Let's Make Robots!

Building the perfect robot power supply

One thing most robots need is a power supply that can put out a reasonable amount of current for servos and other small motors as well as run a processor and sensors. Probably the best setup is 5x 1.2V NiMh cells to provide 6V for motors and servos with a low dropout regulator providing 5V for the processor and sensors. Unfortunately this isn't always practical. Sometimes you have motors that need higher voltages or maybe the only battery you have lying around is a 7.2V battery from a RC model. In many of these cases there are ways around the problem which I demonstrated in a walkthrough on voltage regulation.

There are instances however when these solutions just won't work or will be very unefficent. In these instances you can't beat a good DC-DC converter. Unfortunately suitable DC-DC converters can be hard to find and very expensive so I've decided to build my own. I made a wish list of what I wanted:

1. Input voltage between 9V and 24V

2. Output voltage of 5V or 6V preferably adjustable

3. At least 3A of current output for driving lots of big servos.

4. Light weight / small size

5. Reasonable price

 

I then searched the internet for suitable controller IC's. Eventually I found the TPS5450 from Texas Instruments. This is a great little IC that can handle input voltages up to 36V and can switch currents up to 5A. It needs very few external components.

The only problem I could find was that it was a surface mount device. This was good from the small lightweight point of view but bad from the difficult / fiddly point of view. What the heck, if it works then the super small size will be great.

Designing the circuit was a snap since the datasheet included a sample that was perfect for my needs (figure 11). I just added a pot so that the voltage can be varied between 5V and 6V.

DC-DC_converter_MkII_Schematic__small_.jpg

So I've ordered the parts, most have arrived, damn they're small! Time to design the circuitboard. (No I'm not using eagleCAD MaltiK!)  Below is a printout with the components I do have sitting on top. This is to help me find any faults in the design such as pins not aligning. I've also got to workout how will I solder all these parts. I'm using solder paste but I need to make sure I can access the tracks with the soldering iron. I might have to allow more space between components.

power_jigsaw__small_.jpg

This board will end up being aprox. 30mm x 25mm (1.25 inch x 1 inch). It is costing me less than $20 AUD.

 


4-3-2009

 

The rest of my parts have arrived. I've made a few changes to my layout. After seeing PirCapAdi's home made switchmode I realised that I hadn't allowed for mounting holes. My original idea was to have it plug into a breadboard like a three terminal regulator but it's starting to get to big for that. PirCapAdi also mentioned a ground plane. Since I had to increase the size for mounting holes this was easily included.

power_jigsaw_2__small_.jpg

 This is the solderpaste.

solderpaste.jpg

The solder paste, blank PCB etc I got from Jaycar Electronics because they're only a short drive away.

The surface mount components I got from Farnell. The parts arrived next day. 

 1x 151-1850  TPS5450 DC/DC converter IC

 1x 154-7056  15uH 6A inductor

 1x 129-9291  SK54C schottky diode

 1x 969-5613  330uF low ESR capacitor

 1x 135-8526  330uF tag tantulum

 2x 969-4455  4.7uF low ESR capacitor

 2x 157-2637  4.7uF multilayer ceramic

 3x 940-6425  10nF capacitor

 1x 117-4302  10K trimpot

 


5-3-2009

 

I printed the mirror image pattern onto a sheet of "Press-n-Peel" circuit board transfer film which creates an iron on transfer of better quality than just printing on paper or transparency. After transferring the pattern I touched up the pattern and etched the board.

board_etched.jpg

You can see a few dark blue spots where I touched up the pattern with a pcb marker prior to etching. I then cleaned up the board and gave it a light coat of circuit board lacquer. Apart from protecting the board from corrosion, the lacquer acts as flux during soldering.

Now for the fun bit.

IC_solder1__small_.jpg

Using a toothpick, I dabbed solderpaste carefully on the tracks to be soldered and on the powerpad under the chip. Because this chip has a built in power FET it gets hot and needs a heaksink. The power pad underneath MUST be soldered to the ground track for heat dissapation. I then carefully placed the chip on the board, aligned it carefully with the toothpick and heated the ground track with the soldering iron at full temperature. The reason for cranking up the temperature was because the ground track was acting as a heat sink. I kept the heat on until I could see the flux bubbling at the base of the chip. Getting this solder joint right without overheating the chip is critical. I practised on a scrap of blank PCB first to see how quickly the solder melted. The bubbling flux from under the chip is your best indicator.

done__small_.jpg

This is the finished product. I tinned the tracks to boost their current carrying / heat sinking ability as well as protecting them from corrosion. In retrospect I should have done all the tracks prior to soldering the components. If you look carefully at the base of the components near the front edge you'll see copper that I could not tin. Click on the image for a larger picture.

terminal_solder.jpg

Since I didn't have plate through holes for my terminal block I raised it above the board enough for me to feed in fine resin core solder while I heated the leg from the other side. I'll pack this gap later with two part epoxy resin to strengthen it. Now to test and callibrate it.

calibration__small_.jpg

This is running from 12x 1.2V AA NiMh batteries (14.4V). I can adjust my output between 1.21V and 7.1V. For now I've set it to 5V for testing.

This little regulator is rated at 5A continuous (6A peak) making it ideal for running lots of servos from a 12V or even a 24V source.


Testing under load

I made a 1ohm resistor out of resistance wire and tested the regulator. It did maintain 5V but I was only getting 4A and the two 4.7uF capacitors and the chip got quite hot.

Reading through the datasheet again I think the problem is that the capacitors do not have a low enough ESR (equivalent series resistance). For now I'm not going to push the prototype beyond 1.5A

Once I replace the two capacitors all should be good.

 


7-3-2009

I've now discovered another mistake in my design. I really didn't read the datasheet carefully enough plus I'm not use to SMD parts. When I saw 4.7uf and 330uF I automatically ordered low ESR electrolytic capacitors. For this application due to the frequency (500KHz) electrolytics won't work very well. The upside of this is that with the new parts the supply can be made smaller. I'll probably redesign the board to take advantage of this.

 


13-3-2009

 

I haven't had a lot of time for this lately. Here's the prototype with ceramic 4.7uF and a tag tantulum 330uF.

With_new_parts.jpg

 

 

 

 

 

 

 

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I understand what you are saying. There was no mention of this in the notes unless I missed it. Even the sample PCB layout was missing it. If it's a problem then my next one will be on a double sided board. If this needs it then I'll have to put some foil on the back.
I recently went through the same exercise, but decided to use the LM2596S. It has a very wide input voltage 8V to 40V, 3A output and is pretty simple to use. About 10mins on Farnell’s website and 1h in eagle and this is the resulting design:

schematic-1.jpg

board-1.jpg

 

I also copied the sample circuit :-) With the build beieng:

IMG_0543.jpg

As you can see my soldering skills are still a bit lacking...

I am running this on my current dev bot consuming up to 500mA with out it getting hot at all.

I am now working on an enhanced version with a 3.3V output using a LDO, voltage reference output, simple divider, and current output using a ST current sensor. I am also toying with the idea to include a charger circuit.

 

That's exactly the type of result I'm hoping for. Unfortunately SMD components aren't easy to solder with normal resin core solder which is why I forked out $27 AUD for 80g of solder paste in a syringe that has to be stored in the fridge.
Given the difficulty of getting precise adjustment on a trimpot, wouldn't a jumper and a fixed resistor have been better to give either 5v or 6v?

Yes it would but I like the ability to vary the output, especially with a prototype. This output should actually vary between 1.21V and 6.71V.

Since it was designed for 5V output, it may loose some efficiency at the other voltages. Ideally I'd like to use jumpers to select between 3,5,6,7.5,9 and 12V. This means it could drive a wide range of circuits and motors as well as servos. To make it efficient over that range Jumpers would probably needed to be used to switch in/out different components to maintain efficiency.

At this stage I've been lazy and just copied the sample circuit. Later I'll go through the math if necessary.

Looks pretty cool Oddbot. That little power-pad under the reg chip can be difficult to get to, you can drill the spot beneath the chip to solder through to that connection. Was there no 2.2k resistor in surface mount, or you just wanted to use some you had?

The power pad isn't a problem. I'll put solder paste on all the pads and sit the chip on top. By heating the track near the chip, the heat will travel to the paste and melt it. I've done this with solder paste before.

As for the resistor, for some reason they only sold the resistors in reels, not individually. Another shop I tried was the same. It wasn't worth the hassel looking any further just for the prototype.