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what core type to use for SMPS
Hiemal:
Designing a buck boost converter and trying to figure out what core type to use.
I have some ferrite ETD cores made of N87 material (http://static6.arrow.com/aropdfconversion/a9ab4ea59bcd9f7c75538730d5f24721fd40243e/2789680714527620etd_49_25_16.pdf)
Two of the ferrite cores have spacing 2 mm, the other two have 0.2 mm.
and several toroidal cores made of Kool-mu (https://www.mag-inc.com/Media/Magnetics/Datasheets/0077438A7.pdf). They're big.
Which would be better to use for this? The Kool-mu cores have more loss, but have a distributed air gap and should saturate less easily.
The buck boost converter will have an input of 12 volts, and output anywhere from 0 to 30 volts at 10 amps. Which means, at 30 volts and 10 amps it should have a peak inductor current of about 30 amps (given efficiency losses and whatnot)...
The main IC of the buck/boost will be an LM5175... which can be set to anywhere between 100 kHz to 600 kHz.
Hiemal:
I apologize for bumping this so soon, but I'm curious to know what ya'll think.
I can obviously combine the two spacing varieties of ferrite too.
I've been told to avoid the Kool-mu toroid since the material is quite lossy compared to ferrite, but again want to know if it's something I should be concerned about or not.
T3sl4co1l:
Probably any of them is suitable.
Do you have any idea yet, how much inductance will be required, based on your choice of core and controller?
The Kool-Mu may be on the lossy side, if it's a higher-mu variety. That would suggest a higher inductance, and an average-current-mode controller. You can put some turns on it to test, and look up the A_L and size in the datasheet to confirm its properties.
The 2mm gap is quite large, implying low mu_avg and a high ripple fraction, suitable for a peak-current-mode controller.
The 0.2mm gap is probably not enough for the required current, but you can always add more.
Tim
Hiemal:
Inductance required for this should be about 10 uH from what the web-bench designer suggests. The value doesn't necessarily need be precise and has a lot of wiggle room.
The Kool-mu core has an A_L of 281, and a u of 125.
"The LM5175 regulates the output using valley current mode control in buck mode and peak current mode control in boost mode." straight from the datasheet.
And I could combine the two gaps to get a middle man if you think that would work better than the Kool-mu.
MagicSmoker:
--- Quote from: Hiemal on May 09, 2019, 12:26:46 am ---I apologize for bumping this so soon, but I'm curious to know what ya'll think.
--- End quote ---
You didn't give nearly enough information to even begin to help you which is why no one bothered replying. At least do some of the preliminary design work rather than just stating you have 3 different core sets and asking if they are appropriate for a buck boost converter delivering 0-30V at up to 10A. I mean, really - what kind of half-assed design spec is that?
That out of the way, Kool Mu material is way too lossy for use at 100kHz if exposed to significant AC flux swing, which is bound to occur at some combination of input vs. output voltage and load current with a peak current-mode control scheme. You could design the buck-boost (aka "non-isolated flyback") converter to stay in continuous conduction mode (CCM) over most of the operating range, but that combination of topology and operating mode is notoriously difficult to stabilize.
Note that datasheets for gapped ferrite cores give AL values assume the use of one gapped and one ungapped core half; using two gapped halves generally cuts the AL value in half. As Tim already mentioned, a 2mm gap is quite extreme, and as has yet to be mentioned, an ETD49 core would be massively oversized for this application even at 50kHz, much less 100kHz or higher. Still, using a bigger core than necessary doesn't really hurt, and can help quite a bit in fact as fewer turns for each winding for the same flux swing will be needed.
And for the really useful part of this post, I highly recommend the below web site to people starting off in SMPS design as it does a creditable job of selecting core and winding parameters given input/output voltage, output current and switching frequency. It defaults to solving for operation at the boundary between discontinuous and continuous conduction modes which is a reasonable place to start, though you'll likely want to lower the inductance value to force operation in discontinuous mode with a flyback/buck-boost.
http://schmidt-walter-schaltnetzteile.de/smps_e/smps_e.html
EDIT - I plugged your sketchy parameters into the above site - 12V in, -30V @ 10A out, 100kHz - and an ETD 49 core with a 2mm gap was one of the recommendations it spat out, saying just 6 turns would be needed. Like I said, massively oversized for the job!
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