Core Electronics Forum

Substitute Inductor choice

Hello,
I have a buck converter circuit I am using for a project. A PCB using the circuit will be made and assembled by JLCPCB (I have already used them for another PCB I designed and had them make with very satisfactory results). The design for the new board (from Texas Instrument WEBENCH Design Report and using a TLV62569DBV IC) calls for an Abracon ASPI-0530HI-2R2 35.0 mOhm Inductor

This item is not available from JLCPCB so a substitute I have identified from the JLCPCB (LCSC) parts list is the Bourns

OR the TDK

Both of these have very similar characteristics in some areas but vary in others and I am not sure if I am reading the results properly so hope someone may be able to help me understand.
The DCR (or RDC??) appears to be 35 mOhm and inductance is 2.2uH both as required. But the Saturation Current (Isat) and Tmp Rise Current (Irms) appear to be significantly lower than the Abracon ASPI-0530HI-2R2. Should I be too concerned about these differences as long as the DRC and Inductance are OK or are they a vital part of the spec and must be met? If they are vital then I will have to continue my search and currently there appears to be a shortage of components so finding an available, adequate substiute is difficult.

Bob I am hoping you may have a comment on this.
regards
Les

@Robert93820

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Hi Leslie
Had a quick look at these data sheets. You put the bourns one up twice instead of TDK.
The 2 main differences between the 2 you have linked is the shape and saturation current. The shape may not matter. The sat current is a bit suspicious, one is 9.0A the other is 0.98A. I wonder if this is a typo. What will affect this is the core material but if the size, inductance and resistance (RDC and DCR are the same = DC resistance) I would expect the core material and thus the sat current to be pretty much the same too. I looked up Element 14 and they do have a couple of 2R2 Abracon inductors but not that particular one.

All parameters are pretty important. The inductance needs to be pretty correct to get the device to work at all, the sat current will come into play as you draw more dc current from the device and once saturation starts the inductance will change radically and the device will probably quit. I think both DCR and core material have a bearing on temperature rise.

I would not trust the saturation current figures here as they seem wrong. Post a link to the TDK device and we can have another look.
Cheers Bob
PS. I note one of the Element 14 devices has a sat I of 5.7A so maybe this figure is correct.

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Hello Bob, thank you yet again for your help.
Silly of me to put the wrong data sheet up but then I also discovered since writing the post that neither of those Inductors are in stock so a little background. I was developing the schematic and PCB in Eagle/Fusion. My problem however was the Eagle libraries are difficult to manage and find parts in and also the found part has no connection to availability. I am now using the JLCPCB App EasyEDA which works very well and has a direct connection to the JLC and their parts companion LCSC and makes ordering the PCB extremely easy for a novice like me.
https://jlcpcb.com/
https://lcsc.com/
https://easyeda.com/?_ga=2.115579718.1539379176.1637651318-1569248801.1635749621
In searching the parts site I have found 2.2uH inductors in stock but only with around 1800mA Isat. I have ID’d a Bourns inductor with 9A Isat and 2.2uH (SRP5020TA-2R2M) but not available so I am stuck for the moment.
I am getting a new appreciation for the engineers and their ability to wade through all the products and the haphazard approach to any sort of standardised data sheet to ID an appropriate part for a design to be made into a populated PCB.
I will keep searching however.
Regards

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Hi Leslie
All part of the learning curve.
Cheers Bob

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The TLV62569DBV looks like it will handle up to 2A. But this may not be the current you are designing for. In which case maybe an inductor with a lower Isat will work for you. I found This document very useful in understanding the design parameters for a buck converter. I bought supposed 3A module with a switching frequency of something like 200kHz and an inductance of 10uH. I only wanted a few hundred mA so the current arrived as pulses. I bought some 7×7×4mm SMD 100uH Shielded Power Inductors off eBay and substituted one and everything was sweet. The Isat (saturation current) should not be exceeded at any point, otherwise the inductor turns into a short circuit. The resistance is not so important, more resistance results in more heat. The core material also affects the heat generated, this rises with switching frequency. There’s a bit to think about but not too tricky.

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@Alan73922 @Robert93820

Thank you Alan for that information I will study it and hopefully gain a better understanding of what I am trying to do. In the meanwhile I have found some good help from the TI WEBENCH schematic with some recommended limits for the Inductor in the circuit.

And after using those to create some filters in the parts library of the PCB manufacturer I have identified an Inductor that appears to fall within the limits - or at least I think and hope it does. I have attached a copy of the data sheet and would appreciate any comments or thoughts on my choice as I am sure to have missed something.

1rating-2001152110_cjiang-Changjiang-Microelectronics-Tech-FXL0518-2R2-M_C475907.pdf (2.0 MB)

I may not have found a substitute yet but I have certainly learnt a lot so far in this process particularly learnt of the poor quality of some Datasheets usually from Chinese manufacturers.
Hope to hear any comments.
Regards

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The links don’t link :thinking:

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Sorry, I think I have fixed it.
Cheers

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Alan, short question but probably requires a long answer. I have really enjoyed reading the info in the link you sent to the voltage supply and at least having some understanding of the component selections in such a circuit. But, what part does the Diode play in the circuit. Given it orientation couldn’t it just be left out?
Not on topic I know but thought i would ask.
Next task for me is an understanding of “ripple” for capacitors.
Cheers

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Hi Leslie
That is pretty close.
If you go down the pages a bit further you will see one that is closer to the one you were after in the first place.
FXL0618-2R2-M. DCR 28mΩ, Idc 7A, Isat 8A.

I personally try to stay away from anything Chinese. If a product must be made in China for me it has to be available locally. The Chinese mail system really sucks. you can be waiting months.
Cheers Bob

Edit, the link is a download

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@Leslie62519 since you are using a synchronous buck converter, I assume you are talking about the diode in the circuit on page 13. It is explained on page 14. The discussion is based on external MOSFETs for switching. Where the MOSFETs are incorporated into the converter chip (as in the TLV62569DBV), I assume the designers have taken the body diode problem into account. The specification doesn’t say.

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@Robert93820 @Alan73922
Thank you Alan, I will read on and hopefully learn to read further before asking questions:)
Bob, your comment re the FXL0618-2R2-M. DCR 28mΩ, Idc 7A, Isat 8A. is an extremely good one and that Inductor is available from the PCB house. Interestingly the specs for that item and for the original Inductor specified by WEBENCH show an IDC of 9A but the WEBENCH Limits recommendations show a range of 4.375A-5.5A so their chosen Inductor falls outside that range. Maybe that doesnt really matter for this design. I am now wondering which one to choose the one closest to the original or the one that fits within the limits??? Any thoughts would be really helpful.

The PCB house I used for two previous PCBs have proven to be quite reliable so far and delivery of the PCBs has been extraordinary with 7-9 days total from order placement and payment to delivery of the finished items to my door in Perth Western Australia. Certainly a different experience from other items I have ordered from China where delivery takes weeks so I understand your lack of interest in buying from there in most circumstances.
Regards

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Hi Leslie

I noticed that. I think as long as the discrepancy is in the right direction it should be OK. You need to stay out of saturation area. As Alan pointed out previously if full saturation occurs inductance is lost and the device becomes a piece of wire. Then the whole thing stops working.

If the original is from a working device I would personally go for the closest one. If it is only a design on paper and untested this could be anybody’s guess. I would still go for the closest as somebody has done the hard work. The close one being available should be a game changer in these days of reduced supply. If it is proven wrong for some reason it can always be changed provided the physical size fits. The one originally chosen may have been influenced by availability at the time too.
Cheers Bob

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@Robert93820
Thanks Bob,
Your comments make perfectly good sense as always. Certainly availability is the controlling factor in many of the designs I have looked at. This design is one I found from TI using an IC that is available at the moment and looks so similar to so many buck converter designs particularly the Adafruit ones I figured for my purposes it would work. I wanted an output voltage of 2.9v so did the calculations from the datasheet for the IC for the 2 in series resistors and those calcs showed 384 for R1 when using a 100 kOhm for R2 and when I looked at the WEBENCH design it showed similar numbers so I believed it. My design uses 390kOhm for R1 as that is a standard and available. The slight voltage change will be negligible I think and the output voltage is not critical as long as it is close.
If I am wrong it is not a huge investment as 5 PCB manufactured and assembled with all components is $35 plus $18 for delivery and I figure if it has problems I will have a learning experience for next time.
Cheers

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Hi Leslie
A good approach and a good learning experience if you are prepared to write off mistakes.
If any mistakes are only component value changes all is not lost. The boards will be re-useable and if you have 5 and only want 1 you certainly will have some spares.
Cheers Bob

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Idc is the maximum current before the device cooks itself. I don’t know anything about WEBENCH, does it really put an upper limit on Idc, or is it a range of minimums based on different usage? The difference from the unobtanium inductor and the FXL0618-2R2-M is the FXL has a lower resistance (28 v 35 mΩ) so should generate less heat. That’s a good thing. I can’t think why a maximum Idc is a design parameter.
I was worried the FXL0618-2R2-M wasn’t rated for high switching frequencies but the spec say up to 5MHz so should be OK.

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Hi Alan
The original put forward by Leslie quotes typical 29mΩ and Max 35mΩ so the FXL is very close.
Cheers Bob
Edit. FXL device quotes typical 28mΩ and Max 35mΩ

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