Hi Luka these designs are very well known and there are a few variants around, basically divided into the ones that have the pull down transistor and those who don't, and those who use a NE555 (yuk) and those who don't.
The TO-92 package at the bottom left is presumably a 78L05 (guessing because of the +5V terminal on the right, and I can just see the "5" poking out, and the fact that there is a 100nF cap close to it). If that is the case, then the board basically matches the schematic. So here are the things that I see questionable about this design: 1) The IRFD220 is under-rated a) It has a power dissipation of 1W, and a current capability of 1A, spike 6.4A. We're going to be pushing about 12A through it at turn on, dropping to 6A when it is hot. Naughty. b) It has a high positive temperature coefficient of Rds (resistance from drain to source). This means that it will heat quickly if you drive it too hard, and the heating effect will snowball. It doubles from 1 Ohm at 30 degrees to 2 Ohms at 130 degrees. You also can't heat sink it easily, so you are only left with the other option I mentioned earlier. This will drop the power when it gets hot. I think this is what you are seeing. c) The thermal conductance of the package is 120 degrees/W. This means at rated load we expect the temperature to be 120 degrees above ambient. We are working outside of this normal range of this device. It will probably have a short and unhappy life, especially if you put it in a box (ambient temperature will then rise to 30-35 degrees). 2) There is no pull down on the FET gate: This is the problem I first thought. This will reduce the efficiency of the circuit. The 330 Ohm (written on the board, but look, there is a 270 Ohm resistor installed) will discharge the gate in a given time. The IRFD has a 6nC gate charge. The 270 Ohm resistor will be able to discharge the gate in 6E-9 S * 270 = 1.62 uS. During this discharge, the drain current reduces linearly, meaning that the turn if is "smooth", and not sharp like we need. This is a issue particular to the 34063: it can only source current but not sink it. 3) The 34063 is running without a current sense resistor, and this doesn't help the efficiency either. The data is from here: http://datasheetcatalog.com/datasheets_pdf/I/R/F/D/IRFD220.shtml I see that the guy claims it can drive IN-18, good luck with that... So, what does that all mean? Well, before complaining too loudly, you need to be sure that your input power supply is capable of driving this board: It needs to be 12V (not 16V or 10V) and have the capacity to drive the inrush current (up to 12A spikes for a few uS). The input smoothing capacitor (200uF) will not cover all of this. I'm sure it can work with *small tubes* if you get everything *just right*, but it's not at all tolerant. With the addition of a pull down and a more robust MOSFET, it would work much better. We're talking maximum 30 cents difference in production price. Regards Ian On Tuesday, 22 September 2015 00:48:57 UTC+2, Luka C wrote: > > Thank you both. > > @gregebert > I would be very thankful if you could explain it or send a link here so I > can see how I could implement it into my design. My clock uses 4x IN-14 > tubes + 4x INS-1 tubes > > @Ian > I can't say for sure since the supply is on a very small board so the > whole thing gets very hot, but yes, I'd say the MOSFET is the hottest > component. And, to be honest, I bought the PS from eBay from EU seller > because I though Chinese ones could be problematic (such irony!) and there > was only the picture I attached here. After I noticed the problems > occurring in the PS, I inspected the PS and concluded it should be the one > from the schematic I posted in my previous message (unfortunately). > > > > > > <https://lh3.googleusercontent.com/-gQNC8HToUCg/VgCJSGVW2GI/AAAAAAAAAFE/8MpZNA93z28/s1600/IMG_20150921_185752.jpg> > > > > > > Dana ponedjeljak, 21. rujna 2015. u 22:22:52 UTC+2, korisnik gregebert > napisao je: >> >> I stick with linear power supplies, especially for the anodes >> (high-voltage), whenever possible. They are much simpler to design, >> least-likely to overheat, and very reliable. In fact, the main reliability >> concern is the HV filter cap. You can mitigate that risk by over-design. If >> anyone's interested, let me know and I'll post my best-known-methods that I >> use. >> > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To post to this group, send an email to [email protected]. To view this discussion on the web, visit https://groups.google.com/d/msgid/neonixie-l/51a63ec4-6573-46df-9420-4ec91e73f2d9%40googlegroups.com. For more options, visit https://groups.google.com/d/optout.
