Difference between inductors, that lets smoke out? - Page 1
Forum section: electronics / projects

author: Artlav
author_rank: Frequent Contributor
timestamp: August 21, 2014, 06:42:22 am
content: I have an LED-driving buck converter, 24V input to about 9V, at 0.2A average, 0.5A max.At first i used this inductor:http://www.digikey.com/product-detail/en/CD54NP-470LC/308-1865-1-ND/3925621And it worked well, getting slightly warm at worst.So i wanted to make 30 of them, for which i got this inductor:http://www.digikey.com/product-detail/en/TYS5040470M-10/240-2716-1-ND/4733625Which is superior to the previous one by all parameters i can see, including cost.However, the second one was getting hot enough to smoke, even at 0.2A !The two are virtually the same size and construction.I checked the datasheets and it's not just digikey showing wrong numbers.What am i missing here?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2014, 06:49:42 am
content: Neither one specifies core loss. I wouldn't touch either!Tim

author: bktemp
author_rank: Super Contributor
timestamp: August 21, 2014, 07:06:45 am
content: Even with non optimal core losses, it should not get so hot. There must be something wrong in your circuit.

author: Niklas
author_rank: Frequent Contributor
timestamp: August 21, 2014, 07:23:01 am
content: Check the circuit as bktemp wrote.The currents you listed, are they the average value displayed on a DMM? What is the current threshold on the swicher IC? For a stepup it could be much higher than average for small currents in pulse skip mode.

author: coppice
author_rank: Super Contributor
timestamp: August 21, 2014, 07:28:11 am
content: Quote from: Artlav on August 21, 2014, 06:42:22 amWhat is yourpeakcurrent through the coil? The coil which works better doesn't specify its saturation current, but the poor one saturates at 1.1A. That seems rather close to its rated current of 1A, so they aren't allowing a lot of margin in their specs. Your design could be working well below 1A for its output requirement, but not knowing how you have built it I wouldn't make assumptions.Quote from: Artlav on August 21, 2014, 06:42:22 amThey may look similar, but ferrite characteristics vary enormously, from very soft to very hard saturation.Quote from: Artlav on August 21, 2014, 06:42:22 amNeither device seems to be well specified for how it behaves as it saturates.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 21, 2014, 07:47:23 am
content: Quote from: coppice on August 21, 2014, 07:28:11 amThat's true of a closely gapped design, but it's usually something like -20% inductance at saturation for this type of (bobbin core) construction, with a 'fully saturated' value of maybe 20-50%, i.e., it only saturates by up to 1/5th or thereabouts. Yes, sharper than the saturation of a powdered iron core, but not at all like the saturation of a nearly full core, like an E-E or whatever shape with a relatively thin gap (where the ratio of L(@ no DC) to L(fully saturated) is usually >30).In the application... 20-50% of nominal inductance will definitely cause problems regardless, so if it is saturating, regardless of material or construction: ...yeah.Tim

author: Artlav
author_rank: Frequent Contributor
timestamp: August 21, 2014, 09:29:35 am
content: Details.The converter is based on ZXLD1360 LED driver.Schematic:Layout:The uC gives out a PWM that should set the current to 0.2A, which was confirmed by a multimeter on the output.Quote from: coppice on August 21, 2014, 07:28:11 amIf i done that math right, the peak current should be 0.4-0.6A.Frequency is about 350KHz, duty cycle of about 0.5.Quote from: Niklas on August 21, 2014, 07:23:01 amIt's a step down only.Quote from: bktemp on August 21, 2014, 07:06:45 amCD54 works fine and cool, so i doubt the circuit is that bad.

author: penfold
author_rank: Frequent Contributor
timestamp: August 21, 2014, 09:38:17 am
content: Whilst it shouldn't get hot enough to smoke, it should get hot. If it doesn't then you've just over spec'd and ironically enough you'll most likely be working at sub-optimal efficiency. Just to reiterate, I am not saying it should get hot enough to smoke.

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 21, 2014, 10:21:25 am
content: Quote from: Artlav on August 21, 2014, 06:42:22 amIf you have access a scope build a pulse generator using a FET and a 1 ohm resistor (to reset the core) a non inductive would be preferred , any simple controller like a TL494, SG3525A will do, then allow the following. A PWM from 0 to 100%, and variable frequency say from 5khz to 200khz (TL494 limited to 400Khz SINGLE ENDED) or SG3525A (500Khz)Here you can see a Arnold core being tested noticed the uniform current ramp with a gentle core reset. with this data i was able to obtain the inductance, core operating frequency and core saturation.You would typically use a rig like this for cores with unknown parameters and it works very well for me.

author: bktemp
author_rank: Super Contributor
timestamp: August 21, 2014, 10:45:22 am
content: The layout looks good. Since the ZXLD1360 uses current control, it should limit the peak current. If the real circuit matches the schematic, non of both inductors should get hot.Did you check the dc resistance of the inductor? Maybe digikey send you a wrong part with a much lower current rating.

author: skrap
author_rank: Contributor
timestamp: August 21, 2014, 05:36:55 pm
content: I know diyaudio already suggested this but I would also check the inductor with a pulse generator.elm-chan built a rather nice one.http://elm-chan.org/works/lchk/report.htmlI switched to a dust core after having similar problems in a past project and it worked a lot better for that circuit, probably due to high peak currents.

author: eneuro
author_rank: Super Contributor
timestamp: August 21, 2014, 06:57:20 pm
content: Quote from: Artlav on August 21, 2014, 06:42:22 amWhat are dimensions of this thing?In this photo wires looks very thick and not too many turns and copper loses at 0.2A looks like 15mWmax only, so how it broke?Ferrite core disapeared?

author: georges80
author_rank: Frequent Contributor
timestamp: August 21, 2014, 07:17:04 pm
content: Do note that you have 5V feeding a 1k/1k divider to the ADJ pin. That means you have a 0V - 2.5V PWM signal, at 2.5V at the ADJ pin you will be driving 200% of the drive current as set by the Rsense resistor.Since you are not filtering the PWM (to create a poor man's D/A) it will switch to 2.5V. So, regardless of your PWM/average current, your peak currents will be 200% of 0.1V/0.47ohm (per your schematic).Measuring current with a multimeter is not a good way to evaluate actual inductor currents in a switching regulator.cheers,george.

author: Artlav
author_rank: Frequent Contributor
timestamp: August 21, 2014, 08:39:03 pm
content: Quote from: diyaudio on August 21, 2014, 10:21:25 amQuote from: skrap on August 21, 2014, 05:36:55 pmNice, that would be quite useful for a lot of things.Thanks for the link.Quote from: eneuro on August 21, 2014, 06:57:20 pmIt didn't, i turned it off once it's exterior started smoking.It was still working at the time.Quote from: bktemp on August 21, 2014, 10:45:22 amSeems to be the right part - both inductance and DCR is within spec.Quote from: georges80 on August 21, 2014, 07:17:04 pmZXLD1360 have an internal low-pass filter, that at 16KHz PWM frequency results in a smooth current control, rather than a chopped up one a low frequency PWM would give.0-2.5V signal is by design - it's 0.2A nominal (0.1/Rsense), with an ability to go to 0.4A.Quote from: georges80 on August 21, 2014, 07:17:04 pmActually, i just realized i can measure it precisely, thanks to the sense resistor built in.This is with a56uHCD54 inductor, running at 41% duty cycle PWM input into the driver, measured across the sense resistor:Seems to add up to about 0.17A average, and 0.24A peak, which is a bit less than i expected, but within range of what i need.I tried a slightly larger CD54 inductor, just in case it would increase efficiency somewhat.As far as i can tell, it's maybe a few degrees cooler - the LED heat overrides it anyway.In any case, the current is well within the rating of either inductor.

author: bktemp
author_rank: Super Contributor
timestamp: August 21, 2014, 08:48:18 pm
content: How does the waveform look like with the other inductor? If the inductor absorbs a lot of energy, the waveform should be quite different.

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 21, 2014, 09:46:49 pm
content: Quote from: skrap on August 21, 2014, 05:36:55 pmThats a good one as well, mine was built from junk box components in a day.

author: MagicSmoker
author_rank: Super Contributor
timestamp: August 21, 2014, 09:57:03 pm
content: Quote from: Artlav on August 21, 2014, 06:42:22 amNo way to tell without some destructive testing. First is the area of the center part of each core. Second is the number of turns on each core.Aside from that, you really need to know the peak-to-peak value of the ripple current as well as the actual core loss coefficients (usually called "Steinmetz parameters") to pin down an exact answer, but my guess, as of now, is that the second inductor uses a much lossier core material, has a smaller center post area, and fewer turns. All this is good for noise filtering purposes; not so good for switchmode power converters.

author: Artlav
author_rank: Frequent Contributor
timestamp: August 22, 2014, 08:33:57 pm
content: Quote from: MagicSmoker on August 21, 2014, 09:57:03 pmQuote from: bktemp on August 21, 2014, 08:48:18 pmI don't quite want to risk blowing up another board, so i'll try it out on an inductor testing rig linked above, once i get to making one.Being able to characterise inductors at home sound GOOD. The last mysterious component understood at last.For now, thanks all for advice and links.

author: rob77
author_rank: Super Contributor
timestamp: August 22, 2014, 10:36:20 pm
content: did anyone else noticed it ?good one:Frequency - Test 2.52MHzbad one:Frequency - Test 100kHzOP's circuit => ~ 350kHzso definitely the core of the "bad" inductor is not designed for higher frequencies

author: bktemp
author_rank: Super Contributor
timestamp: August 23, 2014, 06:04:50 am
content: Quote from: rob77 on August 22, 2014, 10:36:20 pmThe same manufacturer uses 1kHz for 100uH and above. So these are only useable at audio frequencies? I doubt that...In most cases the manufactorer simply uses a frequency that works for all inductors of the series (like the second one).The datasheet mentions DC DC converters in modern products, even automotive as typical usage. Almost no modern switching converter uses frequencies below 100kHz. Therefore I can not image the inductor beeing not useable for a 350kHz stepdown.I have used a lot of inductors from a few mW upto the kW range. The core material effects the efficiency, yes, but not as drastic as here:The stepdown runs at 24V and converts it to 9V 0.2A. Thats only 1.8W output power! To heat the inductor until it smokes needs a lot of power, probably >1W even for such a small inductor. I have never seen any ferrite material beeing so bad. So there must be something else going wrong here.@ArtlavIf you power your circuit with a lower input voltage the frequency should start at 0Hz for <9V and increase with the input voltage. This puts less stress on the inductor. Therefore it should be safe to measure with a lower voltage. Go as high as possible until the inductor starts getting hot.

author: Artlav
author_rank: Frequent Contributor
timestamp: August 23, 2014, 06:22:22 pm
content: Ok, i made the rig, measured both inductors, and the result is not too clear.Red one is TYS5040, the bad, green is CD54, the good.Sense resistor is 0.1 Ohm, input voltage is 12V.What i can see is that the bad one saturates much sharper, and looses more inductance when it does.However, they both start to saturate at 1-1.5A, well above the limits in my circuit.On an LCR meter at 100KHz they getexactlythe same DCR, 46 and 52uH of inductance, 40 and 50 Q.Once again, not much of a difference.So, i'm back to square one.What is different between the two, that makes one heat up a lot, and other - not?

author: bktemp
author_rank: Super Contributor
timestamp: August 24, 2014, 06:02:40 am
content: The only thing I can image is, that for some reason your peak current in the circuit is much higher and is driving the inductor into saturation. Then the much sharper current rise can cause many problems including an overheating inductor.Try to measure the current with the bad inductor in the real circuit. This is the only way to find out why it is not working.

author: skrap
author_rank: Contributor
timestamp: August 25, 2014, 10:54:06 am
content: My apologies for posting and not being able to contribute anything new to this thread. I tried to come up various crazy reasons to why it might happen, even considered curie temperature but it always just ends in pure speculations with no real clue to what might be wrong.It might be a bit selfish but I'm just really curious to what the underlying problem turn out to be.I don't want this thread to disappear into the deep archives just yet...

author: PChi
author_rank: Frequent Contributor
timestamp: August 25, 2014, 12:07:15 pm
content: I am with the core loss theory being the problem.The idea could be tested by increasing the value of the currrent sense resistor R3 to prevent damage to the PCB and looking at the waveform across the current sense resistor.If funds were unlimited the inductors could be tested with a Wayne and Kerr Precision Magnetics Analyzer - 3260B,http://www.waynekerrtest.com/global/html/products/woundcomponent/3260.HTMIt can measure inductance and loss with dc bias currents and at different frequencies.

author: nukie
author_rank: Frequent Contributor
timestamp: August 25, 2014, 12:55:15 pm
content: Shielded inductors are not as easy to work with especially when dealing with high current. Stick to unshielded inductors for a more stable performance. Those shielded inductors may look great with high inductance and shorter copper windings that has less resistance but when put to work, the inductance drops as soon as the current increases. And inductance also drop quickly when they heat up. This is why I call them variable inductors. If you wish to use shielded inductors make sure you spec a big one with high current capability without going through all the suffering of magnetic torture.If you want to test real capability of a shielded inductor you need to pump actual current through the inductor, not just by testing 100khz.

author: eneuro
author_rank: Super Contributor
timestamp: August 25, 2014, 09:51:15 pm
content: Quote from: Artlav on August 21, 2014, 09:29:35 amWhile I'd like to use this nice driver in my laser diode project just read its datasheet and take a look at your schematics with AVR ATTiny85 pin AVref connected to ADJ pin of LED driver where voltage divider is created so when output MCU PB0 is High (5V) ADJ is 2.5Vmax I guess.However in its huge datasheet we have something like this:QuoteAnd recomendation when current is controlled by DC voltage on ADJ pin:QuoteWhatever this(†)means there, when you output PWM high 5V than you have 200% inom which in your case is:IOUTnom =0.1/RS=0.1/0.47R= 0.213A@ 1.25Vrefwhile you used RS=0.47 Ohm,so you should have:425mA @ 2.5V = 5Vpb0/2if this driver were DC controlled.Did you tested those "good" inductors with DC voltage applied not PWM from MCU?We should get typical 4% output current accuracy, so when PWM is switched off and AVR in reset mode PBO should have 2.5V volt so output current should be in your case about 0.425A.BTW, does it makes any sense to test for currents above 400mA while its datasheet says about 350mAmax for this LED driver?I do not know why are you using only 2 pins of this DIP8 AVR Attiny85 and what are requirements for output current regulaltion-some kind of pulses or only predefined constant current, but you coud use ADC of this MCU and simply measure voltage on ADJ pin of this LED driver and adjust MCU PWM on pin PB0/AFREF -> ADJ to get desired output current from this nity LED driver by controlind DC voltage on ADJ.This is what I plan to do-add additional capacitor to ADJ pin which will work as additional soft start tooQuoteRun this MCU in fast PWM mode, measure ADJ pin voltage and adjust PWM to get calculated DC voltage needed to controll ZXLD1360 output current.Of course this solution depends what you need, but it should be easy even using potentiometer adjust PWM and set ADj DC voltage to get expected output current and see if it is within range it should be using this forumula from its datasheet:QuoteSo to get 200mA we need:QuoteThan use 100uH, 47uH inductors and see what happends, but ensure that it is NOT AVR MCU PWM trigering problems with overheating choosen inductor, while it looks like it is more tricky choose correct PWM than DC voltage on ADJ pin of this LED driver to get desired output current

author: Artlav
author_rank: Frequent Contributor
timestamp: August 25, 2014, 10:27:59 pm
content: Quote from: bktemp on August 24, 2014, 06:02:40 amQuote from: skrap on August 25, 2014, 10:54:06 amMe curious as well.I got 8 more PCBs made, will probe the bad inductor on a real board soon.Quote from: eneuro on August 25, 2014, 09:51:15 pmAs i said above, it is driven by 16KHz PWM corresponding to 0.2A level, and ZXLD's low pass filter ensures that this 16KHz PWM is turned into a smooth constant current control, not a 2.5V to 0V to 2.5V chopped one.Quote from: eneuro on August 25, 2014, 09:51:15 pmYes -https://www.eevblog.com/forum/projects/temperature-tolerance-of-an-led-regulator/In short, at 0.4A the controller gets much hotter than the good inductor, which stays only slightly warmer than the PCB:Quote from: eneuro on August 25, 2014, 09:51:15 pmHuh? The driver chip is supposed to be capable of 1A under ideal circumstances.Quote from: eneuro on August 25, 2014, 09:51:15 pmBecause i only need two pins?The requirement is to have a dozen of computer-controlled LEDs on a one-wire bus.On, off, half, dim, blink, ramp, etc.

author: mzzj
author_rank: Super Contributor
timestamp: August 26, 2014, 04:46:10 am
content: Artlav,Have you tested more than 1 of the smoky inductors in-situ? And did you do the measurements for the exact invidual that was smoking or was it different invidual?Asking in case that you got 1 bad inductor that let the snoke out and rest of them measureand work good?

author: eneuro
author_rank: Super Contributor
timestamp: August 26, 2014, 10:08:38 am
content: Quote from: Artlav on August 25, 2014, 10:27:59 pmWhat those "ideal circumstances" do you mean: -30*C operating temperature or planning to use water cooling or aditional heatsinks with fans?This temperature graph @ 0.4A only is a confirmation what is wrote in its datasheet: 0.350Amax and at its maximum current very well designed PCB copper heatsinks.This is first two sentences of its datasheet:QuoteThere is also something else-there is no diode protecting driver input pin ADJ on your schematics recomended in datasheet for some reasonSo, I will simply use this method of driving this pin from MCU or opamp, add capacitor on ADJ pin, use a few inductors between 47uH-100uH of course with known core specs and will keep output current below 200mA with large bottom copper heatsink.

author: bktemp
author_rank: Super Contributor
timestamp: August 26, 2014, 10:35:01 am
content: Quote from: eneuro on August 26, 2014, 10:08:38 amHe is using a ZXLD1360 not ZXLD1350

author: diyaudio
author_rank: Frequent Contributor
timestamp: August 26, 2014, 04:02:23 pm
content: Quote from: Artlav on August 23, 2014, 06:22:22 pmDont you have duty cycle adjust on your rig? if so yours is at 99%, you may have saturated the core already, slowly adjust your duty cycle from 1% to x% and start doing incremental measurements again, also lower the frequency, you operating at 100Khz is this your circuits operating frequency ?The red trace show a obvious deviation at 6us.

author: eneuro
author_rank: Super Contributor
timestamp: August 26, 2014, 04:14:51 pm
content: Quote from: bktemp on August 26, 2014, 10:35:01 amThx for this note"Unkle Google" completed my search when was looking for its datasheet and it got ZXDL1350However, even in this 1A LED driver ZXDL1360 they are still talking about this clamping diode and erratic behaviour in its datasheet, but it is written slightly diffrent than in 1350 datasheet:It is interesting what they put inside this chip, but 55V IRFZ44N 0.018 RDSON @ 1A should be cold with 18mW losess with good PWM driving