Solar-Powered LED Lighting Power-Conversion Techniques

Electronics Technical ArticlesSouth-East European INDUSTRIAL Мarket - issue 4/2014

Stephen Stella,
Microchip Technology Inc.

Performance improvements in Light-Emitting Diodes (LEDs) and photovoltaic solar cells due to developments in technology improve the performance of the end-application. In applications that employ both of these core technologies in combination, such as solar-powered lighting, there is the potential to deliver substantial end-application performance improvement.

For example, it’s possible to reduce the solar-cell area as higher-efficiency solar cells convert more of the Sun’s energy into electricity and the high-efficiency LEDs run longer and brighter into the night. Solar-lighting solution manufacturers seek to leverage these advances quickly and cost effectively; one way to achieve this is using a power-conversion strategy which enables rapid development and deployment of solutions that utilise the latest technologies. This article will review the components needed to develop such a system, and analyse the impact of this approach.

Solar powered lighting ranges from solar-powered lanterns used as a night-time reading lamp in areas with an unreliable power grid to the deployment of full-scale, community-grade street lighting. The diverse applications for solar/LED lighting systems are broad-based and global; they only differ in the scale of the end application.

The core components for each of these systems are: Solar cells - energy collectors; Battery - energy storage; LEDs - energy emitters.
As demonstrated in the system configuration diagram Figure 1a.
For this implementation to work, the behaviours of each element must be compatible, which 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.

Component Overview
Figure 2 provides an overview of the performance characteristics of each component. While each component can be made to behave close to each other within a limited configuration set, it is almost impossible to guarantee performance. The 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.

A system that interfaces all of these components directly has significant limitations, as well as ramifications for the overall system efficiency and its robustness.

Figure 1b presents an alternate system diagram which addresses these limitations. The 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 optimised. The microcontroller is not essential; a standalone battery-charger integrated circuit (IC) can address the needs of the NiMH charging profile, and LED driver ICs can convert the battery voltage into a constant current source.

However, the flexibility of a configuration without a microcontroller is limited; the devices are likely to have a fairly narrow operating range, which limits their ability to respond to changes. If the solar-cell configuration is changed, the battery-charging IC will need to be replaced.

Both the battery-charging IC and the LED-driver IC will need to be replaced in the event that the energy-storage technology or configuration is changed. Finally, if the LED type or configuration is changed, then the LED-driver IC will need to be reconfigured. Given the pace of innovation, standard flexibility allows faster responses to changing requirements and new opportunities. Flexibility in the system comes from the fact significant hardware changes which require intensive redesign and requalification.

A discrete-based solution would have difficulty keeping up with the pace of innovation without a system optimisation component. A generic battery charging IC would not maximise the output of the solar cell in the same way as a solution within a microcontroller which also had a Maximum Peak Power Tracking (MPPT) algorithm included.

Proposed Implementation
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 a proposed implementation.
There are three advantages to this approach.

Quick and easy system optimisation. There are four primary systems within this solution: LED, battery, solar-cell and power electronics. The battery-charging profile should be controlled to enhance both the charging efficiency as well as its lifetime, but 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 SolarаElectricity power conversion.

The result is a reduction in the size of the solar array while still achieving the charge objectives.The reduction in size impacts the product’s form factor, and provides options to the designer to enhance visual appeal. Light quality may be a critical characteristic in the target application, as would be the case if used for reading.

Light quality can be attributed to the current waveform, driving a tight tolerance for the LED drive current or including dimming capability. The implementation with a microcontroller allows design engineers to optimise everything from the component efficiency to the system’s overall robustness and lifetime.

Scalable and functional across a broad power range. A single solar cell, off-the-shelf rechargeable NiMH batteries and a few LEDs using 20-75mA of drive current can power a compact, portable lantern for reading. Replacing the powertrain components, including readilyavailable power MOSFETs and transformers ensures thatthis 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 350mA of drive current can be used.

Platform flexibility to adapt to rapid technology change. 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.The flexibility therefore moves beyond performance and into diagnostic functionality which allows a device to predict and communicate when it will require maintenance.

A microcontroller-based power-converter solution offers extensive flexibility when coupling two emergent technologies such as PV solar and LEDs. The solution allows for fast implementation of improvements which satisfy customer needs and dovetail with the advances of this emerging technology.