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LM5160A: A proper inductor selection

xyz xyz
Intellectual 1000 points
Part Number: LM5160A
Other Parts Discussed in Thread: LM5160, , SN6501, TPS65130

HI

I have designed a Fly-Buck PSU based on the LM5160. I have reviewed the datasheet and some application notes about the IC such as TIDA-01427

page 18 of the LM5160's datasheet mentions some notes about the inductor selection. but I have a question in my application:

Vin: 4.5 to 5.5 Volt

dVi=75 mV (input Ripple)

Ron=332K Ohm for 260KHz

or 169K for 500kHz.

(which one is better?)

Vo1=-8

Vo2=+8

Io2=180mA

Io1=180mA

I have calculated the Inductor's currents as following according to LM5160's datasheet (page 18, 8.2.1.2.3 Inductor Selection):

for Fsw=260 KHz, Lmin=83uH

for Fsw=500 KHz, Lmin=42uH

IL1 = 1.056
IL2 = 0.18
IL1_pk = 1.12029577464789
as you know in Fly-Buck with LM5160 the Transformer is configured in series mode.
now I want to choose an inductor in Digikey or Mouser.
for example SRF1260-101 is good or not?
Could you please help me?
Best Regards
  • 0
    TI__Mastermind 49597 points
    Hello,

    Yes, you can use a coupled inductor as mentioned. Just ensure that the leakage inductance is reasonablly low and the sat current is above the maximum current limit of the LM5160A. Various magnetic solutions are referenced in the LM5160A documentation and related reference designs. All of this content is available from the LM5160A product folder. The reference designs specifically are located under "Tools and Software".

    Regards,
    Tim
  • 0 in reply to Timothy Hegarty
    Intellectual 1000 points

    Thanks

    I checked many reference designs for LM5160 before.

    for my application I have mentioned some parameters such as (LM5160's datasheet, page 18, 8.2.1.2.3 Inductor Selection):

    for Fsw=260 KHz, Lmin=83uH

    for Fsw=500 KHz, Lmin=42uH

    IL1 = 1.056
    IL2 = 0.18
    IL1_pk = 1.12029577464789
    now for SRF1260-101:
    1-which parameters I should consider in my design? Parallel parameters or Series parameters in above Table?
    as you know in FlyBuck PSU with LM5160 we use the transformer in series mode
    2-you mentioned a note about leakage Inductance? do you mean leakage inductance due to PCB layout?
    3-Actually I have a big problem with LM5160.
    Actually I wanted +/-5.5V but I increased it to +/-8V due to drop-out voltage
    but output currents should be as my real test:
    Io+ =450mA
    Io- =200mA
     
    with above load my inductor gets hot and sometimes I can hear some noise from the PSU like a whistle. (I'm very stressed)
    on the other hand the load regulation is not interesting, usually I have 2~3V drop-out voltage.
    I have used SRF1260-101 (based on the TIDUD38 document) but I think it's not proper for my application.

    I really need your help.

    http://www.ti.com/tool/lm5160dntfbk-calc

    I have set the Vpri=8 and Vsec=7.5 (Actually my Vsec=-7.5 but please compare above pic and the pic in the Excel file)

    LM5160-test.xls

     

     

    Kind Regards

  • 0 in reply to xyz xyz
    TI__Mastermind 49597 points
    Hello,

    I wouldn't recommend the coupled inductor you mentioned as it's only rated for 1.1A for one winding (such as the primary), wheras the LM5160A peak primary current at current limit inception is 2.875A (max).

    Note that your schematic is configured for an inverting buck-boost where the primary-side output voltage is negative. The quickstart file you sent is then not suitable (hence the negative values for currents, etc.). Try using the universal quickstart calculator, www.ti.com/.../flybuck-flyback-design-calc, or use WEBENCH for your calculations.

    PS: other options here include the SN6501/5 push-pull converter if the load reg spec is not tight. We also have parts for bipolar outputs if isolation isn't required. Do you need the positive output isolated?

    Regards,
    Tim
  • 0 in reply to Timothy Hegarty
    Intellectual 1000 points

    Thanks for your attentions

    actually I have build two LM5160DNTR kits. now I want to replace the inductor and some components to work fine.

    I have assembled two kits of LM5160. I can divide the loads on the outputs of two kits as below:

    load 1 (+230mA and -20mA) --->LM5160 Kit 1

    load 2 (+180mA and -180mA) --->LM5160 Kit 2

    I'd be grateful if you could help me to modify my kits.

    Best Regards

  • 0 in reply to xyz xyz
    Intellectual 1000 points

    plus to my previous reply.

    I found some inductors in the below link:

    when the universal calculator calculates the inductance and current rating as below:

    (I have set the switching frequency  to 800KHz)

    can I use a 22uH Inductor from above link?

    (I'm skeptical about 800KHz Fsw and 22uH inductance)

    Thanks

  • 0 in reply to xyz xyz
    TI__Mastermind 49597 points
    Hello,

    Your design seems feasible with the 22uH mag inductance even though 800kHz is quite high for a Fly-Buck / Fly-Buck-Boost converter (tradeoff of size vs. efficiency, high Fsw reduces light-load efficiency in particular).

    Note that Coilcraft and Wurth also have 1 : 1 coupled inductors as well as dedicated transformers for the LM5160 on their websites. Please send your completed Execl file for review.

    You may also be interested in this blog:
    e2e.ti.com/.../powering-bipolar-rails-with-wide-vin-fly-buck-converter

    Regards,
    Tim
  • 0 in reply to Timothy Hegarty
    Intellectual 1000 points

    Thanks

    I have reviewed your article.

    LM5160-ver1.xlsx

    Actually I wanted +/-5.5V at the outputs to supply TPS7A series LDOs (I have set the TPS7A LDOs at 5V, 3.3V and 1.8V) and if you see I set the outputs at +/-8V in the Excel file bcoz I'm concerned about the drop-out voltages.

     

    All possible situations in my real loads:

    +410mA and -200mA or

    +360mA and -20mA or

    +200mA and -200mA

    I found a note. when I set the Iout+=200mA instead of 450mA the minimum Inductor in the calculator is 46.88uH. (in case the deviation in my loads)

     

     

    if a 22uH Inductor is adequate, I found SRF1280-220 Inductor or CoilCraft Inductors (I listed from 15uH to 47uH, MSD1514's rating is very good):

    https://www.digikey.my/product-detail/en/bourns-inc/SRF1280-220M/SRF1280-220MTR-ND/2458545

    https://www.mouser.com/Coilcraft/Passive-Components/Inductors-Chokes-Coils/Coupled-Inductors/_/N-afrbkZgjdhub?P=1yzcfvtZ1yzcfvhZ1yzcffoZ1yzcfvnZ1yzcfvdZ1yzcfw4Z1yyt81kZ1yzcffhZ1yyt8i6&Rl=afrbkZer9dZ1z0wpl0Z1z0wr0ySGT&Ns=Maximum%20DC%20Current|1

    in my real circuit I set RFB1=10K and RB2=3.3K but in the Excel file the calculator calculated 20K and 5.714K respectively (the result is 9V not 8V)

    I set Rr=100K Cr=10nF and Cf=100nF (Type III configuration) in my real circuit based on a TI document.

    Best Regards

  • 0 in reply to xyz xyz
    TI__Mastermind 49597 points
    The MSD1514 is quite large. My understanding is that Coilcraft has MSD coupled inductors in smaller footprints that are suitable.

    Also, note that if VIN < 5.5V, the TPS65130 family of split-rail converters may be suitable.

    Regards,
    Tim
  • 0 in reply to Timothy Hegarty
    Intellectual 1000 points
    you are right, I found MSD1583 series in smaller size.
    Also TPS65130 is a good choice.
    Did you review my Excel file? which inductor is better for my application? 22uH or 33uH or 47uH? if the calculator says me 20uH is good which size I should consider? 1.5 times larger or 2 times larger? or a little larger?
    if IL-peak=2A how much current rating should be larger for my application? 1.5 time larger is good?
    I want to choose a proper inductor for my application.

    Thanks
  • 0 in reply to xyz xyz
    TI__Mastermind 49597 points
    22uH should be fine. Just ensure that its sat current is above the peak ILIM of the LM5160A. The TPS6513x seems like a good solution if you isolation isn't required and the input voltage meets the spec. EVMs are available to order from TI's estore if you'd like to verify its performance.

    Regards,
    Tim