Abstract:
A high-voltage power source apparatus with a simple construction which outputs a voltage by overlapping a DC voltage with an AC voltage. An alternating current (AC) voltage generator generates an AC voltage and outputs the AC voltage to a secondary coil of a transformer. The AC voltage and the DC voltage are developed and simultaneously overlapped from a common secondary coil of the transformer. Thus, separate AC and DC voltage generators are not necessary to produce the overlapped voltage, simplifying construction. The DC portion of the overlapped voltage may be regulated by feeding back a sample of the output voltage to a control circuit which compares the feedback with a reference and regulates the DC portion accordingly. A plurality of the overlapped voltage circuits may be operated in parallel from a single secondary coil of the transformer.

Description:
BACKGROUND OF THE INVENTION 
   This application claims the priority of Japanese Patent Application No. 2002-193574 filed Jul. 2, 2002, in the Japan Intellectual Property Office, the disclosure of which is incorporated herein by reference. 
   1. Field of the Invention 
   The present invention relates to a high-voltage power source apparatus that is installed in a developing unit of a printer using electrophotography in order to obtain an output voltage that is an overlap of a direct current (DC) voltage with an alternating current (AC) voltage. 
   2. Description of the Related Art 
   In general, a printer, which prints an image using electrophotography, illuminates a laser beam on a photosensitive drum to form an electrostatic latent image thereon, applies toner onto the electrostatic latent image to develop the image, and transfers the image coated with the toner onto transfer paper. A developing unit of such a printer includes a case in which toner is stored, and a developing roller in the case. To develop an electrostatic latent image, the developing unit makes the developing roller contact a photosensitive drum via a 0.2 mm aperture thereof, and then rotates the developing roller and the photosensitive drum in order to adhere the toner covering the developing roller to the electrostatic latent image on the photosensitive drum via the aperture. 
   For instance, a circumference of the photosensitive drum is charged with an electric potential of −50V and the other circumference of the photosensitive drum is charged with an electric potential of −700 V. Next, a voltage is generated by overlapping a DC voltage of −300 V with an AC voltage of 2000 Volts peak to peak (Vp-p) and the overlapped voltage is applied to the developing roller. As a result, the toner cleaves only to the surface of the photosensitive drum which is charged with the electric potential of −50 V. Accordingly, a general printer requires a high-voltage power source apparatus as shown in  FIG. 1  which makes a voltage by overlapping the DC voltage with the AC voltage and supplies the obtained voltage to the developing roller. 
     FIG.1  is a circuit diagram of a conventional high-voltage power source apparatus that includes an AC voltage generator  100  and a DC voltage generator  200 . 
   The AC voltage generator  100  comprises an operational amplifier OP 1 , a push pull output circuit having transistors Tr 1  and Tr 2  which are reciprocally connected to each other via bias resistors R 1  and R 2  and current limiting resistors R 3  and R 4 , resistors R 5  and R 6  which form a voltage divider to bias a negative input of the operational amplifier OP 1 , a feedback resistor R 7  connected between an output of the push pull output circuit and the negative input of the operational amplifier OP 1 , an input bias resistor R 8 , decoupling capacitors C 1  and C 2 , a DC blocking capacitor C 3  and a transformer T 1 . 
   The operational amplifier OP 1  compares a pulse signal ACPWM (or a sine wave voltage) input to an input terminal  1  with a voltage which is obtained by adding a feedback voltage output from the push pull output circuit to a voltage determined by the resistors R 5  and R 6 . Next, the operational amplifier OP 1  outputs the result of the comparison to the push pull output circuit. The output of the operational amplifier OP 1  is amplified by the push pull output circuit and output as an AC voltage to the DC blocking capacitor C 3 . The output AC voltage is stepped up by the transformer T 1  and output as an AC voltage of 2000 Vp-p, which is similar to an input waveform, at a secondary side of the transformer T 1 . The capacitors C 1  and C 2  remove noise from input power sources, indicated as +24V and +5V, respectively. 
   The DC voltage generator  200  comprises a DC-to-DC converter which includes a controller  201  and a blocking oscillator  202 . When a control signal CP is input to an input terminal  2  of the controller  201 , a transistor Tr 32  is switched on or off to cause the blocking oscillator  202  to oscillate or stop oscillating. An operational amplifier OP 2  compares a reference voltage DCVref input through an input terminal  3  with a feedback voltage DCVfb input through an input terminal  4 , and outputs the result of the comparison to a transistor Tr 34 . Then, the transistor Tr 34  is controlled based on the comparison result to cause the blocking oscillator  202  to oscillate a frequency having a circuit constant value. A resistor R 36  and a capacitor C 34  filter the reference voltage DCVref which is input to the negative input of the operational amplifier OP 2 . A capacitor C 35  decouples the positive input of the operational amplifier and diodes D 32  and D 33  limit the amplitude of the voltage DCVfb by clamping the voltage DCVfb to a power supply voltage +5V and to ground. A capacitor C 36  and a resistor R 37  provide feedback between an output and the input of the operational amplifier OP 2 . A resistor R 38  couples the output of the operational amplifier OP 2  to a base of the transistor Tr 34 . A collector of the transistor Tr 34  is interfaced with the blocking oscillator  202  via a resistor R 35  and a transistor Tr 33  so that a collector voltage of the transistor Tr 33  adjusts an internal reference voltage of the blocking oscillator  33  relative to a value established across a zener diode ZD 31 . The zener diode ZD 31  is biased by current flowing from a power supply +24V through a resistor R 31 . A transistor Tr 31  has a base connected to a common connection of the resistor R 31  and the zener diode ZD 31 . An emitter of the transistor Tr 31  is serially connected to ground via a resistor R 33 . A collector of the transistor Tr 31  is protected from extreme negative voltages by a diode D 31  which clamps to ground. A capacitor C 31  provides decoupling of the +24V power supply. The 24V power supply is connected to one end of a primary coil of a transformer T 1  and another end of the primary coil of the transformer T 2  is connected to the collector of the transistor Tr 31 . A capacitor C 32  is connected in parallel with the primary coil of the transformer T 2 . An auxiliary coil of the transformer T 2  has one end connected to ground and another end which feeds back an induced current to the base of the transistor Tr 31  through a capacitor C 33  and a resistor R 32 . Respective polarities of the primary coil and the auxiliary coil are arranged so that the feedback through the transistor Tr 31  results in an oscillatory voltage at the primary coil of the transformer T 2 , which couples an oscillatory output voltage to a coil on a secondary side of the transformer T 2 . 
   The oscillatory output voltage is extracted from the secondary side of the transformer T 2 , rectified and smoothed by a diode D 34  and a capacitor C 40  to provide a DC voltage. The DC voltage is is applied to a capacitor C 5  connected in parallel with the secondary side of the transformer T 1  through a resistor R 43 . The voltage applied to the capacitor C 5  becomes a DC voltage of −300V, is overlapped with an AC voltage output from the transformer T 2 , flows through a protective resistor R 10 , and is output as an output voltage Dev through an output terminal  5 . The output voltage Dev is applied to the developing roller. 
   When an output of the high-voltage power source apparatus of  FIG. 1  is blocked, the resistor R 11  discharges an electric current from the capacitor C 5 . Also, the DC voltage across the capacitor C 40  is sampled by a voltage divider formed of resistors R 41  and R 42  to provide the voltage DCVfb, which is rectified and smoothed, is fed back to the input terminal  4  of the controller  201  as mentioned above. In addition, the pulse signal ACPWM, the control signal CP, and the reference voltage DCVref are respectively input to the input terminals  1 ,  2 , and  3  at predetermined intervals, using a controller (not shown) included in a printer body. 
   As described above, a conventional high-voltage power source apparatus adopted by a developing unit of a general printer requires two high-voltage power source circuits, i.e., the AC voltage generator  100  and the DC voltage generator  200 , thereby complicating the structure of the apparatus. For instance, the DC voltage generator  200  is a DC-to-DC converter including the controller  201  and the blocking oscillator  202  and therefore requires a large number of circuit elements as shown in FIG.  1 . In particular, a color printer needs four DC voltage generators  200  for toners of four colors, i.e., yellow Y, magenta M, cyan C, and black B, thereby complicating the construction thereof and increasing the size and manufacturing costs. 
   SUMMARY OF THE INVENTION 
   The present invention provides a high-voltage power source apparatus with a simple construction, which obtains an output voltage that is an overlap of a direct current (DC) voltage with an alternating current (AC) voltage. That is, a central point of the AC voltage is offset from a reference, such as for example, circuit ground by a value of the DC voltage. 
   According to an aspect of the present invention, a high-voltage power source apparatus comprises an AC voltage generator that generates an AC voltage and outputs the AC voltage to a secondary side of a transformer; a capacitor connected to a secondary coil of the transformer in series; resistors connected to the capacitors in parallel; and a current direction limiting unit that is connected to the secondary coil of the transformer and charges the capacitors with an electric current, which is generated in a particular direction using the AC voltage. The apparatus overlaps the DC voltage charged in the capacitor with the AC voltage in order to obtain an output voltage. 
   More specifically, the AC voltage generator generates an AC voltage and outputs the AC voltage to the secondary side of the transformer, and the current direction limiting unit charges the capacitor with an electric current using the AC voltage. As a result, the AC voltage overlaps a DC voltage charged in the capacitor to generate an output voltage. The magnitude of the DC voltage charged in the capacitor depends on a resistor, which is connected to the capacitor in parallel, and a resistor connected to the current direction limiting unit in series. Thus, the high-voltage power source apparatus according to the present invention does not require a DC voltage generator with a complicated structure, which is adopted by the related art, thereby simplifying the structure thereof. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and/or other aspects and advantages of the present invention will become more apparent by describing in detail embodiments thereof with reference to the attached drawings in which: 
       FIG. 1  is a circuit diagram of a conventional high-voltage power source apparatus; 
       FIG. 2  is a circuit diagram of a high-voltage power source apparatus according to a first embodiment of the present invention; 
       FIG. 3  is a circuit diagram of a high-voltage power source apparatus according to a second embodiment of the present invention; 
       FIG. 4  is a graph illustrating a relationship between an output voltage and a DC component signal which is obtained by dividing and integrating the output voltage; 
       FIG. 5  is a circuit diagram of major parts of a high-voltage power source apparatus according to a third embodiment of the present invention; 
       FIG. 6  is a circuit diagram of major parts of a high-voltage power source apparatus according to a fourth embodiment of the present invention; 
       FIG. 7  is a circuit diagram of major parts of a high-voltage power source apparatus according to a fifth embodiment of the present invention; 
       FIG. 8  is a circuit diagram of major parts of a high-voltage power source apparatus according to a sixth embodiment of the present invention; 
       FIG. 9  is a circuit diagram of major parts of a high-voltage power source apparatus according to a seventh embodiment of the present invention; and 
       FIG. 10  is a circuit diagram of major parts of a high-voltage power source apparatus according to an eighth embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to like elements throughout. [Hereinafter, embodiments of a high-voltage power source apparatus according to the present invention will be described with reference to the accompanying drawings.] 
     FIG. 2  is a circuit diagram of a high-voltage power source apparatus according to a first embodiment of the present invention. Here, elements that are the same as elements shown in  FIG. 1  are indicated with the same reference numerals as shown in FIG.  1  and descriptions of such elements will not be repeated. 
   Referring to  FIG. 2 , the first embodiment comprises an AC voltage generator  100 , which may have a same construction as the conventional AC voltage generator  100  shown in  FIG. 1 , and an AC/DC overlapped voltage generator  300 - 1 . As mentioned above, an AC voltage is output to a secondary side of a transformer T 1 . The first embodiment does not require the controller  201 , the blocking oscillator  202 , the diode D 34 , the capacitor C 40 , and the resistors R 41 , R 42 , and R 43  as shown in FIG.  1 . 
   Further, in the apparatus according to the first embodiment, a charging capacitor C 5  and a discharging resistor R 9  are connected to each other in parallel between one end of a secondary coil of the transformer T 1  and ground. Also, a diode D 9  and a current limiting resistor R 11  are connected with each other in series between another end of the secondary coil of the transformer T 1  and ground. 
   In the AC/DC overlapped voltage generator  300 - 1  shown in  FIG. 2 , a portion of an AC voltage, for example, 2000 Vp-p, which is output from the secondary side of the transformer T 1 , is rectified by the diode D 9  and the capacitor C 5  is charged via the resistor R 11 . When a DC voltage is output at a connection point between the capacitor C 5  and the secondary coil of the transformer T 1 , the DC voltage is overlapped with the AC voltage and the overlapping result is output as an output voltage Dev to an output terminal  5  via the resistor R 10 . The output voltage Dev is supplied to a developing roller of a printer. 
   The magnitude of an electric current charged in the capacitor C 5  depends on a ratio of the resistor R 9  to the resistor R 11 . When the capacitor C 5  is charged with a particular magnitude of an electric current, a particular magnitude, e.g., −300V, of DC voltage is generated. The resistor R 9  immediately discharges the capacitor C 5  when an output of the AC voltage generator  100  of  FIG. 2  is blocked. 
   Accordingly, unlike the conventional high-voltage power source apparatus of  FIG. 1 , the apparatus according to the first embodiment does not include a separate DC voltage generator with a complicated construction, thereby simplifying a construction of an overlapped voltage generator compared with the generator shown in FIG.  1 . 
     FIG. 3  is a circuit diagram of a high-voltage power source apparatus according to a second embodiment of the present invention. Here, elements that are the same as those in  FIG. 1  are indicated with the same reference numerals as shown in FIG.  1  and descriptions of such elements will not be repeated. 
   The second embodiment comprises an AC voltage generator  100  and an AC/DC overlapped voltage generator  400 - 1 . As compared to the high-voltage power source apparatus of  FIG. 1 , the second embodiment uses a transistor Tr 3  as a current limiter, instead of the resistor R 11  of FIG.  2 . Also, an output voltage VDC is divided by resistors R 12  and R 13  which are voltage dividing resistors. The divided voltage is integrated by an integration capacitor C 7  so as to extract a DC component signal VC 7 . The DC component signal VC 7  is used to observe the output voltage VDC. The DC component signal VC 7  is fed back to the transistor Tr 3  so as to control the transistor Tr 3 . 
   When a reference voltage DCVref is input to an input terminal  6  using a controller (not shown), an operational amplifier OP 3  compares the reference voltage DCVref with the DC component signal VC 7  and outputs the result of the comparison. In response to the result of comparison, the transistor Tr 3  varies a resistance between a collector and an emitter of the transistor Tr 3 . 
   A capacitor C 5  and a resistor R 9  are installed at one end of a secondary coil of a transformer T 1 . The operations of the capacitor C 5  and the resistor R 9  of  FIG. 3  are substantially the same as those of the capacitor C 5  and the resistor R 9  of FIG.  2 . The apparatus of  FIG. 3  further comprises a capacitor C 6  for the removal of noise, and resistors R 14  and R 15  which are biasing resistors for the transistor Tr 3 . A varistor ZD 1  conducts current to protect the transistor Tr 3  if an overvoltage is applied across the transistor Tr 3 . 
   As an example of the operation of the apparatus according to the second embodiment, when the output voltage VDC of −300V is divided by the resistors R 12  and R 13  to become a voltage of −3V, a voltage of +5V is applied to the resistor R 12  so as to obtain a DC component signal VC 7  of +2V. Also, as shown in  FIG. 4 , if the output voltage VDC changes between 0V and −500V, the voltage for the component signal VC 7  changes between 0 and +5V. 
   The operational amplifier OP 3  compares the feedback DC component signal VC 7  and the reference voltage DCVref of +2V and controls the collector-emitter resistance of the transistor Tr 3  based on a result of the comparison. For instance, if the output voltage VDC is lower than −300V, the operational amplifier OP 3  increases the resistance between the collector and the emitter of the transistor Tr 3  to lower the level of a voltage charged in the capacitor C 7 . Conversely, if the output voltage VDC is higher than −300V, the operational amplifier OP 2  reduces the resistance between the collector and the emitter of the transistor Tr 3  to raise the level of the voltage charged in the capacitor C 7 . In this way, the DC component signal VC 7  is controlled to be the same as the reference voltage DCVref. 
   Therefore, the apparatus according to the second embodiment stably controls an electric current charged in the capacitor C 5  so as to obtain a stable output voltage VDC. Also, an intensity of a DC voltage, which is applied to a developing roller, may be changed to a desired level. 
   Here, the transistor Tr 3  is an NPN type transistor such as for example, a transistor which is used in a circuit called a dynamic focus in a television receiver. That is, an inexpensive general high voltage transistor may be used as the transistor Tr 3 , thereby reducing costs of manufacturing the high-voltage power source apparatus according to the second embodiment. 
     FIG. 5  is a circuit diagram of major parts of a high-voltage power source apparatus according to a third embodiment of the present invention. The third embodiment comprises an AC voltage generator  10  and an AC/DC overlapped voltage generator  400 - 2 . Elements that are the same as those in  FIGS. 1 and 3  are indicated with the same reference numerals and descriptions of such elements will not be repeated. The AC voltage generator  100  of  FIG. 5  may be the same as the AC voltage generator  100  of  FIGS. 1 ,  2  and  3 . 
   In the case of the conventional high-voltage power source apparatus shown in  FIG. 1 , the AC voltage generator  100  and the DC voltage generator  200  are independently constructed, and thus, the DC voltage is controllable using the controller  201  of the DC voltage generator  200 . In contrast, a high-voltage power source apparatus as shown in  FIGS. 3 and 5 , in which an AC voltage generator and a DC voltage generator are combined, requires an integration circuit, such as for example, the capacitor C 7 , to obtain the feedback DC component signal VC 7  for the control of the transistor Tr 3 . For this reason, when the pulse signal ACPWM is input to the input terminal  1  of the AC voltage generator  100  or the reference voltage DCVref is input to the input terminal  6 , the output of the output voltage Dev is delayed in the integration circuit and thus overshoot occurs in the output voltage VDC. 
   To solve this problem, the apparatus according to the third embodiment further comprises a transistor Tr 4 , which prevents the occurrence of overshoot of an output voltage, in a circuit that feeds back a DC component signal VC 7  generated by a capacitor C 7 . If a control voltage DCOUT is applied to an input terminal  7 , the transistor Tr 4  is switched on or off to suppress the occurrence of overshoot in the output voltage. In other words, the transistor Tr 4  is a device that controls the DC component signal VC 7  by preventing the output voltage from rising to a higher level. When a voltage of +5V is applied to a base of the transistor Tr 4  via a resistor R 17 , the transistor Tr 4  is switched on. 
   In order to obtain an output voltage Dev, the control voltage DCOUT is applied to the transistor Tr 4  to switch off the transistor Tr 4 , and then, the DC component signal VC 7  is controlled to make the output voltage VDC reach 0V, thereby suppressing the occurrence of overshoot. That is, when the output voltage VDC is 0V, the transistor Tr 4  is switched on and the DC component signal VC 7  has a potential of 0V. Under such a condition, when the pulse signal ACPWM is input to the input terminal  1  of the AC voltage generator  100  and the reference voltage DCVref is applied to the input terminal  6 , the control voltage DCOUT is applied to the input terminal  7  to switch off the transistor Tr 4 . In this way, the DC component signal VC 7  gradually rises to a certain level from the potential of 0V, and thus, overshoot is minimized in the output voltage Dev. 
     FIG. 6  is a circuit diagram of the major parts of a high-voltage power source apparatus according to a fourth embodiment of the present invention. The fourth embodiment comprises an AC voltage generator  100  and an AC/DC overlapped voltage generator  400 - 3 . Elements shown in  FIG. 6  that are the same as those in  FIGS. 3 and 5  are indicated with the same reference numerals and descriptions of such elements will not be repeated. 
   The AC/DC overlapped voltage generator  400 - 3  shown in  FIG. 6  comprises a time-constant circuit that uses time-constant resistors R 18  and R 19 , a time-constant capacitor C 8 , and a diode D 2 , instead of the transistor Tr 4  of  FIG. 5 , to suppress the occurrence of overshoot. Referring to  FIG. 6 , a reference voltage DCVref, which is applied to an input terminal  6 , is delayed by a predetermined time constant and then applied to an operational amplifier OP 3 . As a result, the rising of an output voltage VDC to a certain level is deferred to suppress the occurrence of overshoot in an output voltage VDC. 
     FIG. 7  is a circuit diagram of the major parts of a high-voltage power source apparatus (hereinafter, the “apparatus”) according to a fifth embodiment of the present invention. The fifth embodiment comprises an AC voltage generator  100  and an AC/DC overlapped voltage generator  300 - 2 . Elements corresponding to those of  FIG. 2  are indicated with the same reference numerals and descriptions of such elements will not be repeated. 
   Relative to the apparatus shown in  FIG. 2 , the apparatus of  FIG. 7  further comprises a varistor ZD 2  connected in parallel with the capacitor C 5  and which clamps a voltage across the capacitor C 5  at a predetermined value. 
   Referring to  FIG. 7 , when a voltage charged in the capacitor C 5  exceeds a predetermined varistor voltage, the varistor ZD 2  conducts and a current charged in the capacitor C 5  is limited by the varistor ZD 2 . Thus, a voltage across the capacitor C 5  increases until the capacitor voltage reaches the the varistor voltage and then the capacitor voltage stabilizes. With the use of the varistor ZD 2 , the capacitor C 5  stably outputs an output voltage Dev. 
     FIG. 8  is a circuit diagram of the major parts of a high-voltage power source apparatus according to a sixth embodiment of the present invention. The sixth embodiment comprises an AC voltage generator  100  and an AC/DC overlapped voltage generator  300 - 3 . Elements corresponding to those of  FIG. 2  are described with the same reference numerals and descriptions of such elements will not be repeated. 
   Referring to  FIG. 8 , an active clamp circuit replaces the resistor R 9  of FIG.  2 . The active clap circuit comprises a first circuit, in which voltage dividing resistors R 20 , VR 1  and R 21  are connected with one another in series, which is installed in parallel with the capacitor C 5 . A second circuit, in which a zener diode ZD 32  and a clamping transistor Tr 5  are connected in series, is installed in parallel with the first circuit. A collector of the transistor Tr 5  is grounded. A voltage which is divided by the dividing resistors R 20 , VR 1  and R 21  is applied to a base of the transistor Tr 5 . 
   More specifically, a voltage charged in the capacitor C 5  is divided by the resistors R 20 , VR 1  and R 21  and the divided voltage is applied to the base of the transistor Tr 5 . A voltage division ratio δ may be expressed as (R 20 +αVR 1 )/(R 20 +VR 1 +R 21 ), where α has a value of 0 to 1. If the divided voltage is larger than a sum of a base emitter voltage of the transistor Tr 5  and a zener breakdown voltage of the zener diode ZD 32 , the transistor Tr 5  and the zener diode ZD 32  conduct and a current charged in the capacitor C 5  is limited. As a result, a value of the voltage output from the capacitor C 5 , i.e., an output voltage Dev, is almost the same as a value calculated by (the base emitter voltage of Tr 5 +the zener voltage of ZD 32 )×(1/δ). Thus, the capacitor C 5  stably outputs the output voltage Dev. Also, the output voltage Dev is adjustable by controlling the variable resistor VR 1 . 
     FIG. 9  is a circuit diagram of the major parts of a high-voltage power source apparatus according to a seventh embodiment of the present invention. The seventh embodiment comprises an AC voltage generator  100  and an AC/DC overlapped voltage generator  300 - 4 . Elements corresponding to elements shown in  FIGS. 2 and 8  are indicated with the same reference numerals and descriptions of such elements are not be repeated. 
   Relative to the apparatus shown in  FIG. 2 , in the apparatus according to the seventh embodiment, the resistor R 9  is removed and resistors R 20  and R 21  are connected with each other in series between one end of the capacitor C 5  and a power source voltage +VCC. A clamping transistor Tr 6  is connected to the resistors R 20  and R 21  and the capacitor C 5 . A control voltage Ref is input to an input terminal  8  and then applied to an emitter of the transistor Tr 6 . The clamping transistor Tr 6  and the resistors R 20  and R 21  form a voltage clamping unit. 
   Referring to  FIG. 9 , a voltage charged in the capacitor C 5  is divided by the resistors R 20  and R 21  and applied to a base of the transistor Tr 6 . If the voltage is greater than a sum of a base-emitter voltage of the transistor Tr 6  and the control voltage Ref, the transistor Tr 6  conducts and a current charged in the capacitor C 5  is limited. Thus, an output voltage Dev output from the condenser C 5  is almost the same as a value calculated by (the base emitter voltage of Tr 6 +the reference voltage Ref)×(R 20 +R 21 )/R 21 ]. Thus, the voltage Dev is stably output. 
   In the seventh embodiment, the transistor Tr 6  is a PNP type transistor. When the collector of the transistor Tr 6  is supplied with a negative voltage, the power source voltage +VCC is connected to the resistor R 21  so as to apply the control voltage Ref of a positive value to the transistor Tr 6 . 
   Alternatively, the circuit of  FIG. 9  may be constructed such that a signal similar to the signal VC 7  as illustrated in  FIG. 5  may be fed back from the base of the transistor TR 6 . 
     FIG. 10  is a circuit diagram of the major parts of a high-voltage power source apparatus according to an eighth embodiment of the present invention. Here, elements that are the same as in  FIGS. 3 ,  5  and  6  are described with the same reference numerals and descriptions of such elements will not be repeated. 
   The apparatus according to the eighth embodiment is adapted for use in a color printer. In detail, the apparatus includes four high-voltage power source circuits  10 Y,  10 M,  10 C, and  10 Bk, as shown in  FIG. 10 , which correspond to the colors yellow Y, magenta M, cyan C, and black Bk, respectively. Each of the circuits  10 Y,  10 M,  10 C, and  10 Bk may have a same construction as AC/DC overlapped voltage generators  400 - 1 ,  400 - 2  or  400 - 3  shown in  FIGS. 3 ,  5  and  6 , respectively, in which an AC voltage generator is combined with a DC voltage generator. Referring to  FIG. 10 , four capacitors C 5  and four resistors R 9  are connected with a second side of a transformer T 1 . Each capacitor C 5  is connected in parallel with a respective resistor R 9  and each parallel combination is connected in series with a respective resistor R 10 . A diode D 1  and a resistor R 12  in each circuit  10 Y,  10 M,  10 C and  10 Bk interface with a remaining part of a respective one of the AC/DC overlapped voltage generators  400 - 1 ,  400 - 2 , or  400 - 3  as shown in  FIGS. 3 ,  5 , and  6 , respectively. Thus, DC voltages are overlapped with AC voltages to become output voltages YDev, MDev, CDev, and BkDev and these output voltages are output to output terminals  5  of the circuits  10 Y,  10 M,  10 C, and  10 Bk, respectively. 
   As described above, a high-voltage power source apparatus according to the present invention generates a high-voltage output by overlapping a DC voltage with a high-voltage, without a DC voltage generator with a complicated construction. Therefore, the construction of the high-voltage power source apparatus according to the present invention is simplified, thereby reducing the manufacturing costs therefor. 
   While this invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.