Abstract:
According to an embodiments, a power supply device includes: a first voltage generator that operates to output a first voltage; a second voltage generator that operates to output a second voltage higher than the first voltage; a third voltage generator that, from the second voltage generated by the second voltage generator, operates to generate a third voltage equal to the first voltage and to output the third voltage therefrom; and a selector that operates to select as an output whichever has a higher voltage from the output of the first generator and the output of the second generator, wherein each of the output of the second voltage generator and the output selected by the selector are supplied to a load.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2011-050685 filed in Japan on Mar. 8, 2011. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a power supply device that outputs multiple power supplies with different voltages, a power supply device control method, and an image forming apparatus. 
     2. Description of the Related Art 
     There are many conventional electronic devices that need multiple power supplies of different voltages. For example, an inkjet printer that ejects ink from a printer head and performs printing requires multiple power supplies of different voltages including a power supply with a first voltage to drive operational units including a display device, such as a liquid crystal display, and a motor; a power supply with a second voltage to drive the printer head; and a power supply with a third voltage to drive a logical circuit. 
     With such multiple power supplies at different voltages, there may be a limitation on the order in which the power is supplied. In this example, a case will be considered where a power supply device supplies a head driver integrated circuit (IC) to drive the printer head with a first power supply for the head driver IC and a second power supply, to drive the printer head, with a voltage much higher than that of the first power supply. In this case, if the supply of the second power supply to the head driver IC is continued even after the output of the first power supply to the head driver IC is stopped, the through current due to the second power supply flows through the head driver IC via the parasitic diode in the head driver IC, which may damage the head driver IC. 
     For this reason, conventional power supply devices are known that have a function of controlling when multiple power output circuits of different output voltages start or stop their outputs. For example, Japanese Patent No. 4279235 discloses a configuration that monitors the power supplied from an external power supply device by using a photocoupler and that controls when the power output circuit is stopped by using transistor switching and by using the length of time for which the capacitor discharges. The configuration according to Japanese Patent No. 4279235 can control, even if the supply of power from the external power supply device is cut out, when the multiple power output circuits stop their outputs such that they are stopped in a predetermined order. 
     Conventional power supply devices control when the output stops by using the time period for which the capacitor discharges in the power supply device and thus the power output of the power supply device ensures when the output stops. The capacitor connected to the circuit of a device supplied with power, such as a printer head driver circuit, is not taken into consideration. Thus, for example, when a capacitor with a large capacitance is connected to the printer head driver circuit, to which a power supply is supplied, to rectify the waveform, a problem occurs in that when the power supply device stops the output and when the output of the voltage of the device supplied with power is stopped might be different. 
     As an example, a case will be considered where a large-capacitance capacitor is connected to the above-described second power supply in the power-supplied device. In this case, even if the power supply device stops the first and second power outputs simultaneously, the supply of the second power supply is continued also after the supply of the first power supply is stopped due to the operation of the capacitor. Accordingly, there is a risk that the head driver IC might be damaged. 
     The above-described configuration of Japanese Patent No. 4279235 cannot solve the problem that, when a large-capacitance capacitor is connected to the circuit of the power-supplied device, the time point when the output of the power supply device stops and the time point when the power supply to the power-supplied device stops may be different from each other. 
     In order to avoid such a problem, a capacitor for the power supply device may be set in consideration of the capacitance of the capacitor connected to the printer head drive circuit. However, this case has a problem in that design freedom is narrowed. 
     The method of controlling when the output of the power supply is stopped by using the charging or discharging of the capacitor has a problem in that it is difficult, when an irregular operation is performed, e.g., when the power supply is turned off during charging, to cause a given time difference between the time points when the output of power supplies at different voltages stops. 
     As described above, conventional power supply devices have a problem that, when a power supply device is stopped, a power-supplied device cannot follow a power supply sequence specific to the device supplied with power, which might cause instability in the device operations or damage the device. 
     There is a need, when multiple power supplies of different voltages are supplied, to prevent unnecessary high-voltage supplies to a device being supplied with power when the power supply is stopped. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an embodiments, a power supply device includes: a first voltage generator that operates to output a first voltage; a second voltage generator that operates to output a second voltage higher than the first voltage; a third voltage generator that, from the second voltage generated by the second voltage generator, operates to generate a third voltage equal to the first voltage and to output the third voltage therefrom; and a selector that operates to select as an output whichever has a higher voltage from the output of the first generator and the output of the second generator, wherein each of the output of the second voltage generator and the output selected by the selector are supplied to a load. 
     According to another embodiment, a power supply device control method includes: first generating that includes operating to output a first voltage by a first voltage generator; second generating that includes operating to output a second voltage, by a second voltage generator, which is higher than the first voltage; third generating that includes operating to generate, by a third voltage generator, a third voltage equal to the first voltage from the second voltage generated at the second generating, and to output the third voltage; and selecting, by a selector, as an output whichever has a higher voltage out of the output at the first generating and the output at the second generating, wherein each of the output at the second generating and the output selected at the selecting are supplied to a load. 
     According to still another embodiment, an image forming apparatus includes: the power supply device mentioned above; a head to which the second voltage generated by the second voltage generator and the output selected by the selector are supplied as a power supply; and an image forming unit that forms an image using the head. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a front schematic line diagram of an exemplary configuration of an inkjet printer applicable to an embodiment; 
         FIG. 2  is a schematic line diagram of an oblique projection of part of the inkjet printer applicable to the embodiment; 
         FIG. 3  is a block diagram schematically depicting an exemplary configuration of the inkjet printer applicable to the embodiment; 
         FIG. 4  is a block diagram of an exemplary configuration of an inkjet head; 
         FIG. 5  is a schematic line diagram of an exemplary sequence in which power is supplied to the inkjet head; 
         FIG. 6  is a circuit diagram depicting in detail an exemplary configuration of a PSU, an MCU, and a head controller according to the embodiment; 
         FIG. 7  is a schematic line diagram depicting an example of a power supply sequence during a normal operation according to the embodiment; 
         FIG. 8  is a schematic line diagram depicting an example of a power supply sequence during a normal operation according to the embodiment; and 
         FIG. 9  is a schematic line diagram depicting an example of a power supply sequence during a normal operation according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of a power supply device, a power supply device control method, and an image forming apparatus using the power supply device will be described in detail below with reference to the accompanying drawings.  FIG. 1  depicts a front view of an exemplary configuration of an inkjet printer  1  serving as an image forming apparatus applicable to the embodiment.  FIG. 2  depicts a view in oblique projection of part of the inkjet printer  1 . 
     As depicted in  FIG. 1 , a guide shaft  13  and a guide plate  14  are provided in parallel to bridge light and left side plates  11  and  12  of a casing  10  of the inkjet printer  1 . The guide shaft  13  and the guide plate  14  penetrate through a carriage  15  so as to be slidable. An endless belt (not shown) is attached to the carriage  15 . The endless belt is looped over a driven pulley and an undriven pulley (not shown) provided on the left and right in the casing  10 . The undriven pulley is rotated in accordance with rotation of the driven pulley and thus the endless belt runs. Accordingly, the carriage  15  moves left and light as indicated by the arrow in  FIG. 1 . 
     Four color inkjet heads  16   y ,  16   c ,  16   m , and  16   b  (if it is not required to specify which inkjet head, the four inkjet heads  16   y ,  16   c ,  16   m , and  16   b  are represented by inkjet head  16 ) for the four colors yellow, cyan, magenta, and black are arranged in parallel in the carriage  15  along the direction in which the carriage  15  moves. Each inkjet head  16  includes a nozzle row in which multiple nozzles are linearly arranged on the downward-facing nozzle surface. Although not illustrated, the linear nozzle row is provided along the direction orthogonal to the direction in which the carriage  15  moves. 
     When the carriage  15  is in the home position on the right end as depicted in  FIG. 1 , each inkjet head  16  faces a single recovery device  18  provided on a bottom plate  17  in the casing  10 . The single recovery device  18  recovers ink from a nozzle in which an ink ejection error is detected to allow the inkjet printer  1  to independently recover from the liquid ejection error. 
     A sheet feeding table  24  to feed a sheet  23 , which is a recording medium, on a plate-like platen  22  is obliquely provided on the back of the platen  22 . Although not illustrated, a sheet feeding roller to feed the sheet  23  on the sheet feeding table  24  onto the platen  22  is provided. Furthermore, a conveying roller  25  is provided that conveys the sheet  23  on the platen  22  in the direction represented by the arrow and ejects the sheet  23  to the front side. 
     A drive device  26  is further set on the left end on the bottom plate  17  in the casing  10 . The drive device  26  drives the sheet feeding roller (not shown) and the conveying roller  25  and drives the driven pulley to run the endless belt so that the carriage  15  is moved. 
     When recording is performed, the sheet  23  is moved onto the platen  22  according to the rotation by the drive device  26  and is positioned to be at a predetermined position. The carriage  15  is then moved and scans the sheet  23  and, while moving left, ejects ink drops from the nozzles by using the four color inkjet heads  16   y ,  16   c ,  16   m , and  16   b , so that an image is recorded on the sheet  23 . After the image is recorded, the carriage  15  is moved back rightward and the sheet  23  is conveyed in a predetermined distance in the direction represented by the arrow in  FIG. 2 . 
     While being moved left again, the carriage  15  then ejects ink drops from the nozzles sequentially by using the four color inkjet heads  16   y ,  16   c ,  16   m , and  16   b  in the outward trip, so that an image is recorded on the sheet  23 . After the image is recorded, the carriage  15  is moved back rightward and the sheet  23  is conveyed for a predetermined distance in the direction represented by the arrow in  FIG. 2 . The same operation is repeated to record images on the single sheet  23 . 
       FIG. 3  schematically depicts an exemplary configuration of the inkjet printer  1  applicable to the embodiment.  FIG. 3  does not illustrate parts not directly relating to the embodiment, such as the motor drive system. The inkjet printer  1  includes a power supply unit (PSU)  101 , a main control unit (MCU)  102 , a head controller  103 , an inkjet head  104 , an operation unit  105 , a controller (CTL)  106 , and an image processing unit (IPU)  107 . 
     The PSU  101  outputs multiple power supplies with different voltages to each unit of the inkjet printer  1  from a commercial power supply  100 . The MCU  102  includes a CPU  110  and a power supply controller  111 . The CPU (central processing unit)  110  generates various control signals and controls the power supply controller  111  according to a program that is pre-stored in a ROM (not shown). The power supply controller  111  is configured from, for example, an ASIC (application specific integrated circuit). The power supply controller  111  generates some of the supplies of power supplied to the inkjet head  104  and controls the power output from the PSU  101 . 
     The CTL  106  includes a CPU  120 . The CTL  106  controls the power output from the PSU  101  according to the program pre-stored in the ROM (not shown) and controls entire operations of the inkjet printer  1  according to control signals from the operation unit  105 . The operation unit  105  includes a display unit used as a display device, such as an LCD (liquid crystal display), and an operation unit for user operations. The operation unit  105  makes displays in accordance with display control signals generated by the CPU  120  and outputs control signals according to user operations and supplies the CPU  120  with the control signals. 
     The IPU  107  includes an image processor  131  and an IO_ASIC  130 . The IPU  107  runs at a power supply with a voltage of 5 V, which is supplied from the MCU  102 . The image processor  131  and the IO_ASIC  130  are communicably connected to the CPU  110  and the power supply controller  111  in the MCU  102  via a bus  112 . The IO_ASIC  130  is configured from, for example, and ASIC. The IO_ASIC  130  is connected to the CPU  120  via a bus, such as a PCI (peripheral component interconnect) bus, and has a function of adjusting communications with the CPU  120  and the CPU  110  and communications of the image processor  131  via the bus  112 . This allows the CPU  120  and the CPU  110  to communicate with each other. 
     The image processor  131  performs given image processing on image data supplied from an image data input unit (not shown). 
     The head controller  103  generates a power supply suitable for the inkjet head  104  from the power supply from the MCU  102  and supplies the inkjet head  104  with the generated power supply. The head controller  103  controls ink ejection performed by the inkjet head  104  in accordance with a head drive signal VCOM supplied from the power supply controller  111  and a control signal based on the image data processed by the image processor  131 . The inkjet head  104  corresponds to the inkjet head  16  depicted in  FIG. 1  and  FIG. 2 . 
       FIG. 4  depicts an exemplary configuration of the inkjet head  104 . In the example of  FIG. 4 , the inkjet head  104  includes a head driver IC (integrated circuit)  230  and includes a piezoelectric device  231 , an ink tank  232  that stores ink, and a nozzle  233  to eject the ink. The piezoelectric device  231 , the ink tank  232 , and the nozzle  233  constitute the head body. In the head body, a high-frequency voltage is applied to the piezoelectric device  231  and the ink in the ink tank  232  is ejected from the nozzle  233  by using the fluctuation of the piezoelectric device  231  caused by the high-frequency voltage. 
     The head driver IC  230  is supplied with, for example, a power supply 3.3VH with a voltage of 3.3 V to drive the head driver IC  230  and a power supply 37VH with a voltage of 37 V to drive the piezoelectric device  231 . The power supply voltage to drive the head driver IC  230  and the power supply voltage to drive the piezoelectric device  231  are merely examples and power supply voltages are not limited to them. 
     The head driver IC  230  is further supplied with a drive signal VCOM from which the drive waveform of the piezoelectric device  231  originates and with a control signal to control the head driver IC  230 . 
     In general, the power supply voltage to drive the piezoelectric device  231  is a high-level voltage compared to the power supply voltage to drive the head driver IC  230 . In contrast, if the head driver IC  230  is supplied with the power supply 37VH to drive the piezoelectric device  231  before being supplied with the power supply 3.3VH to drive the head driver IC  230 , a through current flows due to the power supply 37VH via the parasitic diode in the head driver IC  230 , which may damage the head driver IC  230 . For this reason, it is necessary to supply the head driver IC  230  with the power supply 3.3VH and the power supply 37VH in accordance with a given sequence. 
       FIG. 5  depicts an exemplary sequence of the power supply to be supplied to the inkjet head  104 . Here, when the head driver IC  230  is not yet supplied with the power supply 3.3VH to drive the head driver IC  230  but is supplied with a power supply 37VH to drive the piezoelectric device  231 , the voltage of the power supply 37VH with which the head driver IC  230  will not be damaged (safe voltage) is 10 V. 
     First, when the power of the inkjet printer  1  is turned on, e.g., when supplying the inkjet head  104  with the power supply 3.3VH and the power supply 37VH is started, it is required that the power supply 3.3VH rises to a specified voltage (3.3 V in this example) before the voltage of the power supply 37VH reaches the safe voltage. Furthermore, when the power of the inkjet printer  1  is turned off, e.g., supplying the inkjet head  104  with the power supply 3.3VH and the power supply 37VH is stopped, it is required that the power supply 3.3VH maintains the specified voltage until the voltage of the power supply 37VH decreases to the safe voltage. 
     Operations of the embodiment will be described here in detail below using  FIGS. 6 to 9 .  FIG. 6  depicts in detail an exemplary configuration of the PSU  101 , the MCU  102 , and the head controller  103  according to the embodiment. In  FIG. 6 , the components in common with those in  FIG. 3  are denoted by the same reference numerals as those of  FIG. 3  and detailed descriptions thereof will be omitted.  FIG. 6  depicts routes and the configuration of the power supplies and power supply control closely related to the embodiment and thus omits other routes and configurations unless otherwise noted. 
     The configuration of the PSU  101  will be described first. The PSU  101  includes A/D conversion circuits  200   a  and  200   b  and switch devices F 1  and F 2 . Each of the switch devices F 1  and F 2  is configured from a p-channel MOSFET (metal-oxide semiconductor field-effect transistor). When a low signal is input to the gate, the on state is kept between the source and drain. 
     Each of the A/D conversion circuits  200   a  and  200   b  includes a transformer, a rectifier and a stabilizer circuit. Each of the A/D conversion circuits  200   a  and  200   b  transforms, rectifies and stabilizes the commercial power supply externally supplied as an AC voltage of 100 V and then outputs the power. The A/D conversion circuit  200   a  generates a DC power supply with a voltage of 5 V and a DC power supply with a voltage of 37 V. The CPU  120  in the CTL  106  controls the on/off operations of the A/D conversion circuit  200   a . The A/D conversion circuit  200   b  generates a DC power supply 5VE with a voltage 5 V to cause the CPU  120  in the CTL  106  to run. 
     The power supply with a voltage of 5 V generated by the A/D conversion circuit  200   a  is input to the drain of the switch device F 1 . The source output of the switch device F 1  is supplied as a power supply 5VG to the MCU  102 . A signal PONENG_N output from the CPU  120  in the CTL  106  is input to the gate of the switch device F 1 . The signal PONENG_N in a low state causes an on state between the source and drain of the switch device F 1 . The signal PONENG_N in a high state causes an off state between the source and drain of the switch device F 1 . In other words, when the signal PONENG_N is in a low state, the power supply 5VG with a voltage of 5 V is output from the switch device F 1  and, when the signal PONENG_N is in a high state, the output of the power supply is stopped. The signal PONENG_N with a voltage of 0 V when in the low state. 
     The power supply with a voltage of 37 V, which is generated by the A/D conversion circuit  200   a , is input to the drain of the switch device F 2 . The source output of the switch device F 2  is supplied as the power supply 37VH to the MCU  102 . A signal HVCONT_N output from the power supply controller  111  in the MCU  102  is input to the gate of the switch device F 2 . The signal HVCONT_N in a low state causes an on state between the source and drain of the switch device F 2 . The signal HVCONT_N in a high state causes an off state between the source and drain of the switch device F 2 . In other words, when the signal HVCONT_N is in a low state, the power supply 37VH with a voltage of 37 V is output from the switch device F 2  and, when the signal HVCONT_N is in a high state, the output of the power supply is stopped. The signal HVCONT_N when in a low state has a voltage of 0 V. 
     The configuration of the MCU  102  will be described here. The MCU  102  includes the power supply controller  111  and the CPU  110  and includes DC/DC converters  210  and  212 , a detecting circuit  213 , a discharge circuit  214 , and a head driver  215 . The power supply 5VG supplied from the PSU  101  to the MCU  102  is output from the MCU  102 , supplied to the head controller  103 , and supplied to other circuits, such as the IPU  107  (see  FIG. 3 ). In other words, the power supply 5VG is a power supply that is commonly used as the power supply with a voltage of 5 V in the inkjet printer  1 . 
     The power supply 5VG is further input to the DC/DC converter  210 . The DC/DC converter  210  converts the voltage of the supplied power supply 5VG to, for example, 3.3 V and then outputs the power supply. The output of the DC/DC converter  210  is supplied as a power supply to the power supply controller  111  and supplied to other ICs (3.3V-IC)  211  that require a power supply with a voltage of 3.3 V. 
     The conversion and output by the DC/DC converter  210  is not limited to the power supply of 3.3 V. It can be a value corresponding to a power supply voltage required by the device to which the power supply is supplied. In this case, if the power supply voltage required by the device to which the power supply from the DC/DC converter  210  is supplied is equal to the voltage of the input of the DC/DC converter  210 , the DC/DC converter  210  can be eliminated. 
     The power supply 37VH supplied from the PSU  101  to the MCU  102  is supplied to the DC/DC converter  212 , the detecting circuit  213 , the discharge circuit  214 , and the head driver  215  in the MCU  102 . The power supply 37VH is output from the MCU  102  and supplied as the power supply 37VH to each inkjet head  104  via the head controller  103 . 
     The head driver  215  generates a drive waveform for the head from the supplied power supply 37VH and outputs the drive waveform as the head drive signal VCOM from the MCU  102 . The capacitors C 1  to C 4  connected to the power supply 37VH are large-capacitance capacitors used in order to stably output the head drive signal VCOM from the head driver  215 . 
     The DC/DC converter  212  converts the voltage of the power supply 37VH to a power supply 5VH of a voltage (5V in this case) equal to that of the power supply 5VG and outputs the power supply 5VH. The power supply 5VH is supplied to the head controller  103 . For the DC/DC converter  212 , the minimum value of an input voltage range in which a specified output voltage (5V) can be output (hereinafter, “input-voltage-range minimum value”) is equal to or less than the safe voltage (10 V in this case) of the head driver IC  230  in the inkjet head  104 . 
     The detecting circuit  213  monitors the voltage of the supplied power supply 37VH and detects whether the voltage of the power supply 37VH is less than a predetermined voltage. In the example in  FIG. 6 , the power supply 37VH is supplied to the cathode of a Zener diode ZD 1  via a resistor device R 1  in the detecting circuit  213 . The voltage extracted from the middle point between the resistor device R 1  and the cathode of the Zener diode ZD 1  is divided into half the voltage by resistor devices R 2  and R 3  and the divided voltage is then input to the analog port of the power supply controller  111 . The power supply controller  111  can detect the voltage input to the analog port. 
     Regarding the Zener diode ZD 1 , a voltage approximately equal to the safe voltage of the head driver IC  230  is selected for a Zener voltage. When the voltage of the power supply 37VH is equal to or more than the safe voltage, the voltage of the cathode of the Zener diode ZD 1  is equal to the Zener voltage, i.e., safe voltage. When the voltage of the power supply 37VH is less than the safe voltage, the voltage of the cathode of the Zener diode ZD 1  is less than the Zener voltage. The power supply controller  111  monitors the voltage of the cathode of the Zener diode ZD 1  and detects whether the power supply 37VH is less than the safe voltage. In practice, the power supply controller  111  monitors the voltage obtained by dividing the voltage of the cathodes by the resistor devices R 2  and R 3 . 
     When the switching device F 2  stops the output of the power supply 37VH, the discharge circuit  214  discharges the supplied power supply 37VH. A signal HVCONT_N output from the power supply controller  111  is inverted by the inverter and input to the gate of the switch device F 3 , which is configured from an n-channel MOSFET, via a resistor device R 5 . The power supply 37VH is input to the drain of the switch device F 3  via a resistor device R 4 . The source of the switch device F 3  is grounded. The middle point between the resistor device R 5  and the gate of the switch device F 3  is grounded via the resistor device R 6 . When the signal HVCONT_N enters a high state and the output of the power supply 37VH from the PSU  101  is stopped, the gate of the switch device F 3  enters a low state and thus the switch device F 3  enters an on state and the power supply 37VH is discharged via the drain-source between the resistor device R 4  and the switch device F 3 . 
     The discharge circuit  214  forces, when the output of the power supply 37VH is stopped, discharge of electrons stored in the capacitors C 1  to C 4  connected to the route of the power supply 37VH, which accelerates the decrease of the voltage of the power supply 37VH. Even outside the MCU  102 , even if the large-capacitance capacitor is connected to the route of the power supply 37VH, the voltage reduction is similarly accelerated. 
     The configuration of the head controller  103  will be described here. The head controller  103  includes a power supply selector  220  and a regulator (REG)  221 . The power supply selector  220  includes a first selection input end and a second selection input end. The power supply selector  220  selects as a power supply whichever has a higher voltage from the power supplies input to the first and second selection input ends and outputs the selected power supply. In the example in  FIG. 6 , the power supply selector  220  includes two diodes D 1  and D 2 . The anode of the diode D 1  is the first selection input end and the anode of the diode D 2  is the second selection input end. The cathodes of the diodes D 1  and D 2  are connected and the output of the power supply selector  220  is extracted from the connecting point. 
     In the power supply selector  220 , the power supply 5VG is input to the first selection input end and the power supply 5VH is input to the second selection input end. The power supply selector  220  selectively outputs as a power supply whichever has a higher voltage from the power supply 5VG and the power supply 5VH. 
     The output of the power supply selector  220  is supplied to the regulator  221 . The regulator  221  stabilizes the supplied power supply at a power supply of a given voltage (for example, 3.3 V) and then outputs the stabilized power supply. The output of the regulator  221  is input to the inkjet head  104  and supplied as a power supply 3.3VH to the head driver IC  230 . 
     The power supply with a voltage of 5 V to be input to the first selection input end of the power supply selector  220  is also supplied to each 5V-IC  222  that requires a voltage of 5 V as a power supply in the head controller  103 . Similarly, the output of the power supply selector  220  is supplied to each 3.3V-IC  223  that requires a voltage of 3.3 V as a power supply. Each 5V-IC  222  and each 3.3V-IC  223  include a controller that controls, for example, a sensor that performs detection regarding head operations. 
     The power supply 37VH and the power supply 3.3VH are supplied to an inkjet heads  104   a ,  104   b ,  104   c  and  104   d  of the respective colors. Similarly, the drive signal VCOM output from the head driver  215  in the MCU  102  is supplied to each of the inkjet heads  104   a  to  104   d  of the respective color. Although not illustrated, control signals based on the image data output from the image processor  131  are supplied to the inkjet heads  104   a  to  104   d  of the respective color. In accordance with each power supply, driver signal VCOM and control signal, each of the inkjet heads  104   a  to  104   d  ejects ink. 
     The power supply sequence according to the embodiment will be described using  FIG. 6  and  FIGS. 7 to 9 . First, the power supply sequence during the normal operation will be described using  FIG. 7 . In  FIG. 7 , ( a ) depicts the power supply 5VE; ( b ) depicts the signal PONENG_N; ( c ) depicts the power supply 5VG (5 V); ( d ) depicts the output (3.3 V) of the DC/DC converter  210 ; and ( e ) depicts the power supply 3.3VH. In  FIG. 7 , ( f ) depicts the signal HVCONT_N; ( g ) depicts the power supply 37VH (37 V); ( h ) depicts the power supply 5VH; and ( i ) depicts the input voltage (DET 37V) to the analog port of the power supply controller  111 . 
     An example of operations when the power is turned on will be described here. For example, if the main switch (not shown) of the inkjet printer  1  is turned on, the A/D conversion circuit  200   b  in the PSU  101  generates the power supply 5VE from the commercial power supply  100  in order to start the CPU  120  in the CTL  106 . 
     In the PSU  101 , along with the generation of the power supply 5VE, the A/D conversion circuit  200   a  generates the power supplies of voltages 37 V and 35 V from the commercial power supply  100  and the power supplies are supplied to the switch devices F 1  and F 2 , respectively. Because the signal PONNENG_N is in a low state when the power is on, the power supply 5VG rises at a time t 1  and accordingly the voltage 3.3 V of the DC/DC converter  210  rises. The rise of the power supply 5VG causes the power supply 3.3VH of the output of the power supply selector  220  to rise. 
     When the voltage 3.3 V of the output of the DC/DC converter  210  rises, the power supply controller  111  in the MCU  102  is started. When the power supply controller  111  is started, it shifts the signal HVCONT_N from the low state to the high state. This turns the switching device F 2  to an off state and thus the output of the power supply 37VH is stopped. For example, at a time point t 2  after a predetermined time from the start, the power supply controller  111  shifts the signal HVCONT_N from a high state to a low state. Accordingly, the switching device F 2  enters an on state and thus the power supply 37VH rises. In accordance with the rise of the power supply 37VH, the power supply 5VH of the output of the DC/DC converter  212  rises. 
     In accordance with the rise of the power supply 37VH, a voltage DET — 37V input to the analog port of the power supply controller  111  increases. When the voltage DET — 37V reaches the Zener voltage (for example, 10 V) of the Zener diode ZD 1  (time point t 3 ), it remains at the Zener voltage. Accordingly, the power supply controller  111  can detect that the power supply 37VH is equal to or greater than the safe voltage of the head driver IC  230  in the inkjet head  104 . 
     During the initial operation and a normal operation (during print operation) after the power is turned on, each power supply and signal (signal PONENG_N, signal HVCONT_N and voltage DET — 37V) does not change. 
     The operations during the standby state will be described here. The inkjet printer  1  shifts to the standby state when, for example, a given time has passed after a user operation on the operation unit  105  or a print operation ends. In the standby state, each inkjet head  104  is capped to prevent it from drying out and thus not able to eject ink. In contrast, in the standby state, the head driver IC  230  and the IPU  107  are in the operating state. 
     When each inkjet head  104  is capped, each inkjet head  104  cannot be driven, i.e., cannot eject ink. Thus, for example, at the time point t 1 , the power supply controller  111  shifts the signal HVCONT_N from the low state to the high state and turns the switch device F 2  to the off state to stop the output of the power supply 37VH (time point t 1 ). Because the electric charges stored in the capacitors C 1  to C 4  are discharged, the output voltage of the switch device F 2  gradually reduces. The operation of the discharge circuit  214  allows discharge of the power supply 37VH in a relatively short time. 
     After the output of the power supply 37VH is stopped, the DC/DC converter  212  maintains the output of the specified voltage 5V until the input voltage becomes lower than the minimum value in the input voltage range of the DC/DC converter  212  (safe voltage of the head driver IC  230 ). At the time point t 12 , when the input voltage becomes lower than the minimum value in the input voltage range, the voltage of the power supply 5VH output from the DC/DC converter  212  drops from the specified voltage of 5 V. Here, because the first selection input end of the power supply selector  220  in the head controller  103  is supplied with the power supply 5VG with a voltage of 5 V, the power supply 5VG is selectively output from the power supply selector  220  after the voltage drop of the power supply 5VH, so that the output voltage of 5 V is maintained. 
     In the detecting circuit  213 , when the power supply 37VH becomes lower than the Zener voltage of the Zener diode ZD 1  (the safe voltage of the head driver IC  230 ), the signal DET — 37V is shifted from a high state to a low state. 
     Operations in the power saving mode will be described here. For example, when a predetermined time has passed after a user operation of the operation unit  105  or since entering the standby state, the inkjet printer  1  shifts to the power saving mode. In the power saving mode, only the power supply 5VE to the CPU  120  in the CTL  106  rises in the inkjet printer  1 . 
     For example, in accordance with an operation of the operation unit  105  instructing a shift to the power saving mode to take place, the CPU  120  shifts the A/D conversion circuit  200   a  in the PSU  101  to an off state to stop generation of voltages of 37 V and 5 V and shifts the signal PONENG_N from a low state to a high state (time t 15 ). Accordingly, the output of the power supply 5VG is stopped, the power supply of the voltage 3.3 V to be supplied to the power supply controller  111  is stopped, and the signal HVCONT_N output from the power supply controller  111  enters a low state. Because both the outputs of the power supply 5VG and the power supply 5VH are stopped, the output of the power supply 3.3VH is also stopped. 
     The above-described operations ensure that, when the power is on, the inkjet head  104  is supplied with the power supply 3.3VH before or simultaneously supplied with the power supply 37VH. Because, when the power is turned off, the output of the power supply 37VH is stopped when the standby state takes place, it is ensured that the output of the power supply 3.3VH is stopped after the output of the power supply 37VH is stopped. Accordingly, the sequence of power supply to the inkjet head is correctly followed, which prevents the head driver IC  230  from being damaged due to a high voltage. In addition, because the output of the power supply 37VH is stopped in the standby state, power saving effects can be obtained. 
     The power supply sequence during an abnormal operation will be described here.  FIG. 8  depicts an example of a power supply sequence in which, during an initial operation or an operation, the main switch enters an off state or the plug of the inkjet printer  1  is pulled out and thus the power is turned off. In  FIG. 8 , ( a ) depicts the power supply 5VE; ( b ) depicts the signal PONENG_N; ( c ) depicts the power supply 5VG (5V); ( d ) depicts the output (3.3 V) of the DC/DC converter  210 ; ( e ) depicts the signal HVCONT_N; ( f ) depicts the power supply 37VH (37V); ( g ) depicts the power supply 5VH; and ( h ) depicts the input voltage (DET — 37V) to the analog port of the power supply controller  111 . 
     When the main switch enters the off state during an initial operation or a normal operation or when the plug to supply the commercial power supply  100  happens to become disconnected, the supply of the commercial power supply  100  is terminated and accordingly all the power supply voltages in the inkjet printer  1  start dropping. In the example of  FIG. 8 , when the main switch enters an off state at the time point t 30 , the voltages of the power supply 5VE, the power supply 5VG and the power supply 37VH output from the PSU  101  starts dropping as depicted in (a), (c) and (f), respectively. Thus, the CPU  120  and the power supply controller  111  running with the power supply 5VE and the power supply 5VG stop their operations and each of the signal PONENG_N and the signal HVCONT_N enters a low state. 
     Due to the effects of discharge from the large-capacitance capacitors C 1  to C 4 , the time required to completely stop the output of the power supply 37VH is longer than the time to stop the output of the power supply 5VH. The voltage of the output of the power supply 37VH due to the discharge is converted to 5 V by the DC/DC converter  212  and then is output as the power supply 5VH. As described above, the minimum value in the input voltage range for the DC/DC converter  212  is equal to or less than the safe voltage of the head driver IC  230 . Because the power supply selector  220  selects as a power supply whichever has a higher voltage from the power supplies input to the first and second selection input ends and outputs the selected power supply, the power supply 5VH output from the DC/DC converter  212  is selected and output when the supply of the commercial power supply  100  is being terminated. For this reason, after the power supply 37VH decreases to a safe voltage, the output of the power supply 3.3VH from the regulator  221  is stopped, which prevents the head driver IC  230  from being damaged. 
     Also in this case, because the discharge circuit  214  shorten the discharge time of the capacitors C 1  to C 4 , all goes well even if the time to switch the main switch from an off state to an on state is short. 
     Next, a power supply sequence will be described in which the operation mode is forced to switch to the power saving mode in an initial operation or a normal operation. For example, the operation mode of the inkjet printer  1  can be shifted to the power saving mode by a user operation of the operation unit  105 , performed during, for example, an initial operation or a normal operation in order for an instruction to be given for a shift to the power saving mode. 
       FIG. 9  depicts an example of a power supply sequence in which the operation mode is forcibly shifted to the power saving mode during an initial operation or a normal operation. Regarding  FIG. 9 , like  FIG. 8 , in  FIG. 9 , ( a ) to ( h ) depict the power supply 5VE, the signal PONENG_N, the power supply 5VG (5V), the output (3.3 V) of the DC/DC converter  210 , the signal HVCONT_N, the power supply 37VH (37V), the power supply 5VH, and the input voltage (DET — 37V) to the analog port of the power supply controller  111 , respectively. 
     The instruction for a shift to the power saving mode made by a user operation of the operation unit  105  is transmitted from the CPU  120  to the CPU  110  in the MCU  102  via the IO_ASIC  130  and the bus  112 . In accordance with the instruction, the CPU  110  instructs the power supply controller  111  to shift the signal HVCONT_N from a low state to a high state. As described using  FIG. 7 , the signal HVCONT_N and the signal PONENG_N are in a low state during the initial operation and the normal operation. 
     In accordance with the instruction from the CPU  110 , the power supply controller  111  shifts the signal HVCONT_N to a high state (time point t 40 ). When the signal HVCONT_N enters a high state, the switch device F 2  enters an off state, the power supply 37VH, which is the output of the switch device F 2 , is stopped (time point t 41 ), and the voltage of the power supply 37VH gradually decreases from 37 V. 
     When the voltage of the power supply 37VH becomes lower than a safe voltage (10 V in this case) of the head driver IC  230  (time point t 42 ), the voltage of the power supply 5VH, which is the output of the DC/DC converter  212 , starts decreasing from the specified 5 V. At the time point t 42 , the switch device F 1  is in an on state and the voltage of the power supply 5VG is 5 V. Thus, the power supply selector  220  selects the output of the power supply 5VG. The output voltage of the power supply selector  220  is converted to a voltage of 3.3 V by the regulator  221  and is then supplied as the power supply 3.3VH to the inkjet head  104 . 
     In contrast, when the voltage of the power supply 5VH starts decreasing from the specified 5V, the voltage of the signal DET — 37V output from the detecting circuit  213  starts decreasing from the voltage based on the Zener voltage of the Zener diode ZD 1  and the power supply controller  111  detects that the voltage of the power supply 37VH becomes equal to or less than the safe voltage (time point t 43 ). The power supply controller  111  transmits the detection result representing that the voltage of the power supply 37VH becomes equal to or less than the safe voltage to the CPU  120  of the CTL  106 . 
     The CPU  120  receives the detection result transmitted from the power supply controller  111  and the CPU  120  controls the A/D conversion circuit  200   a  in the PSU  101  such that it enters an off state and shifts the signal PONENG_N from a low state to a high state (time point t 44 ). Accordingly, the output of the switch device F 1  is stopped and the voltage of the power supply 5VG decreases (time point t 45 ) and accordingly the signal HVCONT_N output from the power supply controller  111  enters a low state and the output of the power supply 3.3VH from the regulator  221  is stopped. 
     The above-described operations cause only the power supply 5VE to rise. As described above, because the power supply 3.3VH is stopped after the power supply 37VH decreases to the safe voltage even during an abnormal operation of a forcible shift to the power saving mode in accordance with a user operation of the operation unit  105  during an initial operation or an operation, the head driver IC  230  can be prevented from being damaged. 
     As described above, in the embodiment, the power supply 3.3VH to drive the head driver IC  230  is generated from the power supply 37VH. Thus, after the power supply 37VH decreases to the safe voltage, the output of the power supply 3.3VH from the regulator  221  is stopped, which prevents the head driver IC  230  in the inkjet head  104  from being damaged. 
     Furthermore, because the power supply 3.3VH is generated from the power supply 37VH, the power supply sequence in the head driver IC  230  when the power supply stops can be satisfied irrespective of the capacitance of the capacitor in the PSU  101  and the capacitance of the capacitors C 1  to C 4  in the MCU  102 . 
     In the embodiment, when multiple power supplies of different voltages are supplied, unnecessary high-voltage supplies to a device supplied with power are prevented when the power supply is stopped. 
     Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.