Set up a lab for Inductor testing  - Page 1
Forum section: electronics / projects

author: opa627bm
author_rank: Regular Contributor
timestamp: October 17, 2016, 09:24:23 am
content: Hi all,I want to set up a component validation lab for inductors.We have experienced some issue with out vendor that they provides us golden sample for validation but when we start mass produce the product, the CM received defected products (less core material) derated ISAT.I am looking at a LCR meter with DC current bias systemInductor range is 80nh~1000nH, ISAT ranged from 10A to 100A.Any suggestions?

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 17, 2016, 11:12:26 am
content: You can bias the inductor with a separate power supply (and a resistor in series, if the PSU doesn't have a true CC mode) then AC couple a generic LCR meter to the inductor. This should be sufficiently accurate for go/no-go testing. If you need to actually plot the dI/dt curve of the inductor (to look for when it actually saturates) then you'll need to make a bit more involved test jig for applying pulses of known voltage and duration across the inductor while measuring the current with a wide bandwidth shunt or CT.

author: BravoV
author_rank: Super Contributor
timestamp: October 17, 2016, 11:15:39 am
content: Check out this post , a dedicated inductor saturation measurement circuit designed by one of our member here ->https://www.eevblog.com/forum/projects/inductor-saturation-tester-alternative-route-to-dump-the-excess-energy/msg181720/#msg181720or just read the whole thread.

author: tszaboo
author_rank: Super Contributor
timestamp: October 17, 2016, 11:20:38 am
content: Some unknown company, making excellent test gear for RLC and transformers:http://www.waynekerrtest.com/Probably it is cheaper to just order higher quality parts.

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 17, 2016, 03:52:11 pm
content: Just a thought: I'm guessing you're buying inductors from less-than-reputed brands. Perhaps buying from an A-brand will be cheaper in the end?

author: Kleinstein
author_rank: Super Contributor
timestamp: October 17, 2016, 04:21:38 pm
content: The constant current source of a typical lab supply may not be good enough for a DC bias, as there often is a parallel capacitance. One way to get around is to have the DC bias through two inductors in series and measure the inductance of the two in parallel. This could be two equal samples or a sample and known (usually larger inductance) one.The values of 80 nH - 1 µH are rather low and 10-100 A is quite some current. So it might get difficult to find a meter that includes the bias current to that level.

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 17, 2016, 10:35:57 pm
content: Quote from: Kleinstein on October 17, 2016, 04:21:38 pmYes, good point - not only does the LCR meter need to be AC coupled to the inductor-under-test, but the DC bias supply needs to be decoupled with inductance, itself; at least 10x the inductance-under-test, but preferably more.But I also agree with the others that said the OP should probably be looking for a new inductor supplier. No way would I tolerate such shenanigans as substituting core area and/or material.

author: opa627bm
author_rank: Regular Contributor
timestamp: October 18, 2016, 05:26:55 am
content: Hi all,The vendor I am dealing with was Eaton-Copper.... I think they are pretty good vendor ...Regards,Li

author: tszaboo
author_rank: Super Contributor
timestamp: October 18, 2016, 08:35:23 am
content: Isat will be dependent on the actual inductance. It can be eg. +/-20% of the nominal inductance. Also, it can change with temperature and a dozen other factors. Are you sure, that you designed the circuit with these variations kept in mind?

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 18, 2016, 11:32:39 am
content: Quote from: opa627bm on October 18, 2016, 05:26:55 amThey are, which raises a few other questions as others mentioned...

author: Kleinstein
author_rank: Super Contributor
timestamp: October 18, 2016, 12:51:56 pm
content: With high permeability cores also the stress from the windings can have a significant effect on the inductance and thus indirectly the saturation current. I had a sample where winding stress reduces inductance by a factor of about 10 (and thus 10 times more saturation current), but this is likely an extreme case, with way to tight winding.

author: fonak
author_rank: Contributor
timestamp: October 18, 2016, 04:00:09 pm
content: HiSee this site:http://www.voltech.com/

author: opa627bm
author_rank: Regular Contributor
timestamp: October 19, 2016, 05:52:02 am
content: Hi all,So the detail of this issue was :1. I company requested us to build a CRB for them.2. After we have the first batch out and everything was great.3. When we MP the product, we got 0 issues from the field.4. DOD customer wants the rating of this board, we stressed it to 105C, after two weeks, it blew up.5. We tried 2 more boards, and they all blew up consistently after two weeks.6. We beat the S**t out of TI for selling us crappy power stages but in the end we found out the ISAT rating on the inductor wasn't in spec.7. We found out they are cutting on core materials so when we not applying bias, it is fine but when we applied bias and thermal stress, it is about 15% off from what they listed.Now we need to build a MIL-STD-202G-A lab to verify all components....

author: Ice-Tea
author_rank: Super Contributor
timestamp: October 19, 2016, 07:20:15 am
content: Quote from: opa627bm on October 19, 2016, 05:52:02 amSo, you stressed the board to 105C ambient? What was the temperature of the coil at that time? Was that within spec you requested from Eaton?

author: Smokey
author_rank: Super Contributor
timestamp: October 19, 2016, 07:26:37 am
content: Um...If you are talking about DOD customers, and MIL standards and and product failing...This is something you work out with your vendor... like immediately...However much fun it would be to throw around on the EEVBlog, this probably isn't the place for this particular problem...

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 19, 2016, 10:02:23 am
content: Sonowyou tell us that this thing is failing after 2 weeks at 105C, a condition it was not designed for originally, apparently.I suppose it needs to be pointed out that most power ferrite materials experience minimum loss around 100C, with losses increasing on either side of that minimum. If ambient is above the minimum loss temperature of the core material then any additional losses from operation with push the core towards thermal runaway (though this is something rarely seen with ferrites; it's much more common with powdered iron).Also note that most commercial off-the-shelf inductors are designed for a max 55C ambient and a 30-40C rise when operated at max rated current... NOT saturation current. I'm going to go out on a limb here and say that no COTS inductors will be suitable. You will have to wind your own, or else consult with your vendor on a custom product. Generally speaking, you will need to design the inductor for a much higher current than necessary (2x or more than the expected peak current is a start) and with as little AC flux swing as possible. Both criteria tend to result in a much larger inductor, which is a common side effect of designing for mil-spec.Actually, this inductor issue is just the first of many you will undoubtedly experience trying to run an existing design at way higher of an ambient than intended. If you've never done this sort of thing before you should strongly consider hiring a mil-spec design specialist firm to help you out. I certainly wouldn't rely on the half-baked opinions of yahoos like me on this forum, especially when we aren't privy to all the key details.EDIT - grammar/spelling

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: October 19, 2016, 10:40:30 am
content: Hi,You probably already have a test set. Use the product, the power supply, itself for the testing. Measure the current through the inductor with a DC current probe and the voltage across it with a differential voltage probe.You can also use a low pass filter to measure average voltage across the inductor. The inductor will have the temperature coefficient of copper (hopefully !!) 0.393% degree C. You can measure the change in resistance of your inductor to determine the temperature rise.Inductor saturation is a poorly defined measurement. It is typically stated as the inductance changes by 30 or 40%. But the measurements methods vary.I like using V=Ldi/dt and looking at the change in slope.The small signal method using an LCR meter and bias current fixture gives different results based on test signal amplitude.Ferrite inductors tend to have a sharper saturation current than powdered iron.It is easy to pick Isat for the ferrite part:For the powered iron, not so easy:Recommended reading:http://www.micrometals.com/thermalaging_index.htmlThis gives you an idea of what is happening in a powered iron core at high temperatures.Here is a link to core ferrite core materials:https://en.tdk.eu/tdk-en/529404/products/product-catalog/ferrites-and-accessories/epcos-ferrites-and-accessories/ferrite-materialsLook at the curves versus temperature.Regards,Jay_Diddy_B

author: tszaboo
author_rank: Super Contributor
timestamp: October 19, 2016, 12:07:32 pm
content: Quote from: opa627bm on October 19, 2016, 05:52:02 amTo be honest, I dont think it is the vendor to be blamed.