By: Stephen Stella, Product Marketer, Analog & Interface Products Division, Microchip Technology Inc. (Germany)
Fast-paced technology developments in Light-Emitting Diodes (LEDs) and photovoltaic solar cells are leading to significant improvements in their performance, which can improve the performance of the end-application. In applications that employ both technologies, such as solar-powered lighting, improvements in these two core technologies combine to offer substantial end application performance improvement potential. In this case, higher-efficiency solar cells convert more of the Sun’s energy into electricity, which can reduce the amount of solar-cell area needed, and the high-efficiency LEDs run longer and brighter into the night. The challenge for solar-lighting solution manufactures’, however, is being able to leverage these advances quickly and cost effectively. One method to maximize the system’s performance is through its power-conversion strategy. A solid power-conversion strategy enables rapid development and deployment of solutions that utilize the latest technologies. In this article, we will review the components, develop a system, and provide a high-level method to analyze its behavior.
There is a wide array of solar-powered lighting examples. Whether you are in an area with an unreliable power grid and use a solar-powered lantern as a night-time reading lamp, or deploy full-scale, community-grade street lighting, the opportunities for a combined solar/LED lighting system are diverse, broad-based and global. The only difference is the scale of the end application (reading vs. general illumination have very different requirements).
The core components in all of these systems are the i) solar cells, ii) battery, and iii) the LED. As an aside, we can adopt a more generalized description of each of these components, including energy collectors (solar), energy storage (battery), and energy emitters (LEDs). While not exactly accurate, it serves to underscore the flexibility of the analysis. Figure 1a shows the most basic of system configurations.
In order for this implementation to work, however, the behaviors of each element must be compatible with each other. In this case, that means the output voltage/current behavior of the solar cell must align with the battery-charging profile, and the battery-discharge profile must match the LED drive requirements. We quickly find that, for the configuration in Figure1(a), they do not.
Reviewing the performance characteristics of each component, as found in the V-I characteristics in Figure 2 (a through d), we find that, while they can be made to behave close to each other within a limited configuration set, it is virtually impossible to guarantee any reasonable level of performance. We quickly see the that maximum solar-cell voltage (per cell) is around 1V, while the NiMH battery operates in a range of .9V to 1.4V, and the LEDs require a constant current source, although their forward voltage is typically above 3V. Further, the NiMH battery has some specific charging requirements to extend its useful life.
While it is possible to develop a system that interfaces all of these components directly, it should be clear that there are significant limitations to that configuration, as well as ramifications for the overall system efficiency and its robustness.
To address these limitations, let us review the alternative system diagram in Figure 1(b). Adding a power electronics interface between each of the three core elements allows a much higher degree of flexibility, and permits the overall system performance to be optimized. The microcontroller is not essential in this configuration. We can find a standalone battery-charger integrated circuit (IC) to address the needs of the NiMH charging profile, and similarly find LED driver ICs that convert the battery voltage into a constant current source.
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 toward 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 optimization-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 maximize the output of the solar cell. A discrete-based solution would have difficulty keeping up with the pace of innovation.
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.
There are three advantages to the implementation in Figure 3. First, all aspects of the system can be optimized 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 optimize 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 fulfill 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 behaviors. 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 optimize its performance, it can also communicate its relative health and predict when it will require maintenance.
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.