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Old 21st December 2008, 05:49 AM   #11
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Hi Guys,

Thanks you so much for your help, it's been really educational

At first I was confused about how you came up with Np = 96 turns for the sample calculation and then I realised that Ae needs to be entered in square meters and so needed to convert up from square millimetres. Seems this is a much simpler method for Np calculation. So my current transformer design with Np=5:0:5 was wrong for 70kHz, but should be happy running f = 48 kHz which should be an interesting test. Will report back.

Just to be sure I really understand this, can we just review the design at 70kHz with the Ae = 1.206 cm2 core?

Np = (12*70000^-1*0.48)/(0.2*(1.206/(100*100))) = 3.41 (rounded to 4 turns)

Ns = 4*(55/12) = 18.333 (rounded to 19)

Thus the new 70kHz 55V + 55V output transformer should be as follows correct?

Np = 4:0:4
Ns = 19:0:19
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Old 21st December 2008, 06:54 PM   #12
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Awesome success today. Just changed the frequency after using the new equation and the SMPS is running like it should. Cool even with moderate loading and only draws about 200mA when idle. I suppose if I used regulation it would be even more efficient when idle. I'm using it at the moment to power a mosfet power amplifier and am very happy indeed :-D

I didn't even change the number of Np windings or the poor winding technique as highlighted previously. Next step will be to wind up a new transformer with the bifilar winding technique and also adjust Ns to get a higher Vout.

Thanks so much! I might have to start thinking about the offline version sooner than expected.
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Old 21st December 2008, 09:52 PM   #13
dtproff is offline dtproff  United States
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I come out with 4 turns on the primary and 18 for the secondary.

However if the voltage can drop to10.5V then you'll need 3 turns and 15 turns.
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Old 22nd December 2008, 02:42 AM   #14
Eva is offline Eva  Spain
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I think that the problem is related to anything but the amount of primary turns.

Could you post pictures of your prototype and transformer? Linear audio amplifiers are almost fool-proof but for SMPS layout matters.

ITL501 inductors seem to be made with gapped ferrite cores and thus are *completely* unsuitable for your purpose. The cores from the other ITL series inductors are not suitable either. Get a straight ferrite toroid core, there are several on-line sources. Most materials will do, for example, I use 3E25 which is intended for signal transformers and EMI filters rather than power transformers but the only penalty is one or two more turns (and a longer dead time is required to achieve Zero Voltage Switching resonant operation due to the higher permability and resulting inductance).
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Old 22nd December 2008, 05:01 AM   #15
dtproff is offline dtproff  United States
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I would alsp suggest that before you crank it to full power you carefully review the primary/secondary leakage since the high current you will have in the leakage will give you a very large leakage inductance spike at turn off.

You need that leakage to be as small as you can get it.

Tony
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Old 22nd December 2008, 10:06 AM   #16
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Hmmm - It's really working quite well seemingly with just a frequency change. Yes the output voltage is lower than hoped, but higher enough to run the amp to make some noise.

The SG3525 is just on a breadboard right now and the Mosfets (F25MN60N) are mounted on a small heatsink.

Sounds like you're very familiar with gapped cores. The ITL501 from Wilco doesn't appear to be gapped at all, at least there's no mention of it in the product information page unless it's implied by the other parameters that are given. The page I'm referring to is here:

http://www.wilcocorp.com/catalog/series.cfm?series=ITL

The page notes explicitly mention ITL501 being a ferrite core so I'm confused by your comments. I must be missing something I suppose, so thanks for your patience :-S

Here are some photos of the amateur setup.

Thanks for looking.
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Old 22nd December 2008, 10:08 AM   #17
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And another...
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Old 22nd December 2008, 11:12 AM   #18
Eva is offline Eva  Spain
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There is no way to get the inductance and saturation current values stated unless the ferrite core is gapped.

Your layout is completely wrong. There is not even an input filter on the 12V line. The input and output filters, MOSFET and transformer require a PCB. Never use point to point for SMPS.

Transformer winding layout is also completely wrong. It results in maximum leakage inductance that way.

There are several threads describing the optimum way to do these things, so you should do some research.
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Old 22nd December 2008, 02:12 PM   #19
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Ah that would be where my inexperience is letting me down. I'm not familiar with inductors parameters and gapped cores etc.

I suppose an unpainted core would make it obvious if there was a gap. Just had a scratch in my stash of stuff and have found some other ferrite cores to play with (photo attached)

A first PCB will be finalised now as I at least have something stable that doesn't threaten to catch fire or deafen me with it's shrieks and whines.

So how does the existing gapped core effect performance and / or design? What is leakage inductance? I'll Google it too.

It seems that even this gapped core is working quite well after correcting the switch frequency following use of the correct formulae. Is there something I can measure to see where this gap is letting me down?
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Old 22nd December 2008, 02:31 PM   #20
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Julian, you're doing great. It isn't possible to learn about SMPS without building some bad ones and blowing them up. Magnetics can be really confusing, as there's lots of contradictory info out there. Two things that have helped me are the old General Radio manuals for their inductance bridges (you can usually download these for free- try the GR Historical pages), and the book Switchmode Power Supply Handbook by Billings (McGraw Hill). It's sort of a loose collection of what might have been course notes or something, but after reading it a few times things begin to make sense. I also have the McLyman book, but it's quite expensive unless on sale, and I haven't had a chance to really read it.

Gaps are interesting. With a powdered iron core, the gap is distributed- it's the space between the iron particles. If you don't have a saturation problem, you don't need a gap, but with high perm ferrites you'll either need a huge core, or a small gap. Gapping is the key to minimizing core size and avoiding saturation. For DIY stuff, that's probably not super important. With commercial stuff it's common to get hundreds of watts out of a very small core.

If you have a signal generator and a scope, it's useful to wind up some test transformers and inductors and measure them, especially if you're working with surplus or unknown parts. The parameters will change (a lot) under power, but it will give you a feel for what you've got.

Another very useful reference, though not aimed at SMPS, are the various Terman electronic texts. F. Terman probably has the best and clearest explanations and formulas for transformers and inductors out there. IMO, everybody else just copied his original texts. Used bookstores or on-line search.
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