MODULAR SINGLE-STAGE STEP-UP PV CONVERTER WITH INTEGRATED POWER BALANCING FEATURE

A modular single-stage photovoltaic step-up converter system with integrated power balancing, the converter system comprising: an input stage comprising at least one converter module, the at least one converter module comprising a single voltage-sensor based maximum power point (MPP) tracking controller and a power circuit; and an output stage comprising an active voltage quadrupler (VQ) circuit for achieving balanced output voltages amongst each of the at least one converter modules, and wherein the least one converter modules are coupled together in a circular configuration.

FIELD

Aspects of the disclosure relate to medium and large-scale photovoltaic (PV) power conversion.

BACKGROUND

Due to the detrimental impact of greenhouse gases on our environment there has been a shift towards the use of renewable energy resources. These resources are abundant in nature and environmentally friendly compared with conventional energy resources such as fossil fuels. Photovoltaic (PV) energy has the highest growth rate among all the renewable energy resources, with its global capacity increased from 5.1 GW to 625 GW from 2005 to 2019 [1-2]. As the output voltage of an individual PV panel is low, multiport modular converters are becoming a popular power distribution structure to match the DC-grid voltage. However, these topologies must be able to achieve a high voltage gain, provide individual maximum power point (MPP) tracking, and balance the output voltage of each module [3-7].

A basic photovoltaic energy based multiport modular MVdc system is shown inFIG.1. This is a two-stage system where the first stage performs maximum power extraction and step-up while the second stage provides an additional step-up while regulating the modules output voltage. This approach requires two DC-DC converters, and therefore increases the size and cost of the system, while lowering the efficiency due to processing the power multiple times [7]. Instead of relying on large step-up converters, one option is a multi-bus approach which utilizes two or more voltage busses to help transfer the power from the input to the load [6].

Another approach is the input-independent, output-series (IIOS) topology shown inFIG.2which typically employs full-bridge converters due to their isolation capabilities and large-step up gain. However, achieving a large gain requires a significant transformer turns ratio, which leads to a large and bulky system. Further, to balance the modules output voltage this approach requires the use of additional power balance unit (PBU) consisting of semiconductor or passive components, adding to the size and cost and increasing the complexity. For example, [3-5] requires the use of two switches and an inductor to perform voltage balancing. To minimize the required components. [7] integrated two switches into a voltage doubler and utilizes an additional inductor to allow for power flow between modules. However, this still results in additional components being required to balance the output voltage.

In order for the topologies shown inFIG.1to extract the maximum power from their input PV panels, MPP controllers are required. These controllers need the input voltage and current of the converter to directly calculate the operating panel power and determine how to move towards the maximum operating point. To ensure accurate measurements the input voltage and current need to be stable, requiring the use of large electrolytic input capacitors. These capacitors are known to have a significantly lower life expectancy than the converter itself which leads to instability, and increased maintenance and increased associated costs.

Obtaining the voltage measurement can simply be performed using a resistive bridge consisting of two resistors where the midpoint is sent to the controller. Measuring the input current is more complex and is typically done through the use of a Kelvin sense resistor or a Hall-effect sensor. Kelvin sense resistors are placed in series with the desired current which results in a voltage drop. This voltage is sent to a controller and with the knowledge the value of the sense resistor, the current can be calculated. The larger this resistance the more accurate the current measurement is, however, this also results in increased power loss. While Hall effect sensors can provide accurate (<1% error) current measurements without power loss, however, the sensors require additional power supplies to operate which results in a larger system and a higher cost.

To circumvent issues with current sensors, several papers in literature have discussed current sensor-less maximum power tracking techniques. In [8, 9] two voltage sensors, known converter parameter values, and the load resistance are used to estimate the input current such that P&O can be employed. In the authors utilize a double capacitor interface to estimate the PV panel current, a method that requires two voltage sensors and knowledge of the input inductance and capacitance. In both these cases it can be understood that the controllers rely on known converter parameters and as a result their MPP algorithms must be tuned for different converters. Also, parameters of passive converter components can change overtime, leading to incorrect estimations and low tracking efficiency. Instead of estimating the operating current, other MPP tracking methods makes use of irradiance, temperature, and voltage sensors [12]. The measured irradiance and temperature are sent into a trained neural network to determine the approximate voltage required to operate at the MPP (Vmp) at each operating condition. To reduce the cost of multiple sensors, tracking methods that consist of only a single voltage sensor have been presented [10,12-13]. By measuring the input voltage and having knowledge of the operating duty cycle, the controller can determine whether the system is at the MPP. However, as with the previously discussed current-senseless techniques, the controllers are designed for a specific power converter which diminishes their range of application.

SUMMARY

In one of its aspects, a modular single-stage photovoltaic step-up converter system with integrated power balancing, the converter system comprising:an input stage comprising at least one converter module, the at least one converter module comprising a single voltage-sensor based maximum power point (MPP) tracking controller and a power circuit, wherein the power circuit in each at least one converter module comprises an integrated boost and a CLL resonant converter which allows for both MPP tracking and soft-switching operation; andan output stage comprising an active voltage quadrupler (VQ) circuit for achieving balanced output voltages amongst each of the at least one converter modules, and wherein the least one converter modules are coupled together by an output inductor, along with duty ratio control of the active VQ circuit, and whereby natural output voltage balancing is achieved without using additional power circuit components.

In another of its aspects, a modular single-stage photovoltaic step-up converter system with integrated power balancing, the converter system comprising:an input stage comprising at least one converter module, the at least one converter module comprising a single voltage-sensor based maximum power point (MPP) tracking controller and a power circuit; andan output stage comprising an active voltage quadrupler (VQ) circuit for achieving balanced output voltages amongst each of the at least one converter modules, and wherein the least one converter modules are coupled together in a circular configuration.

In another of its aspects, a modular single-stage photovoltaic step-up converter system with integrated power balancing, the converter system comprising:an input stage comprising a first converter module and a second converter module, each of the converter modules comprising a single voltage-sensor based maximum power point (MPP) tracking controller and a power circuit; and an output stage comprising an active voltage quadrupler (VQ) circuit coupled to the first converter module and the second converter module.

In another of its aspects, a modular single-stage photovoltaic step-up converter system for a photo-voltaic (PV) array, the converter system comprising:an input stage comprising a first converter module and a second converter module, each of the converter modules comprising a single voltage-sensor based maximum power point (MPP) tracking controller and a power circuit;an output stage comprising an active voltage quadrupler (VQ) circuit coupled to the first converter module and the second converter module; andwherein inputs to the first converter module and the second converter module are connected in parallel, and connected to the PV array.

In another of its aspects, a modular single-stage photovoltaic step-up converter system with integrated power balancing, the converter system comprising:an input stage comprising a first converter module and a second converter module, each of the module comprising a single voltage-sensor based maximum power point (MPP) tracking controller and a power circuit; andan output stage comprising an active voltage quadrupler (VQ) circuit for achieving balanced output voltages amongst each of the converter modules, wherein the VQ circuit comprises switches which are gated asymmetrically, whereby direction of power flow is controllable and the power can be shared with either the first converter module and the second converter module regardless of the recipient module's operating state.

Advantageously, the multi-port PV step-up power converter for MVdc system is capable of independent single-sensor maximum power extraction and output power balancing. The DC-DC converter utilizes a two-switch integrated boost and an asymmetrical pulse with modulated (APWM) CLL resonant converter along with an active voltage quadrupler (VQ), which allows the system to achieve a high step-up gain without the need of a large and bulky transformer. The resonant inductor is coupled with the active VQ to provide isolation between the input and the output, and the output inductor of each stage is coupled together to allow for power flow between the VQs, which in turn allows for the output voltage of each stage to be regulated without the need of additional components. Furthermore, the MPP tracker (MPPT) performs with a single voltage sensor and operates independently of the power balancer.

In addition, the single-stage modular step-up converter comprises an integrated MPPT and does not require separate power balancing units to achieve output voltage balancing among all the converter modules. Furthermore, the PV MPPT control scheme in each module requires only one voltage sensing control loop, as opposed to conventional MPPT technique that requires an additional current sensing loop, in one example. The input filtering capacitance is significantly reduced by at least 20%, and as a result, small size long lifespan film capacitors can be used in the converter. The single-stage modular step-up converter is also capable of loss-less switching for over 90% of the operating range, with >96% efficiency. The multi-port PV step-up power converter may be implemented in PV farms (with medium voltage grid structure) and solar powered EV charging stations.

DETAILED DESCRIPTION

Referring toFIG.3, there is shown a multi-port PV step-up power converter10for a MVdc system capable of independent single-sensor maximum power extraction and output power balancing. The DC-DC converter utilizes a two-switch integrated boost stage and an asymmetrical pulse with modulated (APWM) CLL resonant converter along with an active voltage quadrupler (VQ)12. This allows the system10achieves a high step-up gain without the need of a large and bulky transformer. A resonant inductor is coupled with the active VQ12to provide isolation between the input and the output. Each stage is coupled to PV array14, and the output inductor of each stage is coupled together to allow for power flow between the VQ's, which in turn allows for the output voltage of each stage to be regulated without the need of additional components. In addition, MPP tracker18performs with a single voltage sensor and operates independently of the power balancer.

Now looking atFIG.4, there is shown a photovoltaic energy based multi-port MVdc system20in one implementation. Each module22a-ncomprises input PV array24is connected to an integrated boost and APWM resonant converter26. The duty cycle of both switches S128and S230are controlled to achieve MPPT operation, for example, by MPP tracker18. A CLL resonant converter is utilized and the resonant inductor (Lr) is coupled with an active VQ to allow for isolation and to further increase the overall gain.

One feature of modular system20is equally distributed voltage across the module22outputs. As the outputs are connected in series at the output, they share the same output current. If one module22aoperates at a higher or lower power than another module22b, their output voltage will be mismatched. As a result, to equally distribute the output voltage when the outputs are connected in series, an alternate power flow path is employed. As seen inFIG.5, by coupling the output inductor of the converters modules an alternate path is created and by controlling the VQ switch the power flow can be set such that the voltage across each output module is constant. The green (top) arrow represents the power flow from the top module22ato voltage quadrupler32while the orange (bottom) arrow represents the power flow from the bottom module22bto voltage quadrupler32. The coupling of the output inductor implies the voltage across one modules inductor is a function of the current flowing through the other inductor (1), (2) where Lois the output inductance, ires12is the secondary side resonant current of the top module22a, and ires22is the secondary side resonant current of the bottom module22b. For simplicity it is assumed the turns ratio between the coupled inductor is 1:1.

In a typical VQ the voltage across the front two capacitors are Vo/4. In one exemplary implementation, one of the capacitors is replaced with a switch which means this relation no longer holds true. Instead, the duty cycle of this switch impacts the voltage across the front capacitor. The larger the duty cycle, the larger the voltage across the front capacitor. As a node of this capacitance is connected to the coupled inductor, it can be understood controlling the capacitor voltage will impact the current on each side of the inductor and in turn affect the power flow (3).

The overall gain of a single sub-module22acan be obtained by combining the gain of each individual stage. The output voltage of the boost stage forms the input square voltage of the second stage. (3) provides the gain of the CLL resonant stage where Q is the quality factor (5), k is the ratio between the resonant inductance and the output inductance (6), N is the turns ratio of the isolating coupled inductor, and wris the ratio between the angular resonant frequency and the operating frequency (7). The gain of the boost stage is the same as a regular boost converter. The total gain of a single module22nis provided in (8) and a plot this gain as a function of duty cycle and operating frequency is shown inFIG.6.

Generally, PV panels directly connected to loads are unable to reach their maximum operating point due to impedance mismatch. When a power converter is interfaced between the panel and the load, a controller can vary an operating parameter of the converter such that the maximal amount of PV energy is extracted. Typically, when perturb and observe (P&O) based MPPT controllers are employed, a voltage and current sensor measure the panel's operating voltage and current to determine the panel's power. Based on the power change from the previous state, the controller pushes the operating conditions towards the maximum power point. There are four possible scenarios which are determined by the change in power and the previous perturbation direction. These scenarios are shown inFIG.7and summarized in Table I.

Although the exact operating power is calculated, this value is not required by the controller. From Table I it can be understood that the controller only checks if the difference between Pkand Pk-1is positive or negative. Instead of calculating the operating power, a surrogate signal can be used to indirectly track the maximum power point. The only requirement is that the chosen signal follows the PV MPP characteristics regardless of the operating condition.

Voltage sensing can easily be performed with a voltage divider, which results in minimal cost and losses compared with other sensors such as current, temperature, or light intensity, which makes it a good choice for single sensor implementations [13]. However, it may not be possible to perform MPP tracking only by measuring the panel's output voltage unless the algorithm is designed for a specific power converter such as in [12-13]. As a result, to perform MPP tracking with only a voltage sensor, an alternate voltage parameter must be measured. In some converters, such as the discussed APWM CLL resonant converter, there are voltage parameters that meet this criteria. The peak voltage across the converters switches is given in (9) and can be seen to be a function of the input voltage and the duty cycle. As this voltage is on the x-axis of the V-P curve inFIG.7it is not a candidate. The resonant capacitor voltage is provided in (10) and is simplified in (11). It can be seen that this voltage is a sinusoidal waveform whose peak is a function of the input power and shifted up by a constant. Therefore, this voltage waveform follows the input panel power characteristics.

At the same time, the voltage across the resonant inductor is related to the resonant capacitor and switch voltage. This relation is provided in (12) and (13) when the switch is off and on respectively. Based on this, the voltage across the resonant inductor can also be utilized as an alternative signal to track the MPP. By maximizing this voltage, the input panel power will be indirectly maximized.

The single-sensor MPPT algorithm utilizes P&O based tracking. Instead of using a fixed step-size, it may be more efficient to implement an adaptive step-size algorithm as this provides faster tracking speed and lower oscillations near the MPP. Gradient based methods can be employed for adaptive step-size as they utilize the change in the measured parameter and the operating state. One benefit of this implementation for single-sensor tracking is that it does not require additional sensors or calculations. Such an implementation may help to increase the tracking speed during periods when the operating conditions change, which leads to a more stable system and higher overall extraction efficiency.

FIG.8shows flowchart100outlining the steps for an algorithm for the MPPT algorithm. Generally, the algorithm operates in two states, State I and State II. State I is the tracking state and occurs when the system is not operating near MPP. Each iteration (k), the controller senses the peak operating voltage of the desired component and along with the previous operating state determines the rate of change. When operating away from the MPP, the rate of change is large. The controller utilizes this large value to tune the step-size as shown in (14) to allow for faster tracking. As the operating parameter is perturbed, the rate of change decreases, and in turn the step-size decreases. If the rate of change is much smaller than the measured peak as indicated in (15), the controller transitions to the second state which is oscillation minimization. During State II the step-size is significantly reduced to allow for minimal oscillation around the MPP. If there is a change in the light intensity the calculated rate of change would be large according to (16), which alerts the controller to transition back to State I.

The algorithm is stable as it converges at the maximum power point.FIG.9shows four scenarios these waveforms where green area bounded by dminand dmaxrepresent operating condition where soft-switching is maintained.FIG.9ashows the ideal situation, in which the system arrives at the MPP without an overshoot occurring.FIG.9bshows the second scenario which is when the system overshot the MPP and arrived at a location where the calculated rate of change is close to zero. However, the system does not transition into State II as the change in the perturbed variable is high. This alerts the controller that the MPP was overshot and that the operating side has changed. The third scenario is shown inFIG.9cwhich is when the perturbation parameter approaches zero. As with the previous scenario, the measured rate of change would be close to zero, however the change in the perturbed variable would also be zero. For the algorithm, this scenario may be avoided by having the controller operate with a minimum2, such that the rate of change will only be close to zero when operating very close to the MPP.

To verify the performance of the system20, simulation results were obtained on a 4 kW, 8 kV PV energy system consisting of two modules, the discussed single sensor MPP tracker and the voltage balancing controller. Each module consisted of nine 220 W PV panels in series to allow for an operating power of up to 2 kW.

Table II shows the design specifications of the system.

First, in order to confirm the chosen voltage could be used as a replacement for the input power, the operating duty cycle of the top module was varied from 40% to 70% at different light intensity conditions.FIGS.10aand10bshows plots of both the input power (a) and the chosen voltage (b) at light intensities 600 W/m2, 700 W/m2, 800 W/m2, and 900 W/m2 respectively. Here it can be seen that the peak voltage and the peak power occur at the same duty cycle regardless of the light intensity. This confirms that maximizing the peak inductor voltage will indirectly maximize the input panel power.

In order to test the designed single-sensor MPP controller and the power balancing technique, the top module's PV array light intensity was varied between 600 W/m2and 900 W/m2while the resonant inductor voltage was sent to the MPP controller. The light intensity was varied every 0.05 s. The output voltage of each module was sent to a separate power balancing controller that would vary the VQ's switch to balance the power flow.

FIGS.11a,11b,12a,12band13contains the results of this simulation. FromFIG.11ait can be seen the single-sensor controller successfully brought the operation power to the maximum at all operating conditions even through it did not measure the input parameters of the panel. From the zoom-in provided inFIG.11bthe extraction efficiency was greater than 99% regardless of the light intensity.

This confirms that the oscillation mitigation portion of the designed controller was successful in improving the extraction efficiency.FIG.11cshows the operating power of the second module which was kept constant throughout operation. When the light intensity of the first module changed there was a dip in the operating power of the second module. This makes sense as the output resistance seen by the second module changed due to the increase in the first modules power level. Regardless, the algorithm was able to bring the operating power back to the maximum. The output voltage of each module is shown inFIG.12awhich is at 4 kV at steady state operation.

When the light intensity of the first module change the voltages became unbalanced, however the controller was able to rebalance the voltage.FIG.12bshows that when the operating power of the first module is much greater than the second module the duty cycle of the first aux switch is larger than the second switch to allow for power to flow to the second module.FIG.13contains the first modules switch current waveforms which are shown to operate under turn-on soft switching condition.

In another implementation, modular DC/DC converter system40comprises modules22a-n, in which each front-end of each module22a-ncomprises an isolated resonant converter, and the output inductor of each module22a-nis coupled together to allow for interconnected power flow, as shown inFIG.14. This power flow is regulated with an active voltage quadrupler (AVQ)32comprising an active switch SQ141and a resonant capacitor Cr1142. Similar to the configuration ofFIG.4, the gain of a single module22a-ncomprises the boost, CLL resonant, and VQ stages. The configuration shown inFIG.14allows for a wider range of soft-switching operation as well as control over the peak switch voltage. The resonant capacitor Cr1142allows for the switch SQ141 voltage to reach zero before the turn on gate signal is applied to the switch SQ141while also allowing the peak switch voltage to be a function of the duty cycle. By moving the switch SQ141to the lower left of the AVQ32, the resonant current flowing through the switch SQ141will be negative for an increased period of time for one switching cycle, allowing for a lower peak switch current and a wider soft switching operating range.

The summation of the voltage across the two output capacitors Co1244, Co1346of the voltage quadrupler32are equal to the total module output voltage (17). However, the voltage across these two capacitors Co1244, Co1346are not equal and are a function of the active switch's duty cycle. The voltages across the two capacitors Co1244, Co1346are provided in (18) and (19), respectively. The voltage across the input capacitor Co1148of the voltage quadrupler32is equal to the voltage across capacitor Co1148(19) and from this the peak voltage across the active switch SQ141can be determined as given in (20). The operating waveforms of two modules22a,22bwith the active voltage quadrupler32are shown inFIG.15. When the gate signal is removed from the switch SQ141, the resonant current charges the resonant capacitor Cr1142, and increases the switch SQ141voltage to its maximum. During this time (t0-t1) the resonant current is decreasing and once it crosses zero, the diodes Dr1349and Dr1450change polarity, resulting in the discharge of the resonant capacitor Cr1142(t1-t2). Once fully discharged, the resonant current is split between the capacitor Con48and vsq's anti-parallel diode. As the gate signal is applied before this current is positive, soft-switching operation is achieved (t2-t4).

In order to confirm the accuracy of the system, a two-module system with a rated power of 4 kW and 10 kV output operating at a frequency of 230 kHz was tested in PSIM.

Table III lists the parameters of the utilized modules.

FromFIGS.16aand16bit can be seen that the system40is able to achieve maximum power extraction at different power levels with this circuit configuration for both module 122a(FIG.16a) and module 222b(FIG.16b). It can be seen at 0.2 s that the operating power level of module 222bdrops sharply, however this also had an impact on the power extraction ability of module 122a. This is due to the operation of the voltage balancer as shown inFIG.17. When the power level changes, the voltage balancer changes the operating condition of both active voltage quadruplers32to maintain a balanced output voltage across each module22a,22b. This in turn changed the required duty cycle for the system to extract the maximum power. It can be seen that the system does return to the maximum power level within 0.3 s which confirms that both the maximum power extraction algorithm and the voltage balancer algorithm are able to function independently.

The switch voltage and current of the active voltage quadrupler32are provided inFIG.18aandFIG.18brespectively. The switch SQ141in each module22a,22bis gated such that they transition to their off state at the same time. The additional resonant capacitor Cr1142acts to delay the voltage rise across the switch SQ141which improves the turn off operation of the switch SQ141. It can also be seen that the switch SQ141 voltage reaches zero while the resonant current is negative, which implies at the turn on condition the switch current is negative and flows through its antiparallel diode. This confirms the active voltage quadrupler32can achieve zero voltage switching (ZVS) operation.

The methods described with reference toFIGS.4and14use a linear configuration where all modules22a-nare coupled to one another with the exception of modules M1and MN. As modules M2to MN-1consist of two coupled inductors, to achieve the same voltage level at the input to their active voltage quadrupler32their inductance would need to be four times smaller than modules M1and MN. This implies that the design of each module22a-nis different which increases the difficulty of adding additional modules to an already built system.

In order to simplify the module design and to easily allow for module expansion, an exemplary circular configuration51(i.e. a ring structure with the modular system) is shown inFIG.19. This particular configuration allows for module M122to be coupled to module MN22. As a result, each module22is coupled to two modules22, improving the voltage balancing capabilities and module expansion capabilities.

An example of a single module22is shown inFIG.20, in which two output inductors Lo160and Lo262are coupled to two other modules. As all modules22are identical, an additional module22can be added between any two existing modules22without needing different component or component parameters. Further, the gating technique of the AVQ switch SQ32has been modified to improve over the linear configuration. By gating the switches SQ32such they turn on at the same time, both the time at which the switch SQ32 voltage rises and falls can be controlled with the duty cycle. This allows for more control over the input voltage waveform seen by the AVQ and as a result expands the operating range at which the output voltage can be balanced.

In order to confirm the operation of the circular configuration51, a three-module system was tested in PSIM with both a rated and scaled-down version. The operating power level of each module22is shown inFIG.21a, and the respective output voltage is shown inFIG.21b. It can be seen that all modules22were operating at their respective maximum power level as their input power overlapped their theoretical maximum. The voltage balancer algorithm was delayed until 50 ms to show that under normal operation the voltage across each module is unbalanced. At 50 ms the voltage balancer algorithm is activated and it can be seen that the voltage across each module22became balanced within 40 ms and reached steady state after 60 ms.FIG.22shows contains the AVQ switch SQ32voltage and current of each module. Due to the utilized gating technique, each switch turns on at the same instant. This implies the switches SQ32turn off at different conditions and as a result the voltage across each switch SQ32rises at a different time which can be seen inFIG.22. This can also be seen in the scaled down version's waveforms shown inFIG.23.

As described above, the DC/DC isolated topology essentially comprises a high frequency DC/AC inversion and a high frequency AC/DC conversion via a high frequency transformer. The previous configurations, described above, comprise modifications of the active voltage quadrupler32which forms the output high frequency AC/DC conversion, in which the DC/AC inversion was the same for each configuration, however this stage may be modified to suit the required application, such as higher rated power or higher PV module output voltage. Two such examples are provided inFIGS.24aand24b, respectively.FIG.24acomprises a topology60which utilizes two high frequency DC/AC modules22a.22bthat are individually coupled to the same active voltage quadrupler32. This topology60reduces the number of AVQ modules22needed to provide power to the output, minimizing the number of components and cost of the system. This also allows for the DC/AC portion to be composed of lower rated components which further decreases the cost of the system. An alternative topology70is shown inFIG.24bwhich utilizes a single DC/AC system but includes an additional resonant stage consisting on a resonant capacitor Cr172and an additional winding74added to the coupled inductor. This allows the system to operate at a higher switching frequency while still maintaining the same operating conditions found previously.

As shown inFIG.25, the front-end topology may also be modified to use an interleaved approach. This topology80comprises two high frequency DC/AC modules22a,22bwith their outputs coupled to a voltage quadrupler32, however their inputs are connected in parallel and to the same PV array. The switching signals of the two high frequency DC/AC modules22a,22bare phase shifted by 180, which allows the input current ripple to be minimized. Further, as the PV current is split between the two modules22a,22bthe rated current of the switches decreases and lower rated components can be used.

While the topologies discussed above use an active voltage quadrupler with a switch replacing either the top or bottom capacitor from the front, the capacitor C01and capacitor Cr2may be replaced with switches SQ141and SQ283respectively, as shown in topology82inFIG.26. By gating the switches SQ141, SQ283asymmetrically, the direction of power flow can be controlled such that the power can be shared with either the top or bottom module regardless of the recipient modules operating state.

Embodiments are described above with reference to block diagrams and/or operational illustrations of methods, systems, and computer program products. The operations/acts noted in the blocks may be skipped or occur out of the order as shown in any flow diagram. For example, two or more blocks shown in succession may be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. While the specification includes examples, the disclosure's scope is indicated by the following claims. Furthermore, while the specification has been described in language specific to structural features and/or methodological acts, the claims are not limited to the features or acts described above. Rather, the specific features and acts described above are disclosed as exemplary embodiments.

REFERENCES