluorescent display is an electron tube which accommodates an anode and a cathode in an evacuated container (envelope) having at least one transparent side surface. The vacuum fluorescent display normally has a triode structure having, between the anode and the cathode, a grid to control movement of electrons emitted by the cathode. In this vacuum fluorescent display, the grid accelerates electrons emitted by the cathode to make them collide against phosphor applied onto the anode. Then, the phosphor emits light, and a desired pattern is displayed. 
     The cathode normally uses a filament with an electron emission material applied. Power is supplied to the filament to make it generate heat, thereby generating thermoelectrons. 
     To drive the vacuum fluorescent display, a driving circuit for supplying a filament voltage, a grid voltage, and an anode voltage is necessary. 
     The filament voltage needs to be a low AC voltage of, e.g., about 5 V. However, the grid voltage and the anode voltage need to be high DC voltages of about 50 V. Normally, the grid and the anode use the same voltage. The grid voltage and the anode voltage will collectively be referred to as a “display voltage” hereinafter. 
     Conventionally, when supplying the filament voltage and the display voltage to the vacuum fluorescent display, a voltage doubling circuit doubles and rectifies an AC filament voltage to generate a DC display voltage. This arrangement provides partial commonality of the filament voltage power supply and the display voltage power supply. 
     However, when an AC voltage is doubled and rectified, power loss is large. Additionally, since the voltage doubling circuit becomes hot, the reliability lowers. 
     A driving circuit which reduces loss by pulse-driving a voltage doubling circuit has been proposed (Japanese Patent Laid-Open Nos. 2003-29711 and 2005-181413). 
       FIG. 5  shows an example of the arrangement of a driving circuit which pulse-drives a voltage doubling circuit. Referring to  FIG. 5 , a driving circuit  200  includes a logic power supply  20 , a reference oscillator  21 , a ½-frequency dividing circuit  22 , a filament driver IC  23 , and a boost circuit  24 . 
     The logic power supply  20  generates a DC power supply voltage Vcc from an input voltage (DC voltage) Vi. 
     The reference oscillator  21  includes an inverting amplifier IC, diodes, resistors, and a capacitor, and generates a reference clock signal of about 100 to 200 kHz, as shown in  FIG. 6A . The reference clock signal is input to a terminal SEL of the filament driver IC  23 . The ½-frequency dividing circuit  22  includes a flip-flop and resistors, and generates an external clock signal by halving the frequency of the reference clock signal, as shown in  FIG. 6B . The external clock signal is input to an external clock input terminal EXTCK of the filament driver IC  23 . 
     The filament driver IC  23  switches the input voltage Vi and outputs complimentary differential pulse voltages P 1  and P 2  from output terminals OUT 1  and OUT 2  ( FIGS. 6C and 6D ). The differential pulse voltages P 1  and P 2  from the filament  6  are supplied to a filament  6  so that an AC filament voltage Ef is applied across the filament  6  (between terminals F 1  and F 2 ). When the terminal SEL is at “H” level, an internal clock operation based on an internal oscillator (not shown) of the filament driver IC  23  is performed. When the terminal SEL is at “L” level, an external clock operation based on the external clock signal is performed. 
     The boost circuit  24  is formed from a voltage doubling circuit including diodes and capacitors, and an emitter follower regulator including a transistor, Zener diodes, resistors, and capacitors. The boost circuit  24  boosts and rectifies the differential pulse voltages P 1  and P 2  output from the filament driver IC  23 , and outputs them as a DC voltage VDD 2  for the display voltage. 
     In the above-described conventional driving circuit, however, when the DC power supply Vi varies, the DC power supply voltage Vcc changes, and the effective voltage supplied to the filament also varies. This causes variations in the amount of electrons emitted by the filament, and degrades the display quality, resulting in, e.g., shorter life of the vacuum fluorescent display or flickering display. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to suppress degradation in the display quality of a vacuum fluorescent display when a DC power supply voltage Vcc varies. 
     In order to achieve the above object, according to the present invention, there is provided a filament power supply circuit of a vacuum fluorescent display, comprising an integration circuit connected to a signal input terminal which receives a pulse signal having a magnitude corresponding to a DC power supply voltage, a comparison circuit which is connected to the integration circuit, compares an output voltage from the integration circuit with a reference voltage, and outputs a result, a first filament cathode connection terminal which is connected to one terminal of a filament cathode of a vacuum fluorescent display and applies the DC power supply voltage to the one terminal of the filament cathode, the vacuum fluorescent display including the filament cathode, an anode spaced apart from the filament cathode and having a fluorescent material applied, and an evacuated container that accommodates the filament cathode and the anode, a second filament cathode connection terminal which is connected to the other terminal of the filament cathode to ground the other terminal of the filament cathode via a capacitive element, and a three-terminal element including a first terminal, a second terminal, and a third terminal, the first terminal being connected to the first filament cathode connection terminal, the second terminal being grounded, and the third terminal receiving an output from the comparison circuit so that a path between the first terminal and the second terminal is switched in accordance with the output from the comparison circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a circuit diagram showing the arrangement of a filament power supply circuit for a vacuum fluorescent display according to the embodiment of the present invention; 
         FIGS. 2A to 2E  are timing charts for explaining the operation of the filament power supply circuit shown in  FIG. 1 ; 
         FIGS. 3A to 3E  are timing charts for explaining the operation of the filament power supply circuit shown in  FIG. 1 ; 
         FIGS. 4A to 4C  are timing charts for explaining the relationship between the vacuum fluorescent display lighting timing and the filament driving voltage waveform; 
         FIG. 5  is a circuit diagram showing an example of the arrangement of a conventional driving circuit; and 
         FIGS. 6A to 6D  are timing charts for explaining the operation of the driving circuit shown in  FIG. 5 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The embodiment of the present invention will now be described with reference to the accompanying drawings. 
     Referring to  FIG. 1 , a VFD (Vacuum Fluorescent Display)  30  is formed by accommodating, in an evacuated container made of, e.g., glass, an anode (not shown) formed on a substrate and having a fluorescent material applied, a filament cathode  301  separately arranged above the anode, and a grid electrode (not shown) arranged between the anode and the filament cathode. 
     The VFD  30  includes filament cathode connection terminals F 1  and F 2  to which a filament voltage supplied from a filament power supply circuit (to be described later) is applied, power supply terminals to which a DC voltage VDD for a display voltage and a DC power supply voltage Vcc (about 5 V) are applied, and signal input thermals for receiving various kinds of signals CLK, BK, LAT, and SI supplied from an external device (CPU  10  for VFD driving) for driving and display of the VFD  30 . 
     Note that the VFD  30  of this embodiment is of a matrix type including a plurality of anodes arrayed in a matrix viewed from the upper side. However, in the present invention, the VFD may be of a so-called segment type including an anode with an arbitrary shape as far as it uses a filament cathode as an electron source. 
     The DC voltage VDD for a display voltage may be obtained from the DC power supply voltage Vcc using, e.g., a known voltage doubling circuit. However,  FIG. 1  does not illustrate the voltage doubling circuit or the like, and a detailed description thereof will be omitted. 
     The CPU  10  for VFD driving is a vacuum fluorescent display driving circuit which receives the DC power supply voltage Vcc and outputs the signals CLK, BK, LAT, and SI to drive the VFD  30 . The CPU  10  for VFD driving has a clock signal output terminal  101  to output a pulse-like clock signal which has a period corresponding to an integral submultiple of the period of the VFD driving signal and a peak value corresponding to the DC power supply voltage Vcc. The source oscillation of the clock signal output from the clock signal output terminal  101  of the CPU  10  for VFD driving is the same as that of the VFD driving signal (e.g., CLK) output from the CPU  10 . Hence, the period of the clock signal can accurately be set to an integral submultiple of the period of the VFD driving signal without synchronization with the VFD driving signal. 
     In the filament power supply circuit according to this embodiment, the clock signal output from the clock signal output terminal  101  of the CPU  10  for VFD driving is usable as the input signal, i.e., the clock signal for the filament power supply circuit, as will be described later. 
     [1. Arrangement of Filament Power Supply Circuit] 
     The filament power supply circuit according to this embodiment includes an RC circuit  40  which receives a pulse signal having a magnitude corresponding to the DC power supply voltage Vcc, a comparison circuit  20  which compares the output voltage of the RC circuit  40  with a reference voltage and outputs the result, and a switching transistor TR 1  serving as a three-terminal element which grounds, in accordance with the output from the comparison circuit  20 , the filament cathode connection terminal F 1  of the VFD  30  to which the DC power supply voltage Vcc is supplied. 
     More specifically, the RC circuit  40  includes a resistive element R 1  having one terminal connected to a signal input terminal  a  that receives the clock signal output from the CPU  10  for VFD driving, and a capacitive element C 1  having one terminal connected to the other terminal of the resistive element R 1 , and the other terminal grounded. The RC circuit  40  functions as an integration circuit. 
     The inverting input terminal of the comparison circuit  20  is connected to the node between the resistive element R 1  and the capacitive element C 1 . The voltage across the capacitive element C 1  is applied to the inverting input terminal. The noninverting input terminal of the comparison circuit  20  is connected to the output terminal of a reference voltage circuit  50 . A predetermined reference voltage is applied to the noninverting input terminal. 
     Note that the reference voltage circuit  50  outputs a predetermined reference voltage Vs (=Vref×R 3 /(R 2 +R 3 )) by dividing a predetermined voltage Vref by resistive elements R 2  and R 3 . 
     The first filament cathode connection terminal F 1  is connected to one terminal of the filament cathode  301  of the VFD  30  to apply the DC power supply voltage Vcc to the one terminal of the filament cathode  301  via an inductance L 1 . On the other hand, the second filament cathode connection terminal F 2  is connected to the other terminal of the filament cathode  301  to ground the other terminal of the filament cathode  301  via a capacitive element C 2 . Hence, when the first and second filament cathode connection terminals F 1  and F 2  are connected to the filament cathode  301 , an LC circuit including the inductance L 1  and the capacitive element C 2  is formed. 
     In the switching transistor TR 1 , the drain terminal serving as the first terminal is connected to the first filament cathode connection terminal F 1 . The source terminal serving as the second terminal is grounded. The output from the comparison circuit  20  is input to the gate terminal serving as the third terminal so that the path between the drain and source is switched in accordance with the output from the comparison circuit  20 . 
     [2. Operation of Filament Power Supply Circuit] 
     The operation of the filament power supply circuit according to this embodiment will be described next with reference to  FIGS. 2A to 2E  and  3 A to  3 E.  FIGS. 2A to 2E  and  3 A to  3 E show time-rate changes in voltages at the following points of the filament power supply circuit. 
       FIGS. 2A and 3A : the signal input terminal (point  a  in  FIG. 1 ) of the filament power supply circuit 
       FIGS. 2B and 3B : the inverting input terminal (point b) and the noninverting input terminal (point c) of the comparison circuit  20   
       FIGS. 2C and 3C : the output signal (point d) of the comparison circuit  20   
       FIGS. 2D and 3D : the first filament cathode connection terminal F 1  (point e) 
       FIGS. 2E and 3E  show time-rate changes in a voltage supplied to the filament cathode. 
     [2.1. Basic Operation of Filament Power Supply Circuit] 
     The basic operation of the filament power supply circuit according to this embodiment will be described first with reference to  FIGS. 2A to 2E . 
     As shown in  FIG. 2A , the clock signal output from the CPU  10  for VFD driving and input to the signal input terminal (point  a ) is a pulse-like signal having the peak value Vcc, a period T, and an ON time τ. 
     The clock signal is input to the RC circuit including the resistive element R 1  and the capacitive element C 1 . The voltage across the capacitive element C 1  exhibits a saw-tooth-shaped change, as shown in  FIG. 2B , in accordance with a time constant determined by the resistive element R 1  and the capacitive element C 1 . This voltage is input to the inverting input terminal (point b) of the comparison circuit  20 . 
     On the other hand, the predetermined voltage Vs is input to the noninverting input terminal (point c) of the comparison circuit  20 . As a result, a signal that is ON (ton=t 1 +t 2 ) while the noninverting input of the comparison circuit  20  is larger than the inverting input and OFF (toff=T−ton) while the noninverting input is smaller than the inverting input is output to the output terminal (point d) of the comparison circuit  20 , as shown in  FIG. 2C . 
     VHIGH, VLOW, t 1 , and t 2  of the voltage waveform of the inverting input terminal (point b) are given by
 
 V HIGH= Vcc×{ 1− e   −(τ/R1C1) }/{1− e   −(T/R1C1) }  (1)
 
 V LOW= V HIGH× e   −((T−τ)/R1C1)   (2)
 
 t 1=− R 1× C 1×ln( V LOW/ Vs )  (3)
 
 t 2=− R 1× C 1×ln {( Vcc−Vs )/( Vcc−V LOW)}  (4)
 
     When the signal shown in  FIG. 2C  is input to the gate terminal of the switching transistor TR 1  to turn on/off the path between the drain and source, the change in the potential of the first filament cathode connection terminal F 1  (point e) has a phase opposite to that of the output signal ( FIG. 2C ) of the comparison circuit  20 , as shown in  FIG. 2D . 
     At this time, a potential VDS of the first filament cathode connection terminal F 1  can be represented by a function of a duty D of the output from the comparison circuit  20 , and is given by 
                           V   ⁢           ⁢   D   ⁢           ⁢   S     =       ⁢     Vcc   ×     (     T   /   toff     )                   =       ⁢     Vcc   /     (     1   -   D     )                     (   5   )               
where D=ton/T
 
     Consequently, every time the path between the drain and source of the switching transistor TR 1  is turned on/off, the capacitive element C 2  is repeatedly charged and discharged, and a voltage (filament voltage) shown in  FIG. 2E  is applied to the filament cathode  301 . At this time, a forward voltage Vef 1  and a reverse voltage Vef 2  applied to the filament cathode  301  are given by 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           Vef 
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                           ⁢ 
                           1 
                         
                         = 
                           
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                           VDS 
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                             VF 
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                             ⁢ 
                             2 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
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                           D 
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                             Vcc 
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                               ( 
                               
                                 1 
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                               ) 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   6 
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                     Vef 
                     ⁢ 
                     
                         
                     
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                   = 
                   
                     
                       VF 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       2 
                     
                     = 
                     Vcc 
                   
                 
               
               
                 
                   ( 
                   7 
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     Since charges removed from the capacitive element C 2  via the filament  301  upon turning on the switching transistor TR 1  equal those stored in the capacitive element C 2  via the filament  301  upon turning off the switching transistor TR 1 , the potential of the second filament cathode connection terminal F 2  is given by VF 2 =Vcc. To satisfy VF 2 =Vcc described above, the second filament cathode connection terminal F 2  may directly be connected to the DC power supply voltage Vcc. 
     The effective values of the voltages applied to the filament cathode are given by 
                           ef   ⁢           ⁢   1     =       ⁢     Vef   ⁢           ⁢   1   ×       (     1   -   D     )       1   /   2                     =       ⁢     Vcc   ×   D   ×       (     1   -   D     )       1   /   2                       (   8   )                 ef   ⁢           ⁢   2     =     Vef   ⁢           ⁢   2   ×     D     1   /   2                 (   9   )               
The filament voltage is given by
 
 Ef =( ef 1 2   +ef 2 2 ) 1/2   (10)
 
     As is apparent from the above description, the filament voltage Ef is represented by a function of the clock signal period T, ON time τ, resistance R 1 , capacitance C 1 , predetermined reference voltage Vs, and DC power supply voltage Vcc. 
     The parameter values are set to minimize the variation in the filament voltage Ef caused by the variation in the DC power supply voltage Vcc. This makes it possible to stabilize the filament voltage Ef and suppress degradation in the display quality of the VFD when the DC power supply voltage Vcc varies. 
     [2.2. Variation in DC Power Supply Voltage Vcc and Operation of Filament Power Supply Circuit] 
     The operation of the filament power supply circuit when the DC power supply voltage Vcc varies will be described next with reference to  FIGS. 3A to 3E . 
     When the DC power supply voltage Vcc lowers, as indicated by the broken lines in  FIG. 3A , the voltage across the capacitive element C 1 , i.e., the input voltage signal of the inverting input terminal (point b) of the comparison circuit  20  also lowers, as indicated by the broken line in  FIG. 3B . To the contrary, the potential Vs of the noninverting input terminal (point c) of the comparison circuit  20  is always constant regardless of the value of the DC power supply voltage Vcc. 
     Hence, when the DC power supply voltage Vcc lowers, the time during which the inverting input of the comparison circuit  20  is smaller than the noninverting input becomes longer. Hence, as shown in  FIG. 3C , the duty of the output from the comparison circuit  20  changes. The ON time ton is longer, and the OFF time toff is shorter. 
     When the DC power supply voltage Vcc lowers, the ON time ton of the output of the comparison circuit  20  becomes longer, and the OFF time toff becomes shorter. That is, the duty D of the output of the comparison circuit  20  increases. 
     If the duty D increases, the time during which the path between the drain and source of the switching transistor TR 1  is turned off to apply the voltage Vef 1  to the first filament cathode connection terminal F 1  via the boost coil L 1  shortens, as indicated by the broken line in  FIG. 3E . At the same time, the value of the voltage Vef 1  increases, as is apparent from equation (6). 
     On the other hand, when the DC power supply voltage Vcc rises, the duty D of the output of the comparison circuit  20  decreases. For this reason, the time during which the path between the drain and source of the switching transistor TR 1  is turned off to apply the voltage Vef 1  to the first filament cathode connection terminal F 1  via the boost coil L 1  becomes long, and the value of the voltage Vef 1  decreases. 
     Even when the DC power supply voltage Vcc varies, the filament voltage applied to the filament cathode  301  and its application time vary to absorb the variation in the DC power supply voltage Vcc. It is therefore possible to stabilize the filament voltage even when the DC power supply voltage Vcc varies, and suppress degradation in the display quality of the VFD caused by the variation in the DC power supply voltage Vcc. 
     As described above, the clock signal for the filament power supply circuit, which is supplied from the clock signal output terminal  101  of the CPU  10  for VFD driving, is based on the same source oscillation as that of the various signals CLK, BK, LAT, and SI to drive the VFD  30 . Hence, the period of the clock signal can accurately be set to an integral submultiple of the period of the VFD driving signal. 
       FIGS. 4A to 4C  show the relationship between the VFD lighting timing and the filament driving voltage waveform. In this embodiment, the period T of the filament driving voltage is an integral submultiple of the period of the VFD driving signal. For this reason, a given lighting time Tn always includes an integral number of (m) periods T of the filament driving voltage (T=Tn/m), as shown in  FIG. 4B . Hence, the effective value of the filament voltage is also constant. To the contrary, if the lighting time Tn is not an integral multiple of the period T of the filament driving voltage (T≠Tn/m), as shown in  FIG. 4C , the effective value varies in every lighting time Tn. 
     As described above, the clock signal for the filament power supply circuit, which is output from the CPU  10  for VFD driving and has a period corresponding to an integral submultiple of the period of the driving signal, is used as the input signal. For this reason, the number of clocks of the filament power supply circuit during the lighting timing is always an integer, and a predetermined effective voltage is supplied to the filament during each lighting timing. This improves the display quality. 
     When the output from the CPU  10  for VFD driving is used, no separate oscillation circuit for the filament power supply circuit is necessary. 
     In this embodiment, the clock signal output from the CPU  10  for VFD driving is used as the input signal. In the present invention, however, it is not always necessary to input the clock signal supplied from the CPU  10  for VFD driving. Any other oscillation circuit may supply a clock signal if it can supply a clock signal having a stable frequency and duty. 
     In this embodiment, the RC circuit  40  is used as an integration circuit. However, an integration circuit having another arrangement may be used. 
     It is only necessary that the comparison circuit  20  is designed to output a signal representing the relationship in the magnitude between the reference voltage and the output voltage from the RC circuit  40 , and the switching transistor TR 1  is designed to turn on the path between the drain and source when the output voltage from the RC circuit  40  is lower than the reference voltage, and turn off the path between the drain and source when the output voltage from the RC circuit  40  is higher than the reference voltage. 
     As described above, according to this embodiment, it is therefore possible to stabilize the filament voltage even when the DC power supply voltage Vcc varies, and suppress degradation in the display quality of the VFD  30  caused by the variation in the DC power supply voltage Vcc. 
     The clock signal for the filament power supply circuit, which is output from the CPU  10  for vacuum fluorescent display driving and has a period corresponding to an integral submultiple of the period of the driving signal, is used as the input signal. This makes it possible to accurately set the period of the filament power supply circuit to an integral submultiple of the lighting timing of the vacuum fluorescent display  30  and improve the display quality.