How to Lay out the PCB of Photovoltaic Power Supply
This article discusses the PCB layout of a power circuit that produces a 3.3 V rail from a small solar cell.
The goal of this project is to create a very simple, very compact circuit that can power a microcontroller-based embedded system. This circuit is only effective for a sufficient amount of illumination because the design does not include a capacitor or battery for storing the remaining energy. In this article, We will understand the PCB layout of the circuit from the power schematic.
The PCB layout of photovoltaic power
The following figures show the layout of the top and bottom of the PCB. All components and most of the traces and copper casting are on the top; the bottom is primarily the ground plane.
The dimension of PCB
The micro-controller is EFM8 Sleepy Bee of Silicon Lab, and the (relative) large connector on the left provides a direct connection to the SiLabs USB debug adapter. This connector consumes a significant amount of PCB space, making the overall design look larger than it actually is.
The figure below shows the dimensions of the PCB in inches. The shorter horizontal size is what I am trying to estimate if the debug connector is removed (and other components are rearranged), how small the board can be.
So my guess is that the double-layer boards of all the components on one side may be less than 1.5 square inches. I would say this is very good, especially considering that we are talking about a two-layer PCB.
Also, I don't think I'm using two layers instead of four layers to lose any performance, because the bottom is almost a solid ground plane, and there is enough space on the top for a wide power line and a large ground connection (also because the microcontroller will Run at a very low frequency).
Small and compact, but it may be smaller
Here are some other ways to reduce the size of the board:
1. I chose larger passive component ICs (0805 and 1206) because they are easier to assemble. If you plan to assemble a board professionally, you can consider using 0603 or even 0402 (you may find an acceptable 2.2μF capacitor in the 0402 package, but for 0.1μF capacitors and resistors, you can definitely use 0402).
2.I chose a larger package for the microcontroller; this is a 9mm x 9mm QFP32. The 32-pin leadless package is significantly smaller (5 mm x 5 mm) and has a 24-pin leadless Foot package, smaller size (4 mm × 4 mm). In my opinion, most applications built around this power supply do not need to exceed a few I/O pins, so a 24-pin package may be the best choice. I use 32-pin devices because the microcontroller does not have any other lead (ie, non-lead) packages.
3. I have a high precision 32.768 kHz crystal for real time clock applications; it is about the size of the 0805 component. The microcontroller has an internal low-power oscillator with very low accuracy (±10%), so if you don't need precise timing, you can omit the crystal.
4. The charge pump switching regulator currently has four 2.2μF output capacitors, but only one.
5. The LEDs and their associated resistors are for debugging only; they can be omitted from the final design.
6. You might think that you can eliminate all the circuits (switches, LDOs, and two capacitors) associated with debugging the power supply. I don't recommend this because solar power is not a convenient power source for firmware development and testing.
The final item on how to make a smaller list is to have components at the top and bottom of the board. When I wrote this article, I began to wonder if the entire circuit fits the area corresponding to the size of the solar cell, so you can design a circuit board with only the top solar cell and the bottom. I decided to remove some unnecessary components from the schematic and try this idea, which I found (in inches):
This is a rough approximation, but as you can see, we are very close to the goal of plugging all the circuits into the PCB space occupied by the solar cells.
To create this component placement, I eliminated three of the four output capacitors, the crystal, the LED and the LED resistance. I also switched the microcontroller package to QFN24. The passive components are still 1206 and 0805, but these larger packages can make up for you needing some way to connect the microcontroller to the debug adapter. Of course there is not much routing space, but if you can use a four-layer board (and there is enough space at the top of the solar cell), I don't think this is a serious obstacle.W
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