Abstract:
A converter system adapted to be connected between a photovoltaic power source and a load comprises a converter circuit, a  control circuit, and a PWM generator circuit. The converter circuit is operatively connected to transfer energy from the photovoltaic power source to the load. The control circuit generates a raw control signal based on at least a voltage generated by the photovoltaic power source. The PWM generator circuit is operatively connected to the converter circuit and generates a PWM switch signal based on the raw control signal. The converter circuit transfers energy from the photovoltaic power source to the load based on the PWM switch signal.

Description:
RELATED APPLICATIONS 
       [0001]    This application (Attorneys&#39; Ref. No. P216086) claims benefit of U.S. Provisional Patent Application Ser. No. 61/062,187 filed Jan. 23, 2008. The subject matter of the foregoing related application is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present invention relates the generation of power using photovoltaics (PV) and, more specifically, to systems and methods for maximizing the power output from PV arrays. 
       BACKGROUND 
       [0003]    PV arrays have a general current source behavior with a maximum power point that varies with insolation levels and cell temperature. It is desirable to maximize the power output from photovoltaic (PV) arrays under varying operating conditions such as insolation, cell temperature, and cable length. Although a PV array may be used to charge a variety of different loads, the present invention is of particular significance in the context of a system for charging a battery, and the present invention will be discussed herein in the context of a PV array used to charge a battery. 
         [0004]    The need thus exists for a signal conversion system that can maximize the power obtained from a PV array and which can be used with a variety of different loads. 
       SUMMARY 
       [0005]    The present invention may be embodied as a DC-DC converter system implementing a Maximum Power Point Tracking (MPPT) algorithm that adjusts the PV array&#39;s V-I operating point to maximize the power output of the array at a specific operating condition. Using a DC-DC converter system having a MPPT control algorithm, this form of the present invention can be used to maximize the power output from photovoltaic (PV) arrays. 
         [0006]    The present invention may also be embodied as a DC-DC converter system or method that may be used in conjunction with a PV array. The PV array generates PV output power; the PV output power varies with factors such as insolation levels, temperature of the PV panels forming the PV array, and the lengths of cables carrying the PV output power. A DC-DC converter system implementing the principles of the present invention uses pulse-width modulation (PWM) to generate converter output power based on the PV output power generated by the PV array. 
         [0007]    Another example of a DC-DC converter system of the present invention optimizes generation of the converter output power based on the PV output power under operating conditions such as insolation, cell temperature, and cable length. In an example method implementing the principles of the present invention, the PWM output power signal generated by a PV array is applied to a converter operating based on a PWM signal. The pulse width of the PWM signal is adjusted based on a voltage level of the PV output power to maintain the specified panel voltage at the predetermined maximum power point level. This method of control allows the use of very inexpensive, highly available standard voltage control PWM integrated circuit (IC). 
         [0008]    The converter output power may be used to supply power to a variety of loads. As examples, the converter output power may be used directly as a power source for a device that operates on DC power, indirectly through an inverter as a source of AC power, or to charge an energy storage system such as a battery. The present invention will be described in detail below in the context of a system for charging a battery but may be applied to other loads. 
         [0009]    In a charging application, the output of the DC-DC converter system is normally connected to a battery. The battery voltage changes as the State of Charge (SOC) changes. An energy storage device such as a battery operates under a fairly wide range of voltages; for example, a 48 volt battery system may operate acceptably from 44 volts to 60 volts. However, the PV output power of a PV system may and frequently does drop to below 44 volts. The principles of the present invention are most relevant when used to transfer power from a PV system and a load such as a battery system. 
         [0010]    In the context of charging an energy storage system, the systems and methods of the present inventions also allow the specified panel voltage to be maintained under all conditions of battery SOC. When used in a charging application, a DC-DC converter system of the present invention may also be configured to satisfy secondary design goals such as battery overcharge protection and adapting a battery charging profile to maximize the lifetime of the battery. 
         [0011]    The converter output power of a DC-DC converter system of the present invention thus may be regulated based on the PV output power from the PV panel source and not the converter output power. The present invention thus extracts maximum power from the PV panel regardless of factors such as solar energy level, panel temperature, and cable length. In addition, a DC-DC converter system of the present invention employs direct feed forward control based on the input voltage to the converter (PV output power voltage), yielding fast response to maintain the PV output power at the predetermined maximum power point level. Further, by ignoring the effects of the load, the systems and methods of the present invention eliminates complications that arise from reacting to load transients and attempting to sense the load voltage/current in a noisy environment. 
         [0012]    The PV output power voltage will further vary based on the panel operating temperature, interconnecting impedance of power cable, and connector characteristics. Regulation of this voltage can be implemented using a relatively inexpensive microprocessor and/or a small analog control circuit at a relatively low speed loop and using heavy average current sensing to analyze the direction of change of the PV ouput power voltage level to achieve the maximum power. 
         [0013]    Further, the systems and methods of the present invention can be configured to employ energy storage components that tend to damp load transients. Variations caused by ambient temperature, voltage drop across the power cable at different load levels, and/or changes in the solar energy level are very slow and can be heavily filtered to prevent the false responses due to noisy environment. 
         [0014]    When configured to use input feed forward control of the panel voltage, the present invention can simplify the control of the PV output power signal of the PV system to achieve maximum power or minimum power with fastest response, inexpensive and simple control, and high level of immunity to noisy environment. 
         [0015]    The present invention may thus be embodied as a converter system adapted to be connected between a photovoltaic power source and a load, comprising a converter circuit, a control circuit, and a PWM generator circuit. The converter circuit is operatively connected to transfer energy from the photovoltaic power source to the load. The control circuit generates a raw control signal based on at least a voltage generated by the photovoltaic power source. The PWM generator circuit is operatively connected to the converter circuit and generates a PWM switch signal based on the raw control signal. The converter circuit transfers energy from the photovoltaic power source to the load based on the PWM switch signal. 
         [0016]    The present invention may also be configured as a method of connecting a photovoltaic power source to a load comprising the following steps. A converter circuit is arranged to transfer energy from the photovoltaic power source to the load. A raw control signal is generated based on at least a voltage generated by the photovoltaic power source. A PWM switch signal is generated based on the raw control signal. The converter circuit is operated based on the PWM switch signal to transfer energy from the photovoltaic power source to the load. 
         [0017]    The present invention may also be embodied as converter system adapted to be connected between a photovoltaic power source and a load comprising a converter circuit, a control circuit, an error amplifier, a reference generator circuit, and a PWM generator circuit. The converter circuit is operatively connected to transfer energy from the photovoltaic power source to the load. The control circuit generates a raw control signal based on at least a voltage generated by the photovoltaic power source. The reference generator circuit is operatively connected to the error amplifier and generates a reference voltage signal. The PWM generator circuit is operatively connected to the converter circuit and generates a PWM switch signal based on the raw control signal. The converter circuit transfers energy from the photovoltaic power source to the load based on the PWM switch signal. The error amplifier generates a PWM control signal based on the raw control signal generated by the control circuit and the reference voltage signal generated by the reference generator. The PWM generator circuit generates the PWM switch signal based on the PWM control signal. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0018]      FIG. 1  is a block diagram of a first embodiment of a DC-DC converter for converting the PV output power of a PV system into a DC power output for a load; 
           [0019]      FIG. 2  is a block diagram of a second embodiment of a DC-DC converter for converting the PV output power of a PV system into a DC power output for a load; 
           [0020]      FIG. 3  is a block diagram of a third embodiment of a DC-DC converter for converting the PV output power of a PV system into a DC power output for a load; 
           [0021]      FIG. 4  is a block diagram of a fourth embodiment of a DC-DC converter for converting the PV output power of a PV system into a DC power output for a load; and 
           [0022]      FIGS. 5A-5G  contain a circuit diagram illustrating one example of the fourth embodiment of a DC-DC converter as depicted in  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION  
       [0023]    Referring now to  FIG. 1  of the drawing, depicted therein is a first example of a DC-DC converter system  20  constructed in accordance with, and embodying, the principles of the present invention. The converter system  20  is arranged to convert PV output power from a PV system  22  into a signal appropriate for a load  24 . 
         [0024]    The converter system  20  comprises a converter circuit  30 , a PWM generator circuit  32 , and an input voltage feed forward control circuit  34 . The converter circuit  30  can be configured to comprise a switching circuit (not shown in  FIG. 1 ) comprising a power switch, an inductor, and a diode to transfer energy from the input (PV output power) to the output (converter output power). The components of the converter circuit  30  can be configured to form a step-down (buck), a step-up (boost), or an inverter (flyback) converter. The power switch is opened and closed based on a PWM signal generated as will be described in further detail below. 
         [0025]    The input voltage feed forward control circuit  34  generates a PWM control signal based on the PV output power. In particular, the control circuit  34  generates the PWM control signal based on at least the voltage level of the PV output power. The PWM control signal generated by the control circuit  34  is input to the PWM generator circuit  32 . The PWM generator circuit  32  generates a PWM switch signal that opens and closes the power switch of the converter circuit  30 . 
         [0026]    In the example converter system  20 , the control circuit  34  generates the PWM control signal such that the PWM generator circuit  32  generates the PWM switch signal to: (a) generate the converter output power at a voltage appropriate for the load  24  and (b) optimize power transfer from the PV system  22  to the load  24 . The example converter system  20  does not directly regulate the voltage level of the converter output power. 
         [0027]    Referring now to  FIG. 2  of the drawing, depicted therein is a second example of a DC-DC converter system  120  constructed in accordance with, and embodying, the principles of the present invention. The converter system  120  is arranged to convert PV output power from a PV system  122  into a signal appropriate for a load  124 . 
         [0028]    The converter system  120  comprises a converter circuit  130 , a PWM generator circuit  132 , and an input voltage feed forward control circuit  134 . The example converter system  120  further comprises a reference generator circuit  140  and an error amplifier  142 . 
         [0029]    As with the example converter circuit  30  described above, the converter circuit  130  comprises components that can be configured to form a step-down (buck), a step-up (boost), or an inverter (flyback) converter. In each of these converter configurations, a power switch is opened and closed based on a PWM signal generated as will be described in further detail below. 
         [0030]    The input voltage feed forward control circuit  134  generates a raw control signal based on at least the voltage level of the PV output power. The reference generator  140  generates a reference voltage level that can be preset or can be adjusted based on operating characteristics of the PV system  122  and/or the load  124 . The error amplifier  142  generates a PWM control signal based on a comparison of the voltage level of the PV output power and the reference voltage level. 
         [0031]    The PWM control signal generated by the error amplifier  142  is input to the PWM generator circuit  132 . The PWM generator circuit  132  generates a PWM switch signal that opens and closes the power switch of the converter circuit  130 . 
         [0032]    In the example converter system  120 , the control circuit generates the PWM control signal such that the PWM generator circuit  132  generates the PWM switch signal to: (a) generate the converter output power at a voltage appropriate for the load  124  and (b) optimize power transfer from the PV system  122  to the load  124 . The example converter system  120  does not directly regulate the voltage level of the converter output power. However, the converter system  120  can be configured to compensate for certain variables associated with the PV system  122  and/or the load  124 . 
         [0033]    Referring now to  FIG. 3  of the drawing, depicted therein is a third example of a DC-DC converter system  220  constructed in accordance with, and embodying, the principles of the present invention. The converter system  220  is arranged to convert PV output power from a PV system  222  into a signal appropriate for a load  224 . 
         [0034]    The converter system  220  comprises a converter circuit  230 , a PWM generator circuit  232 , and an input voltage feed forward control circuit  234 . As with the example converter circuits  30  and  130  described above, the converter circuit  230  comprises components that can be configured to form a step-down (buck), a step-up (boost), or an inverter (flyback) converter. The power switch is opened and closed based on a PWM signal generated as will be described in further detail below. 
         [0035]    The example converter system  220  further comprises a reference generator circuit  240  and an error amplifier  242 . In addition, the example converter system  220  comprises an input EMC filter  250 , and output EMC filter  252 , an input power filter  254 , and an output power filter  256 . 
         [0036]    The input EMC filter  250  and the input power filter  254  are arranged in series between the PV system  222  and the converter  230 . The output power filter  256  and the output EMC filter  252  are arranged in series between the converter  230  and the load  224 . The construction, operation, and purpose of the filters  250 - 256  are or may be conventional, and these filters  250 - 256  will not be described in detail herein. 
         [0037]    The example reference generator circuit  240  comprises a processor  260 , output voltage sense circuit  262 , and output current sense circuit  264 . The output voltage sense circuit  262  and output current sense circuit  264  are or may be conventional and generate output voltage and output current signals, respectively, associated with the converter output power. The processor  240  generates a reference voltage level based on the product of output voltage and output current signals. The reference voltage level is thus representative of the converter output power. 
         [0038]    The input voltage feed forward control circuit  234  generates a raw control signal based on at least the voltage level of the PV output power. The reference generator  240  generates the reference voltage level based on operating characteristics of the PV system  222  and/or the load  224  as represented by the converter output power. The error amplifier  242  generates a PWM control signal based on a comparison of the voltage level of the PV output power and the reference voltage level. 
         [0039]    The PWM control signal generated by the error amplifier  242  is input to the PWM generator circuit  232 . The PWM generator circuit  232  generates a PWM switch signal that opens and closes the power switch of the converter circuit  230 . 
         [0040]    In the example converter system  220 , the control circuit generates the PWM control signal such that the PWM generator circuit  232  generates the PWM switch signal to: (a) generate the converter output power at a voltage appropriate for the load  224  and (b) optimize power transfer from the PV system  222  to the load  224 . The example converter system  220  does not directly regulate the voltage level of the converter output power. However, operation of the reference generator circuit  240  allows the converter system  220  to compensate for fluctuations in power associated with the PV system  222  and/or the load  224 . 
         [0041]    Referring now to  FIG. 4  of the drawing, depicted therein is a fourth example of a DC-DC converter system  320  constructed in accordance with, and embodying, the principles of the present invention. The converter system  320  is arranged to convert PV output power from a PV system  322  into a signal appropriate for a load  324 . 
         [0042]    The converter system  320  comprises a converter circuit  330 , a PWM generator circuit  332 , and an input voltage feed forward control circuit  334 . As with the example converter circuits  30 ,  130 , and  230  described above, the converter circuit  330  comprises components that can be configured to form a step-down (buck), a step-up (boost), or an inverter (flyback) converter. The power switch is opened and closed based on a PWM signal generated as will be described in further detail below. 
         [0043]    The example converter system  320  further comprises a reference generator circuit  340  and an error amplifier  342 . In addition, the example converter system  320  comprises an input EMC filter  350 , and output EMC filter  352 , an input power filter  354 , and an output power filter  356 . 
         [0044]    The input EMC filter  350  and the input power filter  354  are arranged in series between the PV system  322  and the converter  330 . The output power filter  356  and the output EMC filter  352  are arranged in series between the converter  330  and the load  324 . The construction, operation, and purpose of the filters  350 - 356  are or may be conventional, and these filters  350 - 356  will not be described in detail herein. 
         [0045]    The example reference generator circuit  340  comprises a processor  360 , output voltage sense circuit  362 , and output current sense circuit  364 . The example converter system  320  further comprises an input voltage sense circuit  370  that generates an input voltage signal representative of a voltage level of the PV output power. 
         [0046]    The output voltage sense circuit  362  and output current sense circuit  364  are or may be conventional and generate output voltage and output current signals, respectively, associated with the converter output power. The processor  360  generates a reference voltage level based on the product of output voltage and output current signals. The reference voltage level is thus representative of the converter output power. 
         [0047]    The input voltage feed forward control circuit  334  generates a raw control signal based at least the voltage level of the PV output power. The reference generator  340  generates the reference voltage level based on operating characteristics of the PV system  332  and/or the load  334  as represented by the converter output power. The error amplifier  342  generates a PWM control signal based on a comparison of the voltage level of the PV output power and the reference voltage level. 
         [0048]    The PWM control signal generated by the error amplifier  342  is input to the PWM generator circuit  332 . The PWM generator circuit  332  generates a PWM switch signal that opens and closes the power switch of the converter circuit  330 . 
         [0049]    In the example converter system  320 , the control circuit generates the PWM control signal such that the PWM generator circuit  332  generates the PWM switch signal to: (a) generate the converter output power at a voltage appropriate for the load  324  and (b) optimize power transfer from the PV system  322  to the load  324 . The example converter system  320  does not directly regulate the voltage level of the converter output power. However, operation of the reference generator circuit  340  allows the converter system  320  to compensate for fluctuations in power associated with the PV system  322  and/or the load  324 . 
         [0050]    The system controller  360  further can be configured to provide the converter system  320  to communicate with external status monitoring and/or data collection systems. The signals generated by the output voltage sense circuit  362 , output current sense circuit  364 , and/or input voltage sense circuit  370  may be represented as data that may be transmitted using a communications signal to any such monitoring and/or data collection system. 
         [0051]    Turning now to  FIGS. 5A-5G , depicted therein is an example of a circuit that may be used to implement the fourth example converter system  320  depicted and described with reference to  FIG. 4 . In particular, the circuit depicted in  FIGS. 5A-5G  comprises the converter circuit  330  ( FIG. 5C ), the PWM generator circuit  332  ( FIG. 5B ), the input voltage feed forward control circuit  334  ( FIG. 5A ), the error amplifier circuit  342  (FIG.  5 D), the input EMC filter  350  ( FIG. 5A ), the output EMC filter  352  ( FIG. 5C ), the input power filter  354  ( FIG. 5A ), the output power filter  356  ( FIG. 5C ), the system controller  360  ( FIGS. 5E and 5F ), the output voltage sense circuit  362  ( FIG. 5C ), the output current sense circuit  364  ( FIG. 5C ), and the input voltage sense circuit  370  ( FIG. 5A ). In addition,  FIGS. 5A-5G  illustrate a power supply  380  ( FIGS. 5B and 5G ), an input protection circuit  382  ( FIG. 5A ), a low voltage disconnect circuit  384  ( FIG. 5B ), and other associated circuits, connectors and interfaces that may be used to construct an example embodiment of the fourth example converter circuit  320  of the present invention. 
         [0052]    Given the foregoing, it should be apparent that the principles of the present invention may be embodied in forms other than those described above. The scope of the present invention should thus be determined the claims appended hereto and not the foregoing detailed description of the invention.