Micro MPPT Solar Charger

Jul 2023 - May 2024

The goal:

UAVs are becoming more common in various industries, and because of this, many solar powered UAVs are being developed. However, these UAVs are extremely large and heavy. This has meant that there has been little need for small and lightweight, yet efficient solar chargers for this application.

My design aims to fill a space in the market which requires the charger to be extremely lightweight while still providing a large power throughput.

Technical Description:

The custom 4-layer PCB implements the MP2731 IC (designed for fast charging single cell LiPo batteries). This was paired with an STM8S microcontroller to implement the MPPT algorithm allowing for efficient solar tracking.

Project Specifications:

1.8 x 1.0 x 0.5 inches in size
3.7V - 16V input voltage
4.5A max output current
Features dynamic power path management (see below)

Applications:

Solar PV in its current state is generally used either for small, low power applications, or extremely large and heavy applications. This charger exists in between these two spaces, providing allowing for small, lightweight applications to still receive a high amount of power. While the board was designed with UAVs in mind, it could be adapted to fit other applications.

Cube Satellites

Delivery UAV

Mobile Laptops

Reconnaissance UAV

UAV Application Example:

The dynamic power path management integrated into this IC allows smart control over power from the input. Compared to the conventional method, this eliminates many issues, allowing charging to happen simultaneously with system operation.

PCB Design:

Schematic and PCB designed in KiCAD. PCB manufactured using PCBWay

Layer 1: Signal, PWR, GND

Layer 3: GND

Layer 2: PWR

Layer 4: Signal, PWR

MCU Design:

MCU

To program the MPPT algorithm into the MP2731 IC, I am using the STM8S903F3, a capable 8-bit microcontroller. I have implemented the MCU onto a custom protoboard, where it interfaces with the Solar Charge PCB via the MCU breakout connection.

Software

Using the ST Visual Develop IDE, the MPPT code is written in the C language, where it is sent to the Cosmic compiler. From there the MCU is flashed using the SWIM interface and bootloader for in-system programming

Program Flowchart (provided by MPS)

Challenges:

The first couple of months I spent working on this project were a complete failure in my eyes. I thought very little about the decisions I made and did very little planning. At first, I was convinced I could cut my own solar cells for this project, even though this was not a part of the project and it was possible to buy them. This alone resulted in at least two months of wasted effort, just for me to resort to buying pre-cut cells.

During this time I did develop a PCB for solar charging, but the design was never tested, as around the time I received them I realized that even if the PCB worked, the rest of my project would not.

At this point I reevaluated the entire scope of the project, making it something that was actually feasible with the skills I had and could learn. I started learning how to plan out a project, set a timeline and due dates for myself.

Although I got better at organization, I still ran into trouble with the scope of my project. I thought it would be easy to develop this solar charger as well as a UAV to demonstrate its performance. But as I have little experience with lightweight gliders, my attempt at creating one was unsuccessful. Another example of needing to narrow my focus on the main goals of the project.

Extra Project Details:

In the summer of 2023, I joined the Information and Smart Systems Lab at KU where Dr. Huazhen Fang assigned me to develop a solar powered paper airplane. This invention would be lightweight, portable, and cheap. Unfortunately, after lots of failure, I was not able to come even close to achieving this. However, drawing from those failures, I decided to continue working, this time on a specific component being the solar charger.

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