Patent Description:
Recent interest in renewable energy has led to increased research in systems for distributed generation of energy, such as photovoltaic cells (PV), fuel cells and batteries. Various inconsistencies in manufacturing may cause two otherwise identical sources to provide different output characteristics. Similarly, two such sources may react differently to operating conditions, e.g. load and/or environmental conditions, e.g. temperature. In installations, different sources may also experience different environmental conditions, e.g., in solar power installations some panels may be exposed to full sun, while others may be shaded, thereby delivering different power output. In a multiple battery installation, some of the batteries may age differently, thereby delivering different power output.

<CIT> discloses that a local power converter may include a controller to manipulate the operating point of a local power converter to cause the power point tracking feature of a central power converter to operate at a point determined by the controller. The local power converter does not always operate according to a maximum power point tracking algorithm.

<CIT> discloses a maximum power point tracking (MPPT) converter of a renewable energy storage system for tracking and extracting a maximum power from a new and renewable energy to provide the maximum power to a DC link. The MPPT converter includes an MPPT controller changing a controlled variable for maximum power point extraction in proportion to a current slope of the new and renewable energy, and an MPPT extractor extracting a maximum power from the new and renewable energy and converting the extracted maximum power in response to a control of the MPPT controller. The controlled variable variation is set to a large value if the current slope is out of a predetermined current slope range, and the controlled variable variation is set to a small value if the current slope is within the predetermined current slope range.

<CIT> discloses a system for combining power from DC power sources. Each power source is coupled to a converter. Each converter converts input power to output power by monitoring and maintaining the input power at a maximum power point. Substantially all input power is converted to the output power, and the controlling is performed by allowing output voltage of the converter to vary. The converters are coupled in series. An inverter is connected in parallel with the series connection of the converters and inverts a DC input to the inverter from the converters into an AC output. The inverter maintains the voltage at the inverter input at a desirable voltage by varying the amount of the series current drawn from the converters. The series current and the output power of the converters, determine the output voltage at each converter.

According to a first aspect, the present invention provides a method as defined in claim <NUM>. According to a second aspect, the present invention provides an apparatus as defined in claim <NUM>. According to a third aspect, the present invention provides a system as defined in claim <NUM>. Preferred embodiments are defined in dependent claims.

The foregoing and/or other aspects will become apparent from the following detailed description when considered in conjunction with the accompanying drawing figures.

Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below to explain examples by referring to the figures.

A conventional installation of solar power system <NUM> is illustrated in <FIG>. Since the voltage provided by each individual solar panel <NUM> may be low, several panels may be connected in series to form a string of panels <NUM>. For a large installation, when higher current may be utilized, several strings <NUM> may be connected in parallel to form the overall system <NUM>. Solar panels <NUM> may be mounted outdoors, and their leads may be connected to a maximum power point tracking (MPPT) module <NUM> and then to an inverter <NUM>. The MPPT <NUM> may be implemented as part of the inverter <NUM>.

The harvested power from the DC sources may be delivered to the inverter <NUM>, which converts the fluctuating direct-current (DC) into alternating-current (AC) having a desired voltage and frequency at the inverter output, which may be, e.g., 110V or 220V at <NUM>, or 220V at <NUM>. In some examples, inverters that produce 220V may be then split into two 110V feeds in an electric box. The AC current from the inverter <NUM> may then be used for operating electric appliances or fed to the power grid. Alternatively, if the installation is not tied to the grid, the power extracted from inverter <NUM> may be directed to a conversion and charge/discharge circuit to store the excess power created as charge in batteries. In case of a battery-tied application, the inversion stage might be skipped altogether, and the DC output of the MPPT stage <NUM> may be fed into the charge/discharge circuit.

As noted above, each solar panel <NUM> supplies relatively very low voltage and current. A challenge facing the solar array designer may be to produce a standard AC current at 120V or 220V root-mean-square (RMS) from a combination of the low voltages of the solar panels. The delivery of high power from a low voltage may utilize very high currents, which may cause large conduction losses on the order of the second power of the current (I^<NUM>). Furthermore, a power inverter, such as the inverter <NUM>, which may be used to convert DC current to AC current, may be most efficient when its input voltage may be slightly higher than its output RMS voltage multiplied by the square root of <NUM>. Hence, in many applications, the power sources, such as the solar panels <NUM>, may be combined in order to reach the correct voltage or current. A common method may be to connect the power sources in series in order to reach the desirable voltage and in parallel in order to reach the desirable current, as shown in <FIG>. A large number of the panels <NUM> may be connected into a string <NUM> and the strings <NUM> may be connected in parallel to the power inverter <NUM>. The panels <NUM> may be connected in series in order to reach the minimal voltage for the inverter. Multiple strings <NUM> may be connected in parallel into an array to supply higher current, so as to enable higher power output.

While this configuration may be advantageous in terms of cost and architecture simplicity, several drawbacks have been identified for such architecture. One drawback may be inefficiencies caused by non-optimal power draw from each individual panel, as explained below. The output of the DC power sources may be influenced by many conditions. Therefore, to maximize the power draw from each source, one may need to draw the combination of voltage and current that provides the peak power for the currently prevailing conditions of the power source. As conditions change, the combination of voltage and current draw may need to be changed as well.

<FIG> illustrates an example of one serial string of DC sources, e.g., solar panels 101a - 101d, and MPPT circuit <NUM> integrated with inverter <NUM>. The current versus voltage (IV) characteristics are plotted (210a - 210d) to the left of each DC source <NUM>. For each DC source <NUM>, the current decreases as the output voltage increases. At some voltage value, the current goes to zero, and in some applications may assume a negative value, meaning that the source becomes a sink. Bypass diodes may be used to prevent the source from becoming a sink. The power output of each source <NUM>, which may be equal to the product of current and voltage (P=I*V), varies depending on the voltage across the source. At a certain current and voltage, close to the falling off point of the current, the power reaches its maximum. It may be desirable to operate a power generating power source (e.g., photovoltaic panel, cell, etc.) at this maximum power point. The purpose of the MPPT may be to find this point and operate the system at this point to draw the maximum power from the sources.

In a typical, conventional solar panel array, different algorithms and techniques may be used to optimize the integrated power output of the system <NUM> using the MPPT module <NUM>. The MPPT module <NUM> may receive the current extracted from all of the solar panels together and may track the maximum power point for this current to provide the maximum average power such that if more current is extracted, the average voltage from the panels starts to drop, thus lowering the harvested power. MPPT module <NUM> maintains a current that yields the maximum average power from the overall system <NUM>. However, since sources 101a - 101d may be connected in series to a single MPPT <NUM>, the MPPT may select a single power point, which would be somewhat of an average of the maximum power points (MPP) of each of the serially connected sources. In practice, it may be very likely that the MPPT would operate at an I-V point that may be optimum to only a few or none of the sources. In the example of <FIG>, each of the sources operate at the same current since the sources are connected in series, but the maximum power point for each source (indicated by a dot on curves 210a - 210d) may be at different currents. Thus, the selected current operating point by MPPT <NUM> may be the maximum power point for source 101b, but may be off the maximum power point for sources 101a, 101cand 101d. Consequently, the arrangement may be not operated at best achievable efficiency.

Turning back to the example of system <NUM> of <FIG>, fixing a predetermined constant output voltage from the strings <NUM> may cause solar panels <NUM> to supply lower output power than otherwise possible. Further, each string <NUM> carries a single current that is passed through all of solar panels <NUM> along string <NUM>. If solar panels <NUM> are mismatched due to manufacturing differences, aging or if they malfunction or placed under different shading conditions, the current, voltage and power output of each panel may be different. Forcing a single current through all of panels <NUM> of string <NUM> may cause individual panels <NUM> to work at a non-optimal power point and can also cause panels <NUM>, which may be highly mismatched to generate "hot spots" due to the high current flowing through them. Due to these and other drawbacks of conventional centralized methods of MPPT, panels <NUM> may be matched improperly. In some cases, external diodes may be used to bypass panels <NUM> that are highly mismatched. In conventional multiple string configurations all strings <NUM> may be composed of exactly the same number of solar panels and panels <NUM> may be selected of the same model and may be installed at exactly the same spatial orientation, being exposed to the same sunlight conditions at all times. Installation according to these constraints may be very costly. During installation of a solar array according to the conventional configurations <NUM>, the installer can verify the correctness of the installation and performance of the solar array by using test equipment to check the current-voltage characteristics of each panel, each string and the entire array. In practice, however, individual panels and strings may be either not tested at all or tested only prior to connection. Current measurement may be performed by a series connection to the solar array such as with a series resistor in the array, which is typically not convenient. Instead, typically only high-level pass/fail testing of the overall installation is performed.

After the initial testing of the installation, the solar array may be connected to inverter <NUM>, which may include a monitoring module, which monitors performance of the entire array. The performance information gathered from monitoring within inverter <NUM> may include integrated power output of the array and the power production rate, but the information lacks any fine details about the functioning of individual solar panels <NUM>. Therefore, the performance information provided by monitoring at the inverter <NUM> may be insufficient to understand if power loss may be due to environmental conditions, from malfunctions or from poor installation or maintenance of the solar array. Furthermore, integrated information may not pinpoint which of solar panels <NUM> are responsible for a detected power loss.

<FIG> illustrates a distributed power harvesting configuration <NUM>, according to an embodiment. Configuration <NUM> enables connection of multiple power sources, for example, solar panels 101a - 101d, into a single power supply. In one aspect, the series string of all of the solar panels may be coupled to an inverter <NUM>. In another aspect, several serially connected strings of solar panels may be connected to a single inverter <NUM>. The inverter <NUM> may be replaced by other elements, such as, e.g., a charging regulator for charging a battery bank.

In configuration <NUM>, each solar panel 101a - 101d may be connected to a separate power converter circuit 305a - 305d. One solar panel <NUM> together with its connected power converter circuit forms a module, e.g., photovoltaic module <NUM> (only one of which is labeled). Each converter 305a - 305d adapts optimally to the power characteristics of the connected solar panel 101a - 101d and transfers the power efficiently from converter input to converter output. The converters 305a - 305d may be buck converters, boost converters, buck/boost converters, flyback or forward converters, etc. The converters 305a - 305d may also contain a number of component converters, for example a serial connection of a buck and a boost converter.

Each converter 305a - 305d may include a control circuit <NUM> that receives a feedback signal, not from the converter's output current or voltage, but rather from the converter's input coming from the solar panel <NUM>. An input sensor measures an input parameter, input power, input current and/or input voltage and sets the input power. An example of such a control circuit may be a maximum power point tracking (MPPT) circuit. The MPPT circuit of the converter locks the input voltage and current from each solar panel 101a -101d to its optimal power point. In the converters 305a - 305d, according to aspects, a controller within converter <NUM> monitors the voltage and current at the converter input terminals and determines the pulse width modulation (PWM) of the converter in such a way that maximum power may be extracted from the attached panel 101a - 101d. The controller of the converter <NUM> dynamically tracks the maximum power point at the converter input. In various aspects, the feedback loop of control circuit <NUM> may be closed on the input power in order to track maximum input power rather than closing the feedback loop on the output voltage as performed by conventional DC-to-DC voltage converters (e.g., MPPT <NUM>). As a result of having a separate control circuit <NUM> in each converter 305a - 305d, and consequently for each solar panel 101a - 101d, each string <NUM> in system <NUM> may have a different number or different brand of panels 101a - 101d connected in series. Control circuit <NUM> of <FIG> continuously maximizes power on the input of each solar panel <NUM> a - <NUM> d to react to changes in temperature, solar radiance, shading or other performance factors that impact that particular solar panel 101a - 101d. As a result, control circuit <NUM> within the converters 305a - 305d harvests the maximum possible power from each panel 101a - 101d and transfers this power as output power regardless of the parameters impacting the other solar panels.

As such, the embodiments shown in <FIG> continuously track and maintain the input current and the input voltage to each converter <NUM> at the maximum power point of the connected DC power source. The maximum power of the DC power source that may be input to converter <NUM> may be also output from converter <NUM>. The converter output power may be at a current and voltage different from the converter input current and voltage. While maintaining the total power given the minor power loss due to inefficiency of the power conversion, the output current and output voltage from converter <NUM> may be responsive to requirements of the series connected portion of the circuit.

In one embodiment, the outputs of converters 305a - 305d may be series connected into a single DC output that forms the input to the load, in this example, inverter <NUM>. The inverter <NUM> converts the series connected DC output of the converters into an AC power supply. The load, in this case inverter <NUM>, may regulate the voltage at the load's input using control circuit <NUM>. That may be, in this example, an independent control loop <NUM> which may hold the input voltage at a predetermined set value, e.g. <NUM> volts. Consequently, input current of inverter <NUM> may be dictated by the available power, and this may be the current that flows through all serially connected DC sources. While the output of the DC-DC converters <NUM> are constrained by current and or voltage regulation at the input of inverter <NUM>, the current and voltage input to power converter circuit <NUM> may be independently controlled using control circuit <NUM>. Aspects provide a system and method for combining power from multiple DC power sources <NUM> into a distributed power supply. According to these aspects, each DC power source <NUM>, e.g. photovoltaic panel <NUM> may be associated with a DC-DC power converter <NUM>. Modules formed by coupling the DC power sources <NUM> to their associated converters <NUM> may be coupled in series to provide a string of modules. The string of modules may be then coupled to inverter <NUM> having its input voltage fixed. A maximum power point control circuit control circuit <NUM> in each converter <NUM> harvests the maximum power from each DC power source <NUM> and transfers this power as output from power converter <NUM>. For each converter <NUM>, the input power may be converted to the output power, such that the conversion efficiency may be <NUM>% or higher in some situations.

Further, the controlling may be performed by fixing the input current or input voltage of the converter to the maximum power point and allowing output voltage of the converter to vary. For each power source <NUM>, one or more sensors may monitor the input power level to the associated converter <NUM>. In some embodiments, a microcontroller may perform the maximum power point tracking and control in each converter <NUM> by using pulse width modulation to adjust the duty cycle used for transferring power from the input to the output. An aspect may provide a greater degree of fault tolerance, maintenance and serviceability by monitoring, logging and/or communicating the performance of each solar panel. In various embodiments, the microcontroller that may be used for maximum power point tracking may also be used to perform the monitoring, logging and communication functions. These functions allow for quick and easy troubleshooting during installation, thereby significantly reducing installation time. These functions may be also beneficial for quick detection of problems during maintenance work. Aspects allow easy location, repair, or replacement of failed solar panels. When repair or replacement may be not feasible, bypass features provide increased reliability. In an aspect, arrays of solar cells are provided where the power from the cells may be combined. Each converter <NUM> may be attached to a single solar cell, or a plurality of cells connected in series, in parallel, or both, e.g., parallel connection of strings of serially connected cells.

In an embodiment, each converter <NUM> may be attached to one or more panels of a photovoltaic string. However, while applicable in the context of solar power technology, the aspects may be used in any distributed power network using DC power sources. For example, they may be used in batteries with numerous cells or hybrid vehicles with multiple fuel cells on board. The DC power sources may be solar cells, solar panels, electrical fuel cells, electrical batteries, and the like. Further, although the discussion below relates to combining power from an array of DC power sources into a source of AC voltage, the aspects may also apply to combining power from DC sources into another DC voltage.

In these DC-to-DC voltage converters, a controller within the converter may monitor the current or voltage at the input, and the voltage at the output. The controller may also determine the appropriate pulse width modulation (PWM) duty cycle to fix the output voltage to the predetermined value by increasing the duty cycle if the output voltage drops. Accordingly, the conventional converter may include a feedback loop that closes on the output voltage and uses the output voltage to further adjust and fine-tune the output voltage from the converter. As a result of changing the output voltage, the current extracted from the input may be also varied.

<FIG> and <FIG> illustrate an operation of the system of <FIG> under different conditions, according to embodiments. An exemplary configuration <NUM> may be similar to configuration <NUM> of <FIG>. In the example shown, ten DC power sources <NUM> /<NUM> through <NUM> / <NUM> may be connected to ten power converters <NUM> / <NUM> through <NUM> / <NUM>, respectively. The modules formed by the DC power sources <NUM> and their connected converters <NUM> may be coupled together in series to form a string <NUM>. In one embodiment, the series-connected converters <NUM> may be coupled to a DC-to-AC inverter <NUM>.

The DC power sources may be solar panels <NUM> and the example may be discussed with respect to solar panels as one illustrative case. Each solar panel <NUM> may have a different power output due to manufacturing tolerances, shading, or other factors. For the purpose of the present example, an ideal case may be illustrated in <FIG>, where efficiency of the DC-to-DC conversion may be assumed to be <NUM>% and the panels <NUM> may be assumed to be identical. In some aspects, efficiencies of the converters may be quite high and range at about <NUM>%-<NUM>%. So, the assumption of <NUM>% efficiency may not unreasonable for illustration purposes. Moreover, according to embodiments, each of the DC-DC converters <NUM> may be constructed as a power converter, i.e., it transfers to its output the entire power it receives in its input with very low losses. Power output of each solar panel <NUM> may be maintained at the maximum power point for the panel by a control loop 311within the corresponding power converter <NUM>. In the example shown in <FIG>, all of panels <NUM> may be exposed to full sun illumination and each solar panel <NUM> provides <NUM> W of power. Consequently, the MPPT loop may draw current and voltage level that will transfer the entire <NUM> W from the panel to its associated converter <NUM>. That is, the current and voltage dictated by the MPPT form the input current Iin and input voltage Vin to the converter. The output voltage may be dictated by the constant voltage set at the inverter <NUM>, as will be explained below. The output current Iout would then be the total power, i.e., <NUM> W, divided by the output voltage Vout.

Referring back to conventional system <NUM>, <FIG> and <FIG>, the input voltage to load <NUM> varies according to the available power. For example, when a lot of sunshine may be available in a solar installation, the voltage input to inverter <NUM> can vary even up to <NUM> volts. Consequently, as sunshine illumination varies, the voltage varies with it, and the electrical components in inverter <NUM> (or other power supplier or load) may be exposed to varying voltage. This tends to degrade the performance of the components and may ultimately cause them to fail. On the other hand, by fixing or limiting the voltage or current to the input of the load or power supplier, e.g., inverter <NUM>, the electrical components may always be exposed to the same voltage or current and possibly have extended service life. For example, the components of the load (e.g., capacitors, switches and coil of the inverter) may be selected so that at the fixed input voltage or current they operate at, say, <NUM>% of their rating. This may improve the reliability and prolong the service life of the component, which may be critical for avoiding loss of service in applications such as solar power systems.

As noted above, according to an embodiment, the input voltage to inverter <NUM> may be controlled by inverter <NUM> (in this example, kept constant), by way of control loop <NUM> (similar to control loop <NUM> of inverter <NUM> above). For the purpose of this example, assume the input voltage may be kept as 400V (ideal value for inverting to 220VAC). Since it is assumed that there may be ten serially connected power converters, each providing <NUM> W, the input current to the inverter <NUM> is <NUM> W/400V=<NUM> A. Thus, the current flowing through each of the converters <NUM> / <NUM> - <NUM> / <NUM> may be <NUM> A. This means that in this idealized example each of converters <NUM> provides an output voltage of <NUM> W / 5A = 40V. Now, assume that the MPPT for each panel <NUM> (assuming perfect matching panels) dictates that the maximum power point voltage for each panel is Vmpp = 32V. This means that the input voltage to inverter <NUM> would be 32V, and the input current would be <NUM> W/32V=<NUM> A.

We now turn to another example, where system <NUM> may be still maintained at an ideal mode (i.e., perfectly matching DC sources and entire power may be transferred to inverter <NUM>), but the environmental conditions may different for different panels. For example, one DC source may be overheating, may be malfunctioning, or, as in the example of <FIG>, the ninth solar panel <NUM>/<NUM> may be shaded and consequently produces only <NUM> W of power. Since all other conditions as in the example of <FIG> are kept, the other nine solar panels <NUM> may be unshaded and still produce <NUM> W of power. The power converter <NUM>/<NUM> includes MPPT to maintain the solar panel <NUM>/<NUM> operating at the maximum power point, which may be now lowered due to the shading.

The total power available from the string may be now <NUM>×<NUM> W+<NUM> W=<NUM> W. Since the input to inverter <NUM> may be still maintained at 400V, the input current to inverter <NUM> will now be <NUM> W/40V=<NUM> A. This means that the output of all of the power converters <NUM>/<NUM> - <NUM>/<NUM> in the string may be at <NUM> A. Therefore, for the nine unshaded panels, the converters will output <NUM> W/<NUM> A=<NUM>. On the other hand, the converter <NUM>/<NUM> attached to the shaded panel <NUM>/<NUM> will output <NUM> W/<NUM> A=<NUM>. Checking the math, the input to inverter <NUM> can be obtained by adding nine converters providing <NUM>. 5V and one converter providing <NUM>. 7V, i.e., (<NUM>×<NUM>.

The output of the nine non-shaded panels would still be controlled by the MPPT as in <FIG>, thereby standing at 32V and <NUM> A. On the other hand, since the ninth panel <NUM>/<NUM> is shaded, assume its MPP voltage dropped to 28V. Consequently, the output current of the ninth panel is <NUM> W/28V=<NUM> A. As can be seen by this example, all of the panels may be operated at their maximum power point, regardless of operating conditions. As shown by the example of <FIG>, even if the output of one DC source drops dramatically, system <NUM> still maintains relatively high power output by fixing the voltage input to the inverter, and controlling the input to the converters independently so as to draw power from each DC source at the MPP.

As can be appreciated, the benefit of the topology illustrated in <FIG> and <FIG> may be numerous. For example, the output characteristics of the serially connected DC sources, such as solar panels, need not match. Consequently, the serial string may utilize panels from different manufacturers or panels installed on different parts of the roofs (i.e., at different spatial orientation). Moreover, if several strings are connected in parallel, it may not be necessary that the strings match; rather each string may have different panels or different number of panels. This topology may also enhance reliability by alleviating the hot spot problem. As shown in <FIG>, the output of the shaded panel <NUM> / <NUM> is <NUM> A, while the current at the output of the unshaded panels is <NUM> A. This discrepancy in current when the components are series connected may cause a large current being forced through the shaded panel that may cause overheating and malfunction at this component. However, by the exemplary aspects of the topology shown, the input voltage may be set independently, and the power draw from each panel to its converter may be set independently according to the panel's MPP at each point in time, the current at each panel may be independent on the current draw from the serially connected converters.

It may be easily realized that since the power may be optimized independently for each panel, panels may be installed in different facets and directions in building-integrated photovoltaic (BIPV) installations. Thus, the problem of low power utilization in building-integrated installations may be solved, and more installations may now be profitable. The described system may also easily solve the problem of energy harvesting in low light conditions. Even small amounts of light may be enough to make the converters <NUM> operational, and they then start transferring power to the inverter. If small amounts of power are available, there may be a low current flow-but the voltage will be high enough for the inverter to function, and the power may indeed be harvested. According to embodiments, inverter <NUM> may include a control loop <NUM> to maintain an optimal voltage at the input of inverter <NUM>. In the example of <FIG>, the input voltage to inverter <NUM> may be maintained at 400V by the control loop <NUM>. The converters <NUM> may be transferring substantially all (e.g., ><NUM>%) of the available power from the solar panels to the input of the inverter <NUM>. As a result, the input current to the inverter <NUM> may be dependent only on the power provided by the solar panels and the regulated set, i.e., constant, voltage at the inverter input.

Conventional inverter <NUM>, shown in <FIG> and <FIG>, may have a very wide input voltage to accommodate for changing conditions, for example a change in luminance, temperature and aging of the solar array. This may be in contrast to inverter <NUM> that may be designed according to aspects. The inverter <NUM> does not utilize a wide input voltage and may be therefore simpler to design and more reliable. This higher reliability may be achieved, among other factors, by the fact that there may be no voltage spikes at the input to the inverter and thus the components of the inverter experience lower electrical stress and may last longer. When the inverter <NUM> may be a part of a circuit, the power from the panels may be transferred to a load that may be connected to the inverter. To enable the inverter <NUM> to work at its optimal input voltage, any excess power produced by the solar array, and not used by the load, may be dissipated. Excess power may be handled by selling the excess power to the utility company if such an option is available. For off-grid solar arrays, the excess power may be stored in batteries. Yet another option may be to connect a number of adjacent houses together to form a micro-grid and to allow load-balancing of power between the houses. If the excess power available from the solar array is not stored or sold, then another mechanism may be provided to dissipate excess power. The features and benefits explained with respect to <FIG> and <FIG> stem, at least partially, from having inverter <NUM> control the voltage provided at its input. Conversely, a design may be implemented, where inverter <NUM> controls the current at its input. Such an arrangement may be illustrated in <FIG> illustrates an embodiment where the inverter controls the input current. Power output of each solar panel <NUM> may be maintained at the maximum power point for the panel by a control loop within the corresponding power converter <NUM>. In the example shown in <FIG>, all of the panels may be exposed to full sun illumination and each solar panel <NUM> provides <NUM> W of power.

Consequently, the MPPT loop will draw current and voltage level that will transfer the entire <NUM> W from the panel to its associated converter. That is, the current and voltage controlled by the MPPT form the input current Iin and input voltage Vin to the converter. The output voltage of the converter may be determined by the constant current set at the inverter <NUM>, as will be explained below. The output voltage Vout would then be the total power, i.e., <NUM> W, divided by the output current Iout. As noted above, according to an embodiment, the input current to inverter <NUM> may be controlled by the inverter by way of control loop <NUM>. For the purpose of this example, assume the input current is kept as <NUM> A. Since it is assumed that there may be ten serially connected power converters, each providing <NUM> W, the input voltage to the inverter <NUM> is <NUM> W/<NUM> A=400V. Thus, the current flowing through each of the converters <NUM> / <NUM> - <NUM> / <NUM> may be <NUM> A. This means that in this idealized example each of the converters provides an output voltage of <NUM> W/<NUM> A=40V. Now, assume that the MPPT for each panel (assuming perfect matching panels) controls the MPP voltage of the panel to Vmpp = 32V. This means that the input voltage to the inverter would be 32V, and the input current would be <NUM> W/32V=<NUM> A.

Consequently, similar advantages have been achieved by having inverter <NUM> control the current, rather than the voltage. However, unlike conventional art, changes in the output of the panels may not cause changes in the current flowing to the inverter, as that may be set by the inverter itself. Therefore, inverter <NUM> may be designed to keep the current or the voltage constant, then regardless of the operation of the panels, the current or voltage to inverter <NUM> will remain constant.

<FIG> illustrates a distributed power harvesting system <NUM>, according to other embodiments, using DC power sources. <FIG> illustrates multiple strings <NUM> coupled together in parallel. Each of strings <NUM> may be a series connection of multiple modules and each of the modules includes a DC power source <NUM> that may be coupled to a converter <NUM>. The DC power source may be a solar panel. The output of the parallel connection of the strings <NUM> may be connected, again in parallel, to a shunt regulator <NUM> and a load <NUM>. The load <NUM> may be an inverter as with the embodiments of <FIG> and <FIG>. Shunt regulators automatically maintain a constant voltage across its terminals. The shunt regulator <NUM> may be configured to dissipate excess power to maintain the input voltage at the input to the inverter <NUM> at a regulated level and prevent the inverter input voltage from increasing. The current which flows through shunt regulator <NUM> complements the current drawn by inverter <NUM> in order to ensure that the input voltage of the inverter may be maintained at a constant level, for example at 400V.

By fixing the inverter input voltage, the inverter input current may be varied according to the available power draw. This current may be divided between the strings <NUM> of the series connected converters. When each converter <NUM> includes a control loop 311maintaining the converter input voltage at the maximum power point of the associated DC power source, the output power of converter <NUM> may be determined. The converter power and the converter output current together may determine the converter output voltage. The converter output voltage may be used by a power conversion circuit in the converter for stepping up or stepping down the converter input voltage to obtain the converter output voltage from the input voltage as determined by the MPPT.

<FIG> illustrates an illustrative example of DC-to-DC converter <NUM> according to embodiments. DC-to-DC converters may be conventionally used to either step down or step up a varied or constant DC voltage input to a higher or a lower constant voltage output, depending on the requirements of the circuit. However, in the embodiment of <FIG> the DC-DC converter may be used as a power converter, i.e., transferring the input power to output power, the input voltage varying according to the maximum power point, while the output current being dictated by the constant input voltage to inverter <NUM>, <NUM>, or <NUM>. That is, the input voltage and current may vary at any time and the output voltage and current may vary at any time, depending on the operating condition of the DC power sources. The converter <NUM> may be connected to a corresponding DC power source <NUM> (or <NUM>) at input terminals <NUM> and <NUM>. The converted power of the DC power source <NUM> may be output to the circuit through output terminals <NUM> and <NUM>. Between the input terminals <NUM> and <NUM> and the output terminals <NUM> and <NUM>, the remainder of the converter circuit may be located that includes input and output capacitors <NUM> and <NUM>, back flow prevention diodes <NUM> and <NUM> and a power conversion circuit including a controller <NUM> and an inductor <NUM>.

The inputs <NUM> and <NUM> may be separated by a capacitor <NUM>, which may act as an open circuit to a DC voltage. The outputs <NUM> and <NUM> may be also separated by a capacitor <NUM> that also acts an open circuit to DC output voltage. These capacitors may be DC blocking or AC-coupling capacitors that short circuit when faced with alternating current of a frequency, which may be selectable. Capacitor <NUM> coupled between the outputs <NUM> and <NUM> may also operate as a part of the power conversion circuit discussed below. Diode <NUM> may be coupled between the outputs <NUM> and <NUM> with a polarity such that current may not backflow into the converter <NUM> from the positive lead of the output <NUM>. Diode <NUM> may be coupled between the positive output lead <NUM> through inductor <NUM>, which acts as a short for DC current and the negative input lead <NUM> with such a polarity to prevent a current from the output <NUM> to backflow into the solar panel <NUM>.

The DC power source <NUM> may be a solar panel, solar cell, string or solar panels or a string of solar cells. A voltage difference may exist between the wires <NUM> and <NUM> due to the electron-hole pairs produced in the solar cells of panel <NUM>. Converter <NUM> may maintain maximum power output by extracting current from the solar panel <NUM> at its peak power point by continuously monitoring the current and voltage provided by the panel and using a maximum power point tracking algorithm. Controller <NUM> may include an MPPT circuit or algorithm for performing the peak power tracking. Peak power tracking and pulse width modulation, PWM, may be performed together to achieve the desired input voltage and current. The MPPT in the controller <NUM> may be any conventional MPPT, such as, e.g., perturb and observe (P&O), incremental conductance, etc. However, notably, the MPPT may be performed on the panel directly, i.e., at the input to the converter, rather than at the output of the converter. The generated power may be then transferred to the output terminals <NUM> and <NUM>. The outputs of multiple converters <NUM> may be connected in series, such that the positive lead <NUM> of one converter <NUM> may be connected to the negative lead <NUM> of the next converter <NUM> (e.g., as shown in <FIG>).

In <FIG>, the converter <NUM> may be shown as a buck plus boost converter. The term "buck plus boost" as used herein may be a buck converter directly followed by a boost converter as shown in <FIG>, which may also appear in the literature as "cascaded buck-boost converter". If the voltage is to be lowered, the boost portion may be shorted (e.g., FET switch <NUM> statically closed). If the voltage is to be raised, the buck portion may be shorted (i.e., FET switch <NUM> statically closed). The term "buck plus boost" differs from buck/boost topology, which may be a classic topology that may be used when voltage is to be raised or lowered. The efficiency of "buck/boost" topology may be inherently lower than a buck plus boost converter. Additionally, for given requirements, a buck/boost converter may need bigger passive components than a buck plus boost converter in order to function. Therefore, the buck plus boost topology of <FIG> may have a higher efficiency than the buck/boost topology. However, the circuit of <FIG> may have to continuously decide whether it may be bucking (operating the buck portion) or boosting (operating the boost portion). In some situations when the desired output voltage may be similar to the input voltage, then both the buck and boost portions may be operational.

The controller <NUM> may include a pulse width modulator, PWM, or a digital pulse width modulator, DPWM, to be used with the buck and boost converter circuits. The controller <NUM> controls both the buck converter and the boost converter and determines whether a buck or a boost operation is to be performed. In some circumstances both the buck and boost portions may operate together. That is, as explained with respect to the embodiments of <FIG> and <FIG>, the input voltage and input current may be selected independently of the selection of output current and output voltage. Moreover, the selection of either input or output values may change at any given moment depending on the operation of the DC power sources. Therefore, in the embodiment of <FIG> the converter may be constructed so that at any given time a selected value of input voltage and input current may be up converted or down converted depending on the output requirement. In one implementation, an integrated circuit (IC) <NUM> may be used that incorporates some of the functionality of converter <NUM>. IC <NUM> may be a single ASIC able to withstand harsh temperature extremes present in outdoor solar installations. ASIC <NUM> may be designed for a high mean time between failures (MTBF) of more than <NUM> years. However, a discrete solution using multiple integrated circuits may also be used in a similar manner. In the exemplary embodiment shown in <FIG>, the buck plus boost portion of the converter <NUM> may be implemented as the IC <NUM>. Practical considerations may lead to other segmentations of the system. For example, in one embodiment, the IC <NUM> may include two ICs, one analog IC, which handles the high currents and voltages in the system, and one simple low-voltage digital IC, which includes the control logic. The analog IC may be implemented using power FETs that may alternatively be implemented in discrete components, FET drivers, A/Ds, and the like. The digital IC may form the controller <NUM>.

In the exemplary circuit shown, the buck converter includes the input capacitor <NUM>, transistors <NUM> and <NUM>, a diode <NUM> positioned in parallel to transistor <NUM>, and an inductor <NUM>. The transistors <NUM> and <NUM> may each have a parasitic body diode <NUM> and <NUM>, respectively. In the exemplary circuit shown, the boost converter includes the inductor <NUM>, which may be shared with the buck converter, transistors <NUM> and <NUM>, a diode <NUM> positioned in parallel to transistor <NUM>, and the output capacitor <NUM>. The transistors <NUM> and <NUM> may each have a parasitic body diode <NUM> and <NUM>, respectively.

<FIG> illustrates another illustrative embodiment of a power converter <NUM>, according to embodiments. <FIG> highlights, among others, a monitoring and control functionality of a DC-to-DC converter <NUM>, according to embodiments. A DC voltage source <NUM> is also shown in the figure. Portions of a simplified buck and boost converter circuit are shown for converter <NUM>. The portions shown include the switching transistors <NUM>, <NUM>, <NUM> and <NUM> and the common inductor <NUM>. Each of the switching transistors may be controlled by a power conversion controller <NUM>.

The power conversion controller <NUM> includes the pulse-width modulation (PWM) circuit <NUM>, and a digital control machine <NUM> including a protection portion <NUM>. The power conversion controller <NUM> may be coupled to microcontroller <NUM>, which includes an MPPT algorithm <NUM>, and may also include a communication module <NUM>, a monitoring and logging module <NUM>, and a protection module <NUM>.

A current sensor <NUM> may be coupled between the DC power source <NUM> and the converter <NUM>, and output of the current sensor <NUM> may be provided to the digital control machine <NUM> through an associated analog to digital converter <NUM>. A voltage sensor <NUM> may be coupled between the DC power source <NUM> and the converter <NUM> and output of the voltage sensor <NUM> may be provided to the digital control machine <NUM> through an associated analog to digital converter <NUM>. The current sensor <NUM> and the voltage sensor <NUM> may be used to monitor current and voltage output from the DC power source, e.g., the solar panel <NUM>. The measured current and voltage may be provided to the digital control machine <NUM> and may be used to maintain the converter input power at the maximum power point.

The PWM circuit <NUM> controls the switching transistors of the buck and boost portions of the converter circuit. The PWM circuit may be a digital pulse-width modulation (DPWM) circuit. Outputs of the converter <NUM> taken at the inductor <NUM> and at the switching transistor <NUM> may be provided to the digital control machine <NUM> through analog to digital converters <NUM>, <NUM>, so as to control the PWM circuit <NUM>.

A random access memory (RAM) module <NUM> and a non-volatile random access memory (NVRAM) module <NUM> may be located outside the microcontroller <NUM> but coupled to the microcontroller <NUM>. A temperature sensor <NUM> and one or more external sensor interfaces <NUM> may be coupled to the microcontroller <NUM>. The temperature sensor <NUM> may be used to measure the temperature of the DC power source <NUM>. A physical interface <NUM> may be coupled to the microcontroller <NUM> and used to convert data from the microcontroller into a standard communication protocol and physical layer. An internal power supply unit <NUM> may be included in the converter <NUM>.

In various embodiments, the current sensor <NUM> may be implemented by various techniques used to measure current. In one embodiment, the current measurement module <NUM> may be implemented using a very low value resistor. The voltage across the resistor will be proportional to the current flowing through the resistor. In another embodiment, the current measurement module <NUM> may be implemented using current probes, which use the Hall Effect to measure the current through a conductor without adding a series resistor. After translating the current measurement to a voltage signal, the data may be passed through a low pass filter and then digitized. The analog to digital converter associated with the current sensor <NUM> may be shown as the A/D converter <NUM> in <FIG>. Aliasing effect in the resulting digital data may be avoided by selecting an appropriate resolution and sample rate for the analog to digital converter. If the current sensing technique does not utilize a series connection, then the current sensor <NUM> may be connected to the DC power source <NUM> in parallel.

In one embodiment, the voltage sensor <NUM> uses simple parallel voltage measurement techniques in order to measure the voltage output of the solar panel. The analog voltage may be passed through a low pass filter in order to minimize aliasing. The data may be then digitized using an analog to digital converter. The analog to digital converter associated with the voltage sensor <NUM> may be shown as the A/D converter <NUM> in <FIG>. The A/D converter <NUM> has sufficient resolution to generate an adequately sampled digital signal from the analog voltage measured at the DC power source <NUM> that may be a solar panel.

The current and voltage data collected for tracking the maximum power point at the converter input may be used for monitoring purposes also. An analog to digital converter with sufficient resolution may correctly evaluate the panel voltage and current. However, to evaluate the state of the panel, even low sample rates may be sufficient. A low-pass filter makes it possible for low sample rates to be sufficient for evaluating the state of the panel. The current and voltage data may be provided to the monitoring and logging module <NUM> for analysis.

Temperature sensor <NUM> enables the system to use temperature data in the analysis process. The temperature may be indicative of some types of failures and problems. Furthermore, in the case that the power source may be a solar panel, the panel temperature may be a factor in power output production.

The one or more optional external sensor interfaces <NUM> enable connecting various external sensors to the converter <NUM>. External sensors <NUM> may be used to enhance analysis of the state of the solar panel <NUM>, or a string or an array formed by connecting the solar panels <NUM>. Examples of external sensors <NUM> include ambient temperature sensors, solar radiance sensors, and sensors from neighboring panels. External sensors may be integrated into the converter <NUM> instead of being attached externally. In one embodiment, the information acquired from the current and voltage sensors <NUM>, <NUM> and the optional temperature and external sensors <NUM> may be transmitted to a central analysis station for monitoring, control, and analysis using the communications interface <NUM>. The central analysis station is not shown in the figure.

The communication interface <NUM> connects a microcontroller <NUM> to a communication bus. The communication bus can be implemented in several ways. In one embodiment, the communication bus may be implemented using an off-the-shelf communication bus such as Ethernet or RS422. Other methods such as wireless communications or power line communications, which could be implemented on the power line connecting the panels, may also be used. If bidirectional communication is used, the central analysis station may request the data collected by the microcontroller <NUM>. Alternatively or in addition, the information acquired from sensors <NUM>, <NUM>, <NUM> may be logged locally using the monitoring and logging module <NUM> in local memory such as the RAM <NUM> or the NVRAM <NUM>.

Analysis of the information from sensors <NUM>, <NUM>, <NUM> enables detection and location of many types of failures associated with power loss in solar arrays. Smart analysis can also be used to suggest corrective measures such as cleaning or replacing a specific portion of the solar array. Analysis of sensor information can also detect power losses caused by environmental conditions or installation mistakes and prevent costly and difficult solar array testing.

Consequently, in one embodiment, the microcontroller <NUM> simultaneously maintains the maximum power point of input power to the converter <NUM> from the attached DC power source or solar panel <NUM> based on the MPPT algorithm in the MPPT module <NUM>, and manages the process of gathering the information from sensors <NUM>, <NUM>, <NUM>. The collected information may be stored in the local memory <NUM>, <NUM> and transmitted to an external central analysis station. In one embodiment, the microcontroller <NUM> may use previously defined parameters stored in the NVRAM <NUM> in order to operate converter <NUM>. The information stored in the NVRAM <NUM> may include information about the converter <NUM> such as serial number, the type of communication bus used, the status update rate and the ID of the central analysis station. This information may be added to the parameters collected by the sensors before transmission.

Converters <NUM> may be installed during the installation of the solar array or retrofitted to existing installations. In both cases, converters <NUM> may be connected to a panel junction connection box or to cables connecting the panels <NUM>. Each converter <NUM> may be provided with the connectors and cabling to enable easy installation and connection to solar panels <NUM> and panel cables.

In one embodiment, physical interface <NUM> may be used to convert to a standard communication protocol and physical layer so that during installation and maintenance, the converter <NUM> may be connected to one of various data terminals, such as a computer or PDA. Analysis may then be implemented as software, which will be run on a standard computer, an embedded platform or a proprietary device.

The installation process of converters <NUM> may include connecting each converter <NUM> to a solar panel <NUM>. One or more of sensors <NUM>, <NUM>, <NUM> may be used to ensure that the solar panel <NUM> and the converter <NUM> may be properly coupled together. During installation, parameters such as serial number, physical location and the array connection topology may be stored in the NVRAM <NUM>. These parameters may be used by analysis software to detect future problems in solar panels <NUM> and arrays.

When the DC power sources <NUM> are solar panels, one of the problems facing installers of photovoltaic solar panel arrays may be safety. The solar panels <NUM> may be connected in series during the day when there may be sunlight. Therefore, at the final stages of installation, when several solar panels <NUM> may be connected in series, the voltage across a string of panels may reach dangerous levels. Voltages as high as 600V may be common in domestic installations. Thus, the installer faces a danger of electrocution. The converters <NUM> that may be connected to the panels <NUM> may use built-in functionality to prevent such a danger. For example, the converters <NUM> may include circuitry or hardware of software safety module that limits the output voltage to a safe level until a predetermined minimum load may be detected. Only after detecting this predetermined load does the microcontroller <NUM> ramps up the output voltage from the converter <NUM>. Another method of providing a safety mechanism may be to use communications between the converters <NUM> and the associated inverter for the string or array of panels. This communication, that may be for example a power line communication, may provide a handshake before any significant or potentially dangerous power level may be made available. Thus, the converters <NUM> would wait for an analog or digital release signal from the inverter in the associated array before transferring power to inverter. The above methodology for monitoring, control and analysis of the DC power sources <NUM> may be implemented on solar panels or on strings or arrays of solar panels or for other power sources such as batteries and fuel cells.

Reference is now made to <FIG>, which illustrates graphically behavior of power output in <FIG> from solar panels <NUM> (and which is input to inverter module <NUM>) as a function of current in conventional system <NUM>. Power increases approximately linearly until a current at which a maximum power point MPP may be found which may be some average over the MPP points of all connected solar panels <NUM>. Conventional MPPT module <NUM> locks (e.g., converges) on to the maximum power point.

Reference is now also made to <FIG> which illustrates graphically power input or power output versus output current from series/parallel connected modules <NUM> or strings <NUM> (<FIG>). It may be readily seen that by virtue of control circuit <NUM> in modules <NUM>, power as a function of current output may be approximately constant. Similarly, power as a function of voltage output may be approximately constant. It is desirable and it would be advantageous to have a system in which modules <NUM> and/or string <NUM> of <FIG> operate with the conventional inverter <NUM> equipped with an MPPT module <NUM> of <FIG>. However, as shown in <FIG>, MPPT <NUM> does not have a maximum power peak (versus current or voltage) on which to lock on to and MPPT circuit <NUM> may become unstable with varying or oscillating current/voltage at the input of inverter module <NUM>. In order to avoid this potential instability, according to a feature, a maximum power at an output voltage or current at least for a time period may be output or presented to conventional inverter module <NUM> equipped with MPPT module <NUM> according to various aspects.

Reference is now made to <FIG> which illustrates in a simplified block diagram of a photovoltaic distributed power harvesting system <NUM> including photovoltaic panels 100a-101d connected respectively to power converter circuits 305a - 305d. Solar panel <NUM> together with its associated power converter circuit <NUM> forms photovoltaic module <NUM>. Each converter 305a - 305d adapts to the power characteristics of the connected solar panel 101a - 101d and transfers the power efficiently from converter input to converter output. Each converter 305a - 305d includes control circuit <NUM> that receives a feedback signal from the input from solar panel <NUM>. Control circuit <NUM> may be a maximum power point tracking (MPPT) control loop. The MPPT loop in converter <NUM> locks the input voltage and current from each solar panel 101a - 101d to its optimal power point (i.e., to converge on the maximum power point).

System <NUM> includes a series and/or parallel connection between outputs of strings <NUM> and the input of a conventional inverter <NUM> with an integrated MPPT module <NUM>. Inverter <NUM> with integrated MPPT module <NUM> is designed to be connected directly to the outputs with series/parallel connections of conventional solar panels <NUM> as in conventional system <NUM> of <FIG>.

Referring back to <FIG>, MPPT algorithm <NUM> of microcontroller <NUM> in converters <NUM> may, in various embodiments, provide a slight maximum input power at a predetermined output voltage or current or conversion ratio into MPPT <NUM>. The input power into MPPT <NUM> may be maximized at a predetermined value of output voltage or current. In one embodiment, as shown in <FIG>, the maximum at the predetermined maximum power point may be very slight with a total variation of just a few percent to several percent over the entire input range of current or voltage of inverter <NUM>. In other embodiments, a circuit <NUM> disposed between panels <NUM> or strings <NUM> and inverter <NUM> may be used to present to MPPT module <NUM> with a maximum power point onto which to lock (e.g., converge).

Reference is now made to <FIG> which illustrates an embodiment of circuit <NUM> for generating a maximum power point at the input of MPPT module <NUM> in configuration <NUM> (<FIG>), according to an embodiment. Circuit <NUM> may be a power attenuator interposed between parallel-connected strings <NUM> and MPPT module <NUM>. Circuit <NUM> may include a non-linear current sink "f" configured to draw a small amount of current at a particular voltage or voltage range from the DC power line connecting strings <NUM> to MPPT module <NUM>. The output of current sink "f" may be fed into the positive input of operational amplifier A1. The output of operational amplifier A1 feeds the base of transistor T1, the emitter of which may be connected and fed back to the negative input of operational amplifier A1. The collector of transistor T1 connects to the positive DC power line. The negative DC power line may be connected to the emitter of transistor T1 through a shunt resistor Rs.

Reference is now made to <FIG>, which illustrates a simplified method for operating modules <NUM> and/or strings <NUM> with inverter <NUM> equipped with an MPPT module <NUM>. Reference is also made again to <FIG> and <FIG>. The output voltage of power converter <NUM> is sensed (step <NUM>) across output terminals <NUM> and <NUM>. Control circuit <NUM> may be configured to set (step <NUM>) the input power received at the input terminals <NUM>/<NUM> to a maximum power for a predetermined output voltage point or voltage range or at a predetermined output current point or current range. The predetermined values may be stored in memory <NUM> and/or <NUM> or may be received through communications interface <NUM>. Away from the predetermined output voltage or predetermined output current, the control circuit may be configured to set (step <NUM>) the input power received at the input terminals to less than the maximum available power (i.e., decrease the input power in response to the difference between the output current and the predetermined current increasing, and increase the input power towards the maximum available power in response to the difference between the output current and the predetermined current decreasing). In certain variations, the predetermined output current values may be selected such that the output power of module <NUM> or string <NUM> is as shown in <FIG>. The predetermined output voltage values versus output power may be selected in a similar way. While <FIG> illustrates one possible embodiment, other embodiments may present MPPT module <NUM> with other output power versus current (or voltage) curves that have one or more local maximum to which the MPPT <NUM> can track and lock (e.g., converge). In this way, maximum power point tracking circuit <NUM>, if present, may stably track (step <NUM>) the voltage and/or current point or range. When a maximum is reached (decision block <NUM>), MPPT tracking circuit <NUM> locks (step <NUM>) onto the power point (e.g., the "predetermined point" in <FIG>).

Reference is now made to <FIG>, which illustrates in a simplified block diagram a photovoltaic distributed power harvesting system <NUM> including photovoltaic panels 101a-101d connected respectively to power converter circuits 905a - 905d. One solar panel <NUM> together with its associated connected power converter circuit <NUM> forms a photovoltaic module <NUM>. Each converter 905a - 905d adapts to the power characteristics of the connected solar panel 905a - 905d and transfers the power efficiently from converter input to converter output. Each converter 905a - 905d includes a control circuit <NUM> that receives a feedback signal from input sensor <NUM>. Specifically, input current sensors and/or voltage sensors <NUM> are used to provide the feedback to control circuit <NUM>. Control circuit <NUM> may also receive a signal from output current and/or output voltage sensors <NUM>.

Inverter <NUM> with integrated MPPT module <NUM> is designed to be connected directly to the outputs with series/parallel connections of conventional solar panels <NUM> as in conventional system <NUM> of <FIG>.

Although photovoltaic modules <NUM> may be designed to be integrated with inverters <NUM> it may be advantageous that each panel module <NUM> may also be integrated with a respective conventional inverter (similar to inverter <NUM>) between the converter <NUM> output and the serially connected outputs of module <NUM> (not illustrated). System <NUM> includes a series and/or parallel connection between outputs of strings <NUM> input to a conventional inverter <NUM> with an integrated MPPT module <NUM>.

Reference is now made to <FIG>, which illustrates another method <NUM> for operating modules <NUM>, and/or strings <NUM> with inverter <NUM> equipped with an MPPT module <NUM>. In step <NUM>, a scan is made by control circuit <NUM> making a variation of the voltage conversion ratio between input voltage and output voltage (Vout) of a power converter circuit <NUM>. During the variation, multiple measurements may be made (step <NUM>) of the input and/or output power (e.g., by measuring input and output current and voltage) of converter <NUM> for different voltage conversion ratios that are set by control circuit <NUM> during the variation. The power measurements made for each different voltage conversion ratio may then be used to determine (step <NUM>) the maximum power point of the connected photovoltaic source. From the determination of the maximum power point of the connected photovoltaic source, the voltage conversion ratio for the maximum point may be used to set (step <NUM>) the conversion ratio for a continued operation of converter <NUM>. The continued operation of converter <NUM> continues for a time period (step <NUM>) before applying another variation of the voltage conversion ratio in step <NUM>.

Reference is now made to flow diagrams of <FIG>, according to various aspects. Power converter <NUM> may control output voltage by varying (step <NUM>) the output voltage from power converter <NUM>. The input voltage to power converter <NUM> may be maintained at the maximum power point. The conversion ratio defined as the ratio of input voltage to output voltage may be varied or perturbed to slowly approach (step <NUM>) maximum power on the output terminals. The term "slowly" as used herein is relative to the response time of MPPT circuit <NUM> associated with load <NUM>. The conversion ratio or output voltage may be selected.

By adjusting the conversion ratio of the power converter, the efficiency of the converter can be adjusted, thereby increasing or decreasing the output power for a received input power. Thus, in one example, while a maximum power point is maintained at the power converter input, the output can be adjusted to increase the output power to provide a maximum power point for MPPT <NUM> (e.g., predetermined point in <FIG>).

Since the output power from power converter <NUM> approaches slowly maximum power, MPPT circuit <NUM> responds accordingly and locks onto the output voltage at maximum output power. Referring now to <FIG>, in the meantime MPPT circuit <NUM> associated with load <NUM> tracks the slow variation of output power from photovoltaic modules <NUM>. In <FIG>, a graph is shown which indicates the slow variation of output power from photovoltaic modules <NUM>, which varies typically over many seconds (DT).

According to various embodiments, the processes of 9a and 9b may be performed in conjunction with other previously described embodiments to move the maximum power point presented to the inputs of MPPT circuit <NUM>. For example, the maximum point illustrated in <FIG> or (other maximum point) may be shifted to a different current and/or voltage such that maximum power is maintained over changing power production and conversion conditions (e.g., light, temperature, faults, etc.) of systems <NUM>/<NUM>/<NUM>/<NUM>/<NUM>. The rate of adapting the system (e.g., moving the peak) is slower than the tracking rate of MPPT <NUM>, such that the MPPT maintains lock (e.g., convergence) on the current/voltage/power at its input of inverter <NUM> within the power peak (e.g., the "maximum point" in <FIG>).

Reference is now made to <FIG>, which together illustrate another process that allows systems <NUM>/<NUM> to be integrated with inverter <NUM> equipped with MPPT circuit <NUM>. In <FIG>, MPPT circuit <NUM> perturbs (step <NUM>) voltage or current across string <NUM>. Control circuit <NUM> senses (step <NUM>) the voltage or current perturbation of MPPT circuit <NUM>. Control circuit <NUM> via sensor <NUM> in step <NUM> slowly maximizes output power at a particular voltage conversion ratio of converter <NUM>. Input power from a photovoltaic panel <NUM> may be maximized. In decision block <NUM>, a maximum output power is being reached and in step <NUM> MPPT <NUM> locks onto the maximum output power.

Claim 1:
A method comprising:
converting input power, received from a direct current power source (<NUM>), at input terminals of a power converter (<NUM>) to output power at output terminals of the power converter;
drawing the input power according to a maximum power point tracking algorithm, wherein the power converter (<NUM>) comprises a control circuit (<NUM>); and characterised by
while drawing the input power according to the maximum power point tracking algorithm, varying, by the control circuit (<NUM>), the output power to present, for a time period, a reduced output power point and then to present a maximum output power point by adj usting the conversion efficiency of the power converter.