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High Voltage Planar Ferrite transformers. ( Intial Experiments )

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Dr. Slack

Sun Sept 21 2014, 09:28PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Patrick wrote ...

OKmm I guess I'll look at 1 turn.

You can have the effect of 1.5 turns if you like, it's just that you'd need a core with two symmetric holes, and a total of 3 turns in 2 parallel windings.

Patrick

Sun Sept 21 2014, 09:54PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Dr. Slack wrote ...

Patrick wrote ...

OKmm I guess I'll look at 1 turn.

You can have the effect of 1.5 turns if you like, it's just that you'd need a core with two symmetric holes, and a total of 3 turns in 2 parallel windings.

I fail to understand, can you diagram this?

Dr. Slack

Sun Sept 21 2014, 10:12PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Too late at night, can't be ar$ed to fire up something to draw it with. Is the specification of 'symmetric' not enough of a hint?

An E core has a fat central limb, and two skinny outer limbs, so two symmetric holes. Put one real turn round each limb, and figure out what their flux linkage is. Alternatively, figure out what the ampere turns is. Then you'll see how to put the two outer limb turns in parallel, with the correct phasing, to give you the effect of half a turn on the central limb. If you haven't got an E core, or a binocular core, then you can't do it.

Ash Small

Sun Sept 21 2014, 11:32PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Patrick wrote ...

I fail to see why I can't use 3500 to 3800 guass?

I think you can, either with extra cooling, or with reduced load.........Losses will increase, though, what percentage efficiency are you after?

Uspring

Mon Sept 22 2014, 11:34AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
Location:
Posts: 711

You could get a symmetric primary by connecting the 3 turns at a short side of the winding and the center tap at the opposite side.

Sulaiman

Mon Sept 22 2014, 06:53PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

it would be unfortunate if you mess up on the turns thing,
I would rather use a higher/lower voltage/frequency than 'optimal'
and have an integer number of turns.
No need to over-stress a first prototype, (it will somehow over-stress itself)
get it working and go to Mk.2 ;)

Patrick

Mon Sept 22 2014, 07:22PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im laying out a new board to etch, to test several primary configurations. it will have a current limited pulse generator with oscilloscope probe points.

Sulaiman

Mon Sept 22 2014, 07:55PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

As far as I know;
for r.f. the transformer could be considered as two ferrite chokes, physically parallel, wired in series
so it would be two half-turns in series per full/integer turn.

at sensible smps frequencies,
you certainly can use 1/2 in turns ratios BUT
one side has one half of a full turn less than the other, 100:99 is negligible
1:2 means 1/2 of one core has to operate at 1/2 'capacity' so that the other half is within limits
the reduced loss in one half allows a little more dissipation in the other,
but you always lose some.

for efficient transformer action (esp. insulation)
the method of using parallel windings on the outside legs as 1/2 ratio transformers
works well but may be harder to insulate and is a slightly less efficient use of copper I think,
and ... rfi will be worse ... UAV + RFI = not good

Side note; a brand of motor drive used a c6VA 'mains' transformer EI with the 'mains' wiring as sense and one turn current-sense wire .. ok
a higher output current version had a '1/2 turn' current sense wire, presumably assuming doubling the transformer ratio ... not entirely ... flux shunting.

Patrick

Tue Sept 23 2014, 07:45AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

ill be starting the etch tomorrow. the test board will have several features and can be configured based on failures and successes. o-scope voltage diveders and triggers will also be used.

so ill try 1 turn first and see if we get the expected v/t out of the secondary.

Dr. Slack

Tue Sept 23 2014, 07:55AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Consider the following diagrams, they are the plan of an E core, showing the copper windings.

The top one is obviously a full turn, though you could consider each pass through a winding window as half a turn, which are then connected in series, to make a full turn.

The middle one is also two half turns in series. Follow the current path and you'll see the effect in the centre limb, Ampere.turns or volts.seconds, is exactly the same as the first diagram. The effect in the outer limbs is exactly the same as well. The same that is, if we assume the core has infinite permeability, and air has zero. That's a good first order approximation, but to the extent that it's only approximate, the second diagram does have a higher leakage inductance than the first to a coil wound on the central core.

The last one takes the two half turns and puts them in parallel. They now have half the voltage and twice the current at the terminals, so they are behaving like half a turn on the centre limb.

You may wonder why the top diagram has a crossover. It's because I drew the middle one first, and wanted to keep the terminal polarity consistent in all three, there's enough confusion to be had without adding to it unnecessarily.

Now if you use only one of these half turn coils, which is what most people talking about half a turn do, its leakage inductance to a coil on the central limb is huge, even if the core permeability is infinite.

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