So I am working on an LED project for a friend, and of course I have to take something relatively simple and cheap and make is complex and expensive. Modules are just no fun
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TL;DR at bottom if you don't want to read all this
I needed a converter to take 9V and output 3.3V at 4.5A full swing. I chose the LM3150 synchronous buck controller from TI, and used their webench online tool to design it for 6A capacity with a 300kHz Fsw.
www.ti.com/lit/ds/symlink/lm3150.pdf
I laid out the PCB, fingers crossed because I could only do single sided at the time. It didn't work well at higher currents at first, but looking at the feedback pin on the scope it looked like a noisy mess. So I added a 220pf bypass as well as another 220pf bypass on the switching node for the same reasons. That seemed to rein it in, and although the voltage was sagging a bit at 4.5A, it was still fine, efficiency was pretty good (93%), and output ripple was a workable 60mV.
First Board:
Full of confidence, I wanted (and needed) to make some changes for the next revision. Mainly to shrink the switching plane and to put the input caps and source of the lower switching MOSFET closer together, both points that are stressed in the datasheet. Also I wanted to make a number of changes to the lower LED control circuitry.
I made a new board and fully populated it this time because of my aforementioned confidence, and once I put in the jumper leads under the board I can't use a hot plate anymore to assist with hot air soldering. Anyway, to cut to the chase, the performance was abysmal for anything above 3A. At 4.5A was getting 2.3V on the output, and it would slowly rise over the course of minutes to 2.6V. I tried all manner of bypass caps allover the place but was only able to get marginal performance gains at best. A 2.2nF on the feedback pin seemed to work best, but again, only marginal gains. I poured over the board looking for irregularities, but came up dry. Efficiency was still OKish at ~86%.
Second Board:
Well now the board is totally dead. I moved away from playing with the buck to test out the AVR. I just flashed a simple LED blink program, but on the first go also pulsed the EN pin on the buck at 1 Hz by accident (both LED and EN are PORTB). I wouldn't think that that would cause damage, but when I went back to the buck circuit later it was totally dead and it's the only thing I can think of that might have done it. The AVR is powered by a separate 5V linear reg.
I'm not about to give up yet though and just get a buck module (I really don't want to do that). So I am hoping that someone here with more experience could make offer some advice on where to go from here design/troubleshooting wise. At least on paper it seems that the second design should perform better not worse, but it does indeed perform much worse. I recently got some double-sided copper clad, and although I would rather drill holes and run jumpers, I'm not totally adverse to the headache of aligning the two toner transfers. Any specific help or general converter design tips are welcome, thanks!
TL;DR: I made two buck converter designs. The first version performs much better than the second version, despite the designing the second version more in alignment with the design guidelines. I can't figure out why that is, and would like to know how to make it so the next design performs at least as well as the first.
Note: I don't have the first design saved anymore, I wrote over it with the second design. (Doh!)
Bonus unpopulated board images
1st board:
2nd board: