Electronic Design - PCB layout of photovoltaic power supply for microcontroller circuit

PCB layout of photovoltaic power supply

All components and most of the traces and copper pouring are on the top; the bottom is mainly the ground plane.

PCB size

The microcontroller is Silicon Labs' EFM8 Sleepy Bee, and the (relatively) large connector on the left provides a direct connection to the SiLabs USB debugging adapter. This connector consumes a lot of PCB space, making the overall design look larger than it actually is.

The shorter horizontal size is my attempt to estimate how small the board can be if the debug connector is removed (and other components are rearranged).

So my guess is that the double-layer board of all components on one side may be less than 1.5 square inches. I would say this is pretty good, especially considering that we are talking about a double-layer PCB.

In addition, I don’t think I lose any performance by using two layers instead of four, because the bottom is almost a solid ground plane, and there is enough space at the top for wide power lines and wide ground connections (also because the microcontroller will Run at a very low frequency).

Here are some other ways to reduce the size of the board:

· I chose larger passive components (0805 and 1206) because they are easier to assemble. If you plan to assemble a circuit 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).

· A larger package was chosen for the microcontroller; this is a 9 mm * 9 mm QFP32. The 32-pin leadless package is significantly smaller (5 mm * 5 mm), and there is also a 24-pin leadless package. Foot package, smaller size (4 mm*4 mm). In my opinion, most applications built around this power supply do not require more than a few I/O pins, so a 24-pin package may be the best choice. I use a 32-pin device because the microcontroller does not have any other lead (ie non-lead) package.

· Provides a high-precision 32.768 kHz crystal oscillator for real-time clock applications; it is approximately the size of an 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.

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

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

· It may be considered that all circuits (switches, LDOs and two capacitors) related to the debugging power supply can be eliminated. This is not recommended because solar power is not a convenient power source for firmware development and testing.

Double-sided selection

The last item on how to make a smaller list is to have components on the top and bottom of the board. When I wrote this article, I began to doubt whether the entire circuit fits in the area corresponding to the solar cell size, so that you can design a circuit board with only the top solar cell and other things on the bottom. I decided to remove some unnecessary components from the schematic and try this idea, this is what I found (dimensions are in inches):

This is a rough approximation, but, as you can see, we are very close to the goal of squeezing 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, LED, and the resistance of the LED. I also switched the microcontroller package to QFN24. Passive components are still 1206 and 0805, but these larger packages can make up for the need for some way to connect the microcontroller to the debug adapter. Of course there is not much room for routing, but if you can use a four-layer board (and there is enough space on the top under the solar cells), I don't think this is a serious obstacle.

in conclusion

We have discussed the PCB layout of the solar microcontroller board I recently designed, and we have also studied a more space-optimized implementation example, where the size of the PCB is close to the size of the solar cell.