Obviousness

Obviousness Analysis under 35 U.S.C. § 103 for US8796884B2

A person having ordinary skill in the art (POSA) in the context of US8796884B2 would be an electrical engineer with expertise in power electronics, renewable energy systems (particularly solar power conversion), and control systems. This POSA would possess knowledge of DC/DC and DC/AC converter topologies, energy storage mechanisms (capacitors, inductors), maximum power point tracking (MPPT) algorithms, and techniques for ensuring power quality and mitigating harmonic distortion in grid-tied applications. They would be motivated to address common challenges in power conversion systems, such as improving efficiency, reducing component size and cost (especially for electrolytic capacitors), and enhancing system reliability and compliance with grid standards.

Based on the provided prior art and the inventive principles of US8796884B2, several combinations of references would render the claims obvious to a POSA.

Combination 1: Constant Power Control Enabling Smaller Energy Storage

References:

• US20080316773A1 (Systems and methods for power conversion).
• US20050219894A1 (Solar inverter with reduced ripple).
• US7564703B2 (System and method for energy harvesting and power conversion).

Rationale for Obviousness:
US20080316773A1 describes power conversion systems that address power oscillation and ripple on DC-link capacitors, proposing techniques to mitigate these problems, including systems that produce a nearly constant average power from a DC power source. US20050219894A1 specifically focuses on reducing ripple in the DC power stage of solar inverters to improve efficiency and component lifespan. Furthermore, US7564703B2 teaches the isolation of power sources from load dynamics and optimizing power transfer.

A POSA, faced with the recognized problems of expensive, bulky, and unreliable electrolytic capacitors used for energy storage in conventional power converters (as highlighted in the background of US8796884B2), would be motivated to combine these teachings. The goal would be to reduce the reliance on large capacitors while maintaining efficient operation. US20080316773A1 and US7564703B2 provide the fundamental concepts of achieving a constant power input from the source and isolating the source from load fluctuations. A POSA would readily understand that if the power source (e.g., a PV panel) is presented with an essentially constant load or power demand (via active control of a converter stage, such as a pre-regulator), ripple at the input to the overall system could be significantly reduced or eliminated. This "constant power control" at the input would isolate the source from downstream power pulsations.

Consequently, the energy storage device further downstream in the power path (e.g., the DC link capacitor before a DC/AC inverter) would then be solely responsible for balancing the pulsating power demands of the AC load. With the input side isolated, a POSA would appreciate that the DC link capacitor could be allowed to operate with larger voltage (or current, for an inductor) fluctuations without impacting the sensitive DC source. Allowing greater voltage swing on the DC link capacitor directly reduces the required capacitance for a given energy storage requirement (ΔE = 1/2 C [Vmax² - Vmin²]), thereby enabling the use of smaller, less expensive, more reliable, and longer-lifespan capacitors (e.g., film capacitors instead of electrolytics), which is a key benefit claimed by US8796884B2. The specific method of maintaining a constant input voltage or current (as described in FIG. 8 of US8796884B2) would be an obvious engineering implementation for achieving the "constant average power" or "isolation" taught by the prior art.

Combination 2: Harmonic Distortion Mitigation with Fluctuating DC Link

References:

• US20080316773A1 (Systems and methods for power conversion).
• US7064985B2 (Grid-connected inverter with maximum power point tracking and energy storage management).
• General knowledge in the art regarding harmonic distortion in grid-tied inverters.

Rationale for Obviousness:
Building upon Combination 1, where a POSA allows the DC link capacitor to operate with larger voltage fluctuations to enable smaller capacitor size, a new problem emerges: these larger voltage swings can introduce significant harmonic distortion into the AC output waveform of the DC/AC inverter, especially when operating into a power grid. US20080316773A1, while focused on ripple mitigation, sets the stage for controlled management of power. US7064985B2 describes grid-connected inverters and energy storage management, inherently implying an awareness of power quality requirements for grid interconnection.

A POSA designing grid-tied inverters would be well aware that the quality of the AC output waveform is critical for grid interconnection, with regulations and specifications limiting harmonic distortion. Given that a fluctuating DC link voltage directly affects the available modulation index for an inverter, a POSA would immediately recognize that the output waveform would be distorted if these fluctuations are large. It would be an obvious design choice for a POSA to apply known harmonic distortion mitigation (HDM) techniques to compensate for this anticipated distortion. Methods such as predistortion (as shown in FIGS. 24-29 of US8796884B2) or direct harmonic current cancellation (as shown in FIGS. 20-23 of US8796884B2) are standard control theory approaches for improving output waveform quality in power converters. Therefore, combining the concept of a constant-power controlled input (allowing a relaxed DC link) with conventional HDM techniques at the inverter output to maintain grid compliance would be an obvious solution for a POSA balancing the benefits of smaller capacitors with the necessity of grid power quality.

Combination 3: Fast MPPT by Selectively Disabling Power Control

References:

• US7064985B2 (Grid-connected inverter with maximum power point tracking and energy storage management).
• US20080316773A1 (Systems and methods for power conversion).
• US7564703B2 (System and method for energy harvesting and power conversion).

Rationale for Obviousness:
US7064985B2 clearly teaches Maximum Power Point Tracking (MPPT) in grid-connected inverter systems to optimize power extraction from sources like PV panels. US20080316773A1 teaches systems for maintaining constant average power and mitigating ripple from a DC source. US7564703B2 describes isolating power sources from load dynamics.

A POSA integrating MPPT functionality (from US7064985B2) into a power conversion system employing constant power control (from US20080316773A1 and US7564703B2) would seek efficient and robust ways to perform MPPT. Traditional MPPT algorithms often rely on perturbing the operating point and observing the power response. US8796884B2 proposes an MPPT technique that leverages the inherent power pulsations of an AC load (e.g., 120 Hz ripple) by temporarily disabling the constant power control loop. A POSA, knowing that constant power control isolates the DC source from these AC load pulsations (as per US7564703B2 and US20080316773A1's aims to reduce ripple), would recognize that suspending this isolation would intentionally expose the power source to the load dynamics.

By temporarily disabling the constant power control (e.g., switching the first stage to a fixed duty cycle as shown in FIG. 35 of US8796884B2), the AC load pulsations would reflect back to the DC source, causing its operating point to sweep along its V-I and power curves. Measuring the resulting voltage and current fluctuations during this sweep would allow a tracking circuit to determine the maximum power point (MPP) or other optimal operating points. This method leverages an existing system characteristic (the AC load pulsation) as the perturbation signal, thereby simplifying the MPPT circuitry and potentially enabling faster tracking compared to slower, externally generated perturbations. The idea of modulating a control loop to observe system dynamics for optimization is a common engineering practice. Thus, a POSA would find it obvious to temporarily disable the input-side constant power control to utilize the inherent AC load pulsations for MPPT, which is a faster and more robust method.

Combination 4: Distributed Inverters (Microinverters) with Constant Power Control and Distortion Mitigation

References:

• US20060012906A1 (Microinverter for photovoltaic applications).
• Elements from Combination 1 (US20080316773A1, US20050219894A1, US7564703B2) for constant power control.
• Elements from Combination 2 (US20080316773A1, US7064985B2, and general knowledge) for harmonic distortion mitigation.
• WO2008070966A1 (Multi-module power converter having a voltage ripple reducer).

Rationale for Obviousness:
US20060012906A1 explicitly teaches microinverters for photovoltaic applications, converting DC power from a single solar module to AC power for an electrical grid. WO2008070966A1 further describes multi-module power converters with voltage ripple reduction.

A POSA, recognizing the advantages of distributed inverter architectures (microinverters) as described in US20060012906A1, would naturally be motivated to apply the same beneficial power conversion techniques developed for central inverters to these smaller, modular systems. The problems of energy storage size, cost, reliability, and power quality are equally, if not more, critical in microinverter applications due to space constraints and the need for long-term reliability in harsh environments. Therefore, integrating constant power control (as made obvious by Combination 1) into each microinverter module would allow each module to optimize power extraction from its individual PV panel and utilize smaller, more reliable energy storage components. Simultaneously, the application of harmonic distortion mitigation (as made obvious by Combination 2) to each microinverter's output would ensure that the combined AC output to the grid meets power quality standards, even with the use of relaxed DC link voltage ripple within each module. The combination of these features within a distributed inverter architecture, as illustrated in FIGS. 50 and 51 of US8796884B2, would be an obvious engineering design choice for a POSA seeking to improve the performance, cost-effectiveness, and reliability of microinverter systems.