Ceramic caps make noise... do film caps make noise too? - Page 1
Forum section: electronics / projects

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 23, 2015, 02:53:52 am
content: Figure-1 shows the schematic of a single-phase SPWM inverter with an output LC filter, which is developed to produce a 50Hz sinusoidal voltage waveform (edited: 240Vrms, 0.75Arms, 180W).The power MOSFETs in the H-Bridge switch at 15kHz.Two tests were conducted:[Test-A]L = 1mH (manufactured by Coiltronics)C = 20uF (manufactured by Panasonic, shown in Fig. 2a and Fig. 2b)[Test-B]L = 1mH (custom-made using toroidal core (Epcos T38), measured using a non-branded LCR meter similar to thishttp://www.tecpel.com.tw/LCR_610.jpg)C = 20uF (manufactured by Panasonic, shown in Fig. 2a and Fig. 2b)InTest-B, an audible noise was observed (we believe it's from the film cap).When the L is replaced with the one made by Coiltronics (i.e. inTest-A),noaudible was observed.Do film caps make noise?What is the possible cause of the audible noise in Test-B?Figure-1: Single-phase SPWM inverterFigure 2a: Film capacitorFigure 2b: Film capacitor

author: singapol
author_rank: Frequent Contributor
timestamp: September 23, 2015, 03:34:19 am
content: Time for a refresher course on RLC circuits.It would be helpful if you also understand how sound is produced. Firstly circuit is switching at 15Khz. this is still within the hearing threshold of humans ofwhich 20Khz. is the theoretical limit, that is why most switching supplies work at minimum 25Khz. and above.If phase changes are right, resonance/ocillation will occur.It simply means RLC conditions allow gain of a particular frequency to maximise and oscillate, if you can hear it it is in the audible range and if you can't hear it, it's quite propable the mosfets will fail sooner than later.Sound is a perturbation of air by electromagnetic waves and the construction of film caps may predispose them to vibrate. After all it's just plastic film rolled into a coil.

author: nickm
author_rank: Regular Contributor
timestamp: September 23, 2015, 04:14:57 am
content: Magnetics will make noise too, especially unvarnished ones, although toroids didn't seem as susceptable. You said you changed the inductor and it fixed it so I would say it's the inductor. The film caps probably won't make noise.

author: EEVblog
author_rank: Unknown
timestamp: September 23, 2015, 04:22:31 am
content: Quote from: onemilimeter on September 23, 2015, 02:53:52 amI've never heard of such a case (no pun intended).Ceramics of course are famously microphonic, you can make them sing like a bird.

author: EEVblog
author_rank: Unknown
timestamp: September 23, 2015, 04:23:07 am
content: Quote from: nickm on September 23, 2015, 04:14:57 amYes, poorly wound inductors can make a noise.

author: mzzj
author_rank: Super Contributor
timestamp: September 23, 2015, 08:38:55 am
content: T38 ferrite is not suitable for this kind of application. Your inductor core is saturating and causes huge peak currents trough capacitors, making your caps/and/or/coil to sing. With enough high peak currents even your cables start to sing.Besides your capacitors seem to be not suitable for this kind of operation"DC ONLY"

author: singapol
author_rank: Frequent Contributor
timestamp: September 23, 2015, 09:00:26 am
content: From: blueskullQuoteOf course you can but you are responsible for the results.Since you are designing DACs whether with off the shelf chips or discreet your selection should reflect the performance, not all mlcc are suitable. A search about the subject would be beneficial.Whether it's audiophoolery or not that's for your customers to decide. If you want science then know that every element in this reality or physical universeHAS a resonant frequencyand it pays to be cognisant of this fact, even planet earthhas a resonant frequency, it's called Schuman's Resonance which is not static but shifting, check it out yourself.Lastly take a leaf from Ken Ishiwata of Marantz now owned by Phuilips, you can see him on Youtubein one of his interviews where he explains his design philosophy: He commission his engineers to do a piece of audio equipment, he is only interested in how all the various components sound in the final quality test. In other words how the various components interact to produce the sound he wants. Of course this is off topic, better to start your own thread where you can share ideas.

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 23, 2015, 09:28:30 am
content: Quote from: mzzj on September 23, 2015, 08:38:55 amYes... the Panasonic capacitor, as prompted by several forum members, is not suitable for this application... I will buy those film caps designed for AC filtering...You mentioned an interesting point: "T38 ferrite is not suitable for this kind of application"... I agree with you that the T38 inductor core may have been saturated and therefore the huge current ripple caused the film-cap or the coil to sing...By the way, could you please suggest a suitable core material (e.g. those available in Farnell/Element14/RS/Mouser which I can purchase the item from) for my application?Thanks.

author: mzzj
author_rank: Super Contributor
timestamp: September 23, 2015, 10:22:49 am
content: Quote from: onemilimeter on September 23, 2015, 09:28:30 amToroid cores: Molypermalloy or iron powder core with a 30....100 relative permeability. Or plenty of split ferrite cores (almost any type) gapped to give 50....100 relative permeability.Size can vary from 1/2" tiny ring to car tyre size depending on your requirements. Or from $0.5 to $1000 per piece.You should really do your own home/groundwork and study some theory to avoid beginner mistakes like the core choice here.But lets guess that your inverter is something like 1kW of power: I am also too lazy to do the math, so ballpark estimate is roughly 3" diameter molypermalloy core or several smaller. Or 1 to 3 EE65 ferrite cores of any ferrite type gapped to 6mm air gap or so. Wire diameter according to 3A/mm2 and "plenty" of turns.( calculate or measure if you know what you need)

author: metacollin
author_rank: Contributor
timestamp: September 23, 2015, 01:03:54 pm
content: Quote from: singapol on September 23, 2015, 03:34:19 amUh, no it's not. Sound is a mechanical pressure and displacement wave that propagates through a compressible medium. In other words, sound propagates through all matter (solid, liquid, gas, even plasma), not just 'air'. Electromagnetic waves and sound have no direct relation. Electromagnetic waves are a change in the local electric field which causes a change in the local magnetic field, which in turns causes another change in the local electric field...back and fourth, the far-field version of a resonant LC tank. In this way it can propagate without a medium, or, put a better way, it propagates through spacetime itself as a medium. Theynevercause sound. Not without the energy in the wave being converted to another form (like moving charge carriers that move a speaker's voice coil. But charge carriers are charge carriers, not electromagnetic waves).Quote from: singapol on September 23, 2015, 03:34:19 amThey are not anymore disposed than any other capacitor to vibrate, and they are not just rolled into coils. That would not be very useful, it's either a really crappy split-winding inductor with tons of self-capacitance, or a really crappy capacitor with tons of self-inductance. Well, its both really. Which makes it crappy at everything, which is why no such thing is made. Some film capacitors are rolled into a coil as part of their manufacturing, but this is not their final construction. Once rolled, alternating edges of the coil are joined to end caps, forming a 'fingers intertwined' like pattern, pretty much identical to ceramic capacitors.Or for more familiar box caps:There is no coil.The force from the electric field could cause vibration of course, but to be 'singing' would also destroy the dielectric, the thin plastic film does not withstand mechanical strain that severe.Class II ceramics are made of ferroelectric ceramic. All ferroelectric materials are also piezoelectric and pyroelectric. They deform their static dimension slightly in response to an externally applied electric field. Conversely, they will generate an electric field in response to being deformed. This is what makes them microphonic, as well as sing. The other takeaway from this is that the effect is not dependent on ripple current, 10mA or 10A of ripple makes no difference. It's purely in response to electric fields - so it is dependent on ripple voltage, and not dependent on current. The ferroelectric effect itself can also cause mechanical strain, but it too is purely a voltage effect (which is why DC bias causes that drop in capacitance, but ripple current doesn't do anything to the capacitance).Inductors on the other hand are a bunch of things undergoing direct magnetic force (two adjacent windings of a coil have fields that are repelling each other) and all inductors will gladly squeal to their hearts content if subjected to ripple current, and in proportion to the current. That is, if the windings are able to move. If there is any give in the winding anywhere, it can make sound. As Dave said, it really does just come down to being poorly wound/made. I usually lock down windings on a toroid with superglue or the like, something that gets really hard, and that always seems to quiet obnoxious toroids.Quote from: onemilimeter on September 23, 2015, 09:28:30 amHow much output power? Or output current? You haven't mentioned that yet. I'd recommend N92 core material it has the best field strength to permeability combo, and it is specifically designed for high power 100% AC applications like power transformers. Even though this is an output inductor, a transformer core and this inductor core will be experiencing similar AC current, so what's good for one is good for the other. Depending on output power, 1mh may not be realistic. You need to increase your capacitor size as much as is practical to get your inductor size down. Also, you want your cutoff frequency to be as far from 50hz as it is from your PWM frequency. ?(50*15000)= 866Hz. Are you sure you need to switch at 15kHz? A little more Hz could make things easierYou will almost certainly need to airgap your core too, so toroids are out. You'll need some kind of airgapped core and bobbin, or a absolutely massive ferrite toroid to keep it from saturating. But I think a ferrite toroid that large would be temperature rise limited anyway (surface area to volume catch-22).

author: mzzj
author_rank: Super Contributor
timestamp: September 23, 2015, 01:11:33 pm
content: Quote from: metacollin on September 23, 2015, 01:03:54 pmFerrite toroids are not usable but there is iron powder, molypermalloy, sendust, coolmu and variety of other magnetic materials suitable for toroid inductor.

author: f5r5e5d
author_rank: Frequent Contributor
timestamp: September 23, 2015, 02:28:52 pm
content: film caps can have electro-mechanical conversion - loose winding, especially oval caps flattened after removing the mandrel can have air gaps - I'm surprised Dave hasn't heard old 'scope switching supply oval Mylar caps "sing"the "electrostriction" is from the electrostatic attractive force on the plates applied to any compliance in the dielectric (or those air gaps) - but may film caps are very solid construction and the actual compression of the capacitor dielectric plastic is film is generally negligiblebut you can apply electrodes to squishy plastic and make electrostrictive actuators

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 23, 2015, 06:39:09 pm
content: Not just most materials used in capacitors and inductors, but all, really, have electrostriction and magnetostriction, as a nonlinear aspect of their inherent permittivity or permeability. The coefficients might be very near 1, so that they have very little effect indeed, but it's nonetheless an effect that's present.More practically, many common types of ferrite, iron and other magnetic core materials, and dielectrics from ceramics (including piezoelectric and electret effects, which are related but distinct from electrostriction) to plastics, exhibit enough of an effect that they're audible.But here's an interesting question for you... was the audible tone 50Hz, a harmonic, a spike, or what? The various effects are distinguished by their response. Striction is a purely square-law effect, so that it produces only 2nd harmonic under zero bias, and fundamental as well when under DC bias.I've heard film caps buzz before (suggesting perhaps not so much electrostriction as a more mundane cause, like loose windings flapping around), so it's not at all unusual.Tim

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 24, 2015, 01:48:29 am
content: Quote from: mzzj on September 23, 2015, 10:22:49 amMany thanks... really useful...

author: onemilimeter
author_rank: Frequent Contributor
timestamp: September 25, 2015, 04:40:46 am
content: Quote from: metacollin on September 23, 2015, 01:03:54 pmThanks metacollin.The inverter is expected to supply a rated output power of 180W (Vrms = 240V, Irms = 0.75A, Ipk = 1.0607A, f = 50Hz).By the way, if f1 is the fundamental frequency (e.g. 50Hz), and fsw is the switching frequency (e.g. 15kHz), is thisfc = sqrt(f1 x fsw) = sqrt(50*15000) = 866Hzthe only way to decide or select the cut-off frequency of the LC filter?Yes... it's possible to switch at higher frequency... probably I will increase the switching frequency to25kHz.Kindly advise.

author: metacollin
author_rank: Contributor
timestamp: October 01, 2015, 10:49:37 am
content: Yeah, you can make virtually anything into a "loudspeaker" if you shove enough current through it. I suppose I shouldn't discount the effects of electroconstriction, it could be the cap. Just in my own anecdotal personal experiences, inductors or ceramic caps tend to be the only things that make noise loudly when there isn't necessarily destructive amounts of current buzzing through them.Oh, that wasn't any proper formula I used for finding that 866Hz center frequency. I was just finding the frequency that would give the best combo of attenuating one frequency without distorting the other. Filters are exponential in nature, and so I was simply finding the geometric mean between the two frequencies. It's the nth root of n values multiplied together, since I had two values, I took the square root. Had it been 3, the cube root would be taken and so on.It was one of those things they tried to teach us in middle or high school school then we never used. Until filter design!Geometric means are just like averages (arithmetic means), only the find the value where the average rate of change for an exponential growth/decay of something is. All exponential functions can be pu in terms of the continuum limit, euler's number, so the geometric mean is really computed as such: e^(ln(v1*v2*....vN)/N)But, this of course algebraicly simplifies to just taking the Nth root of N things multiplied together, so it's the same thing ultimately.Oh yeah, I had a brain fart and was avoiding mentioning powder cores due to the 200% ripple current the output inductor will see and the significant hysteresis losses that come with that, forgetting this was 200% ripple at 50Hz. And even the PWM ripple is not that fast and much less in magnetude, so the best core, like someone else mentioned after my post (sorry on mobile and too lazy to check who) is a powder core. Kool mu/sendust (they're different names for the same material) is my favorite but it's expensive. I would suggest a hydrogen reduced power iron core, the yellow and white material, I forget the number but it's fine and also the cheapest.