Re: Smps Toroids on a Ground Plane

Paul Mathews wrote:

On Apr 19, 10:28 am, D from BC <myrealaddr...@xxxxxxxxx> wrote:

On 19 Apr 2007 06:45:48 -0700, Paul Mathews <o...@xxxxxxxxxxx> wrote:






On Apr 18, 1:59 pm, D from BC <myrealaddr...@xxxxxxxxx> wrote:

On 18 Apr 2007 13:04:50 -0700, Paul Mathews <o...@xxxxxxxxxxx> wrote:

On Apr 18, 12:40 pm, D from BC <myrealaddr...@xxxxxxxxx> wrote:

On Wed, 18 Apr 2007 20:03:45 +0100, John Devereux

<jdREM...@xxxxxxxxxxxxxxxxxx> wrote:

D from BC <myrealaddr...@xxxxxxxxx> writes:

I recall somewhere an author recommended that the inductors be placed
on a ground plane..

That's probably nice for little inductors..Like buttons.
But my toroid is 1/2" tall and about 1.5" wide..

I don't believe putting this toroid flat down on a ground plane is
going to help reduce EMI emissions. Correct?
All the copper windings are too far from the ground plane..

Would it help to Faraday shield the toroid to reduce EMI... ?
(I'm trying to avoid killing my neighbors AM radio reception.)
Could I blanket my toroid with some foil and ground the foil?
Or don't bother...?

Toroid Conditions
f=100khz
I=2A average, 200mA ripple
V=170V peak, 0.4 duty square wave (hard switching)

This toroid is in a earth grounded metal box. The smps power ground is
not connect to earth ground. The toroid is part of an offline
unisolated smps..If the two grounds connect.. .Poof!!!
So ..I'm guessing a faraday shield will be connected to power ground.
That would make the shield "hot" relative to earth ground. Shocking to
touch when live but safe in a closed earth grounded box.
D from BC

In my experience toroids don't emit much anyway; their flux is
contained in the core. Have you any reason to think otherwise? I would
pay more attention to the PCB layout, e.g. minimising the area of high
dI/dT current loops. Also make sure input and output wires are
filtered.

Yes..I know toroids are good at keeping in a magnetic fields but
there's an electric field too ..
When high switching voltages exist across a toroid..can the electric
field be a problem...?
I don't know much about electric field interference.
D from BC- Hide quoted text -

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Yes, indeed. Most inductors in switching circuits get connected to a
high dv/dt node at one end and a lower dv/dt node at the other.
Placement of the high dv/dt pin(s) should be away from potential EMI
and cross-talk victims. Many inductors can also be placed in either of
2 orientations on the PCB (due to pin symmetry), and sometimes it
makes a big difference which placement you happen to use. This has to
do with the way that the 1st turn on one end comes from inside the
core and vice versa, which means that the highest dv/dt locus moves a
bit. There is also a extra 'turn' on most toroids that generates field
that is not in the core at all. This is the turn that the current
takes as the coil turns progress around the core. We'll probably have
Mr. Sloman chime in on this fine point. A few standard toroid
inductors are wound with a crossing turn halfway around, so that this
effect is mostly cancelled. You can also have inductors wound with 2
windings and accomplish the crossover turn on your PCB. In any case,
the ground place can help by concentrating the E field in a region
around the high dv/dt turns, thereby reducing parasitic currents to
other structures. Of course, this can also 'inject' noise into your
ground system...push down here and it pops up there.
Paul Mathews
Paul Mathews

Good colloquial "push down here and it pops up there" :)

Crossing turn halfway around?? I don't understand yet..
Maybe after some more coffee.... :)

Still thinking...
D from BC- Hide quoted text -

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Imagine a toroid with 10 turns. Beginning on the inside of the core,
wind 5 turns, covering about 160 degrees around the core and ending
outside the core. Then, cross over the the core, that is, route the
wire on a diameter right over the core central axis to a point near
the first turn (about 10 degrees to the bare side of the 1st turn) and
continue the remaining 5 turns. You'll end up with a crossing turn
that goes over the top of the entire core, with 5 turns progressing CW
around the core and 5 turns progressing CCW (or ACW for some folks).
There are many variations on this approach that can be used to
accomodate multiple windings, minimize parasitic capacitance, etc. A
few suppliers of power toroids do this as standard practice.
Paul Mathews

ok..I got the structure now..
To help me understand what's going on I'm going to try this trick:

I'm going to pretend the wound toroid is a power reostat.
Let's say I put that in my smps..(but it still acted like an inductor)
In my app..one end of the inductor is 170VDC and the other has 300Vpk
100Khz square wave.
Moving the wiper along the core (normally wound) and the wiper has an
increasing square wave amplitude. I'm imagining the electric field is
like this too.

Now I switch to that special winding technique..
Turning the reostat from the beginning and the waveform amplitude
starts off small...increases....jumps to max...then decreases to half.

But I'm still a little fuzzy on how this minimizes electric field
interference with neighboring components...
....I'm gonna have to have some more coffee :)

D from BC- Hide quoted text -

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Magnetic field effect: The parasitic turn from normal toroid winding
around the circumference in one direction produces field outside of
the core. The effect of the field depends on what loops might couple
to it, and ordinary shielding is ineffective at preventing such
coupling. You cancel this field with the crossover turn approach.

Electric field effects: The usual winding approach brings opposite
ends of the coil near to one another. High dv/dt of one end relative
to the other, along with their close proximity, means current flow
through a relatively high parasitic capacitance. As you suggest, dv/dt
varies continuously around the turns. Using the crossover-turn winding
technique, you end up with the low dv/dt end close to the middle of
the coil rather than the end, where it is adjacent to half the dv/dt.
It takes a complex model to approximate the overall effect, since
you'd need to consider all of the turns and not just the ends.
However, by simply measuring the self-resonant frequency, it's easy to
demonstrate that the crossover-turn technique can raise resonant
frequency by a factor of 4 or so. At the higher resulting ring
frequency, you can use much lower snubbing capacitance, as an example
of one benefit. These used to be considered 'RF' techniques, but
switchmode power supplies have entered that realm.

Side note: Best read for offline power switchmode is anything by
Sanjayit Maniktala.


In case someone uses a search engine the spelling of his name would be
Sanjaya Maniktala.

Also very good reading are the older Unitrode app notes, now TI and hopefully still on their server. If you have the "Unitrode IC Data Handbook" from around 1990 don't ever think about tossing it. Tons of valuable SMPS info in there.

--
Regards, Joerg

http://www.analogconsultants.com
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