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SkyDvv , 02-05-2024, 01:07 AM

Hello everyone, Hope you're okay.
I'm working on a buck converter with high current 20 continuous amperes driving Stepper motors modules, 4 of them each 5A.
So I'm stuck at choosing suitable inductor for the buck..
datasheet is saying (Check the inductor datasheet to ensure that the saturation current of the inductor is well above the peak inductor current of a particular design)
so as they are saying, for 20 Amps DC and maximum ripple of 40%
I_L(Peak) = I_out + delta(I_L)/2
I_L(Peak) = 20 + (20)(0.4)/2 = 24A?
So I need inductor with more than 24A in saturation
There's alot of inductors on LCSC with different prices but i don't know how to choose one.
There's https://www.lcsc.com/product-detail/Power-Inductors_COILMX-MS1360-1R0M_C380085.html
has I_sat of 30A
and there's https://www.lcsc.com/product-detail/Power-Inductors_cjiang-Changjiang-Microelectronics-Tech-FXC1265-1R0M_C208109.html
with I_sat of 49A
https://www.lcsc.com/product-detail/Power-Inductors_Chilisin-Elec-MHCC12050-1R0M-R7_C329553.html
I_sat of 37A but less than half of the price :"")
all of them has close R_DC, what is the trade off? (There's a huge number of different inductors with the same price and different maximum current, or cheaper with high feature)
the question is there any other parameters should I look for?

QDrives , 02-05-2024, 09:19 PM

Inductors have two 'current' specs: one for saturation and one for thermal.
A higher SRF (Self Resonant Frequency) is better, but most of the parameters are up to you.

SkyDvv , 02-05-2024, 11:55 PM

thanks so much I'll look into it

SkyDvv , 02-07-2024, 12:21 PM

after using WEBBENCH power designer of TI, For this part of COMP pin, I looked on datasheet and found out that it's for control loop, If i messed up the suggested values a little bit like using 2.2k for Rcomp1, or using 150pF for Ccomp3 is that gonna do anything bad?
I found out that those are for control loops and found transfer functions and poles and zeros and I really want this buck for doing easier job for driving a stepper motors, should it bother me or that small diffrences will not matter for easy uses.
this is the datasheet link: https://www.ti.com/lit/ds/symlink/lm5145.pdf?ts=1707305418609&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLM5145%253Futm_source%253Dgoogle%2526utm_medium%253Dcpc%2526utm_campaign%253Dapp-null-null-gpn_en-cpc-pf-google-eu%2526utm_content%253Dlm5145%2526ds_k%253DLM5145%2526dcm%253Dyes%2526gad_source%253D1%2526gclid%253DCjwKCAiA8YyuBhBSEiwA5R3-EwyZWLHHnkHqtvLDyrkiM8x_KTTRjHFVWW_d2_2zYP4B9pYbkuVDNRoCT50QAvD_BwE%2526gclsrc%253Daw.ds

QDrives , 02-07-2024, 09:34 PM

Are you using this buck converter to actually drive the motors?
What is the input voltage and what is the output?
Why not use the supply voltage directly?
Anyhow, changing Rcomp1 from 2.43k to 2.2k will not be such a problem.

SkyDvv , 02-08-2024, 12:31 PM

Yes, I want to drive 12V stepper motors yes each 5A total
Power input will be from battery pack of 10s, so input range is 39-42v and output is constant 12V
So using supply directly will burn the stepper I guess.

SkyDvv , 02-08-2024, 12:31 PM

And for the control loops what is the worst that can happen!

QDrives , 02-08-2024, 10:26 PM

Does the DC-DC converter burn too?
The stepper motor has inductance. Probably more so than L1 in the schematic above. So switching it fast enough (>= 20kHz) will not cause an overcurrent. In fact, most motor drivers work like that.

As for the control loop: either some instability at certain frequencies or lower bandwidth.

SkyDvv , 02-09-2024, 12:12 AM

thanks alot sir <33