Switching Power Supply

My magnetometer sensor is going to run off a 12 volt lead acid battery charged with a solar panel. The main power needed is 5 V DC so a regulator to convert the battery voltage to 5 V DC is needed. I will use a switching regulator to keep the power needs low.
Texas Instruments has a switching power supply design application, WEBENCH,  on their web site that lets you set the requirements and then provides several designs that meet those requirements. Of course they use TI semiconductors.


Power Supply Schematic

The WEBENCH application generates a schematic, bill of material and performance data for the power supply design. This design only has 7 components.

I am not going to let surface mount parts get in the way of building circuits. The only leaded integrated circuits available today are older designs. If you want to build something with modern parts you are forced to use surface mount parts. So I set about getting up to speed building circuits with surface mount parts.  The surface mounted discrete parts capacitors, resistors and inductors are less expensive than the leaded versions anyway.  I ordered 10 each of the discrete parts for this board to help build up my inventory.


Board Layout

One thing about surface mounted parts is that you are pretty much forced to make a printed circuit board to mount them. I used the free version of CadSoft Eagle to do the board layout. Since I already had a schematic from the WEBENCH application and there are only 7 parts I went directly to board layout, skipping the schematic entry part of Eagle. I lucked out and there is only one place where a circuit trace passes under a component. So there was no need for a double sided board or jumpers. Eagle will let you print out the board patterns actual size or with a fine scale factor if needed to match the printer output.

To make the board I used a product from Techniks called press-n-peel blue. To use this you laser print the copper pattern actual size and mirrored onto the press-n-peel sheet and then iron the pattern onto onto a piece of copper clad. The laser tone acts as an adhesive to bond the blue coating on the press-n-peel onto the board. I had to try it a couple of times before I got the ironing technique down. There were still a couple of places where the pattern did not stick but it did not affect the circuit. After that the board is etched in a warm ferric chloride bath.

To solder the parts on a microscope is essential at least with my eyesight. I apply liquid soldering flux to the board. This not only makes the solder flow well it also helps hold the parts in place while soldering. I consider the use of the flux essential. A fine tip temperature regulated soldering iron, very small diameter solder and two steady hands are also needed.  After the parts are soldered on I like to wash off the flux and inspect the solder joints under the microscope. I have been known to put my boards in the dishwasher to clean the flux but in this case I just washed it off with some lacquer thinner.


Completed Circuit Board

In the photo of the board you can see that I did not get all the flux washed off. You can also see the effect of the printer dots in the edges of the traces. The small traces here about as small as I would want to go with the press-n-peel method.

There is a lot of discussion on the WEB about using reflow ovens and solder paste to build boards like this but for making just one board hand soldering works just fine. In the future I might try hand soldering using solder paste instead of the wire solder because the paste would hold the part in place before soldering better than the flux. To do that one would place some solder paste on the pads with a syringe, stick the part down and heat the pads with the soldering iron.  On problem with the paste is that it has a shelf life and should be kept refrigerated until use.

I hooked up the completed power supply to a load resistor and battery to check it out. It does indeed work as designed putting out 5 V DC at 100 ma. Now I have a nice little 5 V power supply to use in bread boarding my sensor designs. In the final circuits for the sensors I will probably incorporate this circuit into a larger circuit board containing the rest of the sensor circuitry.

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