Smps Ferrites, Frequency and Loss

I'm guessing you've already looked at Pressman pg. 242.

This may be of interest:

pdf

Take to investigate intellectual property rights if you pursue the above approach.

700 kHz not really at the bleeding edge these days, although you'd definitely be more comfortable an octave or 2 lower. You need to design for flux levels considerably below saturation, since core losses will tend to be the determining factor. In other words, the core ends up being larger than you'd predict by scaling up from a lower freq. Note, too, that core loss specs are usually stated in watts/volume unit, so, for a given flux level, a larger core dissipates more power. It makes an interesting design optimization exercise, particularly when you factor in skin and proximity effects. My experience has been that you're better off, unless you have to achieve very high power density, going down in frequency to the region where everything just fits in the available volume. Paul Mathews

Nope never saw that book.. However, I did found it in Google books.. Yup..that's the one. P.242. Fig. 6.18 Perhaps the most insane topology to pick when beginning to learn smps design...At my rate of understanding, the patents might expire by the time I figure out the magnetics. :P

One time I spotted (Epcos) loss specs in terms of kW/m^3. ..Who's using a cubic meter of core these days.. Why not Megawatts/cubic kilometer :P Geez... Expressed with mW/cm^3 is better when cores are way below 100 cubic centimeters.

D from BC British Columbia Canada.

Kw/m^3 = 1E6mW/ (1E2cm)^3 = mW/cm^3

RL

How about Terawatts/cubic kilometer..

1TW/km^3 = 1E15mW/1E15 cm^3 = mW/cm^3

D from BC British Columbia Canada.

No this doesn't work. To make it scientistic you need to make all the numbers in non-multiplied units. Thus I suggest that the only reasonable units would be Watts per cubic light second.

Just so long as you can recognize what you're looking at.

RL

Most can recognize TWatts Best regards, Spehro Pefhany

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Clever, LOL!

It might not be as bad as you think.

The isolation transformer in a Cuk has to be AC-coupled (input and output chokes are in series - switch shows up across the primary of the isolation transformer), and doesn't do any energy storage, You're working with total reversal around zero.

This gives you a double flux swing for the same loss, as loss is supposedly computed as being a product of peak flux, rather than pk-pk swing.

It's a formula for a smaller transformer than might immediately be expected, anyways.

RL

Better than being recognized as one.

RL

If Tera and Giga got married, what would the kids be called? I'm working on a punch line. :)

D from BC British Columbia Canada.

Yup..that's something I've been thinking about. The Cuk transformer is not stuck in one quadrant of the B-H characteristic.

For example Material F (Amidon sells it.)

On the core loss graph is has: "These curves are determined from AC data" Great!...That's what I want.. The cores have been tested by sine wave drive with no offset. (It's square wave in smps app so..some calc error.)

By the looks of it...I can get ~300mW/cm^3 if the AC flux density is at 300 gauss (coincidence). f= 700khz.

If I got this right... Using faradays law, this is around 13 turns on a 1cm^2 cross section. (Erms=4.44*B*Ac*N*f*10E-8)

But I don't know the math between loss density,toroid size and temp rise.

I'd hate to see how much damage thermal runaway to the Curie point can do.. :P

D from BC British Columbia Canada.

Peak density at f gives core loss in mw/cm^3 to be picked off of the mfr's core loss chart.

Temperature rise is 1 degree C per milliwatt per cm^2 surface area for the shape of transformer chosen. (+/- 20%). Don't forget copper loss.

Ferrites with high resistivity, used almost without exception for power conversion above 50KHz, also exhibit currie temperatures significantly in excess of most commodity safety isolation systems.

If you see a currie temperature below 150degC, you've picked a signal transformer or EMC (intentionally lossy) material.

RL

Neato...

1deg C __~___ /1mW / /__ __/ 1 cm^2

With 20% error.. :)

Yup... Was looking at wrong mixes too. I looked at Steward toroid cores (material 35 is available from Digikey). Curie temp 150 Resistivity 100 ohmcm But I think the big clue was "Common Mode Material" is written at the top of the datasheet :)

Not a choice material for power conversion :P

D from BC British Columbia Canada.

Maybe, but too many poorly educated twits would confuse the resulting numbers with zero.

Maybe their initials would be I.I., M.C., and D.C.