[TowerTalk] Litz wire

Jim Lux jimlux at earthlink.net
Mon Mar 14 14:52:37 EST 2005

At 11:02 AM 3/14/2005, Tom Rauch wrote:
> > Ah yes.. but manufacturing technology has advanced quite a
>bit since the
> > days of Terman.  It's all well and good to assert that
>1000kc was the
> > practical limit in Terman's day.  However, I wonder (not
>too much, but just
> > curious) what today's practical limit might be.
>The nice thing about my job is I've always been paid to
>measure things like this, and manufacturers and vendors give
>me sample parts.
>As I've said, I've never found a case above 300kHz (the
>lower limit of my RF impedance test sets) where Litz wire
>has decreased impedance, ESR, or increased Q in the same
>form factor when replacing solid copper wire.     Never.
>I can't imagine stranded wire being better than solid wire
>for ground impedance when the diameter is the same, because
>it never is better for RF from the BC band on up. It is
>never better at DC either for a given OD.

All true, if you're OD limited.  But there are non-OD limited cases that 
are of substantial economic importance, so that must be driving the use and 
manufacture of litz wire (otherwise, they'd not be making it in industrial 

>As a matter of fact when I modified a KW Hustler mobile
>loading coil for 75M by rewinding it with the same diameter
>solid enameled wire instead of the Litz wire and make no
>other changes Q increases about 1.6-1.7 times.
>I've seen Litz wire used in small LF switching transformers,
>but never measured one A-B with enameled wire the same
>diameter for ESR. So I don't know where Litz wire improves
>things. I do know the DC resistance (for a given wire
>diameter) is higher with Litz wire and the 300kHz and higher
>resistance is higher in any application I've tested, but
>there might be a sweet spot in some applications.

There seems to be a fair amount of interest in it.
Here's a paper by someone proposing something better than Litz (or foil), 
but it shows that Litz wire is better than plain round copper.

Most of the application data for new PWM chips refer to the use of Litz 
wire in the magnetics.

These are all typically at several hundred kHz and higher.
A reference from TI talks about using Litz wire to maintain a reasonable 
Rac/Rdc, trading off the decreased Rac for the increased Rdc (due to less 
copper in a given cross section).  I think a lot of the design process  has 
to do with what the relative levels of ripple current and DC current are, 
which in turn has a lot to do with the dynamic range required of the power 

Everybody seems to cite the same reference (Unitrode Design Seminar SEM-400)

>My transformer book claims it only helps reduce eddy
>currents in the copper, nothing else. I tend to thing that
>is correct based on what I've measured.

In a transformer application, the inductance of the wire is immaterial, of 
course, since the permeability of the core will dominate. Then it would 
come down to a AC loss vs DC loss issue of eddy currents and skin effect, etc.

The lower inductance would be relevant for isolated conductors (not wound 
in a coil).. perhaps in a fast pulse discharge system (but there, you might 
be better off using a transmission line)

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