> I have a supply that kind of works, but I get a large sawtooth-like ripple on > the output. The ripple is 17V peak-to-peak at 170V and the frequency is > 23-80Hz (depending on the load).
There are four things I would look at: current limiting, feedback oscillation, power supply problems, and thermal issues. The MAX1771 has built-in current limiting, which could cause the whole shebang to cycle on and off, which could appear as a sawtooth after the output filtering. Make sure you have solid, low impedance connections to the current sense resistor and back to the CS input. Power supply problems are the easiest - see if there's any similar oscillation at the power supply input pin. Nick's diagram shows separate power inputs for the inductor and the MAX1771 - are you powering them separately? Either way, look at the input power, if it's a lab supply that's going into current limit and retrying, you could get behaviour like this. If you're not powering them separately, it might be worth a try. For feedback oscillation, you'd have a phase shift happening somewhere in the voltage regulation loop - or something that pretends to be phase shift. I'd look closely at the feedback resistors and make sure there aren't any parallel capacitors inadvertently hooked to them. I'd also carefully check the output capacitors - both the electrolytic and the high frequency one. If they're not doing their job of absorbing rapid spikes and hash, the controller chip can get confused about what's really going on. Scoping those points is tricky, due to all the high voltage, high current stuff going on nearby, which will tend to couple into the probe lead, upsetting the regulation further and confusing the scope display. The last thing that occurs to me is a thermal issue. The MAX1771 will throttle down its on-time when it gets hot, which could lead to thermal cycling. You might need an external FET driver to switch your FET properly. Shottkey diodes get leaky when they're hot, but it doesn't seem to me that this would cause oscillation, I'd expected runaway instead. Your FET could change its characteristics when it gets warm, see if changing its heatsink changes the behaviour. And you could have a defective component that's just switching between working and not-working. - John -- 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.
