Nixie boost converter circuit does not want to behave - Page 1
Forum section: electronics / projects

author: ferenczyg1
author_rank: Newbie
timestamp: January 17, 2021, 11:36:00 am
content: Hi Everyone,I became fascinated by Nixie clocks a few years back and decided to create my own design with a little help from my brother, who is an electrical engineer. I am a software engineer and back then I didn't know much about electronics so I asked him to design an arduino compatible 4 digit nixie clock, which he did. It worked as it should, but I had a few problems with the design (it was just a prototype anyway). The PCB was bigger than I would have liked, and I changed my mind about wanting to use an arduino. Instead, I wanted to use a simple TM4C MCU.So I thought its time to start learning this stuff, and started designing my own version of the nixie clock with a few changes:The first design used these old K155ID1 driver ICs, the new uses SN75468 transistor arrays with a clamping diode.The first design used "old school" through hole components, the new one almost exclusively uses smd components.The new design has RGB LEDs under the nixie tubes.And here comes the problem. On the new PCB the boost circuit behaves really weirdly. It pumps up the output voltage to around 280V (poor output cap...). There is no load on the output. I have a 0.5A current limit set on my power supply. If I try to run the circuit from 12V (as it is intended), then the input voltage drops to around 9V with the current limit maxed out. This is around 4-5W of power dissipated somewhere. The only thing I can feel warming up is the FET. The original design works like a charm in the same conditions.Although I changed the FET to an SMD part in the new design, I couldn't use it, because it went out of stock just before I wanted to order it. So I soldered onto the SMD pads an IRF644 which was used in the original design. Because of this, the two boost circuit designs are virtually the same, apart from the actual layout.The LDO IC is not mounted at the moment, so only the boost circuit is powered up.Some troubleshooting I already did:Measured all connections and compared it to the first design, all seems to be connected properly.Measured the feedback loop resistances, everything seems to be fine.Checked the diode with a multimeter, seems to be doing just fine.I used a slightly different physical size inductor in the new design (but still 100uH), so just to exclude every difference, I changed it to one from the first design.I replaced the MAX1771 boost IC (I ordered two from everything), but the behavior didn't change.Some noteworthy observations:When I measure the feedback pin of the boost IC with a multimeter, I see around 2.6V. This is strange, because the inner reference voltage of the boost IC is 1.5V, so it should regulate the output so that the feedback is also 1.5V.Its even more strange, that when I place the probe on the FB pin, it seems that the boost IC shuts down. Or at least the current consumption drops to 0 on my power supply display. If I remove the probe from the FB pin, it starts to work again.I attached everything I thought could be important to analyze the issue. Tell me if you need anything else.Although I am mostly interested in solving this problem, I welcome any critic regarding the schematic or the PCB layout. But please take note, that this is not my profession, and this is my first ever electric design.Regards,Gergely