Micro-inverter Topology with Soft-Switched DC-DC Stage
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Solution Overview
Problem
Existing micro-inverters for photovoltaic systems face issues such as high DC input voltage safety hazards, fire risks, reduced energy harvest due to shading, limited operating lifetime of electrolytic capacitors, and inability to sink AC current out-of-phase with the grid voltage, which limits their functionality in stabilizing the utility grid and accommodating new utility requirements.
Innovation Solution
A micro-inverter topology using non-electrolytic capacitors with a soft-switched DC-DC converter design that operates at lower voltages, capable of sourcing and sinking AC current out-of-phase with the grid voltage, incorporating a two-stage converter with a soft-switched DC-DC stage and a DC-AC full bridge inverter, which reduces switching losses and allows for reactive power compensation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If electrolytic capacitors are used in micro-inverter design, then the inverter can be compact and cost-effective, but the operating lifetime is severely limited due to high temperature degradation
Solution Approach 1:
The patent replaces expensive, long-lived non-electrolytic capacitors with cheaper, shorter-lived electrolytic capacitors. The DC-DC converter continuously regenerates the DC bus voltage, effectively resetting the capacitor charge cycle and extending the operational life of the electrolytic capacitor beyond its normal limitations.
Solution Approach 2:
The patent changes the operating parameters of the electrolytic capacitor by maintaining it at a fixed, regulated voltage level through the DC-DC converter. This voltage regulation prevents the capacitor from experiencing the voltage stress and temperature cycling that normally limit its lifetime, allowing it to operate reliably for extended periods.
2Object-generated harmful factors
If a high-voltage DC bus is used to reduce current and minimize EMI, then EMI is reduced, but switching losses on power semiconductors increase significantly
Solution Approach 1:
The patent changes the voltage parameter by using a low-voltage DC bus (24V-48V) instead of high voltage. This increases the current required for the same power level, but the soft-switching technique compensates by minimizing switching losses through resonant charging and discharging of the DC bus capacitor.
Solution Approach 2:
The patent uses soft-switching techniques that employ resonant oscillations (vibrations) to charge and discharge the DC bus capacitor. This resonant charging/discharging process minimizes the hard switching transitions that cause energy losses, allowing efficient operation at low voltage with higher current.
3Temperature
If series-connected photovoltaic panels are used to provide high DC voltage, then the voltage to string inverters is high, but all panel power goes to zero when a single panel is shaded
Solution Approach 1:
The patent segments the photovoltaic system into independent single-panel units, each with its own micro-inverter. This segmentation allows each panel to operate independently, so shading of one panel does not affect the output of other panels, maximizing overall energy harvest while maintaining manageable voltage levels.
4Device complexity
If most micro-inverter topologies are designed to only source current into the grid, then the design is simple, but they cannot accommodate new utility requirements to produce reactive power
Solution Approach 1:
The patent designs the micro-inverter with a universal architecture that can perform multiple functions: sourcing active power to the grid, sinking active power from the grid, and providing reactive power compensation. The DC-DC converter and DC-AC inverter stages can operate in different modes to achieve these diverse functions, making the inverter adaptable to various grid requirements.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution provides a reliable, efficient, and cost-effective micro-inverter topology that operates at lower voltages, extends component lifespan, and enables reactive power compensation, addressing safety and functionality limitations of existing micro-inverters while minimizing EMI and switching losses.
Implementation Method 1
a first magnetic component comprising at least two windings, a core, and a flux path
Implementation Method 2
return energy stored in the first magnetic component to the direct-current power source
Implementation Method 3
a first electronic component comprising a capacitance
Implementation Method 4
using a resonance of the first electronic component in such a way as to reduce switching loss
Data Source
Figure 1A
Figure 1B
Figure 2A
AI summary
A DC-to-AC power converter is disclosed. The power converter has a DC input to receive DC power from a photovoltaic device, an AC output configured for direct connection to an AC mains power supply line, six semiconductor switches, one isolated high-frequency power transformer, two high-frequency inductors, a small resonant capacitor, and a large non-electrolytic (e.g. film) capacitor energy storage component. One of the semiconductor switches located on the primary side of the transformer operates to regulate the voltage across the non-electrolytic capacitor energy storage component. A second semiconductor switch located on the primary side of the transformer provides a resonant reset for the energy stored in the transformer and allows the first semiconductor switch to operate with nearly zero-voltage-switching. The other four semiconductor switches and a high-frequency inductor coupled to the ac output operate with a variable switching frequency to produce a sinusoidal current into the ac output such that the sinusoidal current may be either in phase or out of phase with the ac output voltage.