Power supply project - Page 1
Forum section: electronics / beginners

author: damn_dirty_ape
author_rank: Contributor
timestamp: June 13, 2017, 07:07:48 pm
content: Hi,I'm planning on making a small power supply as a project to learn some more about PCB layouts and SMD soldering.I'm looking at the design example (attached) and have a few questions.C1 through C5 are all the same, if I can get a single 50uF cap can I use it, or is there a reason to have an array of 5 10uF?Same question for C11 and C12, except they have two connections drawn to ground instead of one. Is that some kind of specialty capacitor or am I supposed to make the traces extra big or what?Also, the mosfet they are reccomending has 4 drain pins, but only one of them is shown, while three pins are used on the source. What is the explanation for this?Thanks.

author: Paul Price
author_rank: Super Contributor
timestamp: June 13, 2017, 08:09:27 pm
content: The answer is no, it would not be a good idea to combine all the bypass capacitors into a single unit, either at the input or output of a switcher..The capacitors are connected in parallel to optimize for lowest ESR, series inductance and to achieve their longest life while working at high currents. This paralleling of multiple caps minimizes capacitor stress and heating and makes for a lower profile PCB than using a single capacitor.The multiple source and drain pins on the MOSFETs are connected in parallel to provide the lowest resistance and lowest inductance to the MOSFET source and this is accomplished by using as many available pins in parallel as its package makes available. The person who created this schematic tries to emphasize the need to use all paralleled connections, but didn't bother to show the actual multiple connections to the drain of the MOSFET but you can be sure the actual PCB would have connected in parallel all available drain pins on its package.A properly designed high-current switcher is much like a HF RF circuit in that inter-component and bypass inductance and resistance should be minimized for best circuit performance.

author: damn_dirty_ape
author_rank: Contributor
timestamp: June 13, 2017, 08:19:18 pm
content: Good explanation, thanks.What about C11 and C12 showing separate traces to ground? What would that physically look like on the board? Is it a larger trace or two separate traces?

author: Paul Price
author_rank: Super Contributor
timestamp: June 13, 2017, 08:41:20 pm
content: It is a single trace, a ground plane PCB layer if PCB possible, and drawn this way to emphasize the need to minimize inductance by creating the widest traces/ground planes/power planes for all circuit connections carrying high currents used by this switcher.

author: damn_dirty_ape
author_rank: Contributor
timestamp: June 13, 2017, 08:45:11 pm
content: Got it, thanks again !!!

author: floobydust
author_rank: Super Contributor
timestamp: June 13, 2017, 08:52:19 pm
content: This is a demanding PCB layout.For learning, I would suggest studying theTPS40303 eval board PCB design.It's just that mistakes routing traces and the 10A output current, things will not run well, even with one trace done incorrectly.

author: damn_dirty_ape
author_rank: Contributor
timestamp: June 14, 2017, 01:44:38 pm
content: Yeah, I don't think this is going to be the chip that I end up using. I just saw the schematic and immediately had questions. I would like to have something that I can plug into the wall, step down with a transformer, rectify etc. and end up with a variable voltage supply.If there are any buck controller ICs, or something else that you would recommend, let me know.