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Old 28th June 2002, 01:17 AM   #1
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Where can I get small ferrite toroidal cores for switching transformers? I'm looking for something that will work for up to a 350W switching supply.
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Old 28th June 2002, 01:29 AM   #2
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The most important question to be asked is "What Frequency?" This will determine what type of material is needed. If you are going to be running at 15-150kHz ferrites are the best types to use. However, there are many different types of ferrites to choose from. Magnetics is a manufacturer of excellent "soft" magnetic materials. www.mag-inc.com. If you cannot get small quantitites from them directly, then a distributor is Allstar magnetics. You also have to make sure that you minimize your core losses and balance them with your copper losses or the transformer will not give good performance. For a DC-DC converter make sure also that some sort of current-mode control is used to prevent the B-H curve of the transformer from going into the saturation region. If this happens you WILL loose you FET switches. GOOD LUCK!!!! I can provide guidance in this area if you so desire....

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Old 28th June 2002, 02:22 AM   #3
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I will be running about 50kHz. It's for a DC-DC converter for a car amp. It's from ESP (where else? ). He gives a winding procedure for the transformer. He does not, however, say anything about layout, and he doesn't mention current-mode control and balancing of losses, so I will need some assisatnce in these areas.

Thank You.

P.S. When I tried your link, it said "You are not authorized to view this page"
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Old 28th June 2002, 02:28 AM   #4
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For the uninformed, lower frequency gives higher efficiency buy higher frequency gives higher current density.
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Old 28th June 2002, 04:09 AM   #5
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Of course if you don't get EMC compliance you will have the law after you...

Have a look at ETD43 or ETD49 size bobbins and cores. They're the '3055 of magnetics. Also 3C85 or 3C90 core material for 50 to 100Khz should be ok. Try and use schottky diodes for rectifiers if the voltage is not too high.

GP.
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Old 28th June 2002, 06:22 AM   #6
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Are you talking about toroids with the EM emitions? I thought toroids minimized that, and many, if not most car amps use them. I certainly couldn't use anything that emits EM noise, it will be within an inch of an audio amp.
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Old 28th June 2002, 08:59 AM   #7
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SMPS are NOISY! quite a bit of the overall design effort of a SMPS often goes on measures to get the EMI/EMF under control.
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Old 28th June 2002, 12:51 PM   #8
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Default AudioFreak is dead right.

Toroids are no quieter than any other and they are harder to wind. I make a living working as an engineer's dogsbody in a SMPS design lab all day every day. Believe me, SMPS's are NOISY .
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Old 28th June 2002, 02:51 PM   #9
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SMPS are extremely noisy no matter what type of core you use. The noise will be much higher with an ETD style core because of the radiation which appears between the two halves. A toroid is commonly used because of its lower EMI but also because it doesn't require any additional hardware to mount (bobbin, etc). The weapon against EMI is containment. By enclosing the entire system inside a completely conductive chassis, it minimizes the magnitude of radiated energy. However, keep in mind that as the propogating E-Field hits the conductive chassis, it becomes a surface current and must have a low impedance path to a reference ground. In car audio, the small amount of noise which may be injected into the audio amplifier circuitry is acceptable only because the automobile is the noisiest audio enviornment possible....electrically and acoustically.

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Old 28th June 2002, 05:27 PM   #10
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Use extensive filtering on the power line going into the amp. Otherwise the power line outside the amp will act as an antenna and radiate all around.

That and the low impedance chassis are probably the best ways to keep the EM radiation down.

/Marcus
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