Abstract:
A converter circuit providing multiple current bypass routes between the output leads to provide reliability in a series connection of several converters. If the converter malfunctions due to component failure, the current bypass routes provide a path for the current that views the malfunctioning converter as substantially a short. Diodes prevent backflow into the power source connected to the converter. Redundancy is provided in the bypass portions of the converter circuit that provides alternate parallel paths in case a defective component in one of the paths opens the circuit along that path. In one example, the converter is implemented as a buck plus boost converter where either the buck or the boost portion or both are operative responsive to a controller controlling the switches of both portions. Most of the converter circuit may be implemented in an integrated circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority to U.S. Provisional Patent Applications, Ser. No. 60/868,851, filed Dec. 6, 2006, and titled “Distributed Solar Array Monitoring, Management and Maintenance,” Ser. No. 60/868,893, filed Dec. 6, 2006, and titled “Distributed Power Harvesting System for Distributed Power Sources,” 60/868,962, filed Dec. 7, 2006, and titled “System, Method and Apparatus for Chemically Independent Battery,” Ser. No. 60/908,095, filed Mar. 26, 2007, and titled “System and Method for Power Harvesting from Distributed Power Sources,” and Ser. No. 60/916,815, filed May 9, 2007, and titled “Harvesting Power From Direct Current Power Sources,” the entire content of which is incorporated herein by reference. Further, this Application is related to ordinary U.S. Patent Applications titled “Distributed Power Harvesting Systems Using DC Power Sources,” “Monitoring of Distributed Power Harvesting Systems Using DC Power Sources,” “Removable Component Cartridge for Increasing Reliability in Power Harvesting Systems,” “Battery Power Delivery Module,” and “A Method for Distributed Power Harvesting Using DC Power Sources” that are filed in at the U.S. Patent and Trademark Office on Dec. 4, 2007 and incorporates the entire content of these applications by this reference. 
    
    
     BACKGROUND 
     1. Field of the Invention 
     The field of the invention relates generally to management of distributed DC power sources, and even more particularly to maintaining reliability in distributed DC power sources, such as fuel cells, solar array applications, etc., in the case of component malfunction. 
     2. Related Arts 
     Distributed power harvesting system generally comprise several DC power sources, such as, e.g., batteries, fuel sells, solar panels, etc., that are connected together to form a power supply. Batteries with numerous cells or hybrid vehicles with multiple fuel cells are examples of DC power sources whose power is accumulated through a series connection in a distributed power harvesting system. Another example is photovoltaic (PV) cells or solar panel power system. Solar energy is obtained from solar cells that provide a clean alternative source of energy. Solar installations include solar panels that convert the light energy to electric power and electronic power harvesting systems that harvest the electric power from the panels and convert it for domestic use. The electronic system is generally referred to in the art as balance of system (BoS). 
     Each of the individual DC power sources usually provides low voltage (e.g., batteries normally provide less than 3 volts) and low current. Consequently, DC-DC converters are used together with various series and parallel topologies to convert the DC power provided from the DC power sources into the required voltage and/or current. In some applications, each DC power source is connected to its own DC-DC converter, and the outputs of the converters are coupled in series or parallel topology. 
     Maintaining reliability in both series and parallel connections is important. Malfunction of one may disturb the operation of the entire installation. For example, in series connections an open circuit malfunction in one converter may stop the flow of current in the entire series connection. On the other hand, in parallel connection a short malfunction in one arm of the circuit would reduce the voltage between the parallel nodes to zero. 
       FIG. 1  illustrates one possible architecture for distributed power harvesting system. In the system of  FIG. 1 , each DC power source  101 , for example, battery, fuel cell, solar panel etc., is connected to its own associated AC module  109 . The AC module  109  may include a DC-to-DC converter  105  and an inverter  114  (when the load requires alternating current). The converter  105  is used for DC to DC conversion of the input voltage—usually as means of maximizing power output from DC source by staying at maximum power point. The inverter  114  is used for inversion of the DC input to an AC output. As such, the power conversion and inversion is distributed within the circuit as opposed to being performed on a centralized collection of the power from the entire circuit. The input of each AC module  109  is connected to one of the panels  101 . Each AC module may be used independently and individually. Alternatively, outputs of the AC modules  109  may be connected in parallel to an AC bus  111 . The AC bus  111  may be connected to the load, such as, for example, the electric box of a house. 
       FIG. 2  illustrates another possible architecture for distributed power harvesting system using multiple DC power sources. In the system of  FIG. 2 , each DC power source  101 , e.g., battery, fuel cell, solar panel, etc., is connected to its own associated DC-DC converter  205 . As such, the power conversion is distributed within the circuit as opposed to being performed on a centralized collection of the power from the entire circuit. The converters  205  are connected in series to provide a string of serially connected DC converters. The output from the series connection of the converters  205  is provided to the common inverter  104 . The converters  205  are DC-to-DC converters and the DC current and voltage are converted to an alternating current at the inverter  104 . 
     In power harvesting from distributed sources, if one of the components in a series-connected group of power sources fails, the circuit is liable to become open and disconnect the current. If one of the components in a parallel-connected group of power sources fails, the circuit is liable to short the current through the entire parallel connection and take the voltage to zero. The reliability of the components is crucial to the success of distributed installations. The cost of parts and labor for maintenance and replacement of parts are burdensome, especially when considering the fact that the components may be located on roofs and other hard-to-get locations. Therefore, there is a need to increase the overall reliability of the components in distributed power harvesting systems. 
     SUMMARY 
     The following summary of the invention is provided in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention, and as such it is not intended to particularly identify key or critical elements of the invention, or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
     According to aspects of the invention, there is provided a current bypass in converters used in distributed DC power sources system, such as, e.g., photovoltaic panels installation, to increase the overall reliability of the power harvesting circuit in the case of component failure, and allow other series-connected circuits to keep on functioning as normal. 
     According to aspects of the invention, a distributed power harvesting system is provided, comprising: a plurality of power sources each providing DC power; a plurality of converters, each converter comprising: input terminals receiving input voltage from, and coupled to, a respective power source from the plurality of power sources; output terminals for providing an output voltage; circuitry receiving and converting the input voltage to the output voltage; and at least one bypass path providing a path bypassing at least part of the circuitry. The circuitry may comprise: a buck portion providing a stepped down output voltage; a boost portion providing a stepped up output voltage; and a controller selectively engaging the buck portion or the boost portion or both. The distributed power harvesting system may further comprise: a plurality of maximum power point tracking circuits, each coupled to a respective power source for tracking power input from the respective power source; and wherein at each of the converters, the controller selectively engages the buck portion or the boost portion or both in response to signal from the maximum power point tracking circuit. The bypass path may circumvent the buck portion and the boost portion. The bypass path may pass through the buck portion. The bypass path may pass through the boost portion. 
     The bypass path may comprise: a first current path circumventing the buck portion and the boost portion; a second current path passing through the buck portion; and a third current path passing through the boost portion. The buck portion and the boost portion may share an inductive coil; wherein at least one of the first, second and third, current paths passes through the inductive coil; and, wherein at least one of the first, second and third, current paths circumvents the inductive coil. The buck portion and boost portion may share an inductor; and wherein the buck portion comprises a buck switching circuitry and a buck switching bypass path; and wherein the boost portion comprises a boost switching circuitry and a boost switching bypass path. The buck switching circuitry may comprise a plurality of buck switching elements and the buck switching bypass path comprises a plurality of current paths, each bypassing one of the buck switching elements; and the boost switching circuitry may comprise a plurality of boost switching elements and the boost switching bypass path comprises a plurality of current paths, each bypassing one of the boost switching element. The output terminals are coupled in series to at least one other converter, to thereby generate a series connection of the plurality of converters, the distributed power harvesting system may further comprise an inverter coupled to the series connection of the plurality of converters and changing a direct current input from the converters to an alternating current output. The system may further comprise: an inverter coupled to each of the plurality of converters and forming an AC module together with the converter, wherein the AC modules are coupled in parallel to provide a collective current from the distributed power harvesting system, and wherein each converter comprises at least one current blocking element for preventing a short through the converter in a reverse direction. A portion of each of the converters may be implemented in an integrated circuit. 
     According to further aspects of the invention, a DC-to-DC power converter for converting an input voltage from a DC power source to an output voltage is provided, the DC-to-DC power converter comprising: a buck portion providing a stepped down output voltage; a boost portion for providing a stepped up output voltage; an inductor coupled to the buck portion and the boost portion; and a controller selectively engaging the buck portion or the boost portion or both. The converter may further comprise a maximum power point tracking (MPPT) circuit for providing a MPPT input signal, and wherein the controller selectively engages the buck portion or the boost portion or both in response to the MPPT input signal. The converter may further comprise a boost bypass path providing a current path bypassing the boost portion through the buck portion. The converter may further comprise: a diode coupled in parallel with a first switch of the buck portion, wherein a current through the diode and a current through a parasitic diode associated with the first switch are parallel. The converter may further comprise a buck bypass path providing a current path bypassing the buck portion through the boost portion. The converter may further comprise: a diode coupled in parallel with a first switch of the boost portion, wherein a current through the diode and a current through a parasitic diode associated with the first switch are parallel. A portion of the DC-to-DC power converter may be implemented in an integrated circuit. 
     According to other aspects of the invention, a distributed power harvesting system is provided, comprising: a plurality of power sources each providing DC power; a plurality of converters, each converter comprising: a buck portion providing a stepped down output voltage from the DC power; a boost portion for providing a stepped up output voltage from the DC power; an inductor coupled to the buck portion and the boost portion; at least one bypass path providing a path bypassing at least on of the buck portion and the buck portion; and a controller selectively engaging the buck portion or the boost portion or both. The distributed power harvesting system may further comprise: a plurality of maximum power point tracking circuits, each tracking DC power from a respective power source; and wherein the controller of each of the plurality of the converters independently selectively engages the buck portion or the boost portion or both in response to a signal from a respective maximum power point tracking circuit. Each of the plurality of power sources may comprise a solar panel. Each of the plurality of power sources may be a fuel cell. Each of the plurality of converters may further comprise a boost bypass path providing a current path bypassing the boost portion through the buck portion. Each of the plurality of converters may further comprise a buck bypass path providing a current path bypassing the buck portion through the boost portion. The distributed power harvesting system may further comprise an inverter coupled to a series connection of the plurality of converters and changing a direct current input from the converters to an alternating current output. 
     According to further aspects of the invention, a method for providing one or more current bypass routes in a series connection of power cells is provided, the method comprising: coupling each of the power cells to a corresponding converter; coupling output leads of the converters in series; and providing a plurality of current bypass routes in each of the converters, wherein the current bypass routes provide routes between the output leads of each of the converters from a negative output lead to a positive output lead and prevent current flow from the positive output lead to the negative output lead. The method may further comprise: providing a buck portion in each of the converters; providing a boost portion in each of the converters; and selectively activating either the buck portion or the boost portion or both using a controller included in each of the converters, wherein at least one of the current bypass routes is provided in the buck portion and at least one of the current bypass routes is provided in the boost portion. The method may further comprise providing overall bypass routes in each of the converters, the overall bypass route passing outside the buck portion and the boost portion. The method may further comprise: providing redundancy by forming some of the current bypass routes parallel to portions of other current bypass routes. 
     According to yet other aspects of the invention, a distributed DC photovoltaic power harvesting system is provided, comprising: a plurality of solar panels, each converting solar energy into electrical current; a plurality of converters, each coupled to one of the solar panels, and each providing converted output voltage; and an inverter coupled to a series connection of the converters and changing a direct current input from the converters to an alternating current output; wherein each of the converters includes: a negative input lead and a positive input lead; a negative output lead and a positive output lead; a first diode coupled to the negative output lead; a second diode coupled to the positive output lead; a maximum power point tracking circuit for tracking power input from the solar panel; a buck portion for providing a stepped down output voltage from the converter; a boost portion for providing a stepped up output voltage from the converter; an inductor coupled between the first diode and the second diode, the inductor being shared by the buck portion and the boost portion; and a controller for determining whether the buck portion or the boost portion or both are operating at a given time responsive to the maximum power point tracking circuit; wherein a first current bypass route passes from the negative output lead to the first diode, to an inductor, to the second diode and to the positive output lead, wherein the first diode and the second diode prevent current flow from the positive output lead to the negative output lead, wherein the buck portion includes: a first switch coupled between the positive input lead and the inductor and being controlled by the controller; and a second switch coupled between the negative input lead and the inductor and being controlled by the controller, wherein a second current bypass route passes from the negative output lead to the second switch, to the inductor and to the positive output lead; wherein a third current bypass route passes from the negative output lead to the first diode, to the inductor, to the second diode and to the positive output lead, and wherein the first diode is parallel with the second switch; wherein the boost portion includes: a third switch coupled between the inductor and the positive output lead and being controlled by the controller; and a fourth switch coupled between the negative input lead and the inductor and being controlled by the controller, wherein a fourth current bypass route passes from the negative output lead to the fourth switch, to the third switch and to the positive output lead, wherein a fifth current bypass route passes from the negative output lead to the first diode, to the inductor, to the second diode and to the positive output lead, and wherein the second diode is parallel with the third switch. 
     According to yet other aspects of the invention, a solar panel is provided, comprising: one or more solar cells, each converting solar energy to electrical energy; one or more switching devices, each connected across a respective solar cell thereby forming a bypass path. The switching device may comprise a transistor. The solar panel may further comprise one or more diodes, each coupled across a respective solar cell to form a second bypass path. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
         FIG. 1  illustrates one possible architecture for distributed power harvesting system. 
         FIG. 2  illustrates another possible architecture for distributed power harvesting system using multiple DC power sources. 
         FIG. 3  shows a distributed power harvesting system using DC power sources according to aspects of the invention. 
         FIG. 4  is a power converter module, according to aspects of the invention. 
         FIG. 5  shows a buck portion of one aspect of the power converter module of  FIG. 4 . 
         FIG. 6  shows a boost portion of one aspect of the power converter module of  FIG. 4 . 
         FIG. 7  shows one current bypass path in the power converter module of  FIG. 4 . 
         FIG. 8  shows another current bypass path in the power converter module of  FIG. 4 . 
         FIG. 9  shows a third current bypass path in the power converter module of  FIG. 4 . 
         FIG. 10  shows a distributed power harvesting system using AC modules formed from DC power sources, according to aspects of the invention. 
         FIG. 11  illustrates an arrangement of a solar panel according to the prior art. 
         FIG. 12  illustrates an arrangement according to an embodiment of the invention for reducing the power loss in solar strings. 
         FIG. 13  illustrates another arrangement according to an embodiment of the invention for reducing the power loss in solar strings. 
         FIG. 14  illustrates an arrangement according to an embodiment of the invention for bypassing a solar string. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present invention provide a DC-to-DC converter that includes both current bypass paths and current blocking paths. The current bypass paths are for preventing the converter to open a series connected circuit. The current blocking paths are for preventing the converter from shorting a parallel connected circuit. 
     Aspects of the present invention provide a current bypass mechanism for the electrical power converters that are connected together in series in a distributed power harvesting system. According to aspects of the invention, each converter has one or more current bypass paths on failure. As a result, upon failure of one of the electrical power converters, current still flows through the failed electrical power converter and does not cut current from the entire series connection of the power sources. While described in the context of solar power technology, the reliability enhancing aspects of the present invention may be used in converters used in any distributed power network utilizing converters. For example it may be used to increase the reliability of batteries with numerous cells or hybrid vehicles with multiple batteries or fuel cells on board. The use of solar panels in the following is to provide better understanding by way of a concrete example. 
     Distributed power harvesting systems, according to embodiments of the present invention, provide a system for combining power from multiple direct-current electrical power sources. The power sources are each connected as inputs to one of multiple electrical power converters. In this particular example, each electrical power converter converts input power to output power by monitoring and controlling the input power at a maximum power level. Outputs of the electrical power converters are connected into a series-connected direct-current output. If an AC current is ultimately desired, an inverter may be used to invert the series-connected direct-current output of the converters into an alternating-current output from the inverter. The inverter may operate according to conventional inverter operation, i.e., control the output according to the specification dictated by the load. However, in this particular example, the inverter maintains its input voltage at a previously-determined voltage by varying the amount of current drawn from the series-connected converters. 
     For each electrical power converter, substantially all the input power is converted to the output power, and the controlling is performed by fixing the input current or voltage to the maximum power point of the DC power source, and allowing output voltage to vary. In each converter, a controller may perform the controlling by adjusting duty cycle using pulse width modulation, thereby transferring power from the input to the output. The controller may be a digital or an analog controller. The direct-current electrical power sources may be solar cells, solar panels, electrical fuel cells, electrical batteries, and the like. For each power source, one or more sensors provide data needed to perform the monitoring of the input power level. 
     In one aspect of the invention, each of the electrical power converters, used in the distributed power harvesting system, has a current bypass path. As a result, upon a failure in one of the electrical power converters which will prevent power harvesting from the module, current from the other modules in the string still flows through that failed electrical power converter. 
       FIG. 3  illustrates a distributed power harvesting and conversion configuration  40 , according to aspects of the present invention. Configuration  40  enables connection of multiple power sources, for example solar panels  401 , to a single power supply. The series coupling of all of the solar panels is connected to an inverter  404 . Instead of the inverter, a DC charge and discharge circuit may be used. 
     In configuration  40 , each solar panel  401  is connected to a separate power converter circuit  405 . The solar panel  401  and its associated power converter circuit  405  together form a module. Power converter circuit  405  adapts optimally to the power characteristics of the connected solar panel  401  and transfers the power efficiently from input to output. Power converters  405  can be buck converters, boost converters, buck/boost converters, flyback or forward converters. The converters  405  may also contain a number of component converters, for example a serial connection of a buck and a boost converter. 
     Each converter  405  includes a control loop that receives a feedback signal, not from the output current or voltage, but rather from the input coming from the solar panel  401 . An example of such a control loop is a maximum power point tracking (MPPT) loop in solar array applications. The MPPT loop in the converter locks the input voltage and current from each solar panel  401  to its optimal power point. The MPPT loop of the converter  405  operates to perform maximum power point tracking and transfers the input power to its output without imposing a controlled output voltage or output current. 
     Converters  405 , or the modules including the panels  401  and their associated converters  405 , can be connected in series to form strings and the series connection of the modules are coupled in parallel to form arrays. 
     In conventional DC-to-DC voltage converters, the controller regulates the output voltage by monitoring the current or voltage at the input, and the current and voltage at the output. The controller determines the appropriate pulse width modulation (PWM) duty cycle to fix the output voltage to the predetermined value increasing the duty cycle if the output voltage drops while varying the current extracted from the input. In converters  405 , according to embodiments of the present invention, the controller monitors the voltage and current at its input and determines the PWM in such a way that maximum power is extracted, dynamically tracking the maximum power point at its input. In embodiments of the present invention, the feedback loop is closed on the input power in order to track maximum power rather than closing the feedback loop on the output voltage as performed by conventional DC-to-DC voltage converters. 
     The outputs of converters  405  are series connected into a single DC output into the inverter  404 , which converts the series connected DC output to an alternating current power supply. 
     The circuit of  FIG. 3  provides maximum power available during continuous operation from each solar panel  401  by continuously performing MPPT on the output of each solar panel to react to changes in temperature, solar radiance, shading or other performance deterioration factors of each individual solar panel  401 . As a result of having a separate MPPT circuit in each converter  405 , and for each solar panel  401 , in the embodiments of the present invention, each string  403  in the embodiment shown in  FIG. 3  may have a different number of panels  401  connected in series. Furthermore panels  401  can be installed in different orientations, as solar panels  401  do not have to be matched and partial shading degrades the performance of only the shaded panel. According to embodiments of the present invention, the MPPT circuit within the converter  405  harvests the maximum possible power from panel  401  and transfers this power as output regardless of the parameters of other solar panel  401 . 
     Another aspect of the present invention is to provide a greater degree of fault tolerance, maintenance and serviceability by monitoring, logging and/or communicating the performance of each solar panel  401 . A controller used in the MPPT circuit of the converter  405 , that is used to perform MPPT individually on each of the solar panels  401 , 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 are also beneficial for quick detection of problems during maintenance work. Aspects of the present invention allow easy location, repair, or replacement of failed solar panels  401 . When repair or replacement is not feasible, bypass features of the current invention provide increased reliability. 
       FIG. 4  illustrates an exemplary DC-to-DC converter  405  according to aspects of the invention. DC-to-DC converters are used to either step down or step up a DC voltage input to a higher or a lower voltage output depending on the requirements of the circuit. The converter  405  is connected to a corresponding solar panel  401  at input terminals  914  and  916 . The converted power of the solar panel  401  is output to the circuit through output terminals  910 ,  912 . Between the input terminals  914 ,  916  and the output terminals  910 ,  912 , the remainder of the converter circuit is located that includes input and output capacitors  920 ,  940 , backflow prevention diodes  922 ,  942  and a power conversion circuit including controller  906  and an inductor  908 . 
     The inputs  916  and  914  are separated by a capacitor  920 , which acts as an open circuit to a DC voltage. The outputs  910  and  912  are also separated by a capacitor  940  that also acts an open to DC output voltage. These capacitors are DC-blocking or AC-coupling capacitors that short when faced with alternating current of a frequency for which they are selected. 
     Capacitor  940  coupled between the outputs  910 ,  912  also operates as a part of the buck and the boost converters discussed below. 
     Diode  942  is coupled between the outputs  910  and  912  with a polarity such that current may not backflow into the converter  405  from the positive lead of the output  912 . Diode  922  is coupled between the positive output lead  912  through inductor  908 , which acts as a short for DC current and the negative input lead  914  with such polarity to prevent a current from the output  912  to backflow into the solar panel  401 . 
     A potential difference exists between the wires  914  and  916  due to the electron-hole pairs produced in the solar cells of panel  401 . The converter  405  maintains maximum power output by extracting current from solar panel  401  at its peak power point by continuously monitoring the current and voltage provided by the panel and using a maximum power point tracking algorithm. The controller  906  includes an MPPT circuit for performing the peak power tracking. Peak power tracking and pulse width modulation, PWM, are performed together to achieve the desired input voltage and current. 
     The generated power is then transferred to output terminals  910  and  912 . The outputs of multiple converters  405  may be connected in series, i.e. the positive lead  912  of one converter  405  is connected to the negative lead  910  of the next converter  405 . 
     The converter  405  is shown as a buck plus boost converter. The term “buck plus boost” as used herein is a buck converter directly followed by a boost converter as shown in  FIG. 4 . If the voltage is to be lowered, the boost portion is substantially shorted. If the voltage is to be raised, the buck portion is substantially shorted. The term “buck plus boost” differs from buck/boost topology, which is a classic topology that may be used when voltage is needed to be raised or lowered. The efficiency of “buck/boost” topology is inherently lower then a buck or a boost. Therefore, the buck plus boost topology of  FIG. 4  has a higher efficiency than the buck/boost topology. However, the circuit has to continuously decide whether it is bucking or boosting. The buck and boost portions of the converter  405  are described with reference to  FIGS. 5 and 6 . 
     The controller  906  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  906  controls both the buck converter and the boost converter and determines whether a buck or a boost operation is to be performed. 
     In one implementation, an integrated circuit (IC)  904  may be used that incorporates some of the functionality of converter  405 . IC  904  is optionally a single ASIC able to withstand harsh temperature extremes present in outdoor solar installations. ASIC  904  may be designed for a high mean time between failures (MTBF) of more than 25 years. However, a discrete solution using multiple integrated circuits may also be used in a similar manner. In the exemplary embodiment shown in  FIG. 4 , the buck plus boost portion of the converter  405  is implemented as the IC  904 . Practical considerations may lead to other segmentations of the system. For example, in one aspect of the invention, the IC  904  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 which may alternatively be implemented in discrete components, FET drivers, A/Ds, and the like. The digital IC may form the controller  906 . 
       FIG. 5  shows the buck portion of the power converter module  405 , according to embodiments of the present invention. In the exemplary circuit shown, the buck converter includes the input capacitor  920 , transistors  928 , and  930  a diode  922  positioned in parallel to transistor  928 , and an inductor  908 . The transistors  928 ,  930  each have a parasitic body diode  924 ,  926 . Note, that  FIG. 5  may also represent an embodiment wherein the converter comprises solely of a buck converter. 
     A buck converter is a step down DC-to-DC converter that, in its simplest form, uses two switches, an inductor and a capacitor. The two switches control the flow of current to the inductor. The buck converter alternates between connecting the inductor to the source voltage to store energy in the inductor and discharging the inductor into the load. In a perfect and loss-less circuit, the ratio of the output voltage to the input voltage is equal to the duty cycle of the switch that is providing current to the inductor. The duty cycle of the switch is the ratio of the on-time of the switch to the entire period, T, of the switch. Because this ratio is always less than or equal to one, the output voltage is always less than or equal to the input voltage. 
       FIG. 6  shows the boost portion of the power converter module  405 , according to embodiments of the present invention. In the exemplary circuit shown, the boost converter includes the inductor  908 , which is shared with the buck converter, transistors  948  and  950  a diode  942  positioned in parallel to transistor  950 , and the output capacitor  940 . The transistors  948 ,  950  each have a parasitic body diode  944 ,  946 . Note, that  FIG. 6  may also represent an embodiment wherein the converter comprises solely of a boost converter. 
     A boost converter is a step up DC-to-DC converter that, in its simplest form, also uses two switches, an inductor and a capacitor. The two switches control the flow of current to the inductor. The boost converter alternates between connecting the inductor to the source voltage to store energy in the inductor and discharging the inductor into the load while also storing this energy in the capacitor. In a perfect and loss-less circuit, the ratio of the output voltage to the input voltage is equal to the inverse of the off portion of the switch that is providing current to the inductor. The off portion is one minus the duty cycle of the switch. Because this ratio is always greater than or equal to one, the output voltage is always greater than or equal to the input voltage. 
     Under some operating conditions, either the buck or boost converter, but not both, are used at any given time, at the discretion of the controller  906 . Under some other operating conditions, when the desirable output voltage is similar to the input voltage, both the buck and the boost converters may be used in tandem. 
     The controller  906  is coupled to the transistors  928 ,  930 ,  948  and  950  and controls the operation of these transistors. The controller  906 , therefore, can determine whether the buck converter or the boost converter is being used. If buck conversion is used, transistor  950  is shorted, providing a current path from the inductor  908  to the positive output lead  912 , and transistor  948  is left open, effectively bypassing the boost converter. Similarly, if boost conversion is used, transistor  930  is shorted, providing a current path from the positive input lead  916  to the inductor  908 , and  928  is left open, effectively bypassing the buck converter. 
     In  FIG. 5 , during the on-state of the buck portion, the controller  906  turns on the transistor  930  to connect the solar panel  401  to the inductor  908 . All other transistors may be off or  950  may be shorted to provide a current path from the inductor without having to go through diode  942 . During the on-state of the transistor  930 , the input  916  is coupled to the output  912  through the transistor  926  and the inductor  908  and energy is being stored in the inductor  908 . During the off-state of the buck portion of the converter  405 , the controller  906  turns off the transistor  930  and turns on the transistor  928 . The inductor  908  is cut off from the panel  401  that is providing power to it and the current through the inductor  908  decreases. The period of the buck portion is the sum of the on-time of the switch  930  and the on-time of switch  928 . Over this period, the ratio of the voltage output from the converter  405  between outputs  910 ,  912  to the voltage input to the converter  405  between inputs  914 ,  916  is substantially equal to the ratio of the on-time of the switch  930  to the sum of the on-time of switch  930  plus the on-time of switch  928 . 
     In  FIG. 6 , during the on-state of the boost portion, the controller  906  turns on the transistor  948  to connect the solar panel  401  to the inductor  908 . All other transistors are off except for transistor  926  that is shorted to provide a current path from the panel  401  to the inductor  908 . During the on-state of the transistor  948 , the inputs  914 ,  916  are coupled to the inductor  908  and energy is being stored in the inductor  908 . The inputs  914 ,  916  and outputs  910 ,  912  of the converter  405  are disconnected from each other. During the off-state of the boost portion of the converter  405 , the controller  906  turns off the transistor  948  and turns on the transistor  950 . The inductor  908  is connected to the outputs  910 ,  912  and the energy stored in the inductor is also stored in the capacitor  940 . The period of the boost portion is the sum of the on-time of the switch  948  and the on-time of switch  950 . Over this period, the ratio of the voltage output from the converter  405  between outputs  910 ,  912  to the voltage input to the converter  405  between inputs  914 ,  916  is substantially equal to the inverse of the on-time of the switch  950 . 
     Reliability of the entire system is maintained given the distributed nature of aspects of the present invention. There are numerous converters  405  connected in each installation such that a failure in a single module presents the threat of causing an entire string  403  to malfunction. For example, if outputs  910 ,  912  of a single converter  405  are disconnected and converter  405  ceases to function, there is no longer a closed circuit connection through string  403 . In order to prevent such a global failure, the converter  405  is designed to naturally bypass current in case of a failure in converter  405 . Thus, only the power output from solar panel  401  attached and adjacent to failed converter  405  is affected, and all other solar panels  401  and converters  405  continue to normally provide power. Further, although a buck plus boost structure is shown in  FIG. 4 , other converter topologies, such as push-pull, flyback or forward converters, may be used with the similar capabilities for current bypass on failure. 
     In case of a failure in some other portion of the converter  405 , there are several possible current routes. These current routes permit the current to bypass the faulty converter  405  and maintain a closed circuit in the string. When the converter  405  is constructed and a buck and boost circuit according to embodiments of the invention, at least two bypass circuits are included, one providing a bypass in case of the buck converter failure, and one providing a bypass in case of the boost converter failure. Additionally, bypass is provided in case of failure in the coil. According to one embodiment, the bypass circuit of the boost converter also serves as a bypass circuit in case of the coil&#39;s failure. A further overall bypass circuit is provided in case of failure of both the buck and boost converters. In one embodiment, this overall bypass remains active during all modes of operation, enabling at least part of the current from other power sources in the series to pass there-through. Additionally, when the converter  405  includes switches that are liable to fail, such as, e.g., transistors, each such switch is provided with a bypass, which may be a diode, or any active or passive circuit which serves this purpose. 
     The bypass mechanism (either diode or other) is not electronically stressed during the normal operation of the circuit, and only comes into play once a fault occurs. This is important, since the useful lifetime of these components is not reduced due to stress, and therefore they have a high probability to properly function. 
       FIG. 7  shows one current bypass path in the power converter module of  FIG. 4 . The bypass circuit illustrated in  FIG. 7  is the overall bypass circuit that enables continued operation of the series power system in case of total failure of converter  405 . If IC  904  ceases to function and all its terminals disconnect, there is still a current path from terminal  910 , through diode  922 , inductor  908 , diode  942  and out of terminal  912 . Notably, in this embodiment this path remains active during all modes of operation of converter  405 , i.e., even when converter  405  operates properly. Therefore, at least part of the current coming from up-stream sources may pass through this path. As illustrated in  FIGS. 5 and 6 , this bypass can also be implemented when only a buck or a boost converter is used. 
       FIG. 8  shows another current bypass path in the power converter module of  FIG. 4 . Actually, what is shown in  FIG. 8  is a triple bypass. One possible current path is from terminal  910  to diode  922  to inductor  908  to diode  942  and out of terminal  912 , just as that shown in  FIG. 7 . As for another path, instead of diode  922 , e.g., if diode  922  fails, the current may pass through the body diode  924  that is in parallel with the diode  922 . This path is also available should the buck converter fail, e.g., transistor  928  fails. As for a further path, instead of diode  942 , e.g., if diode  942  fails, the current may pass through the body diode  946  that is in parallel with the diode  942 . As illustrated in  FIG. 5 , this bypass can also be implemented when only a buck converter is implemented. 
       FIG. 9  shows yet another current bypass path in the power converter module of  FIG. 4 . In case the buck converter fails, another possible current path is from terminal  910  to body diode  944  to diode  942  and to terminal  912 . Instead of diode  942 , the current may go through the body diode  946  that is in parallel with the diode  942 . Notably, if inductor  908  fails, e.g., disconnects from the circuit or has one of its winding broken or burned, this bypass path is still available for the current and the current may still flow through body diode  944 . Thus, even if converter  405  fails the rest of the solar array installation continues to function normally and to produce power. As illustrated in  FIG. 6 , this bypass can also be implemented when only a boost converter is used. 
     When the circuit is implemented in an IC, the inductor  908  is not shown as part of the IC  904 . However, in one implementation it may be implemented as a part of the IC  904 . In one implementation automotive silicon processes that are designed to withstand high voltages and high temperatures are used to develop the IC. In one implementation, the transistors may have a current rating of 30 A and voltage rating of over 100V, the capacitors may be ceramic capacitors having a capacitance of 1 μF at 80V, the inductor may be high power 10 μH inductor at 20 A. The diodes may be implemented using power diodes at 20 A and diode voltage of 0.4V, and they may either be implemented inside the IC or outside of it, as discrete components. 
       FIG. 10  shows a distributed power harvesting system using AC modules formed from DC power sources, according to aspects of the invention. In the system of  FIG. 10 , each DC power source  1001  is connected to its own AC module  1003 . The AC module  1003  includes a DC-to-DC converter  1005  and an inverter  1004 . The converter  1005  is used for DC to DC conversion of the collected voltage. The inverter  1004  is used for inversion of the DC input to an AC output. The input of each AC module  1003  is connected to one of the panels  1001 . Outputs of the AC modules  1003  may be connected in parallel to an AC bus  1110 . 
     The converter  1005  used in the AC module  1003  of  FIG. 10 , may be similar to the converter  902  of  FIG. 4  according to aspects of the invention. Then if some of the components of the converter short due to failure, the diodes  942  and  922  prevent a short to occur across the converter. 
     As shown in  FIG. 3  and  FIG. 10  adding electronic elements in the series or parallel arrangement may reduces the reliability of the system, because if one electrical component breaks or fails it may affect the entire system. For a series-connected installation, if a failure in one of the serially connected elements causes an open circuit in the failed element, current ceases to flow through the entire series, thereby causing the entire system to stop function. For a parallel-connected installation, if a failure in one of the parallel connected elements causes a short circuit in the failed element, all current flows through the shorted element, thereby causing the voltage across the parallel nodes to go to zero. Aspects of the present invention provide a converter circuit where electrical elements of the circuit have one or more bypass routes associated with them that carry the current in case of the electrical element fails. For example, each switching transistor of either the buck or the boost portion of the converter has its own bypass. Upon failure of any of the switching transistors, that element of the circuit is bypassed. Also, upon inductor failure, the current bypasses the failed inductor through the parasitic diodes of the transistor used in the boost converter. 
       FIG. 11  illustrates an arrangement of a solar panel according to the prior art. In  FIG. 11 , solar panel  1100  comprises solar cells  1105 , which are grouped into serially connected strings  1110 . The strings  1110  are connected together in series. For each string  1110 , a bypass diode  1120  is provided so that in the event of drop in power output of one string, that string may be bypassed via the respective diode  1120  instead of having the cells enter a negative voltage region, which will lead to power dissipation across them and may cause them to burn. However, when current flows through the diodes, they dissipate energy. For example, if a current of 5 A flows through a conventional diode having 0.7 volt cut-in voltage, the loss is 3.5 W. In practice the loss may easily amount to 10 W. 
       FIG. 12  illustrates an arrangement according to an embodiment of the invention for reducing the power loss in solar strings. In  FIG. 12 , the solar panel  1200  is made of solar cells  1205 , which are grouped into serially connected strings  1210 . The strings  1210  are connected together in series. For each string  1210 , a bypass diode  1220  is provided so that in the event of drop in power output of one string, that string may be bypassed via the respective diode  1220 . Additionally, one switching device, such as FET or IGBT (insulated gate bipolar transistor),  1225  is connected in a by-pass configuration so as to bypass the respective diode. Once it is sensed that current is flowing via one diode  1220  (or once the voltage across string  910  is sensed to be negative), its respective switching device  1225  is activated. This directs the current through the switching device, so that the loss of energy is drastically reduced. The sensing can be done by, for example, sensing the voltage across the string or the current across the diode. 
       FIG. 13  illustrates another arrangement according to an embodiment of the invention for reducing the power loss in solar strings. In  FIG. 13 , the solar panel  1300  is made of solar cells  1305 , which are grouped into serially connected strings  1310 . The strings  1310  are connected together in parallel. For each string  1310 , a bypass switching device  1325 , such as FET or IGBT, is provided so that in the event of drop in power output of one string, that string may be bypassed via the respective switching device  1325 . Once it is sensed that a string  1310  enters reverse bias (whether due to poor lighting or malfunction), the respective switching device  1325  is turned on so that current is flowing via its respective switching device  1325 . The sensing can be done by, for example, sensing the voltage or current of the string. 
       FIG. 14  illustrates an arrangement according to an embodiment of the invention for bypassing a solar string. That is,  FIG. 14  illustrates how a converter, such as, for example, the converter of  FIG. 6 , may be utilized to trigger the bypass of the solar string and/or a diode coupled across a solar string. In  FIG. 14 , the solar panel  1400  is made of solar cells  1405 , which are grouped into serially connected strings  1410 . The strings  1410  are connected together in parallel. For each string  1410 , a bypass diode  1420  is provided so that in the event of drop in power output of one string, that string may be bypassed via the respective diode  1420 . However, as explained with respect to  FIG. 13 , the diodes may be eliminated. Additionally, one switching device, such as FET or IGBT,  1425  is connected in a by-pass configuration so as to bypass the respective string  1410  and/or diode  1420 . Once it is sensed that a solar string enters reverse bias, its respective switching device  1425  is activated by the controller  906 . This directs the current through the switching device  1425 , so that the loss of energy is drastically reduced. The sensing can be done by, for example, sensing the voltage across the string or the current across the diode, as explained with respect to elements  703  and  704  of  FIG. 7 . 
     The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of hardware, software, and firmware will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims and their equivalents.