How to layout the PCB of photovoltaic power supply?

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How to layout the PCB of photovoltaic power supply? [Copy link]

This article discusses the PCB layout of a power supply circuit that generates a regulated 3.3 V rail from a small solar cell. My goal in this project was to create a very simple, very compact circuit that could power a microcontroller-based embedded system. The circuit will be active only during the hours when there is ample lighting, as the design does not include capacitors or batteries to store excess energy.

In this article, I will take a look at the PCB layout of the circuit from the power supply schematic.

PCB Layout for a Photovoltaic Power Supply

The following image shows the layout of the top and bottom of the PCB. All of the components and most of the traces and copper pours are on the top; the bottom is primarily a ground plane.

PCB size

The microcontroller is EFM8 Sleepy from Silicon Labs Bee, the (relatively) large connector on the left provides a direct connection to the SiLabs USB debug adapter. This connector consumes quite a bit of PCB space, making the overall design look larger than it really is. The image below shows the dimensions of the PCB in inches. The shorter horizontal dimension is my attempt to estimate how small the board could be if the debug connector was removed (and the other components rearranged). So my guess is that a two-layer board with all the components on one side would probably be less than 1.5 square inches. I would say that’s pretty good, especially considering that this is a two-layer PCB we’re talking about.

Also, I don't think I lost any performance by using two layers instead of four, since the bottom is almost a solid ground plane and there's plenty of room on the top for wide power lines and a generous ground connection (also because the microcontroller will be running at a very low frequency).

It's small and compact, but it could be smaller

Here are some other ways you could have reduced the size of this board:

● I chose larger passive component ICs (0805 and 1206) because they are easier to assemble. If you plan to have the board professionally assembled, you might consider using a 0603 or even a 0402 (you might be able to find an acceptable 2.2 μF capacitor in a 0402 package, but for the 0.1 μF capacitor and resistor, you can definitely use a 0402). I chose a larger package for the microcontroller; it's a 9 mm × 9 mm QFP32. The 32-pin leadless package is significantly smaller (5 mm × 5 mm), and there's also a 24-pin leadless package that's even smaller (4 mm × 4 mm). In my opinion, most applications built around this power supply won't require more than a handful of I/O pins, so the 24-pin package is probably the best choice. I used a 32-pin device because there aren't any other leaded (i.e., non-leaded) packages for this microcontroller. I used a high-precision 32.768 kHz crystal for real-time clock applications; it's about the size of a 0805 component. The microcontroller has an internal low-power oscillator with poor accuracy (±10%), so if you don't need precise timing, you can omit the crystal.

● The charge pump switching regulator currently has four 2.2μF output capacitors, but only one is needed.

● The LED and its accompanying resistor are for debugging only; they can be omitted in the final design.

● You might think that you could eliminate all the circuitry associated with the debug power supply (the switch, LDO, and two capacitors). I would not recommend this because solar power is not a convenient power source for firmware development and testing.

[font=-apple-system-font, BlinkMacSystemFont,The last item on the list of how to make it smaller is to have components on the top and bottom of the board. As I was writing this I began to wonder if the entire circuit would fit in an area corresponding to the size of the solar cell, so that you could design a board with just the solar cell on the top and everything else on the bottom. I decided to remove some unnecessary components from the schematic and then try this idea, here is what I found (dimensions are in inches):

This is a rough approximation, but, as you can see, we're pretty close to our goal of cramming all of the circuitry into the PCB space occupied by the solar cell.

To create this component placement, I eliminated three of the four output capacitors, the crystal, the LED, and the resistor for the LED. I also switched the microcontroller package to a QFN24. The passive components are still 1206 and 0805, but these larger packages make up for the fact that you'll need some way to connect the microcontroller to the debug adapter. There's certainly not a lot of room for routing, but if you can get by with a four-layer board (and there's still plenty of room on the top side under the solar cell), I don't think this is a serious hindrance.

Conclusion

We have discussed the PCB layout for a solar-powered microcontroller board that I recently designed, and we have also looked at an example of a more space-optimized implementation in which the dimensions of the PCB approach the dimensions of the solar cell. If you have any experience with space-constrained design for low-power embedded devices, feel free to share your thoughts in the comments.

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"]We’ve discussed the PCB layout for a solar-powered microcontroller board I recently designed, and we’ve also looked at an example of a more space-optimized implementation where the dimensions of the PCB approach the dimensions of the solar cell. If you have any experience with space-constrained design for low-power embedded devices, feel free to share your thoughts in the comments.

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"]We’ve discussed the PCB layout for a solar-powered microcontroller board I recently designed, and we’ve also looked at an example of a more space-optimized implementation where the dimensions of the PCB approach the dimensions of the solar cell. If you have any experience with space-constrained design for low-power embedded devices, feel free to share your thoughts in the comments.

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The main reason is that the space is limited. In fact, photovoltaic power sources are the same as general power sources in other places.