Forgot the link, see figure 4 (but this is lower frequency material):
or figure 7 here:
This also illustrates the major impact that the voltage swing has.
-- Regards, Joerg http://www.analogconsultants.com/
how did you arrive at 1uH ?
sticking your numbers and a few guesses into the eq. in the datasheet I come up with 2uH to 8uH
-Lasse
-Lasse
25V at 100kHz is way too much for a 1uH inductor of the size you have.
-- Regards, Joerg http://www.analogconsultants.com/
With two coils in series (2uH) and 270kHz switching, my efficiency has gone up about 10%, to about 75%. I'm going to keep going in this direction.
And, all those coils I ordered are obsolete, unless I can really drive the frequency up.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Make sure to stop at 101%. Remember Tchernobyl ...
You should notice the diminishing core losses. Since cores are mostly black or almost black you can use one of those cheap Harborfreight contactless thermometers.
Eventually the switching losses in the FET are going to get you. Drive it hard, no gate resistors. Worry about that when EMC is to be dealt with.
-- Regards, Joerg http://www.analogconsultants.com/
...or a lot less, depending on the voltage swing across the inductor.
...though it does sound impractical at most useful pwm output voltages.
RL
V=L . di/dt
di = V .t /L
RL
All my concern about the gate resistors more or less went "poof!" when I realized that the coil was the hottest thing on the board. At this point the only part of the simulation I'm disbelieving is the coil efficiency.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
That's where simulations fail. Core losses are very difficult or almost impossible to truly simulate. Mostly you can't even get the material data needed as input. That stuff needs to be tested on the bench, like you are doing now.
There is a reason why I always have a tube of Aloe Vera ointment within immediate reach, on top of a big power supply, so I can turn that off and reach for the tube in one swoop. The CVS house brand is a pretty good deal.
-- Regards, Joerg http://www.analogconsultants.com/
We use tons of the Vishay IHLP and Toko FDSD, DFE, and DFEG powdered iron inductors in 2-3.5MHz SMPS. Their soft-saturation makes them a natural for DC-DC converters.
I did have a problem with these on a 4-phase 400W boost converter[*]. They were getting rather warm with he supply just idling. I switched to CoilCraft SER29s.
[*] The only time one melted off the board was when the bottom FET shorted. Oops!
You know, I can't remember a single one of my high-school guidance counselors or career-training class materials listing "burnt fingertips" as an occupational hazard of circuit design.
And yet, here we are.
-- Tim Wescott Wescott Design Services http://www.wescottdesign.com
Four coils in series, 270kHz, a hair over 78% efficient. Woo hoo! I'm going to throw pride to the winds and put a pot on the frequency pin of the 3757, then twiddle it for best efficiency.
-- www.wescottdesign.com
How much output current?
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Loss goes as B^2 but B goes as 1/f. In an ideal Steinmetz (eddy currents only) material, it's a wash, but it's usually to the advantage (loss going as f^-0.5 or so) in real materials.
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Now you are on the way to success. If you can, try something with over
10uH. Eventually the improvement peters out and then the main losses will be in the diode and the FET. That Harborfreight meter will usually tell, I'd get one. It wasn't expensive, maybe $10-20.-- Regards, Joerg http://www.analogconsultants.com/
A bit over 1A. It's feeding a 25 to 200V boost that needs to put out
100mA, and which is currently a bit under 80% efficient (my next job is seeing if I can improve that any).-- www.wescottdesign.com
Boost converter feeding boost converter? That has to be inefficient. A single flyback would be better, if you can get the transformer somewhere.
There must be a watts-per-pound-of-ferrite number. I did that sort of thing for 60 Hz transformers.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
It was Phil Hobbs's suggestion. I tried a flyback using off the shelf transformers, and got no joy whatsoever. I'm getting much better efficiency overall this way.
If I knew more about magnetic design I could maybe make a custom transformer -- but my customer was unhappy with having to source one on their current product, so for them at least off the shelf is the way to go.
-- Tim Wescott Control systems, embedded software and circuit design I'm looking for work! See my website if you're interested http://www.wescottdesign.com
t
Seems a bit nuts. Custom transformers are wound in small quantities in loca l winding shops. If you want to be paranoid, make sure that there are two m ore or less local shops that can wind satisfactory examples and source batc hes from them alternately.
Off the shelf transformers are obviously less work, but they tend to be spi ll-over's from somebody else's larger batches, so they aren't any more reli ably available in the long term.
This works fine as a long term strategy if you know what's going on - the m onitors on Cambridge Instruments electron microscopes in the 1980s were reg ular domestic TV tubes, made in batches of 100,000.
The catch was that the TV manufacturers never made the same batch twice - t hey were always fiddling with the shape and styling - and the monitor housi ng on the electron microscopes were made big enough and flexible enough to allow us to fit whatever we could buy. Messy, but much cheaper that trying buy exactly what we would have liked.
-- Bill Sloman, Sydney
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