I was looking for a dual voltage power supply initially to power an old HDD with 12V and 5V. I wanted to build a home server based on the Raspberry Pi. I later realized that I had a newer hard-drive that needed only 5V. By then I had already designed and built this one. This power supply has a typical efficiency of 85%+ under real-world conditions and is based on TI's TPS54383 DC-DC converter chip which operates at 300 KHz. A 600 KHz version is also available and is a drop-in replacement for TPS54383.
Edit @08 Feb 2014: Pictures of the new revision added. Also the board shows reverse polarity for Vout-II. This is what happens when you create boards at 1 AM. The '+' and '-' should be switched. You'd see a negative supply on Vout-II with the way the board is as of now.
Edit @25 Dec 2013: Attached are Gerbers and PDFs for a board with USB female connectors should you want to use this power supply to connect to devices such as the RPI or standalone MCU systems. Remember that USB connectors support a maximum of 1A current through its pins.
A piece of advice and a warning of sorts: We are dealing with high currents and voltages here (especially if you use transformers to step down voltages to power this system). Please be careful and I'd hate to see you get zapped. If you are not sure about certain connections, DO NOT make them. Read and research about it. Ask questions! Disconnect all power to the board before assembly. Understand what outputs you'll expect to read and where. Even though the output voltages may seem low they are high-current outputs that can still give you nasty jolts if you touch them and act as the load. Do not short the outputs. Even though this device has some protection built it in it, its never a good to test and see if it works. I'm not responsible for your actions with this project that may land you in the ER.
This particular chip was selected because of its high efficiency. This plays a critical role when the system needs to be always on. This also is very useful when the whole system is powered through a battery. The TPS54383 also features an over-current protection (hard-wired at 3.5A here for Output 2 and 4.5A for Output 1), a soft-start where Output 2 starts after Output 1 has reached full regulation.
Step 1: Design Stage
This output voltages for my particular design were 12V and 5V. This is not a breadboard design project and you will need to get a proper PCB made. Oh and you do need soldering expertise. I have also included the Gerber CAM files for the board. The picture shown above is an older design and a newer design is included here.
First up, the circuit. This is taken from the reference schematic in the datasheet (though some of the values mentioned in the datasheet were a bit off and I had to adjust them in the simulation). I simulated this circuit in TI's TINA9-TI software which is available freely from TI's website (simulation files are included here). I also got a few queries answered through their e2e TI community and they were really helpful and very quick with their responses.
I wanted to power the whole circuit from AC mains and hence you'll see a full bridge rectifier and some smoothing capacitors (C1, C2, C3 and C4). What you don't see here is a toroid transformer that steps down mains AC to about 18V AC. This whole system works with a battery as an input source too. I'd recommend rechargeable NiMh or LiIon ones since they typically have higher current capacities. In fact, the whole point of selecting this chip was its high efficiency (85%+) factor that plays a significant role in always on supplies and when powered with a battery.
As you can see in the image, there is a USB connector here which is not included in the new design. Since my earlier design had to power a Raspberry Pi and a HDD, I though a USB connector would serve as a nice power connector.
Key design components
For each output here are the key components that decide the output voltage,
- L1 and L2 - Inductor values vary according to the output voltage desired (47uH for 12V, 22uH for 5V and 3.3V outputs)
- Feedback network on FB pins, R5, R7 and R6, R8
- For 12V - R5: 51.1K, R7: 3.83K
- For 5V - R5: 20K, R7: 3.65K
- For 3.3V - R5: 20K, R7: 6.34K
Should the need arise the power supply can be switched on and off with a microcontroller through the EN1 and EN2 pins of the chip. These are active low which means they need to be pulled to 0V for them to be active. For an always on supply, these pins need to be grounded (through jumpers on the board). They are marked on the board.
C4 is not required for higher output voltages. However for lower output voltages (below 5V typically) you will need this. Capacitors C9, C10, C13 and C14 are also optional but I've used them here. Same goes for the RC network on the voltage divider on the FB1 and FB2 pins which is primarily used to reduce ringing at the FBx pin of the chip. The PCB anyway has placeholders for these should you want to use them.
Step 2: PCB Design
Switching regulator board design is a science by itself. Careful consideration must be given here and often this is the part that takes the most amount of time. In this instructable a ground plane construction is used so as to reduce noise in the system that leads to optimal performance.
The board has two ground planes that are connected through a jumper. This is primarily done to isolate the signal and power sides. Power grounds might sink a lot of noise and this will interfere with the signal-side of the chip. Hence the two ground planes.
The board shown in the picture here is based on an older design. The newer board included here is smaller in size and follows the same board design considerations as the evaluation kit available from TI for this chip.
Also included here is a slightly altered edition (with the word 'Instructables' in the files) which includes a USB connector footprint for drawing out power. This is especially useful when powering devices such as the Raspberry Pi or standalone MCU systems. You can of-course leave this part unpopulated if you do not need it. This makes the board a tad bigger. Board size is now 3.62 inches x 1.26 inches. Remember that USB connectors support a maximum of 1A current through its pins.
Step 3: Assembly
I usually get my boards made from OSH Park. The boards are of excellent quality and quite inexpensive since this is
a community shared panel. There is some lead time but nothing that will
break your schedule.
If you are new to surface mount soldering then take a look at this wonderful (though slightly long) tutorial. This is part 3 of a 3-part series and the earlier parts also look at soldering through hole components. All in all a wonderful and useful tutorial.
I usually solder all the surface mount components first starting with the main chip. I used a hot-air gun to solder all surface mount components. The chip has a heat pad in the middle which needs to be connected to the Signal ground. A single heat pad on the PCB is enough if you intend to reflow solder this board. I've provided two heat pads on the top and bottom layers just in case you decide to hand solder this chip. The lower pad is primarily used to heat the pad on the top through the vias on it. A great tutorial for doing this is shown here.
All the through hole components are soldered at the end ranging from the shortest at first and the tallest at the end. I could've used surface mount capacitors for the output (C11 and C12) but instead used through hole ones. This is primarily due to cost and the fact that through hole ones are easily available and are quite cheap. You can substitute the inline bridge and make your own bridge with individual rectifier diodes though this is not recommended since all the leads would be exposed (look at the current ratings of individual rectifier diodes here. They are important. The bridge used here is rated at 10A though a 4-6A rated one is fine). The big input capacitor (C1) is rated at 35V but if you are configuring it for lower output voltages a 16-25V rated one would work just as well.
I have a sizeable stock of Molex power connectors (39-28-1023) which have a pitch (spacing between the connector pins) of 4.2mm. This is why there are two input connectors on the board - P2 is a 4.2mm one and P3 a standard 0.1 inch pitch connector. You can use either of them. Note: They are not seperate inputs where you can connect two different sources. So don't do that. They are one and the same with the flexibility of connecting different connectors. Only use either of them.
Use connector P1 if you need to connect a battery as the source. You can skip all the full-rectifier components (P2 and P3, C1 to C4 and the bridge D1) and use that space to physically place the battery there instead. The overall board size is about 3.1 inches x 1.1 inches. All mounting holes are standard M2 sized holes.
Step 4: Testing
I tested this using four multimeters. I dont have an image of this though. Connect two multimeters across the output ports (P4 and P5). Connect two resistors of low values but high power ratings in series with the load and multimeters three and four measure the voltage across these resistors. Based on the voltage and resistor values the current can be deduced. This website has good info on how to measure voltages and currents. Though the device is capable of delivering rated outputs at longer durations, I have not tested this at large lengths of time.
Thats about it for this instructable! Let me know if you find it useful and use the comments space for any questions you may have. Happy building!
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