Ferrite bead on the input and output of an SMPS - Page 1
Forum section: electronics / beginners

author: bsas
author_rank: Regular Contributor
timestamp: November 08, 2021, 10:24:08 am
content: So, I know that is extremely common to use external ferrite beads (those rounded ones) around data cables (very common on USB cables) to filter interference right?So, my question is: you have an SMPS that converts mains voltage to DC (classic example those Mean Well boxes). Is it worth it to put ferrite beads on the main line wires? Is it worth to also put them on the DC output wires? Is there any negative effect to put on both? Any possible theoretical positive effect reducing noise?Thanks!

author: epongenoir
author_rank: Contributor
timestamp: November 08, 2021, 10:46:27 am
content: Ferrite beads are usually used on differential lines to reduce common mode noise and GND loops, because they present a (lossy) impedance only to the common mode currents.Keep in mind that they can also induce ringing (They are basically a lossy inductor)With that said , if you don't strictly NEED them (regulatory/emc) I would'nt bother using them with MeanWell SMPS, there are filters on the inside.G

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 08, 2021, 12:54:15 pm
content: Short story: impossible to say without a setup and measurements.The long is, ferrite beads simply increase the impedance a bit, of whatever they enclose. This could be used common mode (around a group of wires or cable) or differential (single wires, or pairs going opposite directions). The impedance is mostly inductive, with a lot of resistance -- they tend to be used where this resistance acts to dampen resonances, between cables for example. Hence why they're commonly seen on peripherals.They can be used for filtering, but especially when it comes to signal or power filtering, mind that they saturate relatively easily. That is, impedance drops with increasing DC bias current, on the order of a few 10s of mA for small chip beads, to a a few 10s A for large toroids.SMPS already contain quite a lot of CM filtering, namely a, "ferrite bead" as it were, with a good couple dozen turns on it -- this gives a far higher impedance, at switching frequencies (0.1-10MHz) than a single or few turns would, giving effective filtering. Some small capacitors do the rest.A common use-case, with AC-DC SMPS modules, is extra CM filtering at the output. They are typically tested with a load resistor at the output terminals, just floating in space. So, while there might be noise at the output, it doesn't show up on the test because it just dead-ends in an open circuit (the resistor is floating, not grounded). The trouble is, when you connect the output to ground, noise out the power cable may be increased. Or if you don't, then noise is sent down the DC line, to whatever else is connected. So, it can pay to put some filtering there; again, a ferrite bead probably isn't enough, you'll likely need a CM choke proper.On DC converter modules (common ground, like three-terminal PoL regulators), ferrite beads may be a common sight, though they're often misused this way. The trouble is, the DC input/output current causes the FB to saturate, reducing its impedance. Like I said, this can happen in just some 10s of mA, whereas the bead might be rated several amperes -- the current rating is thermal only, it doesn't say anything about the electrical characteristics of the part. Now, on something like a power supply, you can afford much lower impedance filters -- so a single FB can give enough impedance for useful filtering (say >20dB attenuation), along with ~uF capacitors and such. If it's not saturated, I mean.Saturation can be avoided by adding an air gap to the magnetic path -- very small inductors are often made by putting one or a few strips of solid metal through a slotted/gapped ferrite bead (so, 'C' rather than 'O' shaped). Most values are made by winding wire around a bobbin or spool shaped core; which, if you look at the cross section, also has a 'C' shape, revolved (along the 'back' of the 'C') to form a spool shape. The air gap also "dilutes" the losses of the ferrite material, raising the Q (reducing resistance) -- this defeats the absorptive property of the ferrite bead, which is good for efficient filtering applications (like the active element in a switching converter), but not so great for EMI damping applications. As a result, often an R+C is used with inductors and other bypass caps, to get good performance in a power distribution network (PDN) or whatever.Tim

author: radiolistener
author_rank: Super Contributor
timestamp: November 08, 2021, 05:56:00 pm
content: Yes, ferrite filter is strictly required on mains for SMPS in to block noise leakage and radiation by mains.It also helps to block noises from mains.But this is not simple and cheap ferrite clips which is usually used on the cables.You're needs something like that: