eddy currents in SMPS xfrm

On 6 Mar 2005 15:49:34 -0800, Winfield Hill wroth:

But keep the turns ratio the same?

I have a fuzzy recollection of transformer construction details where the fringe-ing magnetic field inside the winding area will cause copper losses. In most transformers, the field is almost totally confined to the core structure. But things like air gaps can result in parts of the field going places where it shouldn't and parts of the winding act just like a shorted turn.

Jim

I read in sci.electronics.design that Adam. Seychell wrote (in ) about 'eddy currents in SMPS xfrm', on Mon, 7 Mar 2005:

Did you measure the secondary output voltage? If it's over 200 V, the secondary is resonating with its own self-capacitance. Note that the actual resonance frequency may be below 200 kHz or above.

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Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
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I read in sci.electronics.design that Terry Given wrote (in ) about 'eddy currents in SMPS xfrm', on Mon, 7 Mar 2005:

The OP said 'unloaded transformer'. Your stuff is good, but appears to be irrelevant.

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

What would cause copper heating in an unloaded transformer constructed the following way.

core: EF20 ferrite (20 x 20mm E core) primary: 8 turns, of 0.3mm wire x 4 strands. secondary: 135 turns, of 0.2mm wire input: 12V 98% duty square wave 200kHz. topology: push pull

When I have only the primary winding the FET+transformer dissipation is around 300mW. As expected, the heating feels mostly from the core material and is acceptable. However when I add the secondary winding the transformer gets very hot as it dissipates a couple of watts. The power consumption rises with frequency, reaching 4W at 350kHz. There is no significant improvement between the order the primary and secondary windings are laid.

What exacly is causing this loss ? Is it the transformer's distributive capacitance of the secondary winding causing loading at high frequencies ? Do I need a bigger E core just to combat this effect , even though the specified power output will remain relativly small ?

Interwinding Capacitance in the Secondary!

Instead of winding it like you probably did:

^^^^^^^^^^^^^^^^^^! ^^^^^^^^^^^^^^^^^^!

You Need to do i like this:

^^^^^^^^^^^^^^^^^^

------------------------! !^^^^^^^^^^^^^^^^^^

All windings the same way.

The Alterneative is a stack of Disc Windings, series Connected. There are coil formers for that.

The real easy alternative is to get a CCFT Transformer, where everything is already done the right way. You do not say what "relatively small" exactly is. And what it is for - If you want DC output, things become easier.

8T x 4 strands * 0.3mm = 32*0.3mm = 9.6mm, should fit on one layer.

132 * 0.2mm = 26.4mm, at least 2 layers (probably three or four)

Proximity effect is what is killing you. At 350kHz, 20C the skin depth in Cu is d = 66mm/sqrt(350kHz) = 0.11mm. Because the primary winding is only one layer, proximity effect does bugger all, ie the effect of having Tcu >> d is negligible (no more than 12% greater power loss).

Very different story with the secondary though. Assuming the primary and secondary are *not* interleaved (primary then secondary, either order) then:

If dia = 0.2mm then area = pi*r^2 = 0.03mm^2. The equivalent rectangular wire is 0.18mm x 0.18mm, so h = 0.18mm. h/d = 0.18mm/0.11mm = 1.61.

[all formulae/charts use equivalent rectangular cross-sectional wire]

Looking at Snelling Fig. 11.14, assuming 4 layers then the ac-dc resistance ratio Fr = 10, so the *actual* AC resistance is 10 times higher than the DC resistance. So the losses will be correspondingly higher, likewise temperature rise.

(4W - 0.3W)/10 = 370mW, which is roughly the primary losses, and no doubt about what you expected. So you probably have 4 layers on the secondary. More layers makes this a *lot* worse.

OTOH if you interleave the windings (1/2 secondary, primary, 1/2 secondary) then there is a line of symmetry thru the centre of the primary winding, and we only need to consider one half of the windings - the primary effectively becomes 1/2 a layer (we still dont care,see above) and the secondary becomes 2 effective layers.

for 2 layers with h/d = 1.61 Fr = 3, so the AC resistance is 3 times the DC resistance. Simply splitting the secondary into 2 separate halves, with the primary in the middle, has reduced the secondary copper losses by a factor of *THREE*

Ideally, Fr = 1.33 for wire (1.5 for foil), so the optimal wire diameter is h = 1.33*d = 0.15mm, Acu = 0.0225mm^2, optimal diameter = 0.17mm

Or, if you dont want to split the layers, use much smaller wire for the secondary - 0.1mm dia wire has 4x the DC resistance of 0.2mm wire, *but* h = 0.008mm^2 so h = 0.088mm. Then h/d = 0.81, so Fr = 2. The DC resistance quadrupled, but the total resistance has gone down by

1-(4*2)/10 = 20% lower than in the original case.

You can clearly see that splitting the secondary into two halves is a

*LOT* more effective than simply reducing wire size.

I have seen transformers that catch fire because of this effect. One in particular had 12 layers of 0.6mm thick Cu foil for the primary, sandwiched between two halves of the secondary winding. It set the UL94V-0 bobbins on fire. I reduced the foil thickness from 0.6mm to

0.1mm (amidst hoots of laughter from the techs, who though I was an idiot). Fr before was about 100, afterwards it was about 1.5. So although the DC resistance got 6 times higher, the overall resistance was (6*1.5)/(1*100) = 11 times *lower* than before. The measured 400C temperature rise dropped to a nice cool 35C. And the techs stopped laughing.

A good reference is: "Soft Ferrites" E.C. Snelling, 2nd ed., Butterworths, ISBN 0-408-02760-6

another is: "Switchmode Power Supply Handbook" K. Billings, McGraw-Hill ISBN

0-07-005330-8

and the Unitrode magnetic design app notes, available free from

(somewhere....I have paper copies)

Cheers Terry

oops