Adam. Seychell said:
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 ?
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
www.ti.com (somewhere....I have paper copies)
Cheers
Terry
