Ferrite Bead picking up noise? - Page 1
Forum section: electronics / beginners

author: bittumbler
author_rank: Contributor
timestamp: February 05, 2022, 03:31:55 pm
content: Hi,i try to design a flyback converter. It basically works.On the secondary output i use ferrite beads (BLM18KG102SZ1D).My problem is, they seem to make ripple/noise worse, instead of filtering it.With an oscilloscope in AC coupling i get <100mV on the input ("1") side of the FB. However after the FBs i get 500-1000mv! (measured with 1000Ohm load on 45V output)My simple understanding of FBs is, they are basically inductors with high loss on high frequencies. However i seem to miss something here.So the big question is: Where does the increase in ripple/noise come from, and what am i doing wrong here?TIAMatthias

author: Kleinstein
author_rank: Super Contributor
timestamp: February 05, 2022, 04:29:02 pm
content: The ferrite beads are lossy inductors, but the loss is not allways high enough to fully suppress resonances at all frequencies. So at some frequencies the bead and capacitance can still resonate and at that requency result in higher amplitude. In some cases additional resistance in parallel to the ferrites may help to increase the damping, though reducing the filter action at high frequencies. One can also dampen the resonace with a lossy capacitor at the output (e.g. with added series resistance to part of the capacitance).In the shown ciruit there is no capacitance at the output and thus a rather high resonant frequency for the output side. The filtering action is mainly effective above the resonance. So it may need additional capacitance at the output.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 05, 2022, 04:58:23 pm
content: Interestingly, I've got similar increase of a noise after a ferrite beard at output of a (dc/dc) circuit when I used two beards in a similar way - in both output wires. Beards were placed near each other about 10-12 mm from each other. With only one beard it became a bit less noisy. I can't say definitely what a problem really was, if it was a parasitic resonanse, a parasitic magnitic coupling, a measurement setup error or something else. I left it without further investigation when achieved output noise low enogh (playing with input filters etc).

author: bittumbler
author_rank: Contributor
timestamp: February 05, 2022, 04:59:43 pm
content: i bodged in a 100nF cap on the output of the ferrite beads. That made it worse, i have now 1.3Vpp ripple.

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 05, 2022, 05:06:44 pm
content: Lossy electrolite caps usually help to dampen parasitics with a small capacitance so they have high enough ESR.I used 10-47 uF caps at the output of a device. Not on the PCB but after the output wires at the device connectors.With lossy electrolites you may use different ceramic caps too, in parallel.

author: bittumbler
author_rank: Contributor
timestamp: February 05, 2022, 06:58:15 pm
content: i replaced the ferrite beads with zero ohm resistors. That gives me ripple of 50-80 mVpp, depending on where/how exactly measured (that is about the limit of my setup).So a possible solution is to drop the FBs. However i still dont understand what is going on. Some resonance?

author: radiolistener
author_rank: Super Contributor
timestamp: February 05, 2022, 07:16:02 pm
content: you're forgot to put capacitors on the output, after normal mode choke coil. Typical RF filter for power supply includes normal mode choke coil and then several ceramic and tantalum capacitors. For example 10 nF + 100 nF + 47 uF in parallel after normal mode choke coil and 100 uF before normal mode choke coil. You cannot replace them with a single capacitor, because electrolytic capacitors with high capacitance has worse ESR and cannot filter high frequency noise, but ceramic capacitors with low ESR has low capacitance and cannot filter low frequency noise. So you're needs several capactiors with different capacitance connected in parallel.Also you're forgot to add common mode choke coil for both lines on the same ferrite core:Common mode choke coil helps to block common mode current noises.So the full filter should looks something like this:or this:Also ferrite bead can reflect impulse back to the source and it can affect your power source and make it more unstable. It also should be taken into consideration.

author: bittumbler
author_rank: Contributor
timestamp: February 05, 2022, 07:55:12 pm
content: Hi radiolistener,you are right, i did not include a common mode choke.But in my understanding, i also did not measure common mode noise. I measured between pos-neg output of the converter, at various stages.To measure common mode noise i would have to measure between output and groud or primary-secondary.My measuring between pos and neg should cancel common mode noise?BTW does the common mode choke have to be a single part, or would it be "good enough" to place two simple inductors next to each other, so that they have some magnetic coupling?

author: Kleinstein
author_rank: Super Contributor
timestamp: February 05, 2022, 08:08:34 pm
content: The inductance for common mode filtering does not have to be a single inductor. However a current compensated choke can be much smaller size to get the required inductance, as the DC current would not saturate to core. So a current compensated inductor can use a high µr matrial, while a simple inductor needs to care about saturation and may need lower µR core or a air gap.100 nF at the output may be too little: this may still result in a resonance in the low MHz range and maybe too high.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 05, 2022, 08:43:03 pm
content: In the before-FB position, common mode current is sunk by the probe's short ground, giving a fairly stable result. Likely a large part of that 100mV is still due to CM-DM mode conversion.In the post-FB position, the GND side FB is dropping all the voltage due to CM current, while the signal side one drops very little voltage against the probe tip's high impedance.Add a 'Y' cap across the transformer, consider reducing slew rate or damping ringing, whatever the case happens to be -- and add a CMC, with a bypass cap at the output side. This will reduce the noise source in the first place, and choke off the CM current, and keep the output voltage stable with respect to whatever the probe is seeing, thus avoiding CM-DM conversion.Tim

author: radiolistener
author_rank: Super Contributor
timestamp: February 05, 2022, 08:45:14 pm
content: Quote from: bittumbler on February 05, 2022, 07:55:12 pmNo. Common mode current can flow through any unexpected path, include mains, equipment case and other.If you want to reject noise, you're needs to use both - normal mode choke and common mode choke together.And don't forgot to use normal mode filter capacitors and common mode filter capacitors.Note that single capacitor is not enough because it cannot have low ESR and high capacitance simultaneously, so you're needs to add several ceramic and electrolytic capacitors in parallel. For example 10 nF + 100 nF + 47 uF. It helps to filter low frequency, middle frequency and high frequency noise.Quote from: bittumbler on February 05, 2022, 07:55:12 pmit should be single part, winding type and polarity is very important.

author: bittumbler
author_rank: Contributor
timestamp: February 06, 2022, 11:51:54 am
content: Did some more testing. It seems the probe is part of the problem, creates a common mode path via its ground. If i reduce the timebase, i can sometimes even see a 50Hz signal overlaid on the ripple.Where should the common mode filter be placed? Near the rectifier diode, or after the other inductors, near the output?Where should the Y cap across the transformer be placed? Between primary-GND and secondary-GND? Primary-VCC and secondary-VCC? Both?Should i add a series resistor to the Y cap to prevent oscillations?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 06, 2022, 08:56:49 pm
content: Wait, does that really have 300uH in each side, between first and second cap? "0.3m" almost seems like a typo...I also realize, as you didn't give a waveform, it's anyone's guess what frequencies are involved here. Is this not just a massive amount of DM ripple because the filter is completely undamped? Such high inductances plus ceramic caps will ring like a bell.Also don't know what the controller/regulator and anything else in front is, the schematic is very small and tightly cropped.Application is unstated, too. Is this a PoE thing? Those output footprints look suspiciously 8P8C...Quote from: bittumbler on February 06, 2022, 11:51:54 amWell, that may be neither here nor there. Is the scope just aliasing the noise? Does it have DPO to smooth over that, or, did you use a "hi res" or digital-LPF mode to ignore HF ripple, showing just frequencies that fit on screen as such?For aliasing, it might not be there at all, and it's coincidence that you see 50Hz. (Typically the aliasing will vary dramatically/unintuitively with time/div.) For DPO, or peak detect mode, it might be AM related to whatever your input supply is (which is also undefined).Oh and is that actually 50Hz, or a harmonic thereof (i.e. 100Hz supply ripple)?Finally, if it is 50Hz in the measurement, not modulation -- maybe your probe is just breaking. I use cheap Chinese probes from time to time, in the expectation that they aredisposable. Get a four-pack every couple of years, y'know. Typically the cable's ground braid fails near the connector, giving a choppy reading at first, eventually breaking almost all the time and the reading is just nonsense. Lack of ground can very well give you a 50Hz component, as the probe picks up ambient E-fields (same as touching the probe tip with your finger, or plugging in an amplifier with RCA or phono plugs and hearing the startlingly loud hum or buzz before the grounds connect).QuoteAnywhere will do, including on the probe cable itself (put a few turns around a ferrite bead, perhaps?). In place of the output inductors will do fine -- CMCs have some DM (leakage) inductance anyway so this is a fine place to get a little extra ripple filtering too. Indeed you may prefer a "power line" type choke, with the windings in separate sections, which gives somewhat more leakage.Separate sections like these:https://www.digikey.com/en/products/detail/sumida-america-components-inc/CPFC805NP-100M05/1879022https://www.digikey.com/en/products/detail/pulse-electronics-power/P0354NLT/2267119Whereas the kind like these or these or these,https://www.digikey.com/en/products/detail/w%C3%BCrth-elektronik/7442335900/4865731https://www.digikey.com/en/products/detail/bourns-inc/SRF0905-500Y/2075952https://www.digikey.com/en/products/detail/w%C3%BCrth-elektronik/744253101/10210479have lower leakage, which may still be enough for filtering purposes, and don't forget about DCR acting in series with that, which might serve to dampen the filter, if its voltage drop isn't a problem.Note that, if they don't give leakage, but they still give DM impedance vs. frequency, that's fine. The LF asymptote corresponds to LL. Also note: if you see DM |Z| bouncing around at HF, the peaks correspond to 1/2 wave resonant modes, and the valleys, 1/4 wave modes (repeating every +N/2 wave), meaning it shows good transmission line behavior -- good for data signal quality (doesn't matter for power).QuoteYes, between P/S GNDs. Or P-VCC and S-GND I guess is the most direct path, being those are what the transformer is directly wired to, but the other side is just a large bypass capacitor away, not a big deal. Sometimes a resistor or ferrite bead is beneficial, depends. Probably values from 100pF to 1nF will show the most improvement, with very large values being less effective; if more attenuation is needed, then consider adding another Y cap between primary GND to after the secondary CMC.Tim

author: bittumbler
author_rank: Contributor
timestamp: February 07, 2022, 09:28:02 am
content: Hi T3sl4co1l,some more background: The converter is based on NCP1031. The rest of the circuit is basically from the datasheet.This is for a home project. I want to power some microcontrollers over some distance. So i use CAT5 for the cabling. Ethernet is not involved, but because of the CAT5, i try to do something "PoE-like".I want/need some isolation between multiple of these connections, so i cannot use actual off the shelve PoE power panels, they are not isolated.Because of the wired connections i want to do some extra filtering, after all, the wires can act like antennas. I dont have the equipment to find the optimal filters, and also didnt give it too much thought: just used something that may be more than needed, but does the job anyway.The actual component values are just my wild guess, then "optimized" to use a low number of different parts. I was expecting the components to maybe behave differently over big spectrum regions, so i used two stages + ferrite beads. And yes, the 0,3mH is actually there.Other than that, this is my first such design, so i have really no experience in this. So i missed common mode filtering...However the filter seems to work, at least until the FBs. See screenshot before the FBs. After the FBs i see an increase, however somewhat different each time i measure. So at this time i think my probing is introducing this.The main frequency of the noise is the swiching frequency. The design is for about 200kHz. I did some experimenting with different switching frequencies, but found no real change (the screenshot shows max frequency of the NCP1031, just for testing).Thanks!Matthias

author: vad
author_rank: Frequent Contributor
timestamp: February 08, 2022, 05:04:06 pm
content: Flyback converter noise is usually a CM noise. Capacitor between primary side ground and secondary side ground is a typical way to mitigate the CM noise here.