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Old 1st Apr 2020, 7:27 am   #21
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Default Re: 'Ferrite Pot Core Pairs'?

The control chip turns on the transistor and current starts to flow in the 10t winding.

Due to its inductance, the current builds as a linear ramp, with a slope V/L amps/second. There is energy stored in the magnetic field created = half*L*(Isquared)

At some time, the control chip turns the transistor off. The inductor won't let the current collapse instantly and it creates a voltage spike to make the current flow somewhere as the field tries to collapse. In a plain boost converter the somewhere would be through a diode to charge a reservoir capacitor. In this case, like a TV set line output transformer, there is an overwind, an extra 80 turns. So the output transient is 9 times (there's still the 10 turn winding in addition to the 80 turns) bigger.

These circuits will make very high output voltages, limited only by losses or things breaking down.

The control chip senses the output voltage, and once it gets high enough, it reduces the amount of time the transistor will be turned on for in the next cycle. This reduces the amount of energy stored in the inductor (think of an inductor with an added winding, don't think of a transformer) and so the rate of charging the output reservoir diminishes. If you take current from the output, the controller will servo-control the on-time to maintain a regulated output.

One rectifier is all you need. But it's important. It also acts to clamp the transient voltage, thus protecting the switch transistor from being destroyed. It really really has to be a very fast diode. Also the inductor has to have quite good inductive coupling between the two windings because it relies on this for the diode to effectively clamp the transient on the transistor.

So that inductor is quite critical. Winding the right number of turns on any old bit of crunchy grey stuff won't work. You need the right amount of inductance else the current won't climp enough in the max on time the controller will give.

The gap dominates the setting of the inductance. As Ian reports, it's critical.

Alternative inductors could be designed around a variety of cores, but the key word is designed. You need to hit the right inductance. You need to design for a high enough saturation current, you need to design for enough peak energy storage capacity. It comes down to reading up on magnetics design and spending a few hours with several catalogues and a scientific calculator.

David
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Old 1st Apr 2020, 11:33 am   #22
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Default Re: 'Ferrite Pot Core Pairs'?

Much appreciated, David. I was reading this article on flyback converters, which sounds very much like what you describe, and looks like the same circuit.

He includes calculations which I will try to apply to the reformer...

It's clear now why that diode is specified as a fast switching one, and the inductance is critical!
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Old 1st Apr 2020, 3:09 pm   #23
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Default Re: 'Ferrite Pot Core Pairs'?

Yes, that gives you the basics.

The Boost, Flyback, and SEPIC converters are all essentially the same thing.

(The SEPIC ties you to a 1:1 winding ratio and allows you to add some capacitive coupling. It makes rectifier snubbing more effective and it allows you to convert to voltages from below to above the input voltage. The flyback converter as shown in that article has an isolated secondary, and so can do mains isolation if an isolated feedback is arranged. The boost converter has an issue that there is a copper path through the inductor and diode to the output so if the input voltage goes above the output voltage straight conduction happens and the output is forced up. For the same reason the output can't be turned off or current limited.)

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Old 3rd Apr 2020, 11:04 pm   #24
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Default Re: 'Ferrite Pot Core Pairs'?

One of the many things I love about this forum is that it often raises questions I didn't know I needed to ask!

This thread raises issues that have intrigued me for a while. Reading post No 18 by David (RW), my head actually exploded! It really did

My enquiry specifically concerns TV line output transformers with ferrite cores. Usually two 'U' section pieces.

1) What exactly is ferrite? When tested with a megger, it seems to have very low electrical conductivity.
2) Presumably, it comes in different types with different magnetic properties?
3) There is usually a very thin piece of something stuck on the ends of the 'U'. I assume this provides a 'gap'.
4) In basic terms, what does the gap do? Is the size of the gap critical? What would be the effect of no gapping piece at all?

If someone could give me a basic overview of ferrites, I would really appreciate it!

Apologies for hijacking the thread and asking my own unrelated questions!

Many thanks
Nick
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Old 4th Apr 2020, 12:04 am   #25
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Default Re: 'Ferrite Pot Core Pairs'?

1100 man - I also had some of your questions. Take with a pinch of salt as from this thread you can see I am all at sea, but I'm swimming towards understanding and in the absence of other replies:

1) Ferrite is a mix of iron oxide dust and various other things including ceramics, so that they are electrically non-conductive but capable of magnetic interaction. Wikipedia has an article I found quite informative.

2) See 1.

3) Yes.

4) In Radio Wrangler's post #18, you can see that the gap permeability is 1 (I suppose 'free space' compared to his assumed minimum permeability of 1200 for the ferrite, because the free space doesn't have any magnetic properties compared to the ferrite. So the gap is important in adjusting how the ferrite is coupled.

Wikipedia also has an article on flyback transformers for TV CRTs
, and their specificities. It took me a while to find the nomenclature, but now I understand the circuit I'm concerned with is a flyback converter, which shares elements with the TV circuit, but in this case uses an autotransformer.

I hope that gives you some leads before the gurus come back!
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Old 4th Apr 2020, 12:43 am   #26
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Default Re: 'Ferrite Pot Core Pairs'?

Ooo-Errr....That's me put on the spot!

1) Ferrite is a sintered powder material, rather like a ceramic made of a mixture of oxides and other compounds. Iron, manganese, zinc and nickel feature.
The finished material can be thought of as a magnetic conductor. Some are electrical conductors, but not very good ones, others are insulators. This lack of conductivity prevents or reduces eddy currents and hence eddy current losses, particularly at high frequencies. So you find them in SMPS transformers, magnetic scanning yokes, RF transformers. They aren't as good a magnetic conductor as iron, which means inductance factors are too low for 50Hz and audio transformer use - that is the domain of iron and steels, whose losses become undesirable at higher frequencies unless fine iron powder is used.

2) different ratios of the base ingredients get milled together and sintered to make a range of ferrites, some good at microwave frequencies, some OK at 10kHz. Some are intentionally lossy and used to absorb unwanted interference. There is a wide enough range of properties that just talking of 'a ferrite' is along the lines of 'a resistor'

3&4) Yes, gaps are interesting and very important. Not all ferrite transformers have a gap, but a gap is needed if you want tighter control of the inductance or want to extend the saturation current.

Let's play it backwards. In most gapped cores, the effect of the gap dominates the setting of the inductance, so let's say that the gap is the real business of the core, and that the ferrite merely serves as a much better mag conductor than the gap to connect one end of the gap to the other! and to collect the magnetomotive force from the coil. Sounds silly, but it's true.

Without a gap, the magnetic circuit is almost a short circuit. Not much magneto motive force (=Ampere turns) will drive a lot of flux circulating in the core. You get a lot of inductance, but it soon saturates at low currents in the coil because the flux soon gets too big for the material. Worse, the inductance isn't terribly well defined and will vary from batch to batch and will show large-ish temperature variation. Fine in uses that don't care about precision or power handling, but it is a limitation.

So there you have it. It's the gap that does the work, that makes the inductor, that defines the inductance. The rest of the core just routes the mag flux to the faces of the gap.

My head exploded years ago, and I'm still looking for some of the pieces

David
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Old 5th Apr 2020, 10:47 am   #27
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Default Re: 'Ferrite Pot Core Pairs'?

Hi David,
Thank you for taking the trouble to explain that in such understandable terms!
The articles linked to by UB were also useful, so thanks for that.

Of course, as my understanding increases, I can feel some more questions brewing which I will ask in due course!

All the best
Nick
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Old 5th Apr 2020, 3:02 pm   #28
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Default Re: 'Ferrite Pot Core Pairs'?

The usual text on ferrites is "Soft Ferrites' by a chap called Snelling, published by Illiffe. It's fairly heavy going not recommended reading for light interest, but it's where to go if things get serious.

A librarian in our lab was asked for a copy of Snelling, Soft Ferrites. She misheard it as Smelling soft ferrites. I wonder what she thought someone was up to?

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Old 11th Apr 2020, 12:59 am   #29
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Default Re: 'Ferrite Pot Core Pairs'?

I imagine an awful lot of things are 'got up to' in electronics labs, David! I'm reminded of the darkroom in 'Gregory's Girl'... (I'm assuming that's an obvious reference for someone living in Fife - if not, an excellent film, and available to rent on BFI Player if you've lost the DVD as I did the other night).

Are you able to proffer the crumbs of a hint on these calculations? I'm confused as I'm trying to work out the inductance of the original circuit to backtrack to how I might make an alternative transformer. The diagram just says 'HT', and the datasheets I've been looking at for the IRF540 don't give values for L.

I know that the circuit should offer 630V output maximum, but how does that relate to the turns ratio? None that I calculate have any relation between 11.4V and 630V. Following the flyback converter page I referenced earlier, he starts with V/L = di/dt but even assuming 20mA maximum I haven't worked out how to obtain a switching time, or relate the current, inductance and voltage into something manageable.

A push in the right direction would be much appreciated.
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Old 11th Apr 2020, 1:25 am   #30
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Default Re: 'Ferrite Pot Core Pairs'?

OK, you can actually get away with a lot here.

Turns ratio.....

If I take an inductor, get some current going in it an then instantly break the current, it will create a huge voltage transient. If I had a second winding with ten times as many turns o it, I'd see a ten times huge transient on it. But huge is already enough! so why bother with the step up winding? It doesn't seem needed?

The step-up winding fixes one limitation of the simple boost converter. The simple boost converter can't make an output voltage bigger than the breakdown voltage of the switching transistor allows. If you have 10:1 ratio (purely a number pulled out of the air for illustration purposes and not out of any consideration of your circuit) Then you can use an FET with lower voltage rating than the output volts would suggest.

There will be leakage inductance transients as well, so design in some margin. You want 630v output, can't remember what that MOSFET type is, so say it's good to X volts. We ought to have some design margin here, so let's run it to no more than half of its rating X/2 volts. So the ratio in the treansformer becomes 630/(X/2) = 1260/X

How much inductance depends on how much power you want to pass and the frequency you want to run at. If you give some figures, a primary inductance can be calculated.

By the way, Linear Technology inc. now part of analog devices inc. did some excellent applications notes written in english by the late, great Jim Williams. A sad loss when Jim died, and Bob Pease the mad scientist at National Semiconductor died on his way home from Jim's funeral when his Beetle went off the road and down a ravine. Look up these characters, they deserve remembering.

David
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Old 11th Apr 2020, 2:09 am   #31
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Default Re: 'Ferrite Pot Core Pairs'?

Hi!

. . .And here it is, for anybody wanting serious brain and headache!

https://archive.org/details/SNELLING...FERRITES__1969

Chris Williams

PS!

There's a very useful table in the back giving all the different grade designation codes for all the various ferrite materials on the go – might be worth printing it out, and Keith Billing's "Switch Mode Power Supply Handbook" (downloadable from various "edu" domains!) has useful magnetics design data in it as well!
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Old 11th Apr 2020, 7:35 am   #32
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Default Re: 'Ferrite Pot Core Pairs'?

Amongst junior engineers the fear factor created by seeing 'Snelling' is roughly 12.5% of the standard level (set by Zverev = 100%). I hope it hasn't scared anyone off.

12.5% is still a lot of terror and it takes a fair bit of desperation for someone to go in far enough to realise that it's an excellent resource.

I found the comparison of different commercial materials to be very helpful.

David
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Old 15th Apr 2020, 12:03 am   #33
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Default Re: 'Ferrite Pot Core Pairs'?

I've been doing some steaming brain this evening. Thank you for that link, Chris, though I'm afraid I haven't dared the 12.5%T of Snelling! I'm still struggling with applying Ohm's Law here.

Back to basics:

If I want 630V out, with an IRF540N with a Drain-Source breakdown voltage of 100V (David's 'X', I believe), then the calculation becomes 1260/100=12.6. If that is the turns ratio, the circuit with its 1:8 ratio has a slightly smaller design margin.

So, from that Simon Bramble site, if V/L = di/dt then am I on the right track if: di is the current delta, and dt the time delta, I can obtain a value for the inductance L by V/(di/dt).

When the IRF540N switches on, the 11.4V is dropped across the 10T winding and the 0.27R resistor to ground through the MOSFET. If that is all across the resistor I get a current of 42A.

The Vishay datasheet I found has a 'turn on delay' a 'rise time' and a 'turn off delay'. I thought these three added together would give an 'on' time of about 1µs.

11.4/(42A/1µs) = 2.7x10-7 which I take to mean an inductance of 270nH/turn for the ten turn winding.

Now, that's not going to be right because the current won't rise to 42A in 1µs, but is this the right approach?

While writing this up I've been considering the 'energy in = energy out', so perhaps I should be looking at the current in the voltage divider to get some figures for the secondary side.

Is the 1.25V reference the voltage applied to the lower side of the voltage divider? If so, what is the 4.7V Zener doing there? Is that to breakdown to save the IC?

At the 630V setting, the lower arm is 588R, which gives 2mA at 1.25V. This current implies 690V across the 300K upper arm, plus a midrange value of 25K for the 50K VR1. This sounds like the right area for the assumptions made.

P.S. David, I hadn't heard of Pease and Williams, though I did read a lot about Bob Widlar after seeing a reference on this forum. I had no idea IC design had so many characters!
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Old 15th Apr 2020, 1:15 am   #34
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Default Re: 'Ferrite Pot Core Pairs'?

The di/dt referred to in the voltage calculation will be confusing. You need to use it twice.

Use the supply voltage and the inductance and V=-Ldi/dt will give you the rate of increase of the inductor current when the switch transistor is on and your power supply is charging up the flux in the inductor.

Use it again AS the transistor switches off, and it gives you the size of the voltage transient generated versus the rate the transistor turns off in amperes per second.

Your sums are missing any info on the timing of the switching.

Let's pick a frequency (no idea what the actual circuit runs at, so it needs looking up) say 20kHz. That gives a period of 50us
Let's say the longest on period we want with the circuit going flat out is 80% of that so the pulse is 0.8x50 =40us.

Another missing parameter is how much power do we want to make at the output. We have to guess. This is an important parameter. it's what decides whether your inductor fits in your pocket or fits in an articulated lorry. Guess 2 watts.

I've forgotten what the supply voltage was in the circuit. so another guess let's say 10v Now to transport 2 watts at 20kHz i need to move 2/20000 Joules on every cycle = 100uJ but my converter isn't going to be perfectly efficient. say 80% so my input needs to be 100/0.8 = 125uJ at the peak charging current. The energy in the inductor will be 0.5xLx(I squared) = 125uJ

Another way of looking at it is that we want 2W out, it's 80% efficient so the input will take 2.5W.

2.5W tells me that the average current from a 10v power supply will be 250mA.

Now the transistor at max welly will be off for 20% of the time to allow time for the inductor to discharge into the output circuit. So the average current of our charging ramp needs to be 250/0.8 mA = 312.5mA

Now this is the average of a linear ramp, so the bottom of the ramp is zero, and the tom must be twice the average 625mA

and I have to reach 625mA in 40us so my charging ramp is 0.625/40u amps per second = 15,625 amps/second

Sounds terrifying?

The inductance value saves us V=-Ldi/dt

V=10v I chose arbitrarily
di/dt = 15625

So L needs to be 10/15625 = 640uH for the primary winding

If your power supply voltage is lower, you'll need proportionally less L, but it'll have to do proportionally more current without saturating
If you run at a higher frequency, you'll need proportionally less L

If you know the Al value of your core, that is the number of nH you get for a 1-turn winding. Inductance increases proportionally to turns squared.

If your core has Al = 630 nH and you want 640,000 nH then turns squared equals 1015. taking the square root says the primary needs 31.8728 turns. Forget partial turns and call it 32 turns

His transformer was a 1:8 step up, was it? now the primary is in there as well, so it's an autotransformer so the secondary needs 32 x7 turns because we count the primary turns as adding to the secondary so 224 turns.

We can wind the secondary in finer wire than the primary. It's doing less current.

You should be able to redo these calculations using your own numbers, but it shows you the process.

If I was doing a formal design, I'd check datasheets and calculate the peak energy a given gapped core could hold to pick the core size for best economy of space and money, then I'd check the necessary turns of appropriate size wire would fit, but for this, anything you can get the wire in ougt to be OK.

Sack time

David
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Old 15th Apr 2020, 7:59 pm   #35
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Default Re: 'Ferrite Pot Core Pairs'?

Thank you, David! I realise having looked at the datasheet of the 34063 that the 1nF capacitor on pin 3 is what sets the timing, rather than some characteristic of the switching MOSFET. It suggests 33kHz is the typical oscillation for this value.

If I work through, then I have the frequency, ramping time etc. and can then obtain the relevant data from your calculations.

Watch this space...
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Old 15th Apr 2020, 8:34 pm   #36
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Default Re: 'Ferrite Pot Core Pairs'?

One gotcha, you'll need a really fast rectifier diode on the output. 1N400x won't have got out of bed before it's time to turn off again. They might overheat, but the circuit relies on the rectifier to clamp the output of the transformer at the output voltage, and through the magic of turns ratio to clamp the MOSFET drain transient. So a poor rectifier will dispatch the MOSFET PDQ to the afterlife.

33kHz, so a period of 30us and 80% of that is 24us so you want to hit 625mA in 24us with say 10v driving (12v supply and some drop in the circuit/switch) 384uH

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Old 17th Apr 2020, 12:11 am   #37
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Default Re: 'Ferrite Pot Core Pairs'?

I'll take your word for the 384µH! I've covered a few pages with head-scratching references to sigma l/A and the interrelationship between µe and AL, without much progress.

I've found some data crumbs on the cores posted earlier by MrBungle, and the most likely suspect seems to be the FX2241. It's close to the diameter of the Jaycar part (which is a clone of the FX2240), is grade A13 which is the same as the FX2240's A5, and has an AL of 5815. The FX3440 would be even better, as Mr Snelling says the A8 material is good for pulse transformers up to 200MHz. However, it's got a diameter of 21mm but an AL of 6450...

I've been looking over your calculations, David, and wondered if there was a way of working back from the existing circuit to do away with the assumptions. Not that I don't believe in your experience, but for the sake of getting to the bottom of the theory I'm interested to know if these values are deducible from what we know about the circuit as drawn, or if this is a fool's errand.

I've made a ham-fisted attempt to link up the input and output currents with the magnetic properties of the existing core, assuming it's an FX2240 and using the values from John BS's useful datasheet, but I'm not convinced by my own work!

In this calculation:

Quote:
Originally Posted by Radio Wrangler View Post
You'll need to read up on magnetics design to see what this does but a quick estimate is an Al value of 6000*(36.6/(36.6+0.12*1200))
what is the algebraic form? AL[guessed mid-range?]*(le/(le+gap*µe)? The gap is 0.12*1 because 4pi x10^-7 * 1 x10^6 is approximately 1?

I have been reading about the gapping and how that affects the permeability, but if a datasheet is not available, how much of the required values can be deduced from the rest of the circuit, and how much has to be measured with the inductor at hand to test?

For example, I know the AL for the FX2241, but not the Ae or le, or the µe (or µr). Calculating the permeability appears to be possible, but not without the former! The theoretical required gap spacing then seems to be out of my reach with the data available.

Keen to learn, but it's tough on the brain!
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Old 17th Apr 2020, 12:47 am   #38
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I tried to edit, but too late. I've just found this interesting site on gapping cores. and the accompanying equation μe = μr . le / (le +(g .μr))
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Old 17th Apr 2020, 12:54 am   #39
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Default Re: 'Ferrite Pot Core Pairs'?

You can take a big short cut. The cores you mentioned have Al values of 5815 and 6450. Those are very big and indicate those particular cores are gapless. If they're in the same material and about the same size as the cores in the magazine design, just replicate his gapping shim and you should be close enough.

It's the gap that dominates the behaviour.

David
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Old 17th Apr 2020, 12:57 am   #40
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Default Re: 'Ferrite Pot Core Pairs'?

As long as they come with gaps! I've not found definitive evidence that they are gapped pairs, but perhaps there isn't another type.

I'm just interested in delving into another area of the hobby I've had no idea about. However, I really should have kept up the maths...I can hardly believe I have an A-level in it when I have to look up 'order of operations'!

I've sent MrBungle a PM about those cores...
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