eddy currents in SMPS xfrm

[Adam. Seychell]

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 ?

 

[Winfield Hill]

Adam. Seychell wrote...

Reduce the number of turns by two, three or four times.


--
Thanks,
- Win

 

[James Meyer]

On 6 Mar 2005 15:49:34 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wroth:

Adam. Seychell wrote...

Reduce the number of turns by two, three or four times.

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

 

[John Woodgate]

I read in sci.electronics.design that Terry Given <my_name@ieee.org>
wrote (in <bIVWd.7513$1S4.795115@news.xtra.co.nz&gt about 'eddy currents
in SMPS xfrm', on Mon, 7 Mar 2005:

Proximity effect is what is killing you.

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

 

[Frithiof Andreas Jensen]

"Adam. Seychell" <invald@invalid.com> wrote in message
news:422b9146$0$1998$afc38c87@news.optusnet.com.au...

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.

 

[Terry Given]

John Woodgate wrote:

I read in sci.electronics.design that Terry Given <my_name@ieee.org
wrote (in <bIVWd.7513$1S4.795115@news.xtra.co.nz&gt about 'eddy currents
in SMPS xfrm', on Mon, 7 Mar 2005:

Proximity effect is what is killing you.


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

oops