Friday, April 27, 2018

VFD Power test board

Both the VFD power and FPGA power test boards showed up today. I decided to assemble one of them this evening, leaving the other for tomorrow.

I chose the VFD power test board because it is really two independent circuits that could be assembled and tested separately. The filament driver I've tested before on a breadboard, so I had reasonable confidence that it'd work if I didn't botch the board layout. The LT3494-based boost circuit was a new (for me) design so I wasn't so confident of that. I'd originally planned to breadboard it, but the effort (and surprisingly, the cost) for that didn't seem worth it.

Here, again, is the layout as it appears in KiCad:

And here's the board with all the components installed:

Please, please... won't someone remind me periodically how small 0603 components are? Don't drink caffeine before handling these things or you'll never get them placed where you want them. Thank physics for surface tension!

The bit that worried me the most was the LT3494. It's tiny, measuring a whopping 3mm x 2mm. It's the small black rectangle above and to the right of its label "U1". It's a DFN with leads spaced 0.5mm apart and a heat sink pad in the middle, and there's no way to visually check whether its leads are properly soldered short of X-ray imaging. Even worse is I didn't spring for a stencil for the solder paste, and it's hard to get slippery solder paste (slippery because of the flux) to stick to smooth gold-plated pads. I was worried at first that I didn't have enough paste on the pads, but you really don't want much.

When I first applied power I got nothing. No output, but no smoke either. I was starting to worry when it occurred to me that I hadn't installed the DIP switch in the lower left corner. R3 holds the LT3494's Enable line low (disabled) until it's driven high by the upper switch, and eventually an FPGA I/O pin. I had a second, briefer scare when the switch didn't do anything before I realized I had only connected the +7.5V power and not the 3.3V power that feeds the switch. With that taken care of I get a nice 30VDC output on J1.

With the upper half circuit working, I removed my only TGM-210NS transformer and SN6505B driver from their respective breadboards and transplanted them onto this board and populated the rest of the circuit. While double-checking everything before applying power I realized I'd installed the SN6505B backward and had to remove and reinstall it.

With everything double and triple checked I again applied power to the board. I used a 30 ohm resistor to load the transformer and got the expected output. Yay!

Another comment on having the right tools: I might have been able to do all this without the hot air soldering system, but I doubt it. Removing even a 6-lead DIP like the transformer would have been a challenge without it. I don't know how I would have soldered an 8-lead (plus heat sink pad) DFN without it. Or a home reflow oven.

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