Reducing SMPS noise

uration, each step switched in or out by a SMT solid state relay. It's ess entially a decade box for a specific application. The relays are controlle d by a micro and all is powered by a boardmount enclosed CUI AC/DC switchin g converter, supplying up to 400mA.

t of noise on the resistance output that can be from nothing other than the SMPS. I'm assuming the power planes on the board are inducing all kinds o f noise into the traces that connect the resistors and solid state relays.

required is a big problem. So I'm looking at filtering the output of the S MPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the SMP S output. I'm looking for any first hand experience with this and dumbed-d own advice that will hopefully get me a solution without days of headaches.

firm that. I've already tried putting several MLCC 1uF caps across the SMP S output but that didn't change anything.

You'll want an indcutor in series with the cap... making a low pass. I think I used 10 uF and 22 uH... (You want the LC frequency to be below 10

You might also try adding some caps to ground on the resistor ladder. That should help all but the first step.

George H.

e:

iguration, each step switched in or out by a SMT solid state relay. It's e ssentially a decade box for a specific application. The relays are control led by a micro and all is powered by a boardmount enclosed CUI AC/DC switch ing converter, supplying up to 400mA.

lot of noise on the resistance output that can be from nothing other than t he SMPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays .

e required is a big problem. So I'm looking at filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the S MPS output. I'm looking for any first hand experience with this and dumbed

-down advice that will hopefully get me a solution without days of headache s.

onfirm that. I've already tried putting several MLCC 1uF caps across the S MPS output but that didn't change anything.

I will give the LC filter a shot.

When you mention caps to ground on the ladder, are you referring to the pow er supply ground? If so, I have to keep that isolated.

If you've got a volt or so to spare, you could use a transistor-based capacitance multiplier, to give you an effective capacitance value quite a bit larger than its physical value. That would make it easier to push the corner frequency of the low-pass filtering down even further.

Check the layout of your PC board, and your grounding arrangements. Try to make sure that current pulses from the SMPS (either the input side or the output side) and voltage jumps aren't being coupled into the output circuitry via either capacitive coupling, or inductive-loop coupling.

In an extreme case you might need to have a shield around one side or the other, broken only for the relays themselves.

Thinking outside the (shielded) box a bit: Using magnetically-loaded "latching" relays might be another helpful option, as their coils could be left un-energized (either open-circuit, or both sides grounded) except for a short pulse to turn them on or off. This would also cut your power supply current demand, and might let you use a smaller/quieter switcher, or a small linear supply with a big output filter/reserve capacitor.

If the noise is in-plain-sight conducted, adding a ferrite bead, or a small inductor, and a cap, can help a lot. I just tried that this morning.

You might need an EMI filter on the switcher input, too. If the coupling is inductive or ground-loop, the fix isn't as simple.

We use mostly wall-wart supplies, to keep the AC line out of our boxes and to better manage noise.

Proper layout of the SMPS is critical. If that's still a problem, you can LC filter the output or follow it with a small LDO.

Be careful with your scope grounding, too. The noise may not be real.

Most likely common mode noise.

Whereas for differential noise, you have the right idea (shunt it with a cap, or make an LC filter), it doesn't do anything for common mode.

Rotate your perspective a little. Suppose the output DC+/- act, together, in parallel, as a single conductor (which is true, at RF, because there's a huge capacitor between them!). Suppose the mains AC H/N behave the same way (for the same reason, usually). Finally, suppose there were a noise source wired between these two connections.

In reality, there are four wires, but it doesn't matter which ones the noise source connects between, because each pair acts the same way at RF.

That noise voltage has to go somewhere. If mains is ultimately near or at ground, then the noise stacks on top, and your circuit is riding on the noise voltage.

When the noise voltage enters a sensitive circuit, you see interference problems. The hallmark of common mode noise is that it's "noise that's not really there", i.e., the circuit (for the most part) keeps on working as it normally does, yet you measure the same noise waveform /anywhere/ in the circuit.

The most illustrative method being: with an oscilloscope and 10x probe, clip the ground clip to the probe tip. Poke the tip into circuit 'GND' (which, because it's being driven by a noise source, is not really "ground", as the oscilloscope sees it!). Note the waveform is not actually zero, despite the probe tip being "shorted" by the ground clip!

Ground clips make very poor RF grounds, and the RF voltage dropped across that ~10cm piece of wire is what you measure.

What to do? Do the same thing as ever: filter it! You use an LC filter exactly the same way as usual: the prime difference is, you can't simply tie a huge capacitor from output to mains. You must use smaller (~1000pF) Y1 rated capacitors, which will withstand hazardous mains voltages without letting through much leakage current.

Since you're filtering pairs of wires, that ultimately aren't identical (they're carrying a DC or low frequency AC voltage, which is the whole point!), you can't simply use one inductor; instead, you use a common mode choke.

If the DC output should be isolated, you should use Y1 type capacitors (as small as practical), from AC H/N to safety GND, and from GND to DC output. If the output doesn't need to be isolated, simply ground it, so you need only the mains Y1 caps.

The ultimate purpose is to make the mains wires, and the output wires, have the same RF voltage: zero. The power supply is allowed to generate its noise voltage, which is dropped across a common mode choke. (The choke can be on either side of the power supply, but it's usually on the primary side because it can be physically smaller.)

Tim

guration, each step switched in or out by a SMT solid state relay. It's es sentially a decade box for a specific application. The relays are controll ed by a micro and all is powered by a boardmount enclosed CUI AC/DC switchi ng converter, supplying up to 400mA.

ot of noise on the resistance output that can be from nothing other than th e SMPS. I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays.

required is a big problem. So I'm looking at filtering the output of the SMPS. I'm a digital and DC analog guy, so this switching noise scares me. Some research and an educated guess tells me to try an LC filter on the SM PS output. I'm looking for any first hand experience with this and dumbed- down advice that will hopefully get me a solution without days of headaches .

nfirm that. I've already tried putting several MLCC 1uF caps across the SM PS output but that didn't change anything.

What's 600 ohms on the FB. Is that it's impedance at 10 or 100 MHz?

I figured out yesterday how to do the ground for a board with multiple CUI smps's. Shunting everyone's noise to ground, made the local gnd "bad". A star gnd should work but I want to test that today.

George H.

Thanks for everyone's responses. I tried some LC filtering and saw no obvi ous improvement. At that point it became obvious to me that I have to use a linear power supply since a switcher will never be quiet enough. This de vice is used to calibrate resistance measuring devices, so a switcher proba bly should have never been in there in the first place.

The convention is Z at 100 MHz. That bead has a low-frequency inductance of a few uH.

One can cut a ground peninsula out of the PCB ground plane and locate things such as to keep switcher ground loop currents out of the rest of the board.

On Wednesday, September 7, 2016 at 3:46:28 PM UTC-4, snipped-for-privacy@yahoo.com wrote :

vious improvement. At that point it became obvious to me that I have to us e a linear power supply since a switcher will never be quiet enough. This device is used to calibrate resistance measuring devices, so a switcher pro bably should have never been in there in the first place.

Is the smps on the pcb or an external thing? I've had great luck with external smps's. (I make power supplies with a few nv/rtHz with them... there is both an LC filter and cap multiplier.)

In some way I think the high frequency is easier to filter than the 60 Hz. (and linear supplies are heavy and spit

You might have common mode noise as Tim W suggested.

Do you have any voltage head room, such that you could put a linear regulator after the smps?

George H.

nfiguration, each step switched in or out by a SMT solid state relay. It's essentially a decade box for a specific application. The relays are contr olled by a micro and all is powered by a boardmount enclosed CUI AC/DC swit ching converter, supplying up to 400mA.

a lot of noise on the resistance output that can be from nothing other than the SMPS. I'm assuming the power planes on the board are inducing all kin ds of noise into the traces that connect the resistors and solid state rela ys.

ace required is a big problem. So I'm looking at filtering the output of t he SMPS. I'm a digital and DC analog guy, so this switching noise scares m e. Some research and an educated guess tells me to try an LC filter on the SMPS output. I'm looking for any first hand experience with this and dumb ed-down advice that will hopefully get me a solution without days of headac hes.

confirm that. I've already tried putting several MLCC 1uF caps across the SMPS output but that didn't change anything.

Thanks, is a bead better than an inductor?.. lower Q or something. (I always worry about making a low pass with a high Q. Resistors are safer that way.)

OK I could try that... I've got 5 supplies, five peninsula's looks like a star. :^)

The HF crud in my lab seems to get nothing but worse. ~20 mV @ ~100 MHz. (with a 60 MHz 'scope, x1 probe) I made a little pomona box LP filter to stick on 'scope. A little active box with some gain would be nice.

George H.

Unfortunately it saturates by 20-50mA or thereabouts (a few 100s mA for the biggest multilayer beads), so it's useless for power applications.

A ferrite bead that's rated for DC bias is called an inductor. Yes, they do actually make that distinction. Even though the ferrite bead technically has higher quality material (high purity = high mu), and they're both inductive components.

Inductors have higher Q, so you usually need to dampen the filter resonance with an R+C. (Or something with ESR, like tantalum, but really, who puts tants on supply rails? Use a ceramic plus resistor for goodness sake.)

Tim

I think this is not likely to do anything effective. The problem is not lower frequency components getting past the filter. The problem is parasitic effects making the filter a pass through for high frequency noise.

Excellent advice. The noise is being coupled across (or around) the isolation by some means. Rather than adding bandaids, better to understand where the problem is coming from.

When the OP says, "I'm assuming the power planes on the board are inducing all kinds of noise into the traces that connect the resistors and solid state relays." it makes me suspect poor layout. The ground plane should not be anywhere near the isolated circuits.

He's using a solid state switch, so I wonder if those are available in a latching form? Still, if it requires a connection to the power supply it might still be coupling noise across the barrier.

I don't think the issue is Q. An inductor has some amount of self capacitance which essentially parallels the inductor. A turn or two or possibly none (just slip the bead over a wire) can add inductance with very little capacitance. If using SMT components they make ferrite filters with a small amount of capacitance to ground and ferrite to add inductance.

We have some 1206 power beads in stock. The chunkiest one is 33 ohms,

Ferrite beads are low-Q by design. Still, the inductance would resonate with a biggish cap at low frequencies.

LT Spice includes a bunch of Wurth bead models, for playing with.

We're in a wooden building in plain sight of this:

so we are sort of an EMI horror lab.

I'm going to bet that 6A is the thermal rating, not the saturation current.

Allan

On Wednesday, September 7, 2016 at 12:46:28 PM UTC-7, snipped-for-privacy@yahoo.com wrot e:

vious improvement. At that point it became obvious to me that I have to us e a linear power supply since a switcher will never be quiet enough. This device is used to calibrate resistance measuring devices, so a switcher pro bably should have never been in there in the first place.

I'm not following that reasoning. A switcher can be a source of power supp ly ripple, or magnetic induced currents (interference), but neither of those are absolute disquali fications. Generating a reference voltage with a linear regulator makes a lot of sense, but 'line ar' doesn't automatically reduce your ripple or induced currents.

There's a lot of induction around a line-frequency transformer. RF goes t hrough a linear regulator and past a big filter capacitor like it has a VIP card.

The data sheet doesn't say anything about saturation. We used it to reduce EMI hazards in the drive to an NMR sample heater. Beads work on RFI the same sort of way that garlic keeps vampires away.