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Smps Toroids on a Ground Plane

D

D from BC

Jan 1, 1970
0
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
 
J

John Devereux

Jan 1, 1970
0
D from BC said:
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.
 
D

D from BC

Jan 1, 1970
0
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
 
P

Paul Mathews

Jan 1, 1970
0
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 -

- Show quoted text -

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
 
D

D from BC

Jan 1, 1970
0
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
 
J

Joerg

Jan 1, 1970
0
D said:
Good colloquial "push down here and it pops up there" :)

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

Probably Whidbey Island speak for "twisted windings". Bifilar, trifilar
and so on.
 
Probably Whidbey Island speak for "twisted windings". Bifilar, trifilar
and so on.

Probably not. "Non-progressive" windings, like the Ayrton-Perry
"bootlace" winding technique, can be applied to single wire windings
or to bifilar, trifilar and rope windings.

Check out "Coaxial AC Bridges" by B P Kibble and G H Rayner, ISBN
0-85274-389-0, which includes a lot of stuff on winding fancy
inductors and transformers for high precision work. The stuff about
non-progressive windings is in section 4.2.

http://www.npl.co.uk/electromagnetic/publications/guides/ac_bridges.html
 
J

Joerg

Jan 1, 1970
0
Probably not. "Non-progressive" windings, like the Ayrton-Perry
"bootlace" winding technique, can be applied to single wire windings
or to bifilar, trifilar and rope windings.

Check out "Coaxial AC Bridges" by B P Kibble and G H Rayner, ISBN
0-85274-389-0, which includes a lot of stuff on winding fancy
inductors and transformers for high precision work. The stuff about
non-progressive windings is in section 4.2.

http://www.npl.co.uk/electromagnetic/publications/guides/ac_bridges.html

Ah, the secrets of RF engineering. Thanks for explaining. That seems to
be one of those books from the days when engineers were self-taught and
knew their stuff by heart.

When I was a kid an old engineer showed me how to build inductive
wideband distributors and combiners. Whenever I asked him why this or
that had to be exactly as shown his answer was: "It just don't work no
other way".
 
P

Paul Mathews

Jan 1, 1970
0
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 -

- Show quoted text -

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
 
P

Paul Mathews

Jan 1, 1970
0
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..

You have the choice whether to mount a toroid horizontally or
vertically and how to orient the high dv/dt vs low dv/dt ends. The
geometry scales: At an equal number of core diameters, the ground
plane has equally beneficial effects, regardless of core size.
Would it help to Faraday shield the toroid to reduce EMI... ?

High dv/dt end will generate parasitic currents according to the
relation: I = C dv/dt. These currents WILL flow, regardless, so
minimize C and dv/dt and provide the shortest possible intentional
paths for such currents. Shields are meant to perform this function,
but you must have a low impedance connection to the common node of the
source of the dv/dt, otherwise the shield itself will have significant
dv/dt. The highest dv/dt is often associated with switching elements
and associated ringing, so rectifier technology, gate drive, and
snubbers are all very important. Shields with large dimensions
relative to the inductor (if you have the room for them) will have a
low capacitance and corresponding low parasitic currents. However, the
larger dimensions then present a challenge to make a sufficiently low
Z connection to the common point. Faraday shields are supposed to
completely enclose a field generator and theoretically neutralize
magnetic and electrostatic fields. A properly wound toroid has very
little external field, so Faraday shields are seldom seen except where
very high isolation from magnetic fields is required.

(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...?

Positioning a shield very close to a switchmode toroid is an excellent
way to generate very high parasitic currents to the shield. Unless the
shield is very well connected to a common return point for the
generated currents, the shield itself will have high dv/dt and radiate
to other structures. See earlier comments.
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.

Safety capacitors can be used to direct parasitic currents through
particular paths, returning them to their sources without much
affecting leakage between mains and other conductively accessible
parts. Of course, this is why there are limits on how much capacitance
you can use for the purpose. Advanced safety topic: study carefully.
D from BC

See some interleaved comments above:
Paul Mathews
 
J

Joerg

Jan 1, 1970
0
Paul said:

Thanks, Paul. In noise critical apps toroid winding can become an art.

Paul Mathews
yes, on Whidbey Island

The closest I came was while fishing with a former boss of mine. Didn't
catch anything, we should have motored on to Langley for a nice
breakfast and a stroll.

Some of my co-workers at ATL in Bothell were from Whidbey Island. Quite
a commute every day with the ferry ride and all. Many had an old car on
the other side (Mukilteo?) and a bicycle on Whidbey Island. Whenever
they talked about the island it sounded like they lived in paradise.
 
D

D from BC

Jan 1, 1970
0
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
 
P

Paul Mathews

Jan 1, 1970
0
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 -

- Show quoted text -

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.
Paul Mathews
 
J

Joerg

Jan 1, 1970
0
Paul said:
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.
 
H

Harry Dellamano

Jan 1, 1970
0
Paul Mathews said:
Side note: Best read for offline power switchmode is anything by
Sanjayit Maniktala.
Paul Mathews
Totally agree Paul, his "Switching Power Supply Design + Optimization"
(ISBN 0-07-143483-6) has a ton of info. Sorry to see him voted off Am.
Idol.
Harry
 
D

D from BC

Jan 1, 1970
0
[snip]
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.
Paul Mathews

Resonance increased by a factor of 4 or more!!! Wow :)

Thanks for the book suggestion.
I'll check out some online bookshops..
D from BC
 
P

Paul Mathews

Jan 1, 1970
0
[snip]






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.
Paul Mathews

Resonance increased by a factor of 4 or more!!! Wow :)

Thanks for the book suggestion.
I'll check out some online bookshops..
D from BC- Hide quoted text -

- Show quoted text -

The general technique of designing windings so that high dv/dt is
maximally far away from low dv/dt can be applied in many different
ways on various kinds of cores. 'Progressive' winding is a fairly well
known example of this, but there are many other less well known
methods, and you can easily dream up your own. Figuring out a way to
make a technique mass producible is the real trick. In my experience,
the majority of magnetics suppliers can't offer much creativity, and
many don't really know much beyond the basics of how to use their
winding machines.
Paul Mathews
 
M

MooseFET

Jan 1, 1970
0
The general technique of designing windings so that high dv/dt is
maximally far away from low dv/dt can be applied in many different
ways on various kinds of cores. 'Progressive' winding is a fairly well
known example of this, but there are many other less well known
methods, and you can easily dream up your own. Figuring out a way to
make a technique mass producible is the real trick. In my experience,
the majority of magnetics suppliers can't offer much creativity, and
many don't really know much beyond the basics of how to use their
winding machines.

It is sometimes easier to design in two inductors in series than to
get one made with special winding methods. On things like EFD
formers, you can get the ones with multiple winding bays. You can
wire the resulting sections in series on the PCB.

You can also specify the winding to be done with "wire rope".
Twisting 7 strands together isn't quite Litz wire but the high
frequency Q is a lot better. Once a winder understands the concept of
how the rope is done, they don't seem to have any problem with doing
it for you. (At least, the winder I use)

Also, if you are going after every % you can get, the snubber can be
made so that some of the energy ends up back in the input supply.
 
T

Tony

Jan 1, 1970
0
Magnetic field effect: The parasitic turn from normal toroid winding
Sorry to have come across this thread late, but what is the "crossover
turn" technique?

I've been trying two methods to minimize susceptibility to axial
fields in Rogowski sensors and tranformers (toroids with non-magnetic
cores):
1) start with a circumferential turn in a groove machined into the
former, then wind on a single layer winding in the opposite direction,
or
2) wind two layers, reversing the feed for the second layer.

Both seem to be OK at reducing axial flux sensitivity, but still
having trouble getting sufficient winding uniformity (another
subject).

But in general, I'd like to know about anything that could raise
self-resonant frequency 2 octaves.

Regards,
Tony
 
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