Saturday, April 28, 2018

FPGA Power test board

Here's the second of my power subsystem test boards being probed:

This one will supply 1.2V for the Xilinx Spartan 6 FPGA's logic core, and 3.3V for the FPGA's I/O interfaces and the 74LVC2G14 Schmitt trigger inverters. The VFD filament transformer driver will also run off this 3.3V supply.

There's only one integrated circuit on this board, the LTC3607 dual synchronous buck regulator. I have it wired so the upper half of this board is the +3.3V regulator and the lower half is the 1.2V regulator, though there's no reason the functions couldn't be swapped by swapping inductors and feedback resistors . The reason there's a big honking aluminum electrolytic capacitor in the lower left corner is that the input is 5 to 8 VAC at 60 Hz, rectified by the bridge rectifier above the capacitor.

During the assembly I discovered that I'd mistakenly ordered a 0.22 µF ceramic capacitor in a 1206 package (0.012 x 0.006 inches) instead of the 0603 package (0.006 x 0.003 inches) the board is laid out expecting. Needless to say there's no way a 1206 is going to fit in the space allocated for a 0603:

Fortunately this was a non-critical component, and I was able to substitute another part I had on hand easily.

Unlike the VFD power test board, which worked the first time, I had some problems with bad solder joints on this one. That's probably because I didn't have solder stencils made, and It's very hard to get any sort of controlled amount of solder paste applied without one. Rather than apply too much and risk shorting leads to the ground pad under the QFN package I opted for too little. I had to go back once to add solder on the top and right edges, and a second time to clear a solder bridge on the right edge. My plan has always been to get a solder stencil with the final board, and this has doubly convinced me of the need to do so.

With the soldering issues resolved the board appears to work as designed. I have a series of tests planned for it, ranging from low load stability to high power capabilities. But those will have to wait for another day.

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