Electronics > Beginners

Adjusting an LC filter while circuit is running: bad idea ?

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ratatax:
By R||L in series you mean like this ?



I don't know the name of this type of filter, could you help me guessing the (approximate) values of those components ?

Btw, thanks for the time you took to answer my posts

T3sl4co1l:
Unfortunately, calculating it would require a model of the regulator.

I would guess maybe 10uH and R for critical damping, and C on the reg side as recommended.

Tim

OwO:

--- Quote from: ratatax on November 17, 2019, 06:17:51 pm ---

--- End quote ---
The inductor will resonate with the input bypassing caps and give you a peak in the power supply impedance vs frequency. That combined with the negative resistance characteristics of the DC-DC converter on the input side (caused by the fact that higher input voltage => less current draw to maintain the same power and vice versa) can cause instability. All DC-DC converters have this negative resistance behavior so you need to design the input circuit carefully always. I would either add a snubber (series RC across the input but please simulate it) or just oversize the input caps so that the Q is limited (depends on the inductor esr).

Al3579:
it has soft start and an input enable, have you tried playing with the enable voltage it has a threshold. Also I didn't see a cap on the soft start.
also the data sheet is vague how is a PWM power supply pseudo constant frequency of 500khz. Must mean the frequency changes with load but their was no graph for that haha.
What is the amperage rating of the wall wart and the load you are suppling. Might be better just getting a common mode choke bridge rectifier caps that will handle the load you are suppling. Looks like it meant to supply 125-150 watts of 5v power for displays and such.

Does anybody know what pseudo constant frequency is?


T3sl4co1l:
It's probably a constant-on-time controller, or something like that, and "pseudo" just means, they measured it at that particular condition and make no claims about it otherwise.  Probably, it's similar at higher loads, but much lower at light load, or something like that.

Tim

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