Abstract:
A power supply apparatus includes a first constant-power power supply that switches and supplies powers of j types (where j is a natural number of two or more), a second constant-power power supply that switches and supplies powers of k types (where k is a natural number of two or more), and a switching controller that selects and switches on the first and second variable-power power supplies, excluding power transition periods thereof, to supply a load with a constant power.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2012-161801, filed Jul. 20, 2012, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a power supply apparatus, an electronic apparatus, and a power-supply control method, which are particularly suitable for an electronic apparatus, such as a digital light processing (DLP [registered trademark]) projector. 
     2. Description of the Related Art 
     Since a large number of electronic apparatuses including various computers require a plurality of stable direct-current voltages from a power supply, DC/DC converters need to be comprised in correspondence with the number of required direct-current voltages. 
     On the other side, a technology for reducing the number of DC/DC converters is considered (for example, see Jpn. Pat. Appln. KOKAI Publication No. 2008-234842). 
     Jpn. Pat. Appln. KOKAI Publication No. 2008-234842 discloses a technology configured to provide a light control unit including: 
     a light source comprising a light emitting part which emits red light, a light emitting part which emits green light, and a light emitting part which emits blue light; 
     a switching unit connected in series with at least one of solid-state light sources of the light emitting parts; 
     a variable constant-voltage power supply which is connected in common to the solid-state light sources and drives the light source; and 
     a switching control unit which switches on/off the switching unit. 
     When the technology described in the Jpn. Pat. Appln. KOKAI Publication No. 2008-234842 is applied to an apparatus which requires a power supply whose voltage sequentially varies in time series, a voltage value is instable during a period in which the voltage value shifts at the time of switching voltages. 
     Therefore, in some cases, a load circuit in a following stage cannot be made to perform a desired operation and, for example, control of temporarily stopping the load circuit is required. 
     Under the circumstances, it is desired to provide a power supply, an electronic apparatus, and a power-supply control method, which are capable of supplying a plurality of rated powers (constant powers) switched at a high speed. 
     BRIEF SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a power supply apparatus comprising: a first constant-power power supply that switches and supplies powers of j types (where j is a natural number of two or more); a second constant-power power supply that switches and supplies powers of k types (where k is a natural number of two or more); and a switching controller that selects and switches on the first and second variable-power power supplies, excluding power transition periods thereof, to supply a load with a constant power. 
     According to another aspect of the present invention, there is provided a power supply apparatus that supplies powers to first through third loads having respectively different driving powers, comprising: a first variable constant-power power supply that cyclically switches and outputs a first constant power for the first load, a second constant power for the second load, and a third constant power for the third load through transition periods among the constant powers; a second variable constant-power power supply that outputs the third constant power, including the transition period from the first constant power to the second constant power of the first variable constant-power power supply, the first constant power, including the transition period from the second constant power to the third constant power of the first variable constant-power power supply, and the second constant power, including the transition period from the third constant power to the first constant power of the first variable constant-power power supply, with the constant powers cyclically switched through the transition periods among the constant powers; and a switching controller that switches outputs of the first and second variable constant-power power supplies such that the first load is supplied with the first constant powers which do not include the transition periods, alternately from the first and second variable constant-power power supplies, respectively, the second load is supplied with the second constant powers which do not include the transition periods, alternately from the first and second variable constant-power power supplies, respectively, and the third load is supplied with the third constant powers which do not include the transition periods, alternately from the first and second variable constant-power power supplies, respectively. 
     According to still another aspect of the present invention, there is provided an electronic apparatus comprising: a power supply unit including: a first constant-power power supply that switches and supplies powers of j types (where j is a natural number of two or more), a second variable constant-power power supply that switches and supplies powers of k types (where k is a natural number of two or more), and a switching controller that selects and switches the first and second variable constant-power power supplies, respectively excluding power transition periods thereof, to supply a constant power; and a load that operates by powers supplied from the power supply unit. 
     According to still another aspect of the present invention, there is provided an electronic apparatus with a power supply unit that supplies powers to a first through third loads having respectively different driving powers, the power supply unit comprising: a first variable constant-power power supply that cyclically switches and outputs a first constant power for the first load, a second constant power for the second load, and a third constant power for the third load through transition periods among the constant powers; a second variable constant-power power supply that outputs the third constant power, including the transition period from the first constant power to the second constant power of the first variable constant-power power supply, the first constant power, including the transition period from the second constant power to the third constant power of the first variable constant-power power supply, and the second constant power, including the transition period from the third constant power to the first constant power of the first variable constant-power power supply, with the constant powers cyclically switched through the transition periods among the constant powers; and a switching controller that switches outputs of the first and second variable constant-power power supplies such that the first load is supplied with the first constant powers which do not include the transition periods, alternately from the first and second variable constant-power power supplies, respectively, the second load is supplied with the second constant powers which do not include the transition periods, alternately from the first and second variable constant-power power supplies, respectively, and the third load is supplied with the third constant powers which do not include the transition periods, alternately from the first and second variable constant-power power supplies, respectively. 
     According to still another aspect of the present invention, there is provided a power supply method comprising allowing a first constant-power power supply to switch and supply powers of j types (where j is a natural number of two or more); allowing a second constant-power power supply to switch and supply powers of k types (where k is a natural number of two or more); and selecting and switching on the first and second variable-power power supplies, excluding power transition periods thereof, to supply a load with a constant power. 
     According to still another aspect of the present invention, there is provided a power supply control method for an apparatus with a power supply unit that supplies powers to first through third loads having respectively different driving powers, the method comprising: allowing a first variable constant-power power supply to cyclically switch and output a first constant power for the first load, a second constant power for the second load, and a third constant power for the third load through transition periods among the constant powers; allowing a second variable constant-power power supply to output the third constant power, including the transition period from the first constant power to the second constant power of the first variable constant-power power supply, the first constant power, including the transition period from the second constant power to the third constant power of the first variable constant-power power supply, and the second constant power, including the transition period from the third constant power to the first constant power of the first variable constant-power power supply, with the constant powers cyclically switched through the transition periods among the constant powers; and switching outputs of the first and second variable constant-power power supplies such that the first load is supplied with the first constant powers which do not include the transition periods, alternately from the first and second variable constant-power power supplies, respectively, the second load is supplied with the second constant powers which do not include the transition periods, alternately from the first and second variable constant-power power supplies, respectively, and the third load is supplied with the third constant powers which do not include the transition periods, alternately from the first and second variable constant-power power supplies, respectively. 
     Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention. 
         FIG. 1  shows a configuration of an electronic circuit and an optical system in a projector apparatus according to a first embodiment of the invention; 
         FIG. 2  shows a specific example of a configuration of the optical system according to the first embodiment; 
         FIG. 3  shows a configuration of connections of a semiconductor light-emitting device and a power supply system according to the first embodiment; 
         FIG. 4  is a timing chart showing states of shifting of electric powers supplied to the semiconductor light-emitting device according to the first embodiment; 
         FIG. 5  shows a configuration of connections of loads and a power supply system according to a second embodiment of the invention; and 
         FIG. 6  is a timing chart showing states of shifting of electric powers supplied to the loads according to the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     (First Embodiment) 
     A first embodiment will now be described with reference to the drawings where the invention is applied to a DLP (registered trademark) projector apparatus. 
       FIG. 1  shows a schematic functional configuration of a projector apparatus  10  according to the first embodiment. 
     An input unit  11  is configured by, for example, a video input terminal of pin jack (RCA) type, an RGB input terminal of D-sub15 type, and a High-definition Multimedia Interface (HDMI) terminal. Analog or digital image signals input to the input unit  11  according to various standards are digitized, if needed, by the input unit  11 , and are then fed to an image converter  12  through a system bus SB. 
     The image converter  12  is also referred to as a scaler or formatter, performs unification of converting input digital-value image data into image data in a predetermined format which is suitable for projection, and feeds the image data to a projection processor  13 . 
     The projection processor  13  drives a micromirror element  14  as a spatial optical modulator to perform display of higher-speed time-divisional driving in which a frame rate of, for example, 120 frames/second, a number of color components, and a number of display gradations in compliance with the predetermined format are multiplied according to the fed image data. 
     The micromirror element  14  turns on/off individually at high speed inclination angles of a plurality of micromirrors which are arrayed on a wide extended graphic array (WXGA: 800 pixels by 1280 pixels) to display an image, and forms an optical image by reflection light thereof. 
     On the other side, light is emitted in primary colors R, G, and B cyclically in a time-sharing manner from the light source unit  15 . 
     Light of the primary colors from the light source unit  15  is totally reflected by a mirror  16  and is irradiated onto the micromirror element  14 . 
     An optical image is formed by the reflection light from the micromirror element  14 . The formed optical image is projected and displayed through the lens unit  17  onto an unillustrated screen which is a projection target. 
     The light source unit  15  includes a semiconductor laser device (LD)  18  which emits blue laser light. 
     The blue laser light (B) which the LD  18  emits is reflected by the mirror  19  and penetrates a dichroic mirror  20 . The light is thereafter irradiated onto a circumferential surface of a fluorescent wheel  21 . 
     This fluorescent wheel  21  is rotated by the wheel motor (M)  22 , and a fluorescent layer  21   g  is formed over the whole annular circumference which is irradiated with the blue laser light. 
     More specifically, the fluorescent layer  21   g  is formed by applying a fluorescent material on the circumference which is irradiated with the laser light of the fluorescent wheel  21 . 
     On the back surface opposite to a surface where the fluorescent layers  21   g  of the fluorescent wheel  21  is formed, an unillustrated reflection plate is provided so as to overlap the fluorescent layer  21   g.    
     When the fluorescent layer  21   g  of the fluorescent wheel  21  is irradiated with the blue laser light, green light (G) is excited as reflection light. 
     The green light is reflected by the dichroic mirror  20  and also by a dichroic mirror  23 , and is then integrated into light flux having uniform luminance distribution by the integrator  24 . The light flux is then reflected by a mirror  25  and reaches the mirror  16 . 
     Further, the light source unit  15  includes a light emitting diode (LED)  26  which emits red light, and a LED  27  which emits blue light. 
     The red light (R) emitted by LED  26  penetrates the dichroic mirror  20  and is reflected by the dichroic mirror  23 , and is then integrated into light flux having uniform luminance distribution by the integrator  24 . The light flux is then reflected by the mirror  25 , and reaches the mirror  16 . 
     The blue light (B) emitted by LED  27  penetrates the dichroic mirror  23 , and is integrated into light flux having uniform luminance distribution by the integrator  24 . The light flux is then reflected by the mirror  25 , and reaches the mirror  16 . 
     As described above, the dichroic mirror  20  allows blue light and red light to penetrate while the dichroic mirror  20  reflects green light. 
     The dichroic mirror  23  allows blue light to penetrate while the dichroic mirror  23  reflects green light and red light. 
     Under control of a CPU  29  described later, the projection processor  13  performs formation of an optical image by display of an image by the micromirror element  14 , light emission of each of the LD  18 , and LEDs  26  and  27 , and rotation of the fluorescent wheel  21  by the wheel motor  22 . 
     The CPU  29  controls all operations of the respective circuits described above. 
     The CPU  29  is directly connected to a main memory  30  and a program memory  31 . 
     The main memory  30  is configured by, for example, an SRAM and functions as a work memory for the CPU  29 . 
     The program memory  31  is configured by an electrically rewritable nonvolatile memory, and stores operation programs and data of various fixed forms to be performed by the CPU  29 . 
     The CPU  29  performs control operation in the projector apparatus  10  by using the main memory  30  and the program memory  31 . 
     The CPU  29  performs a variety of projection operations in accordance with key operation signals from an operation unit  32 . 
     The operation unit  32  includes a key operation unit provided on a body of the projector apparatus  10 , and an infrared-light receiving unit which receives infrared light from an unillustrated remote controller specialized for the projector apparatus  10 . The operation unit  32  outputs, directly to the CPU  29 , key operation signals based on keys which are operated by the key operation unit of the body or by a remote controller of the projector apparatus  10 . 
     The CPU  29  is connected also to an audio processor  33  through the system bus SB. 
     The audio processor  33  includes a sound source circuit, such as a PCM tone generator, and converts audio data supplied through the system bus SB during a projection operation into analog data. The audio processor  33  enhances and outputs the data through a loudspeaker unit  34  or generates a beep sound if needed. 
     Next with reference to  FIG. 2 , a specific example of a configuration of an optical system will be described including the light source unit  15 , micromirror element  14 , and projection lens unit  17 . 
     In  FIG. 2 , the LD  18  is configured as an LD array in which a plurality of devices, for example, a total 24 of 8×4 devices (directions vertical to the surface of the figure), are arranged on a matrix. 
     Blue laser light emitted from each of the devices is reflected by a mirror  19  which is configured by a mirror array where eight mirrors are arranged in steps and are each shaped like a strip. 
     The blue laser light reflected by the mirror  19  is projected onto the fluorescent wheel  21  through lenses  41  and  42 , the dichroic mirror  20 , and lenses  43  and  44 . 
     The green light excited by the fluorescent layer  21   g  (see  FIG. 1 ) of the fluorescent wheel  21  is reflected by an unillustrated reflection plate which is provided on the back surface opposite to a surface where the fluorescent layer  21   g  of the fluorescent wheel  21  is formed. The green light is then reflected by the dichroic mirror  20  through lenses  44  and  43 , penetrates lens  45 , and is then reflected by the dichroic mirror  23 . 
     The green light reflected by the dichroic mirror wheel  23  is further reflected by the mirror  25  through lens  46 , integrator  24 , and lens  47 , and reaches the mirror  16  further through lens  48 . 
     The green light reflected by the mirror  16  is irradiated onto the micromirror element  14  through lens  49 , and an optical image of a corresponding color is formed by the micromirror element  14 . 
     The formed optical image is emitted to the side of the projection lens unit  17  through lens  49 . 
     The red light emitted by LED  26  penetrates the dichroic mirror  20  through lenses  50  and  51 , and is reflected by the dichroic mirror  23  through lens  45 . 
     The blue light emitted by LED  27  penetrates the dichroic mirror  23  through lenses  52  and  53 . 
     Next, a specific circuit configuration of a drive circuit for the LD  18 , LED  26 , and LED  27  which form the light emitting device will be described referring to  FIG. 3 . 
     For example, a predetermined voltage, for example, a direct-current voltage of 5.5 V, is applied to each of a first DC/DC converter  62  and a second DC/DC converter  63  from a direct-current power supply  61  which is configured by an AC/DC converter. 
     Both the first DC/DC converter  62  and the second DC/DC converter  63  are variable constant-voltage power supplies, and generate a voltage Vg for driving the LD  18 , a voltage Vr for driving LED  26 , and a voltage Vb for driving LED  27 , based on control signals from a power controller (voltage/current controller)  64 A in a power supply controller  64  described later. 
     The voltage which the first DC/DC converter  62  generates is applied to an anode of LED  26  through an FET switch SW 1 R, an anode of the LD  18  through an FET switch SW 1 G, and an anode of LED  27  through an FET switch SW 1 B. 
     Each of the cathodes of LED  26 , LD  18 , and LED  27  is grounded. 
     Similarly, the voltage which the second DC/DC converter  63  generates is applied to the anode of LED  26  through an FET switch SW 2 R, the anode of the LD  18  through an FET switch SW 2 G, and the anode of LED  27  through an FET switch SW 2 B. 
     Further, a gate signal for on/off control is supplied from a switching controller  64 B of the power supply controller  64  to a gate terminal of each of FET switches SW 1 R, SW 1 G, SW 1 B, SW 2 R, SW 2 G, and SW 2 B. 
     The power supply controller  64  includes the power controller  64 A and the switching controller  64 B. 
     The power controller  64 A controls voltages which are respectively generated by the first DC/DC converter  62  and the second DC/DC converter  63 , shifting timings and current values thereof. 
     The switching controller  64 B selectively controls light emission timings and supplied powers of the respective light emitting devices by switching on/off FET switches SW 1 R, SW 1 G, SW 1 B, SW 2 R, SW 2 G, and SW 2 B, by signals supplied to the respective gate terminals of these switches. 
     The power supply controller  64  is provided in the projection processor  13  in  FIG. 1 , and causes the power controller  64 A and the switching controller  64 B to perform light emission driving of LED  26 , LD  18 , and LED  27 , based on control instructions from the CPU  29 . 
     An operation of the circuit configuration as described above will now be described below. 
       FIG. 4  is a timing chart which shows voltage waveforms at several points on the circuit configuration shown in  FIG. 3 . 
     The operation will be described with reference to an example in which voltage Vg for driving the LD  18  foregoing drive, voltage Vr for driving LED  26 , and voltage Vb for driving LED  27  satisfy a relation such that Vr&lt;Vg&lt;Vb. 
     In the first embodiment, one frame of a color image to project is supposed to comprise three fields, i.e., a red image (R) field, a green image (G) field, and a blue image (B) field. Light of primary colors is supposed to be emitted by repeatedly performing, on the side of the light source, a similar pattern of driving for each unit which includes a total of two frames, i.e., one frame from time t 1  to time t 2  and one frame from time t 2  to time t 3  as shown in the drawings (hereinafter referred to as a first frame and a second frame). 
     As indicated by the waveform of the first DC/DC converter in  FIG. 4 , the power controller  64 A controls switching so as to sequentially output constant voltages Vr, Vb, and Vg sequentially to the first DC/DC converter  62  during two frames from time t 1  to time t 3 . 
     More precisely, supposing that a voltage output from the first DC/DC converter  62  shifts from voltage Vg to voltage Vr and that a transition period required to become able to supply stable voltage Vr is expressed as Tsgr, the power controller  64 A instructs the first DC/DC converter  62  to make an output voltage variable (or switch) from Vg to Vr at an earlier timing by transition period Tsgr than time t 1 . 
     Alternatively, supposing that the voltage shifts from voltage Vr to voltage Vb and that a transition period required to become able to supply stable voltage Vb is expressed as Tsrb, in synchronization with a field B of the first frame, the power controller  64 A instructs the first DC/DC converter  62  to make the output voltage variable (or switch) from Vr to Vb at an earlier timing by transition period Tsrb than time t 11  when the field B of the first frame starts. 
     Still alternatively, supposing that the voltage shifts from voltage Vb to voltage Vg and that a transition period required to become able to supply stable voltage Vg is expressed as Tsbg, in synchronization with a field G of the second frame, the power controller  64 A instructs the first DC/DC converter  62  to make an output voltage variable (or switch) from Vb to Vg at an earlier timing by transition period Tsbg than time t 12  when the field G of the second frame starts. 
     Further as indicated by the waveform of the second DC/DC converter in  FIG. 4 , the power controller  64 A controls the second DC/DC converter  63  to switch such that constant voltages Vg, Vr, and Vb are sequentially output in a phase which is delayed from an output pattern of the first DC/DC converter  62  by approximately one field concerning color switching, during two frames from time t 1  to time t 3 . 
     More precisely, supposing that the voltage output from the second DC/DC converter  63  shifts from voltage Vb to voltage Vg and that a transition period required to become able to supply stable voltage Vg is expressed as Tsbg, the power controller  64 A instructs the second DC/DC converter  63  to make the output voltage variable (or switch) from Vg to Vr at an earlier timing by transition period Tsbg than time t 21  when the field G of the first frame starts. 
     Alternatively, supposing that the voltage shifts from voltage Vg to voltage Vr and that a transition period required to become able to supply stable voltage Vr is expressed as Tsgr, in synchronization with a field R of the second frame, the power controller  64 A instructs the second DC/DC converter  63  to make the output voltage variable (or switch) from Vg to Vr at an earlier timing by transition period Tsgr than time t 2  when the second frame starts. 
     Still alternatively, supposing that the voltage shifts from voltage Vr to voltage Vb and that a transition period required to become able to supply stable voltage Vb is expressed as Tsrb, in synchronization with a field B of the second frame, the power controller  64 A instructs the second DC/DC converter  63  to make the output voltage variable (or switch) from Vr to Vb at an earlier timing by transition period Tsrb than time t 22  when the field B of the second frame starts. 
     On the other hand, in the first frame, the switching controller  64 B of the power supply controller  64  switches on (or causes to conduct) switch SW 1 R in synchronization with the field R as indicated by the waveform of SW 1 R in  FIG. 4 , switch SW 2 G in synchronization with the field G as indicated by the waveform of SW 2 G in  FIG. 4 , and switch SW 1 B in synchronization with the B field as indicated by the waveform of SW 1 B in  FIG. 4 , continuously without an interval. Accordingly, stable constant voltage Vr which does not include a transition period can be applied to LED  26  during the R field. Constant voltage Vg can be applied to the LD  18  during the G field, as well as constant voltage Vb can be applied to LED  27  during the B fields. So, LED  26 , LD  18 , and LED  27  each can therefore be made to emit light continuously without an interval with desired stable power. 
     Similarly, in the second frame, the switching controller  64 B switches on (or causes to conduct) switch SW 2 R in synchronization with the field R as indicated by the waveform of SW 2 R in  FIG. 4 , switch SW 1 G in synchronization with the field G as indicated by the waveform of SW 1 G in  FIG. 4 , and switch SW 2 B in synchronization with the field B as indicated by the waveform of SW 2 B in  FIG. 4 , continuously without an interval. Accordingly, LED  26 , LD  18 , and LED  27  can be operated to emit light with desired stable power without an interval. 
     Thus, while voltage transition periods of the two DC/DC converters  62  and  63  are controlled so as not to overlap each other, outputs of the converters which are switched alternately are supplied to the side of the light emitting devices. In this manner, three loads  26 ,  18 , and  27  which have respectively different operating voltages are respectively driven sequentially without an interval in a stable state. 
     (Second Embodiment) 
     Next, a second embodiment of the invention will be described with reference to the drawings. 
       FIG. 5  shows a specific circuit configuration where loads are driven respectively at different voltages. 
     In  FIG. 5 , a total of two loads of a first load L 1  and a second load L 2  are driven respectively at different voltages. 
     Specifically, the first load L 1  is driven at a voltage V 1  or V 2 . 
     The second load L 2  is driven at a voltage V 1  or V 3 . 
     For example, a predetermined voltage, for example, a direct-current voltage of 5.5 V is applied to each of a first DC/DC converter  72  and a second DC/DC converter  73  from a direct-current (DC) power supply  71  configured by an AC/DC converter. 
     Both the first DC/DC converter  72  and the second DC/DC converter  73  are variable constant-voltage power supplies, and generate voltages V 1 , V 2 , and V 3  for driving the first load L 1  and the second load L 2 , based on control signals from a power controller (voltage/current controller)  74 A in a power supply controller  74  described later. 
     The voltages which the first DC/DC converter  72  generates are applied to the first load L 1  through an FET switch SW 11 . 
     The voltages which the second DC/DC converter  73  generates are applied to the second load L 2  through an FET switch SW 22 . 
     Further, a gate signal for switching on/off is supplied from a switching controller  74 B of the power supply controller  74  to a gate terminal of each of FET switches SW 11  and SW 22 . 
     The power supply controller  74  includes the power controller  74 A and the switching control unit  74 B. 
     The power controller  74 A controls voltages, which are respectively generated by the first DC/DC converter  72  and the second DC/DC converter  73 , transition timings and current values thereof. 
     The switching controller  74 B selectively controls driving states of the respective loads by switching on/off FET switches SW 11 R and SW 22  by signals which are supplied to the respective gate terminals of these switches. 
     An operation of the circuit configuration as described above will be described below. 
       FIG. 6  is a timing chart which shows voltage waveforms at several points on the circuit configuration shown in  FIG. 5 . 
     The operation will be described with reference to an example in which voltages V 1  and V 2  for driving the first load L 1  and voltages V 1  and V 3  for driving the second load L 2  satisfy a relation such that V 1 &lt;V 3 &lt;V 2 . 
     The second embodiment supposes one cycle from t 31  to t 32  shown in the figure as a unit which is divided by time sharing into a total of four phases: a first phase which takes the first load L 1  as voltage V 1 ; a second phase which takes the second load L 2  as voltage V 3 ; a third phase which takes the first load L 1  as voltage V 2 ; and a fourth phase which takes the second load L 2  as voltage V 1 . The first load L 1  and the second load L 2  are driven repeatedly in the same pattern. 
     The periods of the respective phases need not be equal to each other as shown in the figure. 
     As indicated by the waveform of the first DC/DC converter in  FIG. 6 , the power controller  74 A switches constant voltages V 1  and V 2  to be sequentially output in turn to the first DC/DC converter  72  during one cycle from time t 31  to time t 33 . 
     More precisely, supposing that the voltage output from the first DC/DC converter  72  shifts from voltage V 2  to voltage V 1  and that a transition period required to become able to supply stable voltage V 1  is expressed as Ts 21 , the power controller  74 A instructs the first DC/DC converter  72  to make the output voltage variable (or switch) from V 2  to V 1  at an earlier timing by transition period Ts 21  than time t 31 , which is the end of the last cycle. 
     Thereafter, supposing that the voltage shifts from voltage V 1  to voltage V 2  and that a transition period required to become able to supply stable voltage V 2  is expressed as Ts 12 , in synchronization with the third phase, the power controller  74 A instructs the first DC/DC converter  72  to make the output voltage variable (or switch) from V 1  to V 2  at an earlier timing by transition period Ts 12  than time t 33  when the third phase starts. 
     Further, as indicated by the waveform of the second DC/DC converter in  FIG. 4 , the power controller  74 A controls the second DC/DC converter  73  to switch constant voltages V 3  and V 1  to be sequentially output in turn in a phase which is delayed by approximately one field from an output pattern of the first DC/DC converter  72 , during one frame from time t 31  to time t 33 . 
     More precisely, supposing that the second DC/DC converter  73  shifts from voltage V 1  to voltage V 3  and that a transition period required to become able to supply stable voltage V 3  is expressed as Ts 13 , the power controller  74 A instructs the second DC/DC converter  73  to make the output voltage variable (or switch) from V 1  to V 3  at an earlier timing by transition period Ts 13  than time t 41  when the second phase starts. 
     Thereafter, supposing that the voltage shifts from voltage V 3  to voltage V 1  and that a transition period required to become able to supply stable voltage V 1  is expressed as Ts 31 , in synchronization with the fourth phase, the power controller  74 A instructs the second DC/DC converter  73  to make the output voltage variable (or switch) from V 3  to V 1  at an earlier timing by transition period Ts 31  than time t 42  when the fourth phase starts. 
     On the other side, the switching controller  74 B of the power supply controller  74  continuously switches on switch SW 11  in synchronization with the first phase and the third phase, as indicated by the waveform of SW 11  in  FIG. 6 , as well as switch SW 22  in synchronization with the second phase and the fourth phase, as indicated by the waveform of SW 22  in  FIG. 6 . In this manner, constant voltage V 1  is applied to the first load L 1  in the first phase, as well as constant voltage V 3  to the second load L 2  in the second phase. Constant voltage V 2  to the first load L 1  in the third phase, as well as constant voltage V 1  to the second load L 2  in the fourth phase. Accordingly, the first load L 1  and the second load L 2  can be driven continuously with desired stable electric powers. 
     Thus, while voltage transition periods of the two DC/DC converters  62  and  63  are controlled so as not to overlap each other, outputs of the converters are switched alternately and supplied to the two loads L 1  and L 2  to drive. In this manner, a plurality of operating voltages are sequentially switched, and the loads can be sequentially driven in turn continuously without an interval in a stable state. 
     As described above, each of the first and second embodiments can switch powers of a plurality of constant voltages at a high speed and can supply the power while the number of required DC/DC converters is reduced to two as the least necessary number. 
     Particularly in the first embodiment, any of three voltages Vr, Vg, and Vb which the DC/DC converters  62  and  63  output is continuously supplied to any of the loads. Therefore, the first embodiment can be achieved by repeatedly performing a pattern of driving in units each including two cycles (two frames). Accordingly, control operations can be simplified. 
     In each of the first and second embodiments, the power controller  64 A ( 74 A) performs control to periodically alternately select and switch the first DC/DC converter  62  ( 72 ) and the second DC/DC converter  63  ( 73 ) both of which are variable constant-power power supplies. Therefore, the burden on the power controller  64 A ( 74 A) is reduced by the periodical drive, and the configuration thereof can be simplified. 
     Further, in each of the first and second embodiments, the power controller  64 A ( 74 A) controls the first DC/DC converter  62  ( 72 ) and the second DC/DC converter  63  ( 73 ) in common. Therefore, the control system can be downsized. 
     Though not described in the first and second embodiments, one power supply which is not actually used by the loads among the first DC/DC converter  62  ( 72 ) and the second DC/DC converter  63  ( 73 ) both being variable constant-power power supplies may be controlled so as to temporarily stop in consideration of a transition period until a supplied power is stabilized at a next startup. 
     For example, in  FIG. 4 , the first DC/DC converter  62  can be stopped during a period from t 21  to the beginning of Tsrb. 
     In this manner, wasteful power consumption is suppressed in an apparatus using batteries whose power consumptions are both limited. Accordingly, a power supply can be effectively used. 
     In addition, the switching controller  64 B ( 74 B) controls FET switches SW 1 B, SW 1 G, SW 1 R, SW 2 B, SW 2 G, and SW 2 R (SW 11  and SW 22 ) to conduct shifted from one another in consideration of transition periods of powers. In this manner, an operation of switching powers of a plurality of constant voltages at a higher speed can be easily achieved. 
     Further, the first embodiment has been described in case of driving an LD  18  and LEDs  26  and  27  which are semiconductor light emitting devices as loads. Even when semiconductor light emitting devices switched at high speed of this type are used as loads, a driving state can be maintained stably as a whole continuously without an interval. In this respect, the invention can be used desirably. 
     Each of the first and second embodiments has been described with reference to a case of a constant-voltage power supply which can vary voltages of electric powers majorly supplied to loads. However, the invention is not limited hitherto but is applicable in a similar manner to a constant-current power supply which controls currents supplied to loads. 
     That is, the invention is applicable to a constant-power-regulated power supply. 
     Further, the invention is not limited to the embodiments described above but can be variously modified in practical phases without deviating from the subject manners of the invention. 
     In addition, functions performed by the embodiments described above may be combined as suitably as possible, and may be performed. 
     The embodiments described above further include various stages, and various inventions can be derived by appropriate combination of a plurality of disclosed components. 
     For example, even if several components are removed from all the components disclosed in embodiments, the configuration from which the several components are removed may be extracted as an invention, insofar as effects of the invention are obtained. 
     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.