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lhx
01-26-2009, 03:52 PM
I'm designing a 5V 3A switched power supply. I have narrowed down to LM2576. This is the probably the cheapest 3A switcher in the market.

But the problem is finding a suitable inductor to go with it. The design has to be compact so I'm looking at SMD inductors or through hole components like

PANASONIC - ELC18B101L
http://in.farnell.com/panasonic/elc18b101l/choke-100uh-0-057ohm-3-20a/dp/1308471?_requestid=76507

and

BOURNS - SDR2207-101KL
http://in.farnell.com/jsp/search/productdetail.jsp?sku=1612709

These are way too expensive. Much more expensive than the switcher itself.

Can anyone suggest a place where these/similar components can be cheaply sourced?

The solution has to be compact, cheap. Alternatives are welcome!
BTW: Which components do you use in your switching supplies?

On a separate note: How damaging is the ripple/noise generated by the switcher considering that there's going to be an ADC in the circuit. I'm somehow not convinced that the 1L 1C filter given in the datasheet would suffice.

MohitM
01-26-2009, 04:39 PM
Roll your own inductors. I do that all the while and there is nothing much to it. Just keep the coils tight. 3A will need a thick wire. If you have PC SMPS that's not working, there are a few inductors in it you could use.

> Which components do you use in your switching supplies?
I've used a PIC18F, a MOSFET driver, a MOSFET, an inductor and ceramic capacitors. Ripple was around 5%, and that suited the design.

> I'm somehow not convinced that the 1L 1C filter given
> in the datasheet would suffice.
Is this a gut feeling, or does your calculations show you something that the DS doesn't?

To use a 10-bit ADC, I would suggest ripple less than 1% and a lot of bypass capacitors all over the place. If its a 12-bit ADC then don't use a switcher.

lhx
01-26-2009, 06:10 PM
Thanks for your reply Mohit.

This isn't a one off piece. Its an existing product which uses linear regulators to drop voltage from 12~15V to 5V
So manually winding so many inductors is not an option besides I wont be sure of its characteristics.

The regulator is in a TO3 pack which sits on a big heat sink mounted outside the cabinet. This arrangement alone is dictating the cabinet size and making it unnecessarily large. I want everything to be on a single PCB and make the cabinet as small as possible which means getting rid of the heat, heat sink and TO3 pack.

>I've used a PIC18F, a MOSFET driver, a MOSFET, an inductor and ceramic capacitors
What frequency did you use to switch the MOSFET. What did you use as an inductor? What value? How much current could it provide?

>Is this a gut feeling, or does your calculations show you something that the DS doesn't?
Haven't tested it is a gut feeling :)

Do you know any shop in Mumbai dealing in inductors?

MohitM
01-27-2009, 11:28 AM
What frequency did you use to switch the MOSFET. What did you use as an inductor? What value? How much current could it provide?
Just beyond the human audible range. IIRC, 25kHz. The inductors are around 100uH-120uH along with around ten 0.1uF ceramic capacitors (Note that while designing LC filters, you need to take care of both L & C. If one is large then the other can be small). The SMPS delivered 12V @ 8A for long periods of running and 12V @ 10A for about 15 minutes in running. I did not check it beyond 15 minutes for 10A as the design was 12V @ 8A. It didn't heat up beyond slightly warm even inside the case, so no heat sinks anywhere, and I could not hear anything at all, so no loose coils. The ripple was about 0.5V on the oscilloscope - but that was inside the design margin.



Haven't tested it is a gut feeling :)
Usually datasheets are correct. The designs that they give are tested. You can go along with what it says. At 52kHz, I believe the L and C would be small. But then again, it depends on the ripple you are designing for.


Do you know any shop in Mumbai dealing in inductors?
Unfortunately, no. I do know you can get them rolled in Delhi. Lajpat Rai Market is great in this way. You'll get everything within a 100m diameter, if you know where to look and if you sound professional/experienced enough, then cheap too.

How many do you need? What design - bobbin, air-core, EI, C, O? If its bobbin (or air-core), buy the cores (or a plastic tube) from a power electronics shop... go to a transformer winding guy and ask him to do it for you. They have this motor with a chuck. The bobbin is fixed on the chuck, and the wire is fed as the bobbin rotates and you get a nice tightly rolled inductor. You'll of course have to calculate how many turns and what gauge of wire. The net is filled with calculation sites to help you do this.

Hope this helps!

shashak
01-31-2009, 09:10 AM
can get them rolled in Delhi. Lajpat Rai Market

can you give the contact no/exact location/shop no. who can roll inductor professionally on large scale,

akshay66
01-31-2009, 09:22 PM
I dont know the exact shop no. or contact no. but when you enter the market in the starting only you'll find only 3-4 shops are working on coils,inductors,big transformers etc.Talk to shop owner not to the worker as wont understand those tecnical things.I have a card of one of them.i'll find & post the numbers soon.