How does my SMPS get noise at the input inductor? - Page 1
Forum section: electronics / beginners

author: jnz
author_rank: Frequent Contributor
timestamp: December 19, 2020, 09:53:44 am
content: I have a 3.3V buck SMPS IC, fed with 9V. Design calls for inductor and caps on the input as well as output.I messed up and didn’t realize I had input >> diode >> bypass caps >> inductor >> IC pin and there was no cap between my inductor and input pin. This made EXTREMELY bad noise and didn’t work at all, awful. I fixed it by adding a cap between inductor and pin.1. What is the input inductor really doing? I imagine it’s to keep pulses and spikes from getting into my smps input. My guess here is that the smps can only react so fast to increases in voltage that the inductor is there to slow these down. ... opposite of a bypass gap that is there to fill in gaps, right? Is it also there to prevent noise backfeeding to the input? Because it seemed to work great at that.2. Why was the noise SO BAD between the inductor and input pin? And not at all at the input itself (input side of inductor)? What is the mechanism that not having a cap (tried a 4.7uF MLCC, works great) causes such “feedback”. I was expecting bad behavior at the output instead of also at the input.3. Knowing that I need my cap right at the IC, does the placement of the input 22uH inductor matter a lot? Is it out of the question to put it 35mm away but on a different circuit board that is soldered to this one? I have a space issue I’m working out.Thanks.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 19, 2020, 10:01:08 am
content: Can you show a representative schematic, so we're sure of what you're talking about?Basically, you transposed the inductor and capacitor? Yeah, that's worse than not having an inductor at all. Just short across it.The inductor can be placed anywhere you don't mind noise may reach. Like, if this is a battery powered thing, probably, who cares? If it's going on big long cables, that'll probably make a great antenna, and you may want to be a little more courteous with it.Tim

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 19, 2020, 05:38:02 pm
content: What does this mean:Quote from: jnz on December 19, 2020, 09:53:44 amIf you don't know:Quote from: jnz on December 19, 2020, 09:53:44 amJust copying a design is not going to get you far if you do not understand how it works. So that is a point to work on.For more details of your particular SMPS circuit, posting a schematic will help, and also if it's properly annotated. Making a remark about "R3" is a lot easier then "That resistor in the left corner next to that other thing".

author: jnz
author_rank: Frequent Contributor
timestamp: December 19, 2020, 09:52:36 pm
content: Yea, I get it.The issue is it's pretty complex black box. A TLE9271QX.https://www.infineon.com/dgdl/Infineon-TLE9271QX%20V33-DataSheet-v01_50-EN.pdf?fileId=5546d46268554f4a0168854f82cf6859Page 29 is the only place they seem to show the BUCK circuit without also using the BOOST which I don't use. And yes, I'm very stupid and somehow the schematic was made with the inductor after the caps, real dumb, won't make that mistake again.What I couldn't really understand is why outside of the input pin was showing so much noise. I get internal to the chip, and at the output, sure, but "before" the input suprised me. I guess I would have thought whatever switching they were doing would be filtered after the input pin.QuoteYea, well, that's the problem. The big 9V battery is 10 feet from the device. Then inductor, less than 1", then input caps and SMPS chip. So, I'm not sure what I can get away with here. I'll probably just try and get the inductor on the same board despite the height issues I may have.

author: opampsmoker
author_rank: Regular Contributor
timestamp: December 20, 2020, 01:41:05 am
content: The inductor smooths the current waveform thats drawn from the input line.But yes, the most downstream component of the SMPS input filter must be a capacitor, not an inductor.......this is because an smps needs to "see" a low impedance when "looking back" into its input filter......i think its called middlebrook's theorem.Also, never have an "untethered" inductor in a filter...always have a capacitor either side of it...remember inductors are antisocial devils...they will keep sourcing current as long as they have a magnetic field, and without a capacitor either side to "tame" them , they can raise the voltages of adjacent circuitry very high.Remember V=Ldi/dt tells us that you cannot suddenly break an inductive current unless you impose an extremely high voltage across that inductor.And if you quickly change the current in an inductor, then the inductor will itself produce a very high voltage, and likely spoil your day.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 20, 2020, 02:48:43 am
content: Ah, I see. Well, you can model it as a black box, sure, but it's worth considering what's inside that box there. A buck converter is simply a switch from the input, into an inductor (and a catch diode or something like that, so it...bucks). The inductor current is a ramp, either discontinuous (returns to zero between pulses) or continuous (always ramping up and down). The supply current is exactly the rising slope of that current: it has "100%" ripple, either ramping up from zero to peak, or starting at some initial current and ramping up somewhat from there. In either case the turn-off is a step current, from peak down to zero.So, applying a step current to an inductor requires huge peak voltages, very quickly. The inductor equation is:V = L dI/dtHere, dI is given (turn-off current, say), dt is small (at 450kHz overall frequency, they'll have the edges switched in a fraction of that, probably <20ns), L is given, and V is to be solved for. At dI ~ 500mA and L = 22uH, we might expect around 550V peak... a figure that's obviously not going to happen, so either dt is much longer (it may be longer, but not by that much), or the rest is clamped by electrical breakdown of the switching device, any internal protection, etc.With a 28V rating on that VS pin (a mere ~3x the supply, or only a 2*supply boosting peak required), you may be lucky it didn't simply explode!Tim