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Optoelectronics/Displays  

Power conversion prospects in solar-powered LED Lighting

Posted: 06 Aug 2012     Print Version  Bookmark and Share

Keywords:LEDs  photovoltaic solar cells  battery 

However, there are at least two down sides to this configuration. First, flexibility is limited. The devices selected likely have a fairly narrow operating range, which limits their ability to respond to changes in the system or customer requests. For instance, if the solar-cell configuration is changed, the battery-charging IC will need to be replaced. If the energy-storage technology or configuration is changed, then likely both the battery-charging IC and the LED-driver IC will need to be replaced. Finally, if the LED type or configuration is changed, then the LED-driver IC will need to be reconfigured. Given the pace of innovation with these technologies, standard flexibility allows faster responses to changing requirements. The inclusion of a microcontroller lends itself towards system, and thus solution, flexibility. Instead of significant hardware changes requiring extensive redesign and requalification, most changes can be incorporated inside of the microcontroller.

The second downside is the system optimisation-component. While we can find a generic battery-charging IC, we likely would have difficulty finding one that also had a Maximum Peak Power Tracking (MPPT) algorithm included, to maximise the output of the solar cell. A discrete-based solution would have difficulty keeping up with the pace of innovation.

Proposed implementation
To address the limitations of a custom-designed solution, a microcontroller can enable a designer to take advantage of the increasing performance of each of the core components while allowing the fundamental architecture to be reused. Figure 3 presents the proposed implementation.

Figure 3: Proposed MCU-based architecture.

There are three advantages to the implementation in Figure 3. First, all aspects of the system can be optimised quickly and easily. There are four primary systems within this solution: LED, battery, solar-cell and power electronics. As mentioned, the battery-charging profile should be controlled to enhance both the charging efficiency as well as its lifetime. However, the overall charging efficiency is also dependent on the efficiency of the solar cell. Incorporating an MPPT profile into the power-conversion algorithm should increase the overall efficiency of the Solar-->Electricity power conversion, ultimately allowing the size of the solar array to be reduced while still achieving the charge objectives.

This impacts the product's form factor, and provides options to the designer to enhance its visual appeal. Similarly, the target application may identify the light quality as a critical characteristic, as would be the case if used for reading. Light quality can be attributed to the current waveform, perhaps driving a tight tolerance for the LED drive current or including dimming capability. The proposed implementation allows design engineers to optimise everything from the component efficiency to the system's overall robustness and lifetime.

Second, this architecture is entirely scalable and can work across a broad power range. A compact, portable lantern used for reading may have a single solar cell, off-the-shelf rechargeable NiMH batteries, and a few LEDs using 20-75 mA of drive current. By simply replacing the powertrain components, including readily available power MOSFETs and transformers, this design can quickly scale the power rating to fulfil the needs of commercial and community-based security lighting. The number of solar cells can be increased, off-the-shelf NiMH batteries can be replaced by custom battery packs, and high-intensity, high-current LEDs requiring over 350 mA of drive current can be used.

Finally, the flexibility of the platform allows for a rapid adaptation to changes in the core technologies, or in customer needs or behaviours. Evolving solar cells or a new LED with specific drive requirements may be quickly adopted and new products introduced. As these products are used, customer application feedback may drive additional, non-core requirements, such as communication (i.e., serial-to-wireless interfaces, etc.) as well as predictive diagnostic support. So, the solution can not only adjust to changing conditions and optimise its performance, it can also communicate its relative health and predict when it will require maintenance.

Conclusion
As with most emerging technologies, it is not always clear what direction it will take. Couple two emerging technologies together, such as PV solar and LEDs, and the flexibility offered by a microcontroller-based power converter solution enables the fast implementation of improvements that satisfy customer needs.

About the author
Stephen Stella is a product marketing manager with Microchip's Analog and Interface Products Division in Chandler, AZ. He is responsible for Microchip's global analogue strategy for mid-power controller products. Prior to joining Microchip in 2010, Stephen spent several years in product marketing with Fairchild Semiconductor in San Jose, CA. He earned his Bachelor of Science degree in Electrical Engineering and Master of Science in Mechanical Engineering from Ohio State University (Columbus, OH), and his Master of Business Administration degree from the University of Michigan (Ann Arbor, MI).

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