Over the last year or so I’ve designed, built and tested a standalone solar charger. It performed quite well but as with any complex design there were a number of things that needed to be improved. So I eventually reached the point where I decided to design a revised version. And this is what this post is all about.
Things to improve
Here’s a list of things that I wanted to improve besides fixing the design errors of the old design:
- Downsize the board and give some thought to how to put the whole thing into a case. Consider carefully where to put screw holes, connectors and the like.
- Reduce complexity wherever possible. Reduce the number of distinct components.
- Reduce total component cost without sacrificing performance.
- Reduce height of the assembled board so that it can ship in a padded envelope.
- Turn user interface on by means of a logic signal as opposed to turning its power supply on and off. Ability to control the display entirely via I2C, including backlight brightness and reset.
Let’s look at these points one by one.
Downsize the board
I looked around for a suitable case for my solar charger and finally decided on a 90x115mm cast aluminium case. The specific model chosen is available from Farnell at around 10 bucks which I found very reasonable. With the case chosen the size of the board was pretty much given. The inside shape of the case is rather special since there are also mounting holes for the case itself that take away space for the board. However, I decided to keep the board rectangular so it stays universal and is not just made to fit that particular case. That left me with a board of 85x78mm as opposed to 90x140mm of the last version. That’s about half the size in terms of board area so fitting everything on the new board was really a challenge.
The main space saving came from using much smaller mosfets. The model previously used was rather huge in physical size. I’m now using NXP PSMN6R0-30YL in a LFPAK package that is much smaller while delivering similar if not better performance.
The main inductor is also somewhat smaller. I found that the Coilcraft MSS1210 series delivers pretty much the same performance as the MSS1583 while saving some space.
Apart from that I didn’t downsize any of the components in order to keep everything hand-solderable and hobbyist-friendly. Particularly, all the resistors and ceramic capacitors are at least 0805 in size.
Getting the board made from dirtypcbs.com also allowed me to do a denser layout, mainly by using smaller vias. But I didn’t push it to the limit, all traces are still at least 12 mils (0.3mm) wide with 0.4mm vias and 6 mils clearance.
Now all the power connectors are located at the top of the board with a single 12-pin 200-mil connector. USB is still located at the front and the programming header is at the right edge of the board. External temperature sensors and fan output are now on the left side. The header for the user interface is now located so that the user interface can just be stacked on top of the charger and sits precisely in the middle. There are also mounting holes for the user interface on the charger board.
The obvious place to start was the combination of PIC18F26J50 microcontroller with a I2C multiplexer and a port extender. The same PIC family also includes a 44-pin version that allows replacing all 3 chips with a single chip. I’m now using the top-of-the-line PIC18F46J50. That not only simplifies the board but also the software since we no longer need to care about the mux and which pins are on the PIC and which are on the port extender.
The 4 mosfet drivers for the power outputs have been replaced by 2 MCP14A0153 dual mosfet drivers that also don’t require a reference voltage signal which further reduces complexity somewhat.
The 5V switching converter for USB charging on the last version needed quite a few external components to function. Besides that it got rather hot at 2 amps. I’ve replaced it with a Texas TPS54428 synchronous switcher that is more efficient, uses less external components, delivers up to 4 amps and has a thermal pad at the bottom which helps keeping it cool. The new Coilcraft MSS1048 coil is slightly higher but otherwise the same size and helps keeping power dissipation low.
I’ve tried quite hard to keep the number of distinct capacitor and resistor values to a minimum. You can easily see that the same values such as 10k, 300k and 680k appear over and over again. The only point where there are still a lot of distinct resistor values are the USB charging ports where I’ve tried to emulate an Apple and Samsung charger, respectively. I’m not sure to what extent one can cut corners here without risking that certain devices don’t charge properly.
Overall, the number of distinct components has been reduced from 60 to 47 despite some added features.
Getting rid of the I2C mux and the port extender already saves some cost. The same goes for the dual mosfet drivers and the downsized inductor. There were some unnecessarily expensive components on the last version. There was an 18-bit ADC that we didn’t really need. The Microchip MCP3428 is a very similar model that only has a maximum resolution of 16 bits and is quite a bit cheaper without making any difference to our application. Another component that was somewhat of an overkill was the voltage reference. Note that the voltage reference is only needed for the temperature measurements. The voltage and current measurements are done by the external ADC that comes with a precise on-board voltage reference. I found the Silicon Labs TS6001 which provides a sufficiently precise 2.5V reference.
I’ve also replaced the large and relatively expensive EEPROM with a much smaller 16kbit Microchip 24AA16 EEPROM in combination with a cost-effective serial Flash (Spansion S25FL208K) with 8Mbit of capacity. This way we get much more storage at a similar total cost while still keeping some easy-to-use EEPROM for configurations and the like.
Since the only overly high components on the last version were the input and output capacitors this was an easy task. I decided that I want to use the same model for both input and output which meant that I needed a 25 volt rating. Maintaining the 10mm diameter and limiting the height to 12.5mm left me with a maximum available capacity of 470uF. The ripple current rating of the new caps is also lower so I now have to use 2 caps in parallel at the input to not exceed this rating at 75 watts.
The caps are still the highest components on the board but only slightly higher than the USB connector, the main inductor and the 200-mil connector. The resulting total height of about 15mm is low enough to comfortably ship the assembled board in a padded envelope which is much cheaper than a small parcel.
I’ve already presented a user interface that fulfills most requirements listed. However, I still needed to again revise the user interface for two reasons. First, the previous version would not physically fit inside the cast aluminium case. So I had to somewhat taper off the board towards the side so it would fit. I also had to re-locate the mounting holes inward so that they lay on the charger board. Secondly, I need to be able to shut down the user interface without blocking the I2C interface. This was not an issue with the last version where I had a I2C mux in between. So the new interface now includes a 2-channel bi-directional switch that is controlled via the same signal that turns the interface on and off.
I’m looking forward to my ReflowR hopefullly arriving soon so I can build up the board in an efficient way. As you can see below I have already prepared a setup to apply the solder paste to the board. I have glued three faulty user interface boards to a piece of coated plywood to repeatedly align the board. I have limited experience with reflow soldering but I will hopefully learn quickly and report my progress and lessons learned here.
As you can see from all the photos here the boards have already arrived and I’m excited to try them out. All the eagle files as well as the Gerbers and everything are on github: github.com/soldernerd/SolarChargerHardware.