Thanks for the excellent advice. I will try a toroidal inductor, I don't know why I never thought of that. I am also building an experimental design based on the TPS40210 — that part is much cheaper despite requiring some more passive components, and it has an automotive variant, which means it will be around pretty much forever. We'll see how it behaves, the devil is in the details. Also, I will use an IRFH5025 FET for switching: it's SMD, the specs look nice, and the price is great.
I have one more question -- did you use a potentiometer in any of your designs? This *really* makes a huge difference in my tests. As for the drain being the source of noise -- right, this is the pulsed inductor output, with monstrous voltage peaks and high currents. But there is a limit to how small one can make it and how far away from the feedback network it can be. You might be right about an SMD FET being better here, though. Great tip about several resistors in parallel for RSENSE, too :-) thanks, --J. On 24 sty 2012, at 03:16, misty01a wrote: > Hello. > > In my "high current" MAX1771 PSU design I made several years ago, the > best results I got when I used a toroidal inductor with a current > rating of 10.3 A (Bourns 2200LL-470-RC), and a RSENSE peak current > setting of 5.33 A. I tried other inductors, including a physically > smaller 2100LL series inductor (with a smaller current limit), but the > efficiency was not good as the 2200LL. I don't know why. > > Several notes that may help (based on my personal experience only): > > - The FET drain tab is the source of a great amount of noise. The FET > drain should be placed as far away from the FB pin trace as reasonably > possible. An SMD (D2PAK) FET may be better than a TO-220 one because > of the horizontal placement on the PCB (also for better cooling of the > FET). > - I have never observed any interference from the inductor, but then I > use toroidal inductors which are supposed to be self-shielding. I > once did get good results with an unshielded Bourns 1110 series > inductor, however. > - For RSENSE I use several 0.075 Ohm 1/4 W resistors in parallel. > They are cheaper than a single larger resistor, and it's easier to > adjust the peak current value. > - A diode between the REF and FB pins may help if the MAX1771 has > trouble starting up. The MAX1771 can enter 12 V fixed output mode if > the supply voltage is larger than 12 volts and the voltage at the FB > pin is too low (because of the high input:output ratio of the voltage > divider). On the other hand, many other people's designs seem to be > working fine without this diode. > > My present design uses a Bourns 2200LL-470-V-RC inductor and an > FDB28N30TM FET (0.129 Ohm ON resistance). The maximum power output in > a testbench situation was 24.9 W at 85 % efficiency with 14 V input > and 220 V output. Board size is 50 mm x 40 mm. I use three of these > units in parallel to provide 40 watts at 250 V in a (non-Nixie) > project. > > -- > You received this message because you are subscribed to the Google Groups > "neonixie-l" group. > To post to this group, send an email to [email protected]. > To unsubscribe from this group, send email to > [email protected]. > For more options, visit this group at > http://groups.google.com/group/neonixie-l?hl=en-GB. > -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
