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Hmm, there was some bounce on the transitions with the 33k, I'm not sure lessening the hysteresis would help that. I think the bounce is due to switching noise or spikes, but my old CRO can't see them. There's no practical need to investigate it though as the circuit runs fine. I think the only real issue with my SMPS is inductor sizing and lack of snubbing. I tried adding capacitive hysteresis on the feedback line but it drove the circuit highly unstable.
Anyway in future use I'll play around with positive feedback resistor values.
Registered Member #3215
Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780
did you calculate the integration for the bounce timings or for your signal timings? that's a common trap... what if you increment from 10pF or 100pF to some nanofarads?
@Shrad; Nope! My scope isn't fast enough to measure them, or even trigger reliably. I see some +10v (over Vset) spikes on the output occur approximately when the bounce probably happens. So that's likely the cause. Since it doesn't trigger reliably I can't even guarantee it's happening at the same time as the spikes and not just visual overlay on the CRT with persistence of vision.
Aside from those spikes/bounce the fet runs cold and output regulation is excellent. On a set load I can turn up the input voltage for less current draw, and lower the input voltage for higher current draw, over a large range when the load is reasonable, all without the output voltage moving so much as 100mV. I'm using a tiny output capacitor as well, so that aspect is well within my specs.
As I mentioned earlier, the more pressing problem is the inductor. I'm tempted to make a new thread about it.
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Strange/Poor Response with LM339 Comparator. Help Requested.
