Obviousness

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

To establish obviousness, it must be shown that a person of ordinary skill in the art (POSITA) would have been motivated to combine existing prior art references to arrive at the claimed invention, and would have had a reasonable expectation of success. The patent US8279642B2 generally addresses the conversion of DC power to AC power, particularly for renewable energy sources like photovoltaic cells, while actively mitigating double-frequency ripple power on a power bus.

Here, we consider the following prior art references explicitly mentioned in the US8279642B2 specification:

1. U.S. Patent Publication No. 2008/018338 to Kimball et al. ("Kimball '338"): This publication describes a Maximum Power Point Tracking (MPPT) algorithm, specifically "Ripple Correlation Control Based on Limited Sampling," for photovoltaic applications. It focuses on optimizing power extraction from DC sources like solar panels by utilizing inherent ripple to track the maximum power point.
2. U.S. patent application Ser. No. 11/871,015 to Krein et al. ("Krein '015"): This application, which matured into US7755916B2, is titled "Methods for Minimizing Double-Frequency Ripple Power in Single-Phase Power Conditioners." It describes methods for reducing double-frequency ripple power by controlling an energy storage device through an interface (like a power converter or inverter) and, in some embodiments, shifting an AC waveform by π/4 radians relative to the output AC waveform.

We will analyze the obviousness of independent claims 1, 19, and 31 of US8279642B2.

Analysis of Claim 1

Claim 1 describes an inverter comprising an input converter with a transformer, an output converter, and an active filter coupled to a power bus. The active filter reduces double-frequency ripple power by supplying/absorbing power to maintain a bus voltage to track a desired bus voltage signal, and includes a switching circuit coupled to an energy storage device. [cite: US8279642B2]

Combination: Krein '015 and general knowledge in power conversion.

Krein '015 directly addresses the problem of minimizing double-frequency ripple power in single-phase power conditioners by using an energy storage device coupled to the power conditioner through an interface that controls power flow. This interface can be a power converter or inverter. A POSITA would understand that an inverter for converting DC to AC (as described in US8279642B2) inherently involves an input converter, an output converter, and a power bus. The goal of minimizing ripple power on the bus, as taught by Krein '015, would naturally lead a POSITA to integrate an active filter with an energy storage device and a switching circuit (which is fundamental to active filters and power converters) onto such a bus.

The motivation to combine these elements is clear: Krein '015 explicitly states the problem of double-frequency ripple power in single-phase AC power systems and provides a solution using an active ripple cancellation technique. A POSITA would readily apply this known ripple reduction technique to the power bus of a standard DC-to-AC inverter. The specific feature of maintaining a bus voltage to track a desired bus voltage signal would be an inherent or obvious control objective for such an active filter, as the purpose of reducing ripple is to stabilize the bus voltage.

Analysis of Claim 19

Claim 19 describes an apparatus with a solar panel generating a DC waveform, and an inverter. The inverter has an input converter coupled to the solar cell and a DC bus, converting the solar panel's DC to a second DC waveform on the bus. An output converter converts this second DC waveform to an AC output. An active filter is coupled to the DC bus to reduce double-frequency ripple power. [cite: US8279642B2]

Combination: Kimball '338, Krein '015, and general knowledge in photovoltaic systems.

Kimball '338 teaches using Ripple Correlation Control (RCC) for Maximum Power Point Tracking (MPPT) in photovoltaic applications. This reference highlights the importance of efficient power extraction from solar panels. Krein '015, as discussed, provides methods for minimizing double-frequency ripple power in power conditioners associated with energy sources.

A POSITA in the field of photovoltaic power systems would be motivated to combine the MPPT capabilities of Kimball '338 with the ripple reduction techniques of Krein '015 for several reasons:

• Improved System Performance: Double-frequency ripple power, if reflected back to the DC source, can compromise the performance of photovoltaic cells and the inverter's ability to operate at the maximum power point (MPP). [cite: US8279642B2] Kimball '338 emphasizes the importance of accurate MPPT to exploit the full power capacity of the solar cell. Combining this with Krein '015's active filtering directly addresses the challenge of ripple negatively impacting MPPT, leading to a more efficient and reliable photovoltaic system.
• Known Problem and Solution: The problem of double-frequency ripple in DC-AC conversion for solar applications was well-known, as explicitly stated in the background of US8279642B2. Krein '015 provides a clear solution for managing this ripple using an active filter and energy storage.
• Integration with Standard Inverter Architecture: The described apparatus uses a standard inverter architecture (input converter, DC bus, output converter). It would be obvious to a POSITA to integrate a known MPPT control (Kimball '338) into the input converter and a known active filter for ripple reduction (Krein '015) onto the DC bus, as these are complementary functions that enhance the overall performance of a photovoltaic inverter.

The phrase "supplying power to and absorbing power from DC power bus" in claim 19 is a direct functional description of how an active filter, as taught by Krein '015, would operate to manage ripple power.

Analysis of Claim 31

Claim 31 describes a power inverter with a DC bus, an input converter (including a transformer, a first inverter circuit converting input DC to a first AC waveform, and a rectifier circuit converting a second AC waveform from the transformer to a second DC waveform on the DC bus), an output converter (converting the second DC waveform to an AC output), and an active filter (reducing double-frequency ripple power by supplying/absorbing power from the DC bus). [cite: US8279642B2]

Combination: Krein '015 and common power converter topologies.

This claim details specific components of a common two-stage inverter topology: a DC-AC stage (first inverter circuit and transformer), followed by an AC-DC stage (rectifier) to create the DC bus, and then a final DC-AC stage (second inverter circuit) to generate the grid-tie AC.

Krein '015 teaches the essential concept of using an active filter with an energy storage device to minimize double-frequency ripple power in single-phase power conditioners. This principle is directly applicable to the DC bus of the two-stage inverter described in Claim 31. The input converter (first inverter + transformer + rectifier) serves to provide a DC voltage on the bus, while the output converter (second inverter) converts this DC to AC. A POSITA would recognize that ripple on this DC bus would necessitate filtering, and Krein '015 provides an effective active filtering solution.

The motivation for this combination stems from the widely understood benefits of active filtering over passive filtering for ripple reduction, especially concerning size and performance, as discussed in the background of US8279642B2. Krein '015 offers a specific method for controlling the active filter to achieve this ripple reduction. Incorporating a known active filter into a conventional two-stage DC-AC inverter to address a known problem (double-frequency ripple) would be obvious to a POSITA seeking to improve inverter performance.

Furthermore, the explicit mention in Krein '015 of the active filter AC waveform being shifted by π/4 radians relative to the output AC waveform to improve filtering function provides a specific and well-known control strategy for such active filters. A POSITA would readily implement such a control strategy in the context of the inverter described in claim 31.