one more try at my input filter question

Sorry to get repetitive... I asked a question above, but I'm not sure I got the specificity of answer I was hoping for. Could I try again in more concrete terms please

I have a source of DC, 12 to 24V say. I have a switching PSU running off the DC. The sink takes current in bursts at 2us intervals. The 500KHz is approximate, but reasonably stable and accurate.


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I'd like to be able to draw 15W in bursts, so assume average input current of 1 to 2A on full load.

That causes noise on the wire going back to the supply. That stops me getting CE approval for the box ;-(

I need to kill the noise.

All the PSU chips which run at fixed frequencies say "this makes the noise easier to filter" - Hmmm. how exactly?

I appreciate that there will be noise at 2us and harmonics at higher frequencies. We have already fitted a proprietary input filter, which sadly seems to provide little attenuation as far down as 500KHz.


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It doesn't appear that [ after the filter get to them ] the harmonics are large enough to cause a problem.

Yes I know, shoot the designer who matched that filter with that switcher. The bullet is on it's way.

So I have just the one problem - noise at 500KHz.

I COULD treat it by just building/fitting a 'bigger/better' filter with a cut-off frequency well below 500KHz. But I have a PCB artwork already, and PCBS built, and that would require making new PCBs. I'd like a retrofit mod if I can work one out.

Now, back when I did exams, I seem to remember circuits which would exhibit a high impedance at a single frequency. usually consisting of a cap and inductor.

something like

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Could I "fix" the problem by fitting such a cct? Would the components required fit inside a small building if I did? Should I stop trying to be clever?

I can buy a 2A-rated inductor at about 33uH

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to get the resonant frequency to be 500KHz I'd need a capacitor of

C= 1/( 4 * PI^2 * L * F^2 ) ???


Sounds a bit small... maybe I could use a smaller inductor and a larger cap?

Maybe I should just fit a 1R/inductor in series and change the input cap to 220uF low ESR instead of 22uF.

Is there a better way, maybe by building a suitable notch-filter circuit and fitting it into the DC supply lead outside the box for now?

All help appreciated.


Reply to
David Collier
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Hello David,

Have you tried putting the filter the other way round - it's designed to protect your ciruit from the PSU noise. This may not help with your pcb. Try a serious ceramic capacitor across the input terminals so the inductors in the filter do more good - you can get 10uF in 0805 out of the Farnell catalogue. I think that might go on the back of your board. You haven't said if the noise is common mode or not (ie is the same on both input terminals, in which case the capacitor won't help). Good luck - let us know what happens.

Michael Kellett

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Reply to

Yes, buck regulators produce a lot more noise in the input side than the output side, because the input current is interrupted , but the output current is continuous, because it passes through the inductor, all the time.

So any input filter has to be efficient (have little resistive drop) while passing at least 2 amperes.

You need a good quality (low equivalent series resistance)capacitor directly across the input and common terminals, to supply the input pulses, and a series inductor to allow that capacitor to sag and recharge each pulse cycle (because that is the only way capacitors deliver and recover charge), without letting that capacitor ripple voltage from also producing a similar ripple further up the line.

Beats me.

That does not have enough inductance to produce approximately a constant current into the input bypass capacitor. These two components must have a resonant frequency well below the pulse rate, if they are going to produce a low pass filter that covers the fundamental, as well as the harmonics.

That would help a lot. The data sheets always show the smallest capacitor that might work, to make the application look very cheap and small. But the ESR of the capacitor is as important as the value. I like to parallel a 1 uF stacked film or X5R multilayer ceramic, e.g.

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(for the higher harmonics), in parallel with a low ESR electrolytic, rated for the full ripple current, at least. e.g.
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Note that he capacitance may have to be quite a bit higher than 22 uF to get to a ripple current rating above an ampere.

Then select an inductor rated for at least 2 amperes to smooth the current to that bypass capacitor pair. e.g. CDRH127/LDNP-680MC (look in Digikey) 68 uHy, 2.6 amperes

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The drum core type you asked about is famous for spraying magnetic field to its surroundings, and radiating noise, this way. This warning also applies to the inductor in the output filter of the regulator.

Reply to
John Popelish

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drum core = never, ever use. always have a closed magnetic circuit, pref. toroid or pot core.

Go further:

- ensure your input LC filter is critically damped, even at no load (filter resonances can be a real PITA)

- ensure the impedance seen looking from the smps into the filter (C in parallel with L) is LESS THAN the magnitude of the smps impedance, Vin^2/Pin. This is to prevent the input filter from causing closed-loop oscillations in an otherwise stable smps, as smps have negative input impedances (raise Vin, Iin drops). hence the issue with filter resonance (parallel L-C resonance gives infinite impedance)

that said, its actually pretty easy to design the input filter. you know the shape of the smps input current, so you can Fourier transform it into its harmonics. you also know the emi spec you must meet. So pick a cap that turns the evil current into not much wobbly DC volts, and an inductor that turns the wobbly volts into even less wobbly amps, which feed into 50R (in the LISN) and must be less than the conducted emissions limits.

I normally do this in spice, as its easy to take into account the cap and L parasitics, which are important at EMI frequencies, and far faster than a DIY mathematical analysis.


Cheers Terry

Reply to
Terry Given

To the degree that you work at only one running frequency figuring out the filter is easier.

If the PWM chip had a frequency that varied all over the place, the same marketing person would find a way to say "spread spectrum" in the data sheet.


If this is a big complex PCB with lots of other stuff on it, I suggest you consider this:

If you can identify the footprint of some part that you can use more or less as a connector, you can make a small sub-assembly that plugs on there to carry the better filter.

--   forging knowledge
Reply to
Ken Smith

In article , David Collier wrote: [....]

Is it noise raditation from the wire or conducted noise on the wire that you are concerned with. In the past, I have gone so fars as to run power supply lines as shielded wire or even coax to reduce radiation.


Parallel tuned circuits in the filter are seldom a good idea. The AC current in the inductor is large so you need a big core. If you've got a big core, you could make a larger inductor at a lower ripple current.

Can you place an inductor right at the power entry to the PCB? Some types of inductors, such as "pot cores" are easy to mount to the sheet metal. If you've got the room, this may help.

They make PCB mounting coil formers for pot cores and a few otehr types. You could use one of these to hold a small PCB onto the POT core and chassis. This will give you room for more capacitors.

--   forging knowledge
Reply to
Ken Smith

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