Image Sensor Energy Harvesting via Bootstrap Capacitor
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Solution Overview
Problem
Existing energy harvesting systems for image sensors face challenges in ensuring a robust cold start and efficient operation over a wide range of light intensities, often requiring batteries or off-chip inductors, which increase cost and footprint, and previous designs with maximum input power point tracking do not necessarily lead to maximum power delivery to the load.
Innovation Solution
A sensor system with a pixel array and a monolithically integrated photodiode stack that can operate in both image capturing and energy harvesting modes, using a DC/DC converter to convert energy captured by the pixel array, where the photodiode stack provides power to the converter, enabling efficient energy harvesting without external batteries or inductors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of energy
If a DC-DC converter is used to generate supply rails from incident light, then energy harvesting efficiency is improved, but the system requires stable supply rails which are difficult to achieve during cold start
Solution Approach 1:
A bootstrap capacitor is introduced as an intermediary energy storage element between the photodiode and DC-DC converter. The capacitor accumulates energy from the photodiode during the reset phase and releases it during the conversion phase, enabling the converter to operate with unstable or zero input voltage during cold start. This mediator resolves the contradiction by decoupling the cold start requirement from the stable supply rail requirement.
2Reliability
If external batteries or charge storage devices are used to cold start the system, then reliable operation is achieved, but device complexity and footprint increase
Solution Approach 1:
The system uses its own photodiode to charge the bootstrap capacitor during normal operation, creating a self-sustaining energy storage mechanism. The photodiode continuously replenishes the capacitor, eliminating the need for external batteries or separate charge storage devices. This self-service approach resolves the contradiction by making the system self-sufficient for cold start without adding external components.
3Use of energy by moving object
If the pixel array is used for energy harvesting, then power supply is improved, but image capturing capability is lost
Solution Approach 1:
The sensor system is divided into two separate functional segments: a photodiode array dedicated to energy harvesting and a pixel array dedicated to image capturing. The photodiode stack is positioned adjacent to the pixel array and connected to the DC-DC converter, allowing independent operation of both functions. This segmentation resolves the contradiction by enabling simultaneous or alternating operation of energy harvesting and image capturing without functional interference.
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
This solution achieves a high pixel fill factor and small pitch without sacrificing image quality, enabling efficient energy harvesting and cold-start capabilities using fringe light, reducing the need for external components and improving power delivery under varying light conditions.
Implementation Method 1
The photodiode stack is located adjacent to the pixel array and configured to provide power to the DC/DC converter
Implementation Method 2
The pixel array is configured to operate in an image capturing mode or an energy harvesting mode
Data Source
AI summary
A sensor system includes a pixel array, a DC/DC converter, and a photodiode stack. The pixel array is configured to operate in an image capturing mode or an energy harvesting mode. The DC/DC converter is configured to convert energy captured by the pixel array while in energy harvesting mode. The photodiode stack is located adjacent to the pixel array and configured to provide power to the DC/DC converter.