No bias supply. My circuit uses the PDs in constant-current mode. That should be pretty quiet.
Hang a big cap on the output, of course.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Bill Sloman wrote in news:766e1a24-c8bb-4985- snipped-for-privacy@googlegroups.com:
No. Phil said the inter-winding cap is the cause. In that case, reducing it is the cure as the circuit is set in stone.
Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:
Sure it is for this and for other oscillator borne problems using tiny pot core transformers.
Read it. I said DO NOT USE factory gapped crap.
Polished face cores with a 1 mil tape on the face of one will gap the caroe perfectly, and will gap a thousand of them moree precisely than ANY factory provided punch press gap. Those cores are not faced at all. Trust me, I have seen hundreds of samples from many makers.
You are guessing at any of it at this point. I said to ADD a gap, on ungapped cores, not insure that there is not one on a gapped core.
Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:
The transformer operates the same. EXCEPT for the inter-winding capacitance, which the reduction of is the goal here. And this task definitely works for that.
Tiny ferrite pot core trnasfomrers?
And you were thinking about the wrong kind of factory-gapped crap.
I'd have to wonder where you got them. The gapped ferrite cores I've used from Philips and TDK have ground and polished mating faces. The central non-mating face is ground just as flat as the mating faces around the rim of the core.
It sounds as if you are talking about metal cores stacked up from lots of laminations, which wouldn't be much use at 2MHz.
You seem to be thinking about rather different transformer cores than the ones being discussed.
-- Bill Sloman, Sydney
But you never measured it.
-- Bill Sloman, Sydney
Bill Sloman wrote in news:a28fd65c-3cd1-4751- snipped-for-privacy@googlegroups.com:
Again... I suggested a polished face NON gapped core on both halves.
THEN, the transformer winder places a 1 mil tape on the face of ONE of the core halves.
And in this case ALL factory gapped cores are crap. ALL of them.
Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:
Then there is no gap. A gapped core has the center post of one core piece milled lower than the rim. Manual gapping is far more precise.
Yes, ground. Rough... ruff... ruff. Just as flat, but slightly lower in depth.
I have seen and used cores with mirror polished faces.
Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:
Sometimes, Bill, you just gotta stop guessing and go back and read what I posted.
Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:
He was discussing an inter-winding ringing, and *I* was the one who brought up pot cores. So, decidedly, the ones being discussed were the ones I mentioned.
You went off on some fantasy about what you think I was thinking about.
Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:
Wake up, Bill. Basic fact, more separation, less capacitance.
His goal is reducing that as then the ringing would subside.
But you don't seem to have read the data sheets. Here's one
Page three lists the gapped cores. The first entry in the table can be fles hed out
N48 material
The gap is roughly 0.23mm - set to get the inductance right.
The effective permeability (averaged over all the magnetic paths) is 134.
Part number is 134B65811+0250A048
Getting +/-3% on inductance is pretty precise. What have you got that does better?
-- Bill Sloman, Sydney
You posts aren't exactly detailed. And a "punch press gap" isn't the way gapped ferrite cores get gapped.
-- Bill Sloman, Sydney
That gives the minimum gap that you can get with tape.
The case doesn't seem to be the one that Phil Hobbs started this thread to talk about.
-- Bill Sloman, Sydney
Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:
Yes they are accurate... to a point. 1 mil transformer tape is an order of magnitude more accurate than being ground off after the pressing is done. Every time.
You focussing on the inductance value of a pot core, when the discussion is about negating a performance hit caused by a ringing effect. Gapping the core is not an attempt to gain or detract from the core's native inductance figures. Why you seem to think it even matters is funny. The transformer works the same in both instances right down to the change being so small as to be negligible. The gap helps stop the ringing.
You barking about getting a value to within three percent is funny too. There are rough pressed cores and the are fine grain cores with polished faces. The polished face cores are even more precise than your numbers. Gapping an ungapped core by applying a precision film to it is far more precise and when clamped together, the core center post and the core rim retains equal pressure on everything reducing any chance of a fracture od magnetorestrictive pressures being applied.
Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:
You know f*ck all about how they are made. Much less the MANY ways in which makers gap them when they offer gapped versions.
What do think is being set "accurately" by the 25 micron thick transformer tape?
You gap a pot core in order get a more precise - if smaller - inductance th an you can get with with an ungapped core. The gapped core saturates at hih ger ampere-turns products than an ungapped core.
Putting in a gap does give your more leakage inductance in each winding, wo uld make ringing worse, rather than better.
Gapping the core the dramatically reduces the inductance you can get out of the length of wire that you wind inside the core.
That's the single most obvious effect.
Why you seem to think it even
Would you like to explain how the gap "helps stop the ringing"?
That particular part had faces that looked polished to me - I didn't try to use them as mirrors.
The magnetic path through the RM8 core without a centre hole is 38mm.
With the N48 material, which has a permeability of 1550 time higher than fr ee air, this equates to 25 micron air gap, so the mating faces of an ungapp ed core pair have got to be flat to a micron or better. Your 1 mil transfor mer tape - 25.4 microns thick on the centre pole and the edges adds 50 micr on of air gap, and would cut Al value from 2900 nH per root turn (+30% -20% ) to about 1000nH (with a lot less uncertainty). The gapped version of that core with the smallest gap - 0.1mmm - is rated as 630nH +/-5% per root tur n which more or less fits - the total magnetic path length is 0.125mm (incl uding the path through the core).
The standard core clips compress the core rim and don't put any pressure on the centre post.
People have mounted pot cores with a central hole by putting a bolt through that central hole, but it isn't a good idea.
-- Bill Sloman, Sydney
Ferrite cores are sintered. This doesn't give very flat surfaces, so the mating faces are ground flat.
Perhaps you'd like to spell out a few of the MANY others ways used to the cores you think I should know about?
-- Bill Sloman, Sydney
Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:
Not if the causal element is inter-winding capacitance.
A slightly better saturation figure makes the zero crossing artifacts less, which is one of the ways ringing gets introduced. Reducing those artifacts helps or eliminates it.
If the thing is getting a pure sine wave drive, which I doubt, then it is a resonant charcateristic of the transformer itself.
Most get driven by a couple FETs.
Bill Sloman wrote in news: snipped-for-privacy@googlegroups.com:
Except we are talking about a transformer, not an inductor, and it does NOT affect transformer operation (efficiency)to any degree, but does affect the handling of the waveform it is transforming. Unless you make the gap too large.
Years of characterizing front ends of HV supplies makes me a bit more intimate with the cores than you are, apparently. Core gapping a transformer has desirable effects in some drive circuits. Gapping too much leads to losses which you are apparently stuck on thinking.
I am not talking about a large gap.
This yet another reason why transformers are so hard to model properly in simulation. Folks only want to include a few effects when more is going on that the sim can manage in some cases.
And don't even think about multi winding jobs or say a simple feedback winding.
One of the big fails in the sim realm, IMO.
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