Patent Publication Number: US-7915835-B2

Title: Imaging apparatus, strobe device, and charging-control method

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application JP 2007-102319 filed in the Japanese Patent Office on Apr. 10, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an imaging apparatus including a strobe device, a strobe device, and a charging-control method for the strobe device. More particularly, the present invention relates to an imaging apparatus including a strobe device having a charging circuit of a separately excited oscillation type, a strobe device, and a charging-control method. 
     2. Description of the Related Art 
     In recent years, there have been various types of strobe-charging circuits used for strobe devices or the like. Charging circuits that have been used often lately are of a separately excited oscillation type, and more specifically, of a flyback transformer type.  FIG. 9  is a diagram of a configuration example of a strobe-charging circuit of the flyback transformer type. 
     The strobe-charging circuit shown in  FIG. 9 , which is a circuit that charges a main capacitor C 1  for supplying power to a strobe-light-flashing unit, includes a direct-current power supply  51 , a fuse element  52 , a step-up transformer  53 , a switching element Q 1 , a feedback capacitor C 2 , a rectifier diode D 1 , and the main capacitor C 1 . The strobe-charging circuit also includes a charging-control unit  61 , an oscillation-control unit  62 , and a full-charge-detection unit  63 , which have functions of controlling the strobe-charging circuit. 
     The step-up transformer  53  is a flyback transformer that steps up a flyback pulse, which is generated in a primary coil, and that outputs another flyback pulse to a secondary coil. The primary coil is connected in series with the direct-current power supply  51 , the fuse element  52 , and the switching element Q 1 . The feedback capacitor C 2  is provided between the fuse element  52  and the step-up transformer  53  so as to be connected in parallel to the direct-current power supply  51 . A switching group is configured to have the feedback capacitor C 2 , the primary coil of the step-up transformer  53 , and the switching element Q 1 . 
     The switching element Q 1  is turned on/off on the basis of a switching-control signal from the oscillation-control unit  62 . When the switching element Q 1  is set to be in a conducting state, a current flows thorough the primary coil of the step-up transformer  53 . Then, when the switching element Q 1  is set to be in an interrupting state, a counter-electromotive force is generated in the secondary coil of the step-up transformer  53 . Accordingly, by continuously turning on/off the switching element Q 1 , a flyback pulse is generated in the secondarily coil of the step-up transformer  53 . 
     The rectifier diode D 1  rectifies the flyback pulse that is generated in the secondary coil of the step-up transformer  53 , and supplies a rectified flyback pulse to the main capacitor C 1 . The main capacitor C 1  is charged using the flyback pulse supplied thereto. A charging voltage of the main capacitor C 1  is supplied through a power-supply terminal  54  to the strobe-light-flashing unit (not shown in  FIG. 9 ) serving as a load. 
     The charging-control unit  61  is a processing unit that performs overall control on the strobe-charging circuit. After a strobe-light-flashing operation is performed in the strobe-light-flashing unit, the charging-control unit  61  provides an instruction for starting an oscillation of the switching-control signal to the oscillation-control unit  62 . When the full-charge-detection unit  63  detects a fully charged state of the main capacitor C 1 , the charging-control unit  61  stops an oscillation operation of the oscillation-control unit  62 . The full-charge-detection unit  63  determines whether or not the main capacitor C 1  is in the fully charged state, for example, by detecting a voltage across the terminals of the main capacitor C 1 . 
     In an example of such a strobe-charging circuit of a separately excited oscillation type, when a charging voltage of a main capacitor reaches a predetermined voltage higher than a light-flashing voltage, an oscillation-control unit is reset to stop a charging operation (for example, see paragraphs [0037] to [0042] and FIG. 1 in Japanese Unexamined Patent Application Publication No. 2000-275706). In another example, a value of a current flowing through a primary coil of an oscillation transformer, or a value of a voltage generated by the current, is compared with a value of a normal-operating current, or voltage, and a charging voltage of a main capacitor is compared with a value of a charging voltage in a normal operation, respectively. Then, as an occasional result of the comparison, a series circuit in which a power supply and a fuse are connected in series is shorted to ground by a switching element, thereby blowing the fuse, so that a supply of power is interrupted in the circuit (for example, see paragraphs [0008] to [0012] and FIG. 4 in Japanese Unexamined Patent Application Publication No. 2007-48702). 
     SUMMARY OF THE INVENTION 
     However, in the above-described charging circuit of the flyback transformer type, a power-supply voltage, which is output from the direct-current power supply  51 , is supplied through the fuse element  52  directly to the primary coil of the step-up transformer  53 . The switching element Q 1  has a function of passing and limiting a current supplied from the direct-current power supply  51  to the step-up transformer  53 . Because a high voltage of the flyback pulse is applied directly to the switching element Q 1 , the switching element Q 1  is configured to have a high-voltage-resistant element manufactured in a high-voltage-resistant process. 
     However, for example, when an abnormal voltage resulting from a failure or the like in a peripheral circuit is applied to the switching element Q 1  or when the switching element Q 1  in which a high-voltage-resistant property is diminished performs a switching operation, the switching element Q 1  can be broken to become a condition short-circuited between the drain and source terminals thereof. In this case, it is difficult to limit a supply of power from the direct-current power supply  51  to the step-up transformer  53 . As a result, an abnormal current continues flowing through the step-up transformer  53  and the short-circuited switching element Q 1 . 
     For example, in a case where the switching element Q 1  is broken and a short circuit occurs in the switching element Q 1 , when the resistance of the switching element Q 1  including the short circuit is sufficiently large, the flowing abnormal current becomes small. Accordingly, heat generated by the switching element Q 1  does not become particularly large. In contrast, when the resistance of the switching element Q 1  including the short circuit is sufficiently small, the fuse element  52  blows, whereby the supply of power to the switching element Q 1  is stopped. 
     However, when the resistance of the switching element Q 1  including the short circuit is the middle value between the sufficiently large resistance and the sufficiently small resistance, a current that is not so large as to cause blowing of the fuse element  52  but as large as to allow the switching element Q 1  to generate heat may flow. In such a case, because the supply of power to the switching element Q 1  is not stopped, the amount of heat generated by switching element Q 1  becomes further large. Additionally, in a case where the switching element Q 1  is formed of a semiconductor, when the amount of heat generated by the switching element Q 1  is increased to exceed a certain junction temperature of the switching element Q 1 , the resistance is sharply decreased. As a result, a further large current may flow. 
     Furthermore, generally, as a safeguard against an abnormal current, the fuse element  52  is furnished. However, since recently a request for reducing a charging time has been increased, it is necessary to increase a rated current of the fuse element  52  as much as possible so that a current can be increased in normal use. For this reason, it has been difficult to deal with a matter that an abnormal current not so large as to cause blowing of the fuse element  52  flows. Furthermore, there was a problem that even when a rated current of the fuse element  52  capable of dealing with such a matter can be calculated, the fuse element  52  with such a rated current may not be available. 
     In view of the above-mentioned subject, it is desirable to provide an imaging apparatus including a strobe device having a charging circuit with safety improved for a case where an abnormal current flows through a switching element, a strobe device, or a charging-control method. 
     According to an embodiment of the present invention, there is provided an imaging apparatus including a strobe device having a charging circuit of a separately excited oscillation type. The imaging apparatus includes the following elements: a main capacitor in which charge is accumulated to supply power to a strobe-light-flashing unit; a step-up transformer including at least primary and secondary coils; a switching element that performs a switching operation on the basis of a switching control signal to control a flowing or a non-flowing state of a current supplied to the primary coil of the step-up transformer; a rectifier diode that rectifies a flyback pulse generated in the secondary coil of the step-up transformer on the basis of the switching operation of the switching element to supply a charging voltage to the main capacitor; a power-supply-interrupting circuit that is provided between, the step-up transformer and the switching element, and a power supply and that selectively interrupts power supplied from the power supply; a full-charge detection unit that detects whether the main capacitor reaches a fully charged state; and a power-supply-control unit that controls the power-supply-interrupting circuit so as to set the power-supply-interrupting circuit to be in an interrupting state, in a case where the full-charge-detection unit does not detect the fully charged state within a predetermined time after the power-supply-control unit has set the power-supply-interrupting circuit to be in a conducting state and started the switching operation of the switching element. 
     In such an imaging apparatus, the power-supply-interrupting circuit is set to be in the conducting state to supply the power from the power supply to the primary coil of the step-up transformer and the switching element. The switching element performs the switching operation to generate a flyback pulse in the secondary coil of the step-up transformer. The flayback pulse is rectified by the rectifier diode, and supplied to the main capacitor to charge the main capacitor. Then, when the full-charge-detection unit does not detect the fully charged state of the main capacitor within the predetermined time after the power-supply-control unit has set the power-supply-interrupting circuit to be in the conducting state and started the switching operation of the switching element, the power-supply-control unit controls the power-supply-interrupting circuit so as to set the power-supply-interrupting circuit to be in the interrupting state. As a result, a supply of power to the switching element is interrupted. 
     In the imaging apparatus according to the embodiment of the present invention, when the full-charge-detection unit does not detect the fully charged state of the main capacitor in the predetermined time after the power-supply-interrupting circuit has been set to be in the conducting state and the switching operation of the switching element has been started, the power-supply-interrupting circuit is set to be in the interrupting state. Thus, when an abnormality occurs in the switching element and a current continues flowing while the main capacitor is being charged, the interrupting state of the power-supply-interrupting circuit can prevent the current from flowing through the switching element, thereby preventing the switching element from generating heat. Therefore, the safety can be ensured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a configuration of a principal part of an imaging apparatus according to an embodiment of the present invention; 
         FIG. 2  is a diagram of a configuration of a strobe-charging circuit and control functions thereof; 
         FIG. 3  is a diagram of another configuration of the strobe-charging circuit; 
         FIG. 4  is an example of a graph showing the relationship between the amount of heat generated by a switching element and other parameters; 
         FIG. 5  is a time chart of a first example of a charging-control operation in a normal imaging case; 
         FIG. 6  is a time chart of a second example of the charging-control operation in a normal imaging case; 
         FIG. 7  is a time chart of an example of the charging-control operation when an abnormal current is flowing due to an abnormality occurring in the switching element; 
         FIG. 8  is a time chart of an example of the charging-control operation when noise is superimposed; and 
         FIG. 9  is a diagram of a strobe-charging circuit of a flyback transformer type as a configuration example. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention, in which imaging apparatuses capable of recoding captured images as digital data, such as digital still cameras, are provided as application examples, will be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a block diagram of a configuration of a principal part of an imaging apparatus according to an embodiment of the present invention. 
     The imaging apparatus shown in  FIG. 1  includes an optical block  11 , an imaging device  12 , an analog-front-end (AFE) circuit  13 , a camera-signal-processing circuit  14 , an image encoder  15 , a recorder  16 , a display  17 , a control section  18 , and a strobe device  19 . 
     The optical block  11  includes a lens that gathers light from an object into the imaging device  12 , a driving mechanism in which the lens is moved to adjust the focus of the lens and to perform a zooming operation, a shutter mechanism, and an iris mechanism. The lens, the driving mechanism, the shutter mechanism, and the iris mechanism are driven on the basis of control signals from the control section  18 . The imaging device  12  is a charge-coupled device (CCD), or a solid-state imaging device of a complementary-metal-oxide semiconductor (CMOS) type or the like, and converts the light, which enters the imaging device  12  from the object through the optical block  11 , to an electronic signal. 
     For an image signal that is output from the imaging device  12 , the AFE circuit  13  performs a sample-and-hold operation using a correlated-double-sampling (CDS) process so that the signal/noise (S/N) ratio can be satisfactorily maintained. Additionally, the AFE circuit  13  controls a gain using an auto-gain-control (AGC) process, and performs an analog-to-digital (A/D) conversion to output a digital image signal. 
     The camera-signal-processing circuit  14  performs various types of detection processes, which are used for an auto-focus (AF) process and an auto-exposure (AE) process, on the basis of the image signal output from the AFE circuit  13 . For the image signal, the camera-signal-processing circuit  14  also performs signal processing, such as a white-balance-adjustment process or a color correction process. For the image signal output from the camera-signal-processing circuit  14 , the image encoder  15  performs a compression-encoding process using a predetermined image-data format, such as joint-photographic coding-experts-group (JPEG) format. The display  17  is, for example, a liquid-crystal display (LCD), and displays a captured image based on the image signal from the camera-signal-processing circuit  14 . 
     The control section  18  includes, for example, a microcomputer configured to have a central processing unit (CPU), a read-only memory (ROM), a random-access memory (RAM), and so forth, and executes a program stored in the ROM or the like to perform overall control on each portion of the imaging apparatus. The strobe device  19  includes a strobe-light-flashing unit and a charging circuit described below. A strobe-light-flashing operation of the strobe-light-flashing unit and a charging operation of the charging circuit are controlled by the control section  18 . 
     Next, the charging circuit included in the strobe device  19 , and control functions of the charging circuit will be described.  FIG. 2  is a diagram of a configuration of a strobe-charging circuit and control functions thereof. 
     The strobe-charging circuit shown in  FIG. 2  is a circuit that charges a main capacitor C 1  for supplying power to the strobe-light-flashing unit. The strobe-charging circuit includes a direct-current power supply  21 , a fuse element  22 , a step-up transformer  23 , a switching element Q 1 , a feedback capacitor C 2 , a power-supply-interrupting circuit  24 , a rectifier diode D 1 , and the main capacitor C 1 . 
     The strobe-charging circuit also includes a charging-control unit  31 , an oscillation-control unit  32 , a full-charge-detection unit  33 , and a power-supply-control unit  34 , which have functions of controlling the strobe-charging circuit. In this embodiment, for example, the functions of the charging-control unit  31  and the power-supply-control unit  34  from among the above-mentioned functions are realized by program execution performed by the control section  18 . Additionally, the control section  18  includes an imaging-operation-control unit  40  that controls imaging operations including the strobe-light-flashing operation. 
     For example, a secondary battery can be used as the direct-current power supply  21 . A power-supply voltage, which is supplied from the direct-current power supply  21 , is supplied through the fuse element  22  and the power-supply-interrupting circuit  24  to a primary coil of the step-up transformer  23 . The fuse element  22  is configured to blow when a current equal to or larger than a predetermined value flows. 
     The step-up transformer  23  is a flyback transformer that steps up a flyback pulse generated in the primary coil, and that outputs another flyback pulse to a secondary coil. The primary coil is connected in series with the direct-current power supply  21 , the fuse element  22 , the power-supply-interrupting circuit  24 , and the switching element Q 1 . The switching element Q 1  is configured to have, for example, an n-channel metal-oxide-semiconductor (MOS) transistor. The gate terminal of the switching element Q 1  receives a switching-control signal from the oscillation-control unit  32 . The switching element Q 1  is turned on/off on the basis of the switching-control signal. The feedback capacitor C 2  is provided between the fuse element  22  and the step-up transformer  23  so as to be connected in parallel to the direct-current power supply  21 . A switching group is configured to have the feedback capacitor C 2 , the primary coil of the step-up transformer  23 , and the switching element Q 1 . 
     The power-supply-interrupting circuit  24  is a circuit that selectively interrupts a current flowing from the direct-current power supply  21  to the step-up transformer  23  and the switching element Q 1 . In this embodiment, for example, the power-supply-interrupting circuit  24  includes a p-channel MOS transistor Q 2 , a resistor R 1 , a capacitor C 3 , and an n-channel digital transistor Q 3 . The source terminal of the MOS transistor Q 2  is connected to one end of the fuse element  22 . The source and gate terminals of the MOS transistor Q 2  are connected to each other through the resistor R 1 . The capacitor C 3 , which is used for the prevention of an inrush current, is connected between the gate and drain terminals of the MOS transistor Q 2 . The digital transistor Q 3  is turned on/off on the basis of a base-control signal, which is supplied from the power-supply-control unit  34 , thereby controlling a conducting/non-conducting state between the gate terminal of the MOS transistor Q 2  and the ground. 
     In the power-supply-interrupting circuit  24 , when the digital transistor Q 3  is turned on, a gate potential of the MOS transistor Q 2  becomes the ground potential. As a result, the MOS transistor Q 2  is turned on, and power from the direct-current power supply  21  is supplied to the step-up transformer  23 . When the digital transistor Q 3  is turned off, the MOS transistor Q 2  is also turned off. As a result, a supply of power from the direct-current power supply  21  is interrupted. The configuration of the power-supply-interrupting circuit  24  is not limited to the above-described configuration, and may be implemented with a relay switch. 
     The rectifier diode D 1  rectifies the flyback pulse that is generated in the secondary coil of the step-up transformer  23 , and supplies a rectified flyback pulse to the main capacitor C 1 . The main capacitor C 1  is charged using the flyback pulse that is supplied through the rectifier diode D 1  to the main capacitor C 1 . A charging voltage of the main capacitor C 1  is supplied through a power-supply terminal  25  to the strobe-light-flashing unit (not shown in  FIG. 2 ) serving as a load. 
     The charging-control unit  31  is a control unit that performs overall control on the strobe-charging circuit. On the basis of a full-charge-detection signal S 1  from the full-charge-detection unit  33  and a strobe-light-flashing signal S 2  from the imaging-operation-control unit  40 , the charging-control unit  31  controls each function of the switching element Q 1  and the power-supply-interrupting circuit  24 , thereby controlling a charging function for the main capacitor C 1 . 
     The oscillation-control unit  32  controls a switching operation of the switching element Q 1  on the basis of an oscillation-start/end-control signal S 3  from the charging-control unit  31 . In other words, only in a period when the level of the oscillation-start/end-control signal S 3  is high, does the oscillation-control unit  32  output the switching-control signal to the gate terminal of the switching element Q 1  to allow the switching element Q 1  to perform the switching operation. 
     The full-charge-detection unit  33  detects a voltage across the terminals of the main capacitor C 1 . When the voltage reaches a predetermined value, the full-charge-detection unit  33  determines that the main capacitor C 1  is in a fully charged state, and reports a detection of the fully charged state to the charging-control unit  31 . 
     The power-supply-control unit  34  outputs the base-control signal to the base terminal of the digital transistor Q 3  in the power-supply-interrupting circuit  24  on the basis of the power-supply-control signal S 4  from the charging-control unit  31 . By using the base-control signal, the power-supply-control unit  34  controls the power-supply-interrupting circuit  24  so that the power-supply-interrupting circuit  24  provides/interrupts a supply of power from the direct-current power supply  21  to the step-up transformer  23 . In other words, when the level of the power-supply-control signal S 4  is high, the power-supply-control unit  34  sets the base-control signal to a high level to provide the supply of power from the direct-current power supply  21  to the step-up transformer  23 . When the level of the power-supply-control signal S 4  is low, the power-supply-control unit  34  sets the base-control signal to a low level to interrupt the supply of power from the direct-current power supply  21 . 
     The strobe-charging circuit having the above-described configuration has a feature in which the charging-control unit  31  can control the power-supply-interrupting circuit  24 , which is included in the strobe-charging circuit, to interrupt the supply of power from the direct-current power supply  21  to the switching element Q 1 . More specifically, in a case where the full-charge-detection unit  33  does not detect the fully charged state even in a predetermined time after the charging-control unit  31  starts charging the main capacitor C 1 , the charging-control unit  31  determines that an abnormality occurs in the switching element Q 1 , and interrupts the supply of power to the switching element Q 1 . In a case where the occurrence of the abnormality causes a short circuit between the drain and source terminals of the switching element Q 1 , when an abnormal current, which is not so large as to cause blowing of the fuse element  22 , continues flowing, this interruption controlled by charge-control unit  31  can prevent the switching element Q 1  from generating heat. 
       FIG. 3  is a diagram of another configuration of the strobe-charging circuit. 
     In the above-described configuration shown in  FIG. 2 , the full-charge-detection unit  33  detects the voltage across the terminals of the main capacitor C 1 . However, the fully charged state can be detected using another method other than the above-described method. A full-charge-detection unit  33   a  shown in  FIG. 3  detects a kickback voltage, which is generated in the switching group including the primary coil of the step-up transformer  23 , to detect the fully charged state. A full-charge-detection signal S 1  that the full-charge-detection unit  33   a  outputs to the charging-control unit  31  is the same as the full-charge-detection signal S 1  of the full-charge-detection unit  33  shown in  FIG. 2 . Accordingly, the descriptions below will be provided on the basis of the configuration shown in  FIG. 2 . 
     First, problems in a case where a short circuit occurs in the switching element Q 1  will be described in detail, and then, the detailed operations of the strobe-charging circuit will be described. 
     In a case where the resistance between the drain and source terminals of the switching element Q 1  is decreased due to the occurrence of an abnormality in the switching element Q 1 , when the amount of a current flowing is sufficiently large, the fuse element  22  blows. As a result, the supply of power to the switching element Q 1  is interrupted, thereby immediately preventing the switching element Q 1  from generating heat. However, a case can be supposed in which a current that is not so large as to cause blowing of the fuse element  22  but as large as to allow the switching element Q 1  to generate heat is flowing in accordance with the resistance of the switching element Q 1 . Additionally, in a case where the switching element Q 1  is formed of a semiconductor, when the amount of heat generated by the switching element Q 1  is increased to exceed a certain junction temperature of the switching element Q 1 , the resistance is sharply decreased. As a result, a further large current may flow. 
     Using the relationship of input power Pd, which is input to the switching element Q 1 , a thermal resistance Rθ, and an ambient temperature Ta, an amount Ts of heat generated by the switching element Q 1  can be given by equation (1):
 
 Ts=Ta+Rθ*Pd   (1)
 
     As equation (1) shows, the generated heat amount Ts is increased in proportion to the input power Pd. Suppose that a power-supply voltage is a fixed value, namely, VBATT. In a case where the power-supply voltage VBATT is applied to a short-circuit resistance Rds after the switching element Q 1  is broken, when a current flowing through the short-circuit resistance Rds is represented by Ids, the relationship given by equation (2) can be obtained:
 
 Pd=VBATT*Ids=Rds*Ids   2   (2)
 
Accordingly, using equations (1) and (2), the amount Ts of heat generated by the switching element Q 1  can be represented as a temperature increase given by equation (3):
 
 Ts=Ta+Rθ*Rds*Ids   2   (3)
 
       FIG. 4  is an example of a graph showing the relationship between the amount of heat generated by a switching element and other parameters. 
     In  FIG. 4 , for example, the relationship of the short-circuit resistance Rds to the current Ids and the generated heat amount Ts (a surface temperature of the switching element in this example) is shown in a case where the power-supply voltage VBATT is 4.2 V and the ambient temperature Ta is 60° C. Referring to  FIG. 4 , for example, even when a current not so large as to cause blowing of the fuse element  22  with a rated current of 1 A (an allowable current of 2 A) is flowing, the surface temperature of the switching element Q 1  can become as large as 400° C. The configuration shown in  FIG. 2  can deal with such a surface temperature. 
     In the related art, as a safeguard against an abnormal current, generally, a current fuse is included. The fuse element  22  shown in  FIG. 2  is such a current fuse. As one of factors in determining a rated current of the fuse element, an abnormal-current value can be used. Normally, when a specification of the fuse element is determined, the specification includes a margin twice or larger than twice the abnormal-current value that can be expected. Accordingly, it is necessary to set the upper limit of the rated current of the fuse element to be equal to or smaller than half of the minimum abnormal-current value. 
     Additionally, as other factors in determining the rated current of the fuse element, there are the following two factors: one is the maximum value of a normal-use current flowing in an apparatus in which the fuse element is mounted; and the other is a Joule-integral value of an inrush current flowing in a case where a power supply is connected to an apparatus in which the fuse element is mounted or in a case where a power-supply switch is turned on. 
     The rated current corresponding to a normal-use current value can be determined hereinafter. The maximum value of a normal-use current is corrected by the following two correction values: one is constant derating, which takes the life of the apparatus in which the fuse element is mounted into consideration; and the other is temperature derating, which depends on an ambient temperature of an ambience in which the fuse element is used. As a result, the lower limit of the rated current can be determined. Accordingly, when the rated current is set, it is necessary for the rated current to include a margin which is sufficient to avoid unexpected blowing of the fuse element in normal use under a state where the life of the apparatus and an ambient-temperature change are taken into consideration. 
     The rated current against the inrush current can also be determined hereinafter. The waveform of the inrush current, which is measured in the apparatus in which the fuse element is mounted, is integrated to obtain a Joule-integral value I 2 t. When the rated current is set, it is necessary for the rated current to include a sufficient margin from a characteristic curve showing tolerance against the Joule-integral value I 2 t of the fuse element. 
     However, there is a request that a charging circuit, such as a strobe-charging circuit, have a normal-use current value as large as possible in order to be rapidly charged. For this reason, there is a probability that the lower limit of the rated value, which is determined from the normal-use current value, is larger than the upper limit of the rated current, which is determined from the abnormal-current value. In such a case, it is impossible to set the rated current of the fuse element. 
     Additionally, because discrete values are set for the rated current of the fuse elements on the market, even when the lower limit of the rated current, which is determined from the normal-use current, is smaller than the upper limit of the rated current, which is determined from the abnormal-current value, i.e., even when a state is provided in which the rated current can be set, there is a probability that it is difficult to obtain a fuse element having the rating current meeting the state. 
     With the configuration shown in  FIG. 2 , even when the fuse element  22 , which is an existing device having a normal-use current value estimated as a comparatively high value, is used, the above-described heat generation due to an abnormal current can be avoided. Thus, as a purpose, the safety can be ensured. 
     The above-described technology disclosed in Japanese Unexamined Patent Application Publication No. 2000-275706 and in Japanese Unexamined Patent Application Publication No. 2007-48702 provides a manner in which the switching operation and the supply of power in the circuit are controlled only on the basis of a detected voltage in the secondary or primary coil of the flyback transformer. In this manner, it is difficult to determine whether the switching operation of the switching element Q 1  has an abnormality or, in a normal switching operation, a current as large as an abnormal current is flowing. Accordingly, it is difficult to solve the above-described problems. 
     Next, control operations using the above-described strobe-charging circuit will be specifically described. First,  FIG. 5  is a time chart of a first example of a charging-control operation in a normal imaging case. 
     Referring to  FIG. 5 , at timing T 20 , because the main capacitor C 1  is in the fully charged state, the imaging-operation-control unit  40  controls the strobe-light-flashing unit so that the strobe-light-flashing unit can perform the strobe-light-flashing operation. In this case, the imaging-operation-control unit  40  outputs the strobe-light-flashing signal S 2  to the charging-control unit  31 . Immediately or in a predetermined time after the charging-control unit  31  detects the strobe-light-flashing signal S 2 , the charging-control unit  31  starts charging the main capacitor C 1 . 
     Timing T 21  shown in  FIG. 5  is the timing when the charging-control unit  31  starts charging the main capacitor C 1 . First, the charging-control unit  31  sets the power-supply-control signal S 4 , which is output to the power-supply-control unit  34 , to the high level to turn on the MOS transistor Q 2 , thereby starting the supply of power from the direct-current power supply  21  to the step-up transformer  23 . Next, at timing T 22  when a predetermined time t d1  passes from the timing T 21 , the charging-control unit  31  sets the oscillation-start/end-control signal S 3 , which is output to the oscillation-control unit  32 , to a high level to start the switching operation of the switching element Q 1 , whereby the charging voltage of the main capacitor C 1  is gradually increased. 
     Next, when, at timing T 23 , the full-charge-detection unit  33  detects the fully charged state of the main capacitor C 1 , the full-charge-detection unit  33  sets the full-charge-detection signal S 1 , which is output to the charging-control unit  31 , to a low level. Referring to  FIG. 5 , an example is shown in which the level of the full-charge-detection signal S 1  is low when the fully charged state is detected but the full-charge-detection signal S 1  is automatically returned to the high level after a predetermined time. When the charging-control unit  31  detects that the level of the full-charge-detection signal S 1  is low, at timing T 24  immediately after the detection, the charging-control unit  31  sets the oscillation-start/end-control signal S 3  to a low level to stop the switching operation of the switching element Q 1 . Then, at timing T 25  when a predetermined time t d2  passes from the timing T 24 , the charging-control unit  31  sets the power-supply-control signal S 4  to the low level to stop the supply of power to the step-up transformer  23 . 
     As described above, in a period including the period in which the switching operation is being performed by setting the oscillation-start/end-control signal S 3  to the high level, the level of the power-supply-control signal S 4  is high to provide the supply of power to the step-up transformer  23 . Accordingly, the charging-control unit  31  controls the oscillation-start/end-control signal S 3  and the power-supply-control signal S 4  so as to provide or interrupt the supply of power without particularly influencing the charging operation. 
     Similarly, in a period from timing T 11  to timing T 19  shown in  FIG. 5 , power is supplied to the step-up transformer  23  to charge the main capacitor C 1 . The charging-control unit  31  starts the supply of power at timing T 11 , and starts the switching operation at timing T 12  when the predetermined time t d1  passes from the timing T 11 . Then, after the fully charged state is detected at timing T 17 , the charging-control unit  31  stops the switching operation at timing T 18 , and stops the supply of power at timing T 19 . The above-described operations of the charging-control unit  31  in the period from timing T 11  to timing T 19  are the same as those of the charging-control unit  31  in a period from timing T 21  to timing T 25 . 
     However, in the period from timing T 11  to timing T 19 , the charging-control unit  31  sets the oscillation-start/end-control signal S 3  to the low level in each of a period from timing T 13  to timing T 14  and a period from timing T 15  to timing T 16  to stop the switching operation. In such periods, other operations that consume a comparatively large power are performed in the imaging apparatus. In the periods, a motor may be driven, for example, to move a zoom lens or a focus lens. The charging-control unit  31  receives, for example, a control signal (not shown in  FIG. 5 ) for enabling/disenabling the charging operation from the imaging-operation-control unit  40 . On the basis of this control signal, the charging-control unit  31  performs the switching operation only in a period when the charging operation is enabled. 
     Next,  FIG. 6  is a time chart of a second example of the charging-control operation in a normal imaging case. 
     Referring to  FIG. 6 , in a period from timing T 31  to timing T 43 , power is supplied to the step-up transformer  23  to charge the main capacitor C 1 . As in the case shown in  FIG. 5 , when the whole charging operation is started, the charging-control unit  31  starts the supply of power to the step-up transformer  23  at timing T 31 , and then starts the switching operation of the switching element Q 1  at timing T 32  when the predetermined time t d1  passes from the timing T 31 . When the fully charged state is detected at timing T 41 , the charging-control unit  31  stops the switching operation at timing T 42  immediately after timing T 41 , and then stops the supply of power at timing T 43  when the predetermined time t d2  passes from the timing T 42 . 
     Additionally, in the period from timing T 31  to timing T 43 , the switching operation is stopped at each of timing T 33  and timing T 37  in accordance with, for example, the performance of the above-described operation of moving a lens. Even in the period when the charging operation is being performed, when the imaging-operation-control unit  40  does not send a notification that the charging operation can be performed again to the charging-control unit  31  before the predetermined time t d2  passes after the switching operation is stopped, the charging-control unit  31  sets the power-supply-control signal S 4  to the low level again to interrupt the supply of power (at timing T 34  and timing T 38 ). After the interruption of the supply of power, when the charging-control unit  31  receives the notification that the charging operation can be performed again, the charging-control unit  31  starts the supply of power (at timing T 35  and timing T 39 ), and then starts the switching operation (at timing T 36  and timing T 40 ), as in the case where the charging-control unit  31  operates when the whole charging operation is started. 
     As described above, the charging-control unit  31  can control the switching operation and the supply of power. Accordingly, for example, in a case where an abnormality occurs in the switching element Q 1  due to the performance of an operation that consumes a comparatively large power, such as the operation of moving a lens, when the occurrence of the abnormality causes a short circuit in the switching element Q 1 , a time in which an abnormal current is flowing through the switching element Q 1  can be decreased. As a result, the generated heat amount can be reduced. 
     Next,  FIG. 7  is a time chart of an example of the charging-control operation when an abnormal current is flowing due to an abnormality occurring in the switching element. 
     Referring to  FIG. 7 , at timing T 51 , the charging-control unit  31  starts the supply of power to the step-up transformer  23 . Then, at timing T 52  when the predetermined time t d1  passes from the timing T 51 , the charging-control unit  31  starts the switching operation of the switching element Q 1  to start charging the main capacitor C 1 . Suppose that the switching element Q 1  is broken and a short circuit occurs in the switching element Q 1  at timing T 53 . In this case, a current not so large as to cause blowing of the fuse element  22  continues flowing. However, because the switching operation is not performed, the main capacitor C 1  is not charged. As a result, the fully charged state is not to be detected. 
     In order to deal with such a case, the charging-control unit  31  includes a counter that counts time. When the charging-control unit  31  starts the switching operation at the timing T 52 , the charging-control unit  31  starts a counting operation. The charging-control unit  31  monitors whether the fully charged state is detected, on the basis of the full-charge-detection signal S 1  until a predetermined maximum waiting time t max  is reached. 
     When the fully charged state is detected before the maximum waiting time t max  passes, it is determined that the switching element Q 1  does not have a particular abnormality. Accordingly, as shown in  FIGS. 5 and 6 , the charging-control unit  31  stops the switching operation, and then stops the supply of power to the step-up transformer  23 . In contrast, as shown at timing T 54  in  FIG. 7 , when the fully charged state is not detected before the maximum waiting time t max  passes, it is determined that an abnormality occurs in the switching element Q 1 . Accordingly, the charging-control unit  31  stops the switching operation, and also stops the supply of power at timing T 55  when the predetermined time t d2  passes from the timing T 54 . 
     As described above, the charging-control unit  31  can controls the switching operation and the supply of power using the maximum waiting time t max . Accordingly, the charging-control unit  31  can estimate that a current not so large as to cause blowing of the fuse element  22  is flowing because the switching element Q 1  is broken and a short circuit occurs in the switching element Q 1 . Thus, the charging-control unit  31  can stop the supply of power to the switching element Q 1 , thereby preventing the switching element Q 1  from generating heat. Additionally, even when the rated current of the fuse element  22  is increased in order to reduce the charging time, the above-described occurrence of the abnormal current can be detected, thereby reliably preventing the switching element Q 1  from generating heat. Thus, a strobe-charging circuit having a short charging time and a high safety can be realized. Furthermore, when the rated current of the fuse element  22  is determined, it is not necessary to take the above-described abnormal current into consideration. Thus, a selection range of the fuse element  22  is extended, and an existing fuse such as the fuse element  22  can be used to reduce the manufacturing cost. 
     The charging-control unit  31  performs the operation of counting time until the maximum waiting time t max  is reached at least during a period when power is being supplied to the switching element Q 1  from when the switching operation is started (at timing T 52 ). For example, in a case where the counting operation is performed from timing T 12  in  FIG. 5  as a starting point, the supply of power to the switching element Q 1  is continued even in the period when the switching operation is temporarily stopped (the period from timing T 13  to timing T 14  and the period from timing T 15  to timing T 16 ). Accordingly, the charging-control unit  31  continues the counting operation. 
     In contrast, in a case where the counting operation is performed from timing T 32  as a starting point shown in  FIG. 6 , a period when the switching operation is temporarily stopped (a period from timing T 33  to timing T 36  and a period from timing T 37  to timing T 40 ) includes the period when the supply of power to the switching element Q 1  is interrupted (a period from timing T 34  to timing T 35  and a period from timing T 38  to timing T 39 ). In the period when the supply of power is interrupted, the counting operation may be temporarily stopped. Such a temporary stop can prevent the switching element Q 1  from generating heat. 
     Needless to say, as timing when the counting operation is started, timing when the supply of power is started (for example, timing T 51  shown in  FIG. 7 ) can be used instead of timing when the switching operation is started (for example, timing T 52  shown in  FIG. 7 ). 
     Next,  FIG. 8  is a time chart of an example of the charging-control operation when noise is superimposed. 
     In the example shown in  FIG. 8 , the charging operation is started at timing T 61 . At timing T 62 , the fully charged state is detected, and at timing T 63  immediately after timing T 62 , the supply of power to the step-up transformer  23  is stopped. Then, at timing T 64 , the strobe-light-flashing signal S 2  is output to perform the strobe-light-flashing operation of the strobe-light-flashing unit. 
     Suppose that the strobe-light-flashing operation leads to noise superimposed on the oscillation-start/end-control signal S 3 , which is output from the charging-control unit  31  to the oscillation-control unit  32 . In this case, there is probability that the oscillation-control unit  32  starts the switching operation of the switching element Q 1  in response to the noise. 
     However, since the charging-control unit  31  controls the charging operation as described above, the supply of power from the direct-current power supply  21  is stopped when the main capacitor C 1  is in the fully charged state. Accordingly, even when the switching operation is accidentally started, no current flows through the switching element Q 1 . Thus, for example, because a risk that the switching element Q 1  is broken by the incorrect switching operation or a risk that the main capacitor C 1  reaches an overcharged state can be prevented, the safety in a case where noise occurs can be improved. Additionally, the design flexibility of a path of the oscillation-start/end-control signal S 3  in a circuit can be enhanced. 
     In the above-described embodiments, the direct-current power supply  21  is not limited to a secondary battery. For example, the direct-current power supply  21  may receive a voltage supplied from a commercial-use alternating-current power supply through an alternating current/direct current (AC/DC) converter. The direct-current power supply  21  can use the combination of a secondary battery and a voltage supplied from a commercial-use alternating-current power supply through an AC/DC converter. Furthermore, when a plurality of power supplies are used, the power-supply-interrupting circuit  24  and the fuse element  22  are connected in series between a junction point of power-supply lines from the power supplies and the step-up transformer  23 . 
     In the foregoing embodiments, the strobe device and the imaging apparatus are configured as one unit. However, the strobe device and the imaging apparatus are configured as individual apparatuses. In such a case, for example, the strobe-charging circuit and control functions thereof (realized by the charging-control unit  31 , the oscillation-control unit  32 , the full-charge-detection unit  33 , and the power-supply-control unit  34 ) may be mounted in the strobe device, and the strobe device may receive a signal related to an imaging operation, such as the strobe-light-flashing signal from the imaging apparatus. Alternatively, a part of the control functions (for example, realized by the charging-control unit  31 , the power-supply-control unit  34 , and the like) may be included in the imaging apparatus. 
     Additionally, when a circuit board that the power supply is directly connected to and a circuit board that the strobe-light-flashing unit is mounted on are separated, these circuit boards may be connected to each other through cables and connection terminals. In such a case, for example, the power-supply-interrupting circuit  24  and the fuse element  22  may be mounted on the circuit board that the power supply is directly connected to or on the circuit board that the strobe-light-flashing unit is mounted on. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.