LTC Switcher CAD3 ?

Wrong. Inductance is proportional to the square of the number of turns, as long as they are closely coupled.

-- Bill Sloman, Nijmegen

If L = alpha * N**2, then alpha has units of henries per turn squared. (Of course a turn isn't really a unit, just as a radian and a steradian aren't, but it sure isn't per square root turn.)

Cheers

Phil Hobbs

Don't be silly. Of course I test for basic stuff like saturation, and efficiency vs frequency vs load, and I thermal image everything to make sure nothing is unreasonably warm.

I'm not used to referring to volt-seconds as "flux". That is, to me, flux density x core cross-sectional area, still a magnetic thing. I'd call, what I think you mean, excitation in volt-seconds.

My breadboard has a 0.1 ohm resistor in series with the main transformer primary so I can see the excitation current. Added after the pic I posted.

If I design a driver, how could I not know the waveform I'm making? You can see the scope probe in my picture.

This part is all simple stuff; hours of work, not days. What's more interesting is the diffamp and the 12-bit, 250 MHz ADC just a few inches away from all these switchers. We're running just about 1 LSB RMS noise.

Well that's what it is. The SI defines flux in webers == V.s (a derived unit), which is B * A_e for a core of cross section A_e, give or take a factor of turns N.

Could've fooled me...

Well, I wouldn't care about the sample rate, as long as it, or the analog input bandwidth, includes the switching fundamental and maybe a couple harmonics. Once you get past 100 megs, you're going to see almost all the harmonics anyway. If ordinary noise abatement can't do it (only localized currents on the ground plane, heavy bypassing, ferrite beads, etc.), harmonics can be controlled (ala LT's AN70). I get the impression that you'd only go to quite that extreme if you need >16 bits accuracy; microvolts get tedious, but LT's good at it. Of course, if you needed 16 bits at 250MSa, you'll be paying serious money for it, and putting it on a red carpet at every turn.

I'd claim in return that we're getting a few LSBs accuracy with our similar (albeit slower) ADC inside an induction heater power supply, but since we're using it to measure the active waveforms, crosstalk is in phase and doesn't interfere with the measurements. We have lots of board area, so the meat and potatoes are comfortably on the opposite side of the board. Induction is kind of a weird business, not quite as glacial as power transmission, but top designs in the field are still the ones with boards measured in square feet, loaded with analog and discrete digital circuitry. The newer designs additionally have microcontrollers, and only a few have even touched FPGAs.

Tim

per

Applying dimensional analysis to a manufacturer's design fudge factor is the sort of thing that physicists do.

It's not a particularly useful activity. Reminding junior engineers at every possible opportunity that inductance is proportional to the square of the number of turns in a winding is an extremely useful activity, and saves loads of time at design reviews.

-- Bill Sloman, Nijmegen

Indeed. The common symbol is A_L. I call it "inductivity", with reference to a given core material and geometry. (It can be a fickle measure because, for example, the inductivity of a long rod core varies by 50% from center to end -- as Bill said, a useful definition depends on k --> 1, which depends on nearly-closed loop, high permeability cores.)

Theorem: it further stands to reason that permeability is actually in units of H / (m * t) (t = turn).

Proof:

A. For a continuous core of effective permeability mu, cross sectional core area A_e and average core path length l_e, A_L = mu * A_e / l_e Options for throwing around the factor of t^-2: 1. mu: -2, A_e: 0, l_e: 0 2. mu: -1, A_e: -1, l_e: 0 3. mu: -1, A_e: 0, l_e: 1 4. mu: 0, A_e: -1, l_e: 1 It doesn't really make sense to ascribe a factor of t^-2 for A_e or t^2 for l_e, so those options aren't listed. I would prefer to avoid 3 and 4, because it's less intuitive to count the path length multiple times (though equally valid).

B. The flux per turn you can apply to a core: Phi_pk / N = B_pk * A_e Options for throwing around the factor of t^-1: 1. B_pk: -1, A_e: 0 2. B_pk: 0, A_e: -1

C. Field strength vs. flux density: H = N*I / l_e B = mu * H H is in units of A / m, but engineering sources typically define it as A * t / m, so we have a good excuse to leave it alone. That means either: 1. B: -1, mu: 0 2. B: 0, mu: -1

C excludes A1, without making things uglier. B1 and C1 are inconsistent with A, when the l_e: 2 option is excluded. This leaves A2, B2 and C2. Thus, core area is also per turn.

Note: "Unit" given if the SI specifies a proper name.

Name | Symbol | Unit | Equation | Derived Units

-------------+---------+------+-----------------------+--------------- Inductance | L | H | L = A_L * N^2 | Wb / A Inductivity | A_L | - | A_L = mu * A_e / l_e | H / t^2 Permeability | mu | - | mu = mu_r * mu_0 | H / (m * t) MFD* | B | T | B = mu * H | Wb / m^2 MFI** | H | - | H = N*I / l_e | A * t / m Core Flux | Phi/N | - | Phi/N = B * A_e | Wb / t Flux | Phi | Wb | Phi = (Phi/N) * N | V * s Core Area | A_e | - | - | m^2 / t Core Length | l_e | m | - | m

• Magnetic Flux Density, or just Flux Density

Engineering units useful to switching design: Phi in uWb B in uWb / (mm)^2 H in A / mm mu_0 ~= 1.256 nH / mm A_e in (mm)^2 l_e in mm A_L in nH/t^2 Most manufacturers specify their core properties in mm or cm, which are easy to use or convert by eye.

Incidentally, notice inductance is ohm * s and capacitance is s / ohm. This can be handy when, e.g., tracking an algebra mistake in an unwieldy transfer function.

Tim

Whereas chemists get it wrong at the top of their lungs?

Bill, why is it so hard for you to say, "Oops, quite right."? It really wouldn't kill you, and you'd probably find that you got on better all round.

Cheers

Phil Hobbs

Well, what i have in mind is purposely overdriving driving saturable inductors in a PFN and fiddling with the parameters to see (a) how few stages one needs to get a reasonable effect, and (b) see how much of a speedup one can get on the rise time of a traveling pulse. Now, some ferrites have a rather nasty nonlinear permeability starting with almost flat, then goes up to a high hill and then drops off like a cliff; would be interesting if that "hill" is of use.

Then there is this other case where i have a transformer where the coupling is a bit frequency dependent, the mutual inductance ditto etc & etc with no specs as how it was wound (only Zin and Zout at 1KHz) or what the core was (prolly silicon steel thin laminations as it is for audio use). Plan to singe someones eyeteeth with that..

NOW we are getting somewhere! Thanks!

Engineers, too. My first engineering course at Tulane, first semister freshman year, was an introduction to units and dimensional analysis. It was taught by Dr Johnson, the Dean of the Engineering School, who thought it was important.

Random noise increases as the square root of bandwidth, so we specify it as volts per root Hz.

Inductance increases with turns squared, so we spec a core Al as henries per turn squared.

See the difference?

They were probably polite enough to laugh at you after you left the room.

EPCOS

ed.

an

I don't find it difficult at all, but I've been using the term Henrys per root turn for many years, and it has taken a while for the logic to penetrate my brain. I don't think straight in the wee small hours, when I should have gone to bed, rather than hanging around waiting to see if my current simulation is going in the right direction.

You are right, and the units ought to Henry per turn^-2 - possibly Henry per radian^-2. All we've got to do now is to submit your reasoning to the General Conference on Weights and Measures (CGPM, Conf=E9rence G=E9n=E9rale des Poids et Mesures). You'd need to go through your national representative.

The U.S. member of the CIPM is Dr. Willie E. May, Director of the NIST Chemical Science and Technology Laboratory. The current NIST delegate to the CCU is Dr. Ambler Thompson of the NIST Technology Services. Questions concerning the more fundamental aspects of the SI and subtle aspects of proper SI usage may be directed to him at the following address:

Dr. Ambler Thompson NIST, Bldg. 222, Rm. B243 100 Bureau Drive, Stop 2600 Gaithersburg, MD 20899-2601

Telephone: 301-975-2333 email: snipped-for-privacy@nist.gov

-- Bill Sloman, Nijmegen

PCOS

nH per

d.

It's 9:30am and my brain is working again, so yes - I do.

Perhaps. But they'd got the point - attention-getting devices don't have to be pedantically correct to work. Look at Howard Johnson on high speed signal propagation.

-- Bill Sloman, Nijmegen

Precisely my point.

Phil Hobbs and John Larkin both think that you are right, and so - after mature consideration - so do I.

Unfortunately, it was one of those thinks that I thought I knew that wasn't so.

Thanks for the correction, and my apologies for responding without thinking hard enough.

-- Bill Sloman, Nijmegen

per

Did you PWBC, post without benefit of coffee? I'm doing that now, and it's a great hazard.

He's as dangerous as Professor Moriarty.

(Incidentally, the latest "Sherlock" was stunning.)

e:

- EPCOS

ared.

dian

I don't know about dangerous, but pedagogically horrible. If you compare the lucid exposition of practical physics you get in Ralph Morrison's "Grounding and Shielding Techniques in Instrumentation"

with the incoherent messes that Howard Johnson peddles,you go from the sublime to the despicable.

You mean the BBC televison series? We've been watching it here - we get BBC 1 and 2 on the local cable - and liked it a lot.

-- Bill Sloman, Nijmegen

Contradicts what Phil, Tim, John, and I all seem to agree on.

Henry per turn^2. Lose the minus.

Being of a charitable disposition, I'll put it down to the lateness of the hour ;-)

Graciousness/gratitude duly noted :-)

EPCOS

Yup. It's painful to read, even in the parts where he's technically correct.

There are two running here now. The "classic" with Jeremy Brett, pretty good, and the modern-day one. It's the new one that I was referring to. I read all the Sherlocks several times when I was a kid, and revisit them now and then for a quick read.