Subject: Re: Core gapping techniques and general SMPS magnetics stuff
Date: 16 Dec 2002 05:41:27 -0600
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On Mon, 16 Dec 2002 05:39:37 GMT, Chris Carlen
>John Woodgate wrote:
>>>But in a flyback which I will be using for the Nixie clock, there will
>>>be high peak flux densities associated with energy storage in the core,
>>>so I need to use a smaller Al, balanced against acceptable I2R losses,
>>>and winding size. It seems that if there weren't I2R losses, and size
>>>and EMI issues, an air core would actually be the best for a flyback.
>>>(A superconducting air core inductor!)
>> Well, if it's just a single winding, maybe. But a transformer with an
>> air core would have far too much leakage inductance.
>Certainly. But I have been thinking of a simple inductor here. I am
>still deliberating about whether to use a transformer or simply an
>inductor. I don't need isolation, and the maximum voltage of 300-350V
>is easily tolerable by typical MOSFETs. So, it's pretty much a crap
>shoot. If you can tell me some things about why I might choose a
>transformer vs. simple inductor for a flyback that doesn't need
>isolation, and will be designed to output about 90-350V at up to about
>12W, I'd be very interested in hearing some reasoning processes.
I didn't see anything about your input voltage, but assuming that
it's relatively low, like 12 volts, one reason to use a transformer
rather than a single inductor is that for large step up ratios the
current pulses during the off time of your switch get very large at
the small pulse widths when you use a single inductor. With a
transformer you can get some of the step up from the turns ratio and a
more favorable duty cycle; the output current pulse magnitude is
proportionately smaller. But, at low power levels, this is quite
doable even with a single inductor.
>>>So I will have to gap my cores. How do you folks gap E-cores for
>>>prototype/low volume applications? Obviously it's not that hard to
>>>figure out a way, but I'm curious how others do it too. I'll probably
>>>cut out some shim material, epoxy it to the outer legs, and hold the
>>>halves together gently in a vice until it all sets up. Then I can soak
>>>it in acetone later for a day if I want to disassemble it.
>> That's how you do it physically, but how do you work out what gap
>> thickness you want? It matters A LOT! The traditional method was to use
>> 'Hanna curves', but I don't think I've ever seen a Hanna curve for a
>The Ferroxcube catalog lists various gap sizes and resulting Al values.
> There are also charts that I haven't quite finished figuring out how
>to use to do it graphically. But it doesn't seem that difficult. I can
>do it a bit by trial and error. Just stick some space in their until I
>get the L I want, for the number of turns I want.
>Thanks for the input,