EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: sdbauman3457 on May 01, 2019, 08:56:37 pm
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Hello, my first post here. I have recently gotten back into electronics and have decided to build a switching power supply which takes a 24VDC input and steps it down to any voltage between 3.3 and 20 VDC. I did some research and settled on the LM2576-ADJ for my buck converter. In looking to purchace this IC I found off brand ones on eBay which claim to be the same thing. Are the datasheets and calculations compatible with each other? Is an LM2576 an LM2576? Using the TI datasheet can I correctly size components for a one-hung-lo brand? Help is greatly appreciated!
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Your mileage may vary on the off-brand ones. I have used a few modules from ebay, and I'm pretty sure they were all fakes. They worked, but I didn't check their efficiency or output ripple. From the few posts I've read about such modules, they claim to run at 150kHz but actually run at 52kHz, and often their I/O caps are pretty poor, giving them a noisy output. With that said, often the modules are so cheap it might be worth buying a couple and testing them. If you're just after the switcher IC itself, then the value of the supporting components (inductor, caps etc..) would depend on its switching frequency, which as mentioned, might not be what as described.
The LM2576 is pretty old. That doesn't make it a bad device, just that there are cheaper devices with higher efficiency, and higher switching frequency (= smaller inductor).
What kind of current are we talking about? MPI does quite a few 2-3A switchers running at higher frequencies, and modules using them are about : https://www.ebay.co.uk/itm/172803275439?ul_noapp=true (https://www.ebay.co.uk/itm/172803275439?ul_noapp=true)
Although I believe the IC's are genuine, the input and output caps are pretty hopeless, so I would add extra decoupling. And the "3A" is often exaggerated, they'll probably top out at 2A max.
Building your own SMPS can be tricky (layout is important), but also a great way to learn and get back into things. If you want to use the LM2576 I would just order some, and if they're fake, meh, I'm fairly sure they will "work" just perhaps not guaranteed at the spec for the genuine device.
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I am looking for just the IC for the educational aspect of building from sort of scratch. I did all my calculations based on a data table from ON semiconductor. Do you know of any more modern IC's I should look into with better availability and efficiency?
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LM2576 is a 52 kHz switching frequency IC. LM2596 is supposed to run at 150 kHz.
You can look up chips on sites like Digikey - i prefer this one because it has easy way to filter based on your needs for ex. more than 24v input voltage, switching frequency higher than 100 kHz (probably 300-600kHz would be best to reduce inductor and capacitor sizes), maybe choose through hole or surface mount etc
Here's for example a starting point, some filters pre-applied
Vin max : 26v minimum, step-up or step-down regulators, ~ 150kHz and higher, 1A or higher output current, active (no discontinued, not for new designs etc) : https://www.digikey.com/short/pzp9vz (https://www.digikey.com/short/pzp9vz)
example of some hobby friendly ICs
MIC4576 4v..36v in , 1.23v..33v out ,3A , 200kHz , TO-220-5 package so easy to heatsink and solder : https://www.digikey.com/product-detail/en/microchip-technology/MIC4576WT/576-1218-ND/771687 (https://www.digikey.com/product-detail/en/microchip-technology/MIC4576WT/576-1218-ND/771687)
Datasheet is brief, but it mentions Application Note 15 (from Micrel) which contains several example circuits, recommended components etc : https://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en576961 (https://www.microchip.com/wwwAppNotes/AppNotes.aspx?appnote=en576961)
Micrel was bought by Microchip so above link is from Microchip
LT1372 ... 500 kHz switching frequency : https://www.analog.com/media/en/technical-documentation/data-sheets/13727fbs.pdf (https://www.analog.com/media/en/technical-documentation/data-sheets/13727fbs.pdf)
Sanken NR111D 6.5v .. 31v in, 0.8v..24v out, 4A , 350 kHz, DIP8 package : https://www.digikey.com/product-detail/en/sanken/NR111D/NR111D-ND/4289389 (https://www.digikey.com/product-detail/en/sanken/NR111D/NR111D-ND/4289389)
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I've used the ACT4088US-T successfully in a couple of projects. It is rated at 1.5 amps although I've never used it for more than 1/2 amp. Mouser has them in stock for 0.77 each in single lot quantity.
Schematic is attached that I used in my latest project.
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Might be cheaper / easir to buy 10 of the ebay 2596 modules (about $10)
10 LM2596 from digikey ( about $55)
replace the potentially fake 2596 with the "real" ones from Digikey.
...and ...$65 later you have 10 2596 modules for the whopping price of 6.50 a piece.
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Digikey wants $2.77 Qty 1
https://www.digikey.com/product-detail/en/texas-instruments/LM2576T-ADJ-NOPB/LM2576T-ADJNS-NOPB-ND/148128 (https://www.digikey.com/product-detail/en/texas-instruments/LM2576T-ADJ-NOPB/LM2576T-ADJNS-NOPB-ND/148128)
I might buy a couple of the Chinese modules just to see the layout but when it came time to buy parts, I wouldn't bother with fakes. My time is far too valuable to spend it chasing component problems.
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To the OP. One of the reasons that not many have provided part numbers is there are *loads* of switcher IC's with an internal switch - so they just require inductor, diode, input/output caps and feedback networks. This is both a good thing - more choice - and a bad thing - its surprisingly hard to narrow down the search to a few parts. Manufacturers often have parametric searches available for things like max input voltage, min input voltage, output current, switching frequency etc.. but again, I have found with these I still get 200+ results. Often we narrow the search further with things like - IC package, cost and extra features.
As this is just for educational purposes, I would just use whatever you can easily get hold of. If it doesn't work very well (cant' reach output voltage, or output current) then you can debug, and try something else. If you have already done your calculations for that particular device you mentioned, then I would say just buy it, and tinker. If you have access to an oscilloscope it will help a great deal in debugging (seeing the inductor current, and the output ripple). If not, it might be a case of build-it-and-see.
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And please keep in mind there are MC34063 switchers that are not periodic - you cannot observe the switching on an analog scope. Very low educational value.
Just my $0.02 on "any DC/DC is ok to learn about switching power converters".