Abstract:
A converter for use in a distributed power system for stepping up or down a voltage of a power source connected thereto includes a step up/down circuit that receives the voltage, steps up or steps down the voltage from the power source, and then outputs the stepped up/down voltage; and a control circuit that detects the voltage from the power source and transmits a control signal for stepping up or stepping down the voltage to the step up/down circuit. The control circuit transmits an identification signal that identifies the converter and is configured to receive an identification signal from another converter when the other converter is connected to the converter.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a converter for a distributed power source such as a photovoltaic panel, a fuel cell, or the like. 
       BACKGROUND ART 
       [0002]    With the spread of distributed power sources such as photovoltaic cells (PVs), fuel cells, and so on, various methods for obtaining optimal power from such power sources are being developed. 
         [0003]    In the case where PVs are used as a distributed power source, there is a method that provides a DC/DC converter that carries out MPPT (Maximum Power Point Tracking) control in each PV in order to obtain a maximum amount of power from the PVs. 
         [0004]    Japanese Translation of PCT International Application Publication No. 2010-521720, for example, discloses a DC/DC converter that carries out MPPT control being provided with a communication function, and information obtained from current and voltage sensors and the like is sent to a central analysis station, where the information is monitored, controlled, and analyzed. 
         [0005]    Furthermore, Japanese Patent Application Laid-Open Publication No. 2012-004626, for example, discloses transmitters provided in each of a plurality of PVs transmitting power generation information to a communication parent device over a power line in order to monitor power generated by the PVs. 
         [0006]    In this manner, the concept of providing a communication device in each PV has been explored, but such a configuration also means separately providing a host apparatus for the plurality of PVs, which increases the cost of the overall system by the amount of the host apparatus. Furthermore, although the host apparatus is described as carrying out control, the stated documents do not describe exactly how that control is carried out. 
         [0007]    Meanwhile, there are cases where a plurality of strings, in each of which a plurality of PVs are connected in series, are connected in parallel, and are further connected to a PCS (Power Conditioning System) and used. A PCS converts a DC (Direct Current) output from the PVs into a household-use AC (Alternating Current) output, and output voltages of the respective strings connected in parallel are adjusted to the same value. Here, in the case where the number of PVs connected in series differs from string to string, the output voltage is adjusted for each PV in each string. Such adjustment is thus carried out not only when a photovoltaic power generating system is installed, but also in a case such as where a PV panel or panels have partially failed, and there are no systems that carry out the adjustment autonomously. 
       RELATED ART DOCUMENTS 
     Patent Documents 
       [0008]    Patent Document 1: Japanese Translation of PCT International Application Publication No. 2010-521720 
         [0009]    Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2012-004626 
       SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
       [0010]    Accordingly, it is an object of the present invention to provide a converter that steps up or steps down a voltage of a distributed power source capable of autonomously adjusting an output voltage based on a connection state of the distributed power source. 
       Means for Solving the Problems 
       [0011]    According to a first aspect of the present invention, a converter for stepping up or down a voltage of a distributed power source includes: a step up/down circuit that receives the voltage, steps up or steps down the voltage, and then outputs the stepped up/down voltage; and a control circuit that detects the voltage and transmits a control signal for stepping up or stepping down the voltage to the step up/down circuit, wherein the control circuit transmits and receives an identification signal that identifies the converter. 
         [0012]    Through this, the number of converters connected in series can be specified and an output voltage can be stepped up or stepped down autonomously using a simple configuration. 
         [0013]    Meanwhile, the identification signal is a random number or a pre-set identifier. This makes it possible to eliminate the burden of an installer of the system assigning unique identifiers to each converter during the installation process. 
         [0014]    Furthermore, a photovoltaic power generating system has strings connected in parallel, each of the strings having a plurality of converters, each converter stepping up or stepping down respective voltages generated by the plurality of photovoltaic cells, wherein each of the plurality of converters has a step up/down circuit that receives the voltage from the corresponding photovoltaic cell, steps up or steps down the voltage, and then outputs the stepped up/down voltage to a load, and a control circuit that detects the voltage and transmits a control signal for stepping up or stepping down the voltage to the step up/down circuit, and wherein the control circuit generates an identifier that identifies the converter so as to detect the number of the plurality of converters connected. Through this, the photovoltaic power generating system can be installed easily. 
         [0015]    Furthermore, the plurality of converters transmit and receive the identifiers with each other. This makes it possible to eliminate the burden of an installer of the system assigning unique identifiers to each converter during the installation process. 
         [0016]    Additionally, the control circuit specifies the connection number of converters by repeatedly transmitting and receiving the identifiers and controls the step up/down circuit in accordance with the detected connection number of converters. Through this, the output voltage can be adjusted autonomously. 
         [0017]    Moreover, the control circuit includes a memory that pre-stores prescribed voltages that are respectively set in accordance with the connection number of converters, reads out one of the prescribed voltages that corresponds to the detected connection number of converters from the memory, and steps up or steps down the voltage so that the stepped up/down voltage coincides with the read-out prescribed voltage. This ensures that control based on the specified number of converters is carried out appropriately. 
         [0018]    Furthermore, the control circuit performs control based on the voltage when the detected connection number of converters is less than a predetermined number, and carries out control so that an output voltage of the step up/down circuit is maintained close to a prescribed voltage when the detected connection number of converters coincides with the predetermined number and a predetermined condition indicating an output voltage of the step up/down circuit is stable is met. This enables efficient voltage output while holding the maximum power point voltage to the greatest extent possible. 
         [0019]    Additionally, the identifier is a random number or a pre-set value. The burden of an installer of the system assigning unique identifiers to each converter during the installation process can be eliminated. 
         [0020]    A photovoltaic system according to a second aspect of the present invention has strings connected in parallel, each of the strings having a plurality of converters, each converter stepping up or stepping down respective voltages generated by the plurality of photovoltaic cells, wherein each converter in each of the strings steps up or steps down the voltage and outputs the stepped up/down voltage in accordance with the connection number of the converters in the corresponding string. Through this, the photovoltaic power generating system can be installed easily. 
       Effects of the Invention 
       [0021]    According to one aspect, an output voltage can be autonomously adjusted based on a connection state of a distributed power source. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]      FIG. 1  is a diagram illustrating an overview of a photovoltaic power generating system according to Embodiment 1. 
           [0023]      FIG. 2  is a diagram illustrating an example of the configuration of a microconverter. 
           [0024]      FIG. 3  is a diagram illustrating an example of circuitry in a connection box. 
           [0025]      FIG. 4  is a diagram illustrating an overview of operations performed by the photovoltaic power generating system according to Embodiment 1. 
           [0026]      FIG. 5  is a diagram illustrating an overview of operations performed by the photovoltaic power generating system according to Embodiment 1. 
           [0027]      FIG. 6A  is a diagram illustrating a processing flow expressing details of processing performed by a microcontroller. 
           [0028]      FIG. 6B  is a diagram illustrating a processing flow of a control process executed by a microcontroller. 
           [0029]      FIG. 7  is a diagram illustrating a processing flow of an AD conversion process. 
           [0030]      FIG. 8  is a diagram illustrating a processing flow of a default mode determining process. 
           [0031]      FIG. 9  is a diagram illustrating a processing flow of a low-voltage period process. 
           [0032]      FIG. 10  is a diagram illustrating a processing flow of a previous value updating process. 
           [0033]      FIG. 11  is a diagram illustrating a processing flow of an operating mode selection process. 
           [0034]      FIG. 12  is a diagram illustrating a processing flow of a connection number-related process. 
           [0035]      FIG. 13  is a diagram illustrating a clamp voltage and an OVP voltage set based on a connection number. 
           [0036]      FIG. 14  is a diagram illustrating a processing flow of a ConvRatio adjustment process. 
           [0037]      FIG. 15  is a diagram illustrating a processing flow of an adjustment process. 
           [0038]      FIG. 16  is a diagram illustrating a processing flow of a connection number determining process. 
           [0039]      FIG. 17  is a diagram illustrating an overview of a photovoltaic power generating system according to Embodiment 2. 
           [0040]      FIG. 18  is a diagram illustrating an overview of operations performed by the photovoltaic power generating system according to Embodiment 2. 
           [0041]      FIG. 19  is a diagram illustrating an overview of operations performed by the photovoltaic power generating system according to Embodiment 2. 
           [0042]      FIG. 20  is a diagram illustrating an overview of a photovoltaic power generating system according to Embodiment 3. 
           [0043]      FIG. 21  is a diagram illustrating the configuration of a connection box according to Embodiment 3. 
           [0044]      FIG. 22  is a diagram illustrating an overview of operations performed by the photovoltaic power generating system according to Embodiment 3. 
           [0045]      FIG. 23  is a diagram illustrating an overview of operations performed by the photovoltaic power generating system according to Embodiment 3. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Embodiment 1 
       [0046]      FIG. 1  illustrates an overview of a photovoltaic power generating system serving as an example of a distributed power source system according to the present embodiment. 
         [0047]    In the present embodiment, a reference string in which three PVs  3  to  5  are connected in series and a second string in which two PVs  1  and  2  are connected in series are connected in parallel to a connection box  200 . The connection box  200  is connected to a PCS  300 . 
         [0048]    Each of the two PVs  1  and  2  in the second string is connected to a microconverter  100  that executes primary operations according to the present embodiment. The microconverter  100  has a power line communication function, and communicates with other microconverters  100  connected in series. The microconverter  100  steps up and steps down an output voltage of a corresponding PV based on the output voltage and an output current of the PV and a number of PVs or microconverters  100  connected in series within the same string, as will be described later. 
         [0049]    The PCS  300  according to the present embodiment does not differ from conventional technology and will therefore not be described further here. 
         [0050]      FIG. 2  illustrates an example of the configuration of the microconverter  100 . 
         [0051]    The microconverter  100  according to the present embodiment includes an input voltage detecting circuit connected to the PV, an input current detecting circuit, an H bridge circuit, an output current detecting circuit, an output voltage detecting circuit, an H bridge circuit driving circuit, a microcontroller MCU, a front-end circuit  120 , and a power line communication coupling circuit  110 . 
         [0052]    The input voltage detecting circuit has resistances R 2  and R 3 , and a signal corresponding to an input voltage is inputted into a number 3 pin of the microcontroller MCU. 
         [0053]    The input current detecting circuit has a resistance R 1  and an operational amplifier CA 1 , and a signal corresponding to an input current is inputted into a number 2 pin of the microcontroller MCU. 
         [0054]    The H bridge circuit has four N-channel FETs (Field Effect Transistor; a switching element) (Q 1  to Q 4 ), an inductor L 1  for storing energy, and two capacitors Cin and Cout. 
         [0055]    The output current detecting circuit has a resistance R 4  and an operational amplifier CA 2 , and a signal corresponding to an output current is inputted into a number 5 pin of the microcontroller MCU. 
         [0056]    The output voltage detecting circuit has resistances R 5  and R 6 , and a signal corresponding to an output voltage is inputted into a number 4 pin of the microcontroller MCU. 
         [0057]    The H bridge driving circuit has a FET driver IC (Integrated Circuit, FET-Driver) for driving the FETs, two capacitors CB 1  and CB 2  for bootstrapping, and two diodes DB 1  and DB 2  for bootstrapping. The FET driver IC for driving the FETs is, for example, a 14-pin IC, having four PWM (Pulse Width Modulation) inputs, two PWM high-side outputs, two PWM low-side outputs, and two high-side midpoint potentials (HS 1  and HS 2 ). 
         [0058]    A number 1 pin of the FET driver IC for driving the FETs is connected to a cathode of the diode DB 1  and one end of the capacitor CB 1 , and a voltage Vcc is applied to an anode of the diode DB 1 . The voltage Vcc is applied to a number 2 pin of the FET driver IC for driving the FETs. Number 3 to number 6 pins of the IC for driving the FETs are connected to the microcontroller MCU. A number 7 pin of the FET driver IC for driving the FETs is grounded. A number 8 pin of the FET driver IC for driving the FETs is connected to a gate of the FET (Q 3 ). A number 9 pin of the FET driver for driving the FETs is connected to one end of the capacitor CB 2  and another end of the inductor L 1 . A number 10 pin of the FET driver IC for driving the FETs is connected to a gate of the FET (Q 4 ). A number 11 pin of the FET driver IC for driving the FETs is connected to another end of the capacitor CB 2  and a cathode of the diode DB 2 . A number 12 pin of the FET driver IC for driving the FETs is connected to a gate of the FET (Q 2 ). 
         [0059]    A number 13 pin of the FET driver IC for driving the FETs is connected to another end of the capacitor CB 1  and one end of the inductor L 1 . A number 14 pin of the FET driver IC for driving the FETs is connected to a gate of the FET (Q 1 ). 
         [0060]    The microcontroller MCU is a CPU (Central Processing Unit) that has a RAM (Random Access Memory) and a ROM (Read Only Memory). 
         [0061]    The voltage Vcc is applied to a number  1  pin of the microcontroller MCU, and the number 2 pin is connected to an output of the input current detecting circuit. The number 3 pin of the microcontroller MCU is connected to an output of the input voltage detecting circuit, and a number 8 pin is grounded. The number 4 pin of the microcontroller MCU is connected to an output of the output voltage detecting circuit, and the number 5 pin is connected to an output of the output current detecting circuit. Furthermore, a number 9 pin of the microcontroller MCU is connected to a number 6 pin of the FET driver IC for driving the FETs, and an on-off driving signal (PWM 2 L) of the FET (Q 4 ) is outputted to this number 6 pin. 
         [0062]    A number 10 pin of the microcontroller MCU is connected to a number 5 pin of the FET driver IC for driving the FETs, and an on-off driving signal (PWM 2 H) of the FET (Q 3 ) is outputted to this number 5 pin. A number 11 pin of the microcontroller MCU is connected to a number 4 pin of the FET driver IC for driving the FETs, and an on-off driving signal (PWM 1 L) of the FET (Q 2 ) is outputted to this number 4 pin. A number 12 pin of the microcontroller MCU is connected to a number 3 pin of the FET driver IC for driving the FETs, and an on-off driving signal (PWM 1 H) of the FET (Q 1 ) is outputted to this number 3 pin. 
         [0063]    The power line communication coupling circuit  110  has a coupling capacitor Ccp and a coupling transformer Tcp. The power line communication coupling circuit  110  superimposes a signal used in power line communication from the front-end circuit  120  onto an output of the H bridge circuit, and conversely outputs, to the front-end circuit  120 , a signal outputted from another microconverter  100  and superimposed onto the output of the H bridge circuit. 
         [0064]    The front-end circuit  120  has a power line communication output circuit and a power line communication input circuit. The power line communication output circuit is connected to a number 7 pin of the microcontroller MCU, and has an SPI (Serial Peripheral Interface) circuit, a D/A converting circuit (DAC), a filter (Filter), a PGA (Programmable Gain Amplifier) circuit, and a capacitor C 1 . The power line communication input circuit is connected to a number 6 pin of the microcontroller MCU, and has a capacitor C 2 , a filter (Filter) and a buffer (Buffer) circuit. 
         [0065]    The power line communication coupling circuit  110  and the front-end circuit  120  are circuits normally used in the case where power line communication is carried out, and thus no further descriptions thereof will be given. 
         [0066]      FIG. 3  illustrates a connection relationship between the connection box  200  and the reference string, and between the connection box  200  and the second string. The connection box  200  includes a DC breaker  210  that is connected to the PCS  300  and is a main switch, diodes D 1  and D 2 , and a DC breaker  220 . 
         [0067]    In this manner, an output from the reference string and an output from the second string are both connected to the PCS  300  via the DC breaker  220 , the diode D 1  or D 2 , and the DC breaker  210 . 
         [0068]    Next, an overview of operations of the microconverter  100  according to the present embodiment will be given using  FIG. 4  and  FIG. 5 . 
         [0069]    In the present embodiment, as illustrated in  FIG. 4 , a current does not flow to the PCS  300  prior to the PCS  300  being started, and thus even if power generation using the PVs is started, the PCS  300  stands by with the reference string having three open voltages of 110V in series for a total output of 330V (less than 400V) and the second string having two output voltages of 125V in series for a total of 250V (greater than a maximum power point voltage of 240V (=80V×3) in the case where there are three PVs in series). 
         [0070]    When the PCS  300  is started, a current flows to the PCS  300 , but the current initially flows from the reference string whose output voltage is higher during standby, and thus the voltage drops toward 240V in the series of three having the maximum power point voltage of 80V. When the voltage drops to 250V, a current begins to flow from the second string, and the voltage drops to 240V. When the output voltage of the reference string is 240V, the two microconverters  100  in the second string convert the output voltages of the respective PVs so as to attain a total of 240V. 
         [0071]    Here, when the current begins to flow, the microconverters  100  in the second string exchange signals expressing identifiers of the respective microconverters  100  with each other through power line communication ((1) and (2) in  FIG. 5 ) and automatically recognize that the number of the microconverters  100  connected in series is 2 in the second string, and the microconverters  100  then execute control for converting the input voltages from the PVs. 
         [0072]    Note that by providing the reference string, the voltage outputted to the PCS  300  takes on a constant value (240V in the example described above), and thus the microconverters  100  in the second string operate so that the output voltage of the second string also takes on this value, which stabilizes the overall operations of the photovoltaic power generating system. More specifically, the PVs in the second string are connected in series, and thus the same current flows in each microconverter  100 , and the PVs change the output current without changing the output voltage (80V, for example) based on the state of power generation. In such a case, if the output voltage in the reference string is set, a combination of a current value and a voltage value in the second string at which the maximum power is outputted is uniquely determined, and thus the operations of the microconverters  100  stabilize. 
         [0073]    Next, specific flows of operations performed by the microconverter  100  will be described using  FIG. 6A  to  FIG. 16 . Note that the microcontroller MCU of the microconverter  100  executes a program stored in the ROM and stores mid-processing data in the RAM. 
         [0074]    First, the microcontroller MCU executes an initializing process ( FIG. 6A : step S 1001 ). For example, a present conversion ratio ConvRatio for mode selection is set to  100 , a conversion ratio ConvRatioMp from one time previous is set to 100, and a maximum power value Pmpp from one time previous is set to 0. Other parameters are also initialized as appropriate. The microcontroller MCU then executes a control process (step S 1003 ). The control process will be described in detail later. The microcontroller MCU then determines whether to end the processing (step S 1005 ). The microcontroller MCU determines to end the processing if the supply of power is stopped, for example. The processing returns to step S 1003  in the case where the processing is not ended. However, the processing ends directly in the case where the processing is ended. 
         [0075]    Next, the control process will be described using  FIG. 6B  and so on. First, the microcontroller MCU executes an AD conversion process ( FIG. 6B : step S 1 ). This AD conversion process will be described using  FIG. 7 . 
         [0076]    First, the microcontroller MCU measures an input voltage Vin and an input current Iin (step S 31 ), and measures an output voltage Vout and an output current lout (step S 33 ). Note that the measurements may be carried out several times and an average value of those measurements may be employed. Once the measurements are complete, the processing returns to the original flow. 
         [0077]    Returning to the descriptions of the processing illustrated in  FIG. 6B , the microcontroller MCU executes a default mode determining process (step S 3 ). This default mode determining process will be described using  FIG. 8 . 
         [0078]    First, the microcontroller MCU determines whether an AD conversion value has been defined (step S 41 ). In other words, it is determined whether or not an input voltage value and an input current value, and an output voltage value and an output current value, have been obtained. The processing returns to the original flow in the case where these values have not been obtained. 
         [0079]    However, in the case where the input voltage value and the input current value, and the output voltage value and the output current value, have been obtained, the microcontroller MCU calculates an input power Pin from the input voltage value and the input current value and calculates an output power Pout from the output voltage value and the output current value (step S 43 ). 
         [0080]    Thereafter, the microcontroller MCU determines whether or not a present mode is an idle mode (step S 45 ). If the present mode is the idle mode, the microcontroller MCU determines whether or not conditions for transitioning to a start mode are met (step S 47 ). Specifically, it is determined whether or not a condition that the input voltage exceeds an MPPT operation start voltage (60V, for example), a condition that a temperature indicated by a temperature sensor is less than 70 degrees, a condition that an input power and an output power exceed a reference value (40 W, for example), and a condition that AD conversion has been successfully carried out a predetermined number of times (ten times, for example) are met. In the case where such conditions are not met, the processing returns to the original flow without a transition to the start mode. 
         [0081]    However, in the case where it is determined that such conditions have been met, the microcontroller MCU sets an operating mode to the start mode (step S 49 ). Furthermore, the microcontroller MCU starts a detection standby timer for an MPPT operation start current (step S 51 ). In other words, the microcontroller MCU starts measuring a predetermined amount of time it is estimated to take until the PCS  300  begins operating. Approximately ten minutes are measured, for example. 
         [0082]    Furthermore, the microcontroller MCU sets the conversion ratio ConvRatio to 100 (step S 53 ). The processing then returns to the original flow. 
         [0083]    Meanwhile, if the operating mode is not the idle mode, the microcontroller MCU determines whether or not a condition for transitioning to a stopped mode is met (step S 55 ). Specifically, a condition that the input voltage is less than an MPPT stop-start voltage (30V, for example), a condition that a temperature indicated by the temperature sensor exceeds 90 degrees, a condition that the input power and the output power are less than 20 W, or a condition that the AD conversion has encountered an error a predetermined number of times (ten times, for example) is met. 
         [0084]    In the case where such a condition is met, the microcontroller MCU sets the operating mode to the stopped mode (step S 57 ). The microcontroller MCU then sets the MPPT operation to off (step S 59 ). The processing then returns to the original flow. However, the processing also returns to the original flow in the case where a condition for transitioning to the stopped mode is not met. 
         [0085]    In this manner, the processing results in one of a case of transitioning to the start mode, a case of continuing the start mode, a case of transitioning to the stopped mode, a case of continuing the stopped mode, and a case of carrying out another mode. 
         [0086]    Generally speaking, the stopped mode is set immediately in the default mode determining process. The start mode is transitioned to thereafter. Then, the mode is changed in processing that will be described below. 
         [0087]    Returning to the descriptions of  FIG. 6B , in the case where the mode is a first start mode or the stopped mode (step S 5 : Yes route), the present control process is ended and the processing returns to the original flow. 
         [0088]    However, in the case where the mode is not the first start mode (continues as the start mode) or in the case of a mode aside from the stopped mode (a pass-through mode, a step up mode, or a step down mode) (step S 5 : No route), the microcontroller MCU determines whether or not the input voltage is greater than or equal to a low-voltage determination value (50V, for example) (step S 7 ). 
         [0089]    In the case where these conditions are not met, the microcontroller MCU executes a low-voltage period process (step S 9 ). This is a process carried out when the PV output voltage has dropped, for example. The low-voltage period process will be described using  FIG. 9 . 
         [0090]    First, the controller MCU executes a previous value updating process ( FIG. 9 : step S 61 ). The previous value updating process will be described using  FIG. 10 . 
         [0091]    First, the microcontroller MCU sets the present conversion ratio ConvRatio to ConvRaioMp ( FIG. 10 : step S 71 ). In addition, the microcontroller MCU sets the present input power Pin to the Pmpp one previous (step S 73 ). The microcontroller MCU then sets the output power Pout to an output power Pmppo one previous. The processing then returns to the original process. 
         [0092]    Returning to the descriptions of  FIG. 9 , the microcontroller MCU sets the MPPT operation to on (step S 63 ). This is to handle the case where the MPPT operation is set to off in step S 25  and so on, for example. 
         [0093]    Furthermore, the microcontroller MCU sets the ConvRatio to a predetermined protective conversion ratio (20, for example) used during an input low voltage (step S 65 ). Furthermore, the microcontroller MCU executes an operating mode selection process based on the ConvRatio set in step S 65  (step S 67 ). Note that after this process is executed, the processing returns to the original flow. This operating mode selection process will be described using  FIG. 11 . 
         [0094]    First, the microcontroller MCU determines a range into which the ConvRatio falls (step S 81 ). Specifically, the microcontroller MCU determines whether the ConvRatio=100, and when such is the case, sets the operating mode to the pass-through mode (step S 87 ). 
         [0095]    The microcontroller MCU then sets a PWM pulsewidth used in the pass-through mode (step S 89 ). Furthermore, the microcontroller MCU changes the present PWM pulsewidth to the set PWM pulsewidth (step S 95 ). In the case of the pass-through mode, the PWM pulsewidth is set so as to maintain the input voltage from the PV. Specific control of the PWM pulsewidth with respect to an H bridge circuit is generally known and thus descriptions thereof will be omitted here. 
         [0096]    In addition, the microcontroller MCU determines whether the ConvRatio exceeds 100, and when such is the case, sets the operating mode to the step up mode (step S 91 ). 
         [0097]    The microcontroller MCU then sets a PWM pulsewidth used in the step up mode (step S 93 ). Furthermore, the microcontroller MCU changes the present PWM pulsewidth to the set PWM pulsewidth (step S 95 ). In the case of the step up mode, the PWM pulsewidth is set so as to attain an output voltage equivalent to the input voltage from the PV×(ConvRatio−100)/ConvRatio, for example. 
         [0098]    In addition, the microcontroller MCU determines whether the ConvRatio is less than 100, and when such is the case, sets the operating mode to the step down mode (step S 83 ). 
         [0099]    The microcontroller MCU then sets a PWM pulsewidth used in the step down mode (step S 85 ). Furthermore, the microcontroller MCU changes the present PWM pulsewidth to the set PWM pulsewidth (step S 95 ). In the case of the step down mode, the PWM pulsewidth is set so as to attain an output voltage equivalent to the input voltage from the PV×ConvRatio/100, for example. 
         [0100]    In this manner, the ConvRatio is increased to exceed 100 when circumstances in other processes call for stepping up, and the ConvRatio is reduced to fall below 100 when circumstances in other processes call for stepping down. 
         [0101]    Returning to the descriptions of  FIG. 6B , when it is determined in step S 7  that the input voltage is greater than or equal to the low-voltage determination value, the microcontroller MCU determines whether or not the operating mode is the start mode (step S 11 ). If the operating mode is the start mode, the microcontroller MCU determines whether or not a soft start operation for starting PWM has been started (step S 12 ). In the case where this step is reached during the first start mode, it is determined that the stated operations have not been started. When such is the case, the microcontroller MCU starts the soft start operation (step S 13 ). In other words, a PWM pulsewidth corresponding to the soft start operation is set and outputted. The present control process then ends and the processing returns to the original flow. 
         [0102]    However, in the case where the soft start operation has already started, the microcontroller MCU determines whether or not the soft start operation has been completed (step S 14 ). If the soft start operation is not complete, the present control process is ended and the processing returns to the original flow. 
         [0103]    Meanwhile, in the case where the soft start operation has been completed, the microcontroller MCU sets the operating mode to the pass-through mode (step S 15 ). In the pass-through mode, the FETs (Q 1  to Q 4 ) for the H bridge circuit are caused to switch so that the input voltage from the PV is outputted without being stepped up or stepped down. The present control process then ends and the processing returns to the original flow. 
         [0104]    Meanwhile, in the case where the operating mode is not the start mode, the microcontroller MCU executes a connection number-related process (step S 11 ). The connection number-related process will be described using  FIG. 12 . 
         [0105]    First, the microcontroller MCU determines whether or not the MPPT operation is off ( FIG. 12 : step S 101 ). The processing returns to the original flow in the case where the MPPT operation is on. 
         [0106]    However, if the MPPT operation is off, the microcontroller MCU executes a previous value updating process (step S 103 ). This process is the same as the process of  FIG. 10 , and thus descriptions will be omitted here. 
         [0107]    The microcontroller MCU then determines whether or not the output current has exceeded a predetermined MPPT operation start current (250 mA, for example) (step S 105 ). A current flows toward the PCS  300  when the PCS  300  operates. However, no current flows when the PCS  300  is not operating. In other words, initially, the output current does not exceed the MPPT operation start current. 
         [0108]    In the case where the output current does not exceed the MPPT operation start current, the microcontroller MCU determines whether the detection standby timer for the MPPT operation start current has timed out (step S 119 ). The processing moves to step S 117  in the case where the detection standby timer has not timed out. 
         [0109]    However, in the case where the detection standby timer has timed out, the microcontroller MCU determines whether the output voltage is less than a standby clamp voltage (125V, for example) (step S 121 ). In the case where the output voltage is not less than the standby clamp voltage, the microcontroller MCU reduces the ConvRatio by a predetermined value (step S 125 ). The processing then moves to step S 117 . However, in the case where the output voltage is less than the standby clamp voltage, the microcontroller MCU increases the ConvRatio by a predetermined value (S 123 ). The processing then moves to step S 117 . In other words, the microcontroller MCU operates so as to hold the output voltage at 125V, as described using  FIG. 4 . 
         [0110]    Meanwhile, in the case where the output current has exceeded the MPPT operation start current, the microcontroller MCU determines whether or not the output current has exceeded the MPPT operation start current for the first time (step S 107 ). In the case where the output current has exceeded the MPPT operation start current for the first time, the microcontroller MCU starts the connection number determining process (step S 109 ). Here, communication is carried out among the microconverters  100  connected in series in the second string in order to autonomously identify the number of microconverters (=the number of PVs) connected in series. This connection number determining process will be described later. The processing then moves to step S 111 . 
         [0111]    In the case where the output current has already exceeded the MPPT operation start current, the microcontroller MCU determines whether the connection number determining has been completed (step S 111 ). The connection number determining process takes a certain amount of time (approximately 10 seconds, for example), and thus the processing moves to step S 119  if the connection number determining is not complete. In other words, the processing enters a standby state until the connection number determining is complete. 
         [0112]    Meanwhile, if the connection number determining is complete, the microcontroller MCU sets the MPPT operation to on (step S 113 ). The microcontroller MCU then selects and sets a clamp voltage and an OVP (Over Voltage Protection) voltage based on the number of connections (step S 115 ). For example, a distinction is made between connection numbers of “2” and “3” in the present embodiment, and thus, as indicated in  FIG. 13 , the clamp voltage and OVP voltage for the case where the connection number (that it, the number of PVs in series) is “2” and the clamp voltage and OVP voltage for the case where the connection number is “3” are stored in the ROM in advance; one of these sets of values is read out and set. 
         [0113]    Furthermore, the microcontroller MCU executes an operating mode selection process (step S 117 ). This process is the process illustrated in  FIG. 11 . The processing then returns to the original flow. 
         [0114]    In this manner, standby processing is carried out until communication among the microconverters  100  starts and the connection number is defined, and settings are made based on the connection number once the connection number is defined. 
         [0115]    Returning to the descriptions of  FIG. 6B , the microcontroller MCU determines whether or not the MPPT operation is on (step S 19 ). The present processing ends if the MPPT operation is off. 
         [0116]    However, if the MPPT operation is on, the microcontroller MCU executes a ConvRatio adjusting process (step S 21 ). The ConvRatio adjusting process will be described using  FIG. 14 . 
         [0117]    First, the microcontroller MCU determines whether or not the output voltage is less than the clamp voltage that is set (step S 131 ). In the case where the output voltage is greater than or equal to the clamp voltage, the microcontroller MCU executes a previous value updating process (step S 133 ). This process is the process of  FIG. 10 . The microcontroller MCU then reduces the ConvRatio by a predetermined value (step S 135 ). Thereafter, the microcontroller MCU executes an operating mode selection process based on this ConvRatio (step S 137 ). The operating mode selection process is the process of  FIG. 11 . The processing then returns to the original flow. 
         [0118]    Meanwhile, if the output voltage is less than the clamp voltage, the microcontroller MCU determines whether or not it is presently a timing for carrying out MPPT control (step S 139 ). In the present embodiment, the PWM pulsewidth is changed at predetermined intervals (that is, each MPPT control period), and thus it determined whether or not a timing for changing the PWM pulsewidth has been reached. The processing returns to the original flow in the case where it is not presently a timing for the change. 
         [0119]    However, if the timing for carrying out MPPT control is reached, the microcontroller MCU starts measuring the MPPT control period (step S 141 ). The microcontroller MCU then determines whether or not three microconverters  100  are connected in series based on the connection number of the microconverters in the connected string (step S 143 ). 
         [0120]    In the case where three are not connected in series, or in other words, if there are two or fewer in the series, the processing moves to step S 145 . 
         [0121]    However, if there are three microconverters  100  connected in series in the string to which the microcontroller MCU is connected, the microcontroller MCU determines whether or not an output power change amount is less than a determination value (step S 147 ). The output power change amount is a difference between the present output power Pout and the output power Pmppo one previous, and it is determined whether or not this amount is less than 1 W, for example. In the case where this condition is met, the operations are stable, and thus the processing moves to step S 149 . However, in the case where this condition is not met, the operations are unstable, and thus the processing moves to step S 145 . 
         [0122]    Furthermore, if the output power change amount is less than the determination value, the microcontroller MCU determines whether or not the output voltage is greater than or equal to the maximum power point voltage+α (2V, for example) (step S 149 ). In the case where the operations are stable, this condition is met, and thus the processing moves to step S 151 . However, in the case where this condition is not met, the processing moves to step S 145 . 
         [0123]    When it is determined that the condition is not met in any one of step S 143 , step S 147 , and step S 149 , the microcontroller MCU executes an adjusting process (step S 145 ). When the adjusting process ends, the processing moves to step S 155 . The adjusting process will be described using  FIG. 15 . 
         [0124]    First, the microcontroller MCU measures the input voltage and the input current (step S 161 ). The microcontroller MCU then calculates the input power Pin as the input voltage x the input current (step S 163 ). 
         [0125]    Then, the microcontroller MCU determines whether or not the input power Pin&lt;the input power Pmpp one previous (step S 165 ). In the case where this condition is met, the microcontroller MCU determines whether or not ConvRatio&lt;ConvRatioMp (step S 167 ). In the case where this condition is met, the processing moves to step S 177 . However, in the case where this condition is not met, the processing moves to step S 169 . 
         [0126]    In the case where the condition Pin&lt;Pmpp is not met, the microcontroller MCU determines whether or not ConvRatio&lt;ConvRatioMp (step S 175 ). In the case where this condition is met, the processing moves to step S 169 . However, in the case where this condition is not met, the processing moves to step S 177 . 
         [0127]    In step S 169 , the microcontroller MCU sets the present ConvRatio to the ConvRatioMp one previous. Furthermore, the microcontroller MCU sets the present Pin to the Pmpp one previous (step S 171 ). The present Pout is also set to the Pmppo one previous. The microcontroller MCU then reduces the ConvRatio by a predetermined value (step S 173 ). The processing then returns to the original flow. 
         [0128]    Meanwhile, in step S 177 , the microcontroller MCU sets the present ConvRatio to the ConvRatioMp one previous. Furthermore, the microcontroller MCU sets the present Pin to the Pmpp one previous (step S 179 ). The present Pout is also set to the Pmppo one previous. The microcontroller MCU then increases the ConvRatio by a predetermined value (step S 181 ). The processing then returns to the original flow. 
         [0129]    By executing such processing, the ConvRatio is increased in the case where the input power is decreasing and the ConvRatio has decreased or in the case where the input power is increasing and the ConvRatio is increasing. On the other hand, the ConvRatio is reduced in the case where the input power is decreasing and the ConvRatio has increased or in the case where the input power is increasing and the ConvRatio is decreasing. 
         [0130]    Returning to the descriptions of  FIG. 14 , in the case where the conditions of step S 143 , step S 147 , and step S 149  have been met, the operations are considered stable and the adjusting process is therefore not carried out, and the microcontroller MCU executes a previous value updating process (step S 151 ). This process is the process of  FIG. 10 . The microcontroller MCU then reduces the ConvRatio by a predetermined value (step S 153 ). 
         [0131]    Thereafter, the microcontroller MCU executes an operating mode selection process (step S 155 ). The operating mode selection process is the process of  FIG. 11 . The processing then returns to the original flow. 
         [0132]    Returning to the descriptions of  FIG. 6B , when step S 21  is executed, the microcontroller MCU determines whether or not the input current is less than an MPPT operation stop current (step S 23 ). In the case where this condition is not met, the present processing ends. However, in the case where such a condition is met, the input current from the PV is decreasing, and thus the microcontroller MCU sets the MPPT operation to off (step S 25 ). The present processing then ends. Even if step S 25  has been executed, the connection number value continues to be held assuming that the connection number determining has been completed. 
         [0133]    By executing processing such as that described above, control is executed based on the connection number of the microconverters  100  connected in each string, and the PWM pulsewidth is set so that the appropriate output voltage control is carried out. 
         [0134]    The connection number determining process will be described next using  FIG. 16 . 
         [0135]    First, the microcontroller MCU obtains an identifier generated by that microcontroller MCU itself (step S 201 ). For example, if a unique identifier is already recorded in the ROM, that identifier is read out. However, if a unique identifier is not recorded in the ROM, the identifier is generated using a random number, for example, and is written into the RAM. 
         [0136]    Then, the microcontroller MCU outputs the obtained identifier generated by that microcontroller MCU itself (step S 203 ). Then, the microcontroller MCU determines whether or not a signal superimposed on the output of the microconverter  100  has been received (step S 205 ). In the case where the signal is not received, the processing moves to step S 211 . However, in the case where the signal has been received, the microcontroller MCU extracts the identifier from the received signal and stores the identifier in the RAM (step S 207 ). Furthermore, the microcontroller MCU links the identifier the microcontroller MCU generated itself with the received identifier and outputs the identifiers (step S 209 ). The processing then moves to step S 211 . 
         [0137]    In step S 211 , the microcontroller MCU determines whether or not a predetermined amount of time has elapsed following step S 201 , for example. The processing returns to step S 205  in the case where the predetermined amount of time has not elapsed. There are cases where startup timings differ even within the same string, and thus the identifiers received in the predetermined amount of time are stored in this manner. 
         [0138]    In the case where the predetermined amount of time has elapsed, the microcontroller MCU counts, among the identifiers stored in the RAM, a number of types of identifiers aside from the identifier the microcontroller MCU itself generated (step S 213 ). For example, if an identifier “B” is received first and the identifier generated by the microcontroller MCU itself is “A”, signals “B”, “BAB”, “BABAB”, and so on are received and stored. Accordingly, it can be seen that there is one type of identifier aside from the identifier generated by the microcontroller MCU itself. Note that there are two types of identifiers in the case where signals “B”, “BACB”, “BACBACB”, and so on have been received. 
         [0139]    The microcontroller MCU then sets the number of types+1 as a series connection number for the string to which the microcontroller MCU is connected (step S 215 ). 
         [0140]    In this manner, the series connection number of the connected string is specified. Note that the microcontroller MCU may simply continue to output the identifier generated by the microcontroller MCU itself every predetermined interval. 
         [0141]    According to the present embodiment as described above, the series connection number can be specified automatically through communication rather than setting the series connection number in the microconverter  100  manually, and control can then be executed autonomously based on a result of specifying the series connection number. 
         [0142]    Note that the process illustrated in  FIG. 6B  runs when there is a power supply from the PV, and thus communication is carried out approximately once each day. In the case where a PV in the second string has failed, the series connection number decreases starting on the next day. In the case where three PVs are connected in series but the series has dropped substantially to two due to one of the PVs failing, for example, the series of two PVs is autonomously recognized the following day and the control is executed. 
       Embodiment 2 
       [0143]    The present embodiment describes a case where three PVs are connected in series in the second string, as illustrated in  FIG. 17 . In the example illustrated in  FIG. 17 , a PV  1 , a PV  2 , and a PV  6  are connected in series in the second string. The microconverter  100  connected to each PV is the same as the microconverter  100  illustrated in Embodiment 1. Furthermore, the number of strings is the same as in Embodiment 1, and thus the configuration of the connection box  200  is also the same. 
         [0144]    Next, an overview of operations of a photovoltaic power generating system according to the present embodiment will be given using  FIG. 18  and  FIG. 19 . 
         [0145]    In the present embodiment, as illustrated in  FIG. 18 , a current does not flow to the PCS  300  prior to the PCS  300  being started, and thus even if power generation using the PVs is started, the PCS  300  stands by with the reference string having three open voltages of 110V in series for a total output of 330V (less than 400V) and the second string having three output voltages of 125V in series for a total of 375V (greater than a maximum power point voltage of 240V (=80V×3) in the case where there are three PVs in series). 
         [0146]    When the PCS  300  is started, a current flows to the PCS  300 , but the current initially flows from the second string whose output voltage is higher during standby, and thus the voltage drops toward 240V in the three PVs in series having the maximum power point voltage of 80V. When the voltage in the second string drops to 330V, a current begins to flow from the reference string, and the input voltage of the PCS  300  drops to 240V. When the output voltage of the reference string is 240V, the three microconverters  100  in the second string convert the output voltages of the respective PVs so as to attain a total of 240V. 
         [0147]    Here, when the current begins to flow, the microconverters  100  in the second string exchange signals expressing identifiers of the respective microconverters  100  with each other through power line communication ((1) to (3) in  FIG. 19 ) and automatically recognize that the number of the microconverters  100  connected in series is 3 in the second string, and the microconverters  100  then switch control for converting the input voltages from the PVs. 
         [0148]    The role of the reference string is the same as in Embodiment 1. 
       Embodiment 3 
       [0149]    The present embodiment describes a case where three PVs are connected in series in the second string and two PVs are connected in series in a third string, as illustrated in  FIG. 20 . In the example illustrated in  FIG. 20 , a PV  6  to a PV  8  are connected in series in the second string. Furthermore, a PV  1  and a PV  2  are connected in series in the third string. Note that the microconverter  100  connected to each PV is the same as the microconverter  100  illustrated in Embodiment 1. 
         [0150]    Although the configuration of the connection box is similar to that illustrated in  FIG. 3 , the number of terminals in the switch  220  has been increased for the third string in addition to the reference string and the second string, and a diode D 3  has been added as well, as illustrated in  FIG. 21 . 
         [0151]    Next, an overview of operations of a photovoltaic power generating system according to the present embodiment will be given using  FIG. 22  and  FIG. 23 . 
         [0152]    In the present embodiment, as illustrated in  FIG. 22 , a current does not flow to the PCS  300  prior to the PCS  300  being started, and thus even if power generation using the PVs is started, the PCS  300  stands by with the reference string having three open voltages of 110V in series for a total output of 330V (less than 400V), the second string having three output voltages of 125V in series for a total of 375V (greater than a maximum power point voltage of 240V (=80V×3) in the case where there are three PVs in series), and the third string having two output voltages of 125V in series for a total of 250V. 
         [0153]    When the PCS  300  is started, a current flows to the PCS  300 , but the current initially flows from the second string whose output voltage is higher during standby, and thus the voltage drops toward 240V in the three PVs in series having the maximum power point voltage of 80V. 
         [0154]    When the output voltage in the second string drops to 330V, a current begins to flow from the reference string, and the input voltage to the PCS  300  drops to 240V. Likewise, when an output voltage in the third string drops to 250V, a current begins to flow from the third string, and the input voltage of the PCS  300  drops to 240V. 
         [0155]    When the output voltage of the reference string is 240V, the three microconverters  100  in the second string convert the output voltages of the respective PVs so as to attain a total of 240V, and the two microconverters  100  in the third string also convert the output voltages of the respective PVs so as to attain a total of 240V. 
         [0156]    Here, when the current begins to flow, the microconverters  100  in the second string exchange signals expressing identifiers of the respective microconverters  100  with each other through power line communication ((1) to (3) in  FIG. 23 ) and automatically recognize that the number of the microconverters  100  connected in series is 3 in the second string, and the microconverters  100  then execute control for converting the input voltages from the PVs. Likewise, when the current begins to flow, the microconverters  100  in the third string exchange signals expressing identifiers of the respective microconverters  100  with each other through power line communication ((4) and (5) in  FIG. 23 ) and automatically recognize that the number of the microconverters  100  connected in series is 2 in the third string, and the microconverters  100  then execute control for converting the input voltages from the PVs. 
         [0157]    The role of the reference string is the same as in Embodiment 1. 
         [0158]    Although the foregoing has described embodiments of the present invention, the present invention is not intended to be limited thereto. For example, the processing flows may be changed as long as the results of the processing are the same. Furthermore, the microcontroller MCU may be any type of microcontroller having processing capabilities sufficient to realize the functions described above. 
         [0159]    In addition, although the foregoing assumes PWM control being carries out, the circuitry may employ another control method instead. 
       DESCRIPTION OF REFERENCE CHARACTERS 
       [0160]      100  microconverter 
         [0161]      200  connection box 
         [0162]      300  PCS 
         [0163]      110  power line communication coupling circuit 
         [0164]      120  front-end circuit