Saturating Ferrite

[nickec]

I have been working on a Science Project and several issues have come up. Among them is ferrite saturation.

In the experiment I need to minimize the current needed to saturate soft ferrite.

So I am looking for inexpensive methods to measure saturation. I am passing current through a coil around the soft ferrite.

Intuition tells me that shape and volume are factors.

I also note that a ferrite toroid is a closed magnetic circuit (no air gap) and that a rod is not a closed magnetic circuit.

Thank you for weighing in on this subject.

 

[jpanhalt]

Have you searched on mag amp (magnetic amplifier)? Here are a couple of hits:

**broken link removed**

Magnetic amplifier having a co-axial winding - Patent 4092607

The second seems to be particularly on point. I think you will be better off with a toroid or cylinder (as described in second patent).

John

 

[MrAl]

Hi,


One way is by measuring the inductance and looking for a drop for some current level.

Since the material becomes magnetized when the current flows and the magnetic
field gets stronger as the current increases until saturation, you might use a
hall effect sensor to detect the field near one of the poles and as you apply
more and more current the field will keep increasing until it nears saturation
and then the field will either stop increasing or start to increase much less
for the same increase in current.
Some of the linear hall effect sensors are not that expensive. You'll also need
a small power supply to power it.

There is a problem however when trying to saturate a material that does not have a
closed magnetic path, and that is that the reluctance of the material is very low
but the reluctance of the air is not, and the air becomes part of the magnetic path.
The combination of the magnetic material and the air means the material may look
like it takes a huge huge current to saturate because the magnetic properties
of the entire construction (air and material) appear like a very linear choke because
of the linearizing effect of the air. In fact, air gaps are often introduced into power
chokes for that very reason: to make saturating harder to do (require more current)
and with a huge air gap such as that which would be the same length as the material
itself, i would think it would take a huge current to see anything like saturation.

On the other hand, with a material that can fold back onto itself such as a piece of
material that forms a closed loop, that may saturate quite easily as most of the field
would be concentrated inside the material.
I guess this also says how to minimize the current needed to saturate the material:
construct it into a closed loop with as little air gap(s) as possible.

This reminds me of an interesting experiment i did once...
Take a regular toroid core and wrap some turns of wire around it, then measure the inductance
and the dc current that it takes to saturate it or at least start to saturate it.
Take the turns of wire off,wrap the core in a clean rag, put that into a vise, tighten slowly until
the core cracks. Take the pieces out trying not to loose any, then glue it back together with super glue.
Put the same number of turns of wire back on.
Measure the inductance again and the saturation current, and note that the inductance
goes down and the current needed to enter saturation goes up. The reason being that
the cracks filled with glue now act as gaps, and the way most cores break like this there
ends up being four gaps, and even that thin glue layer four times over creates air gaps
that are effective enough to dramatically change the characteristic of the core.
You can also glue two of the four pieces back together and then vary the gap by
placing the two halves close together, which means two gaps will be larger.
Perhaps some hot glue would help keep them together just to make the measurements.

 

[RCinFLA]

One way is to measure the small signal inductance of a coil that has a variable D.C. bias applied. The tricky part is injecting the D.C. current without impacting the small signal inductance measurement.

The brut force way is to put chokes to the coil inductance to inject the d.c. current on the coil to be measured. You have to know the characteristics of the chokes, primarily that they are not saturating and maintain high impedance relative to the measurement inductance.

A more predictable and simpler way is to put coil in the collector (or drain) of a common emitter (or common source) amplifier that you can change the D.C. bias on. The amplifer has lots of A.C. feedback via emitter (source) degeneration resistor to keep the A.C. transfer gain constant over changes in the device d.c. bias.

You can parallel resonate the collector tank with a known value capacitor and detect the frequency of peaking A.C. swing, keeping it just large enough to detect by adjusting the A.C. signal level on input of amplifier.

Another way, particularly useful for switching power supply coils is to actually measure the V= L di/dt effect. This is done by using a variable pulse width function generator to drive a MOSFET. Drain goes to coil and a scope is placed across a low valued resistor in series with coil at Vcc end of coil to measure the current ramp. The current ramp will start out linear and will start to increase in rate as saturation is getting close. A Schottky diode to a resistor is used to discharge the coil between pulses. You need to start out with short pulses and work your way up to keep things from getting out of control otherwise you will trash a MOSFET. Heat sink on MOSFET as necessary for its Rs and current.

 

[MrAl]

Hi again,


Another way i like to do it is to use an actual buck regulator circuit with the
inductor in the usual place. With this we can vary the load current, and
set point output voltage which allows variation of the duty cycle. We look
for a sharp rise in current near the end of the output pulse if the device
is saturating. This does require a scope really though to observe the
coil current.

 

[RCinFLA]

Keep in mind an ungapped ferrite core will have a soft saturation curve. The effective inductance will go down the higher the current.

Many switchmode buck circuits run into constant current mode at higher current. This effectively puts a D.C. bias current through the inductor, again reducing its effective inductance.

Actually setting up a scope trace with an N-ch Mosfet driver shows the Ldi/dt slope change as current gets higher during the field build up.

 

[MrAl]

Keep in mind an ungapped ferrite core will have a soft saturation curve. The effective inductance will go down the higher the current.

Huh? Did you mean to say that a gapped core will have a softer sat curve
or that an ungapped core will have a harder sat curve?

 

[RCinFLA]

As in ungapped are softer saturation in regard to inductance value variance as flux density increases. Small signal inductance measurements are almost useless on ungapped cores, unless a controlled D.C. bias is also applied.

On a gapped core, the effective permeability is diluted by the gap making nonlinearity of permeability slope less, therefore more constant inductance value over drive current then an ungapped core.

Hard magnetic flux density core saturation remains the same between gapped and ungapped. (it does take a higher magnetizing force to get there on gapped cores).

 

[MrAl]

Hi again,

Ok no problem. In the past i have always talked with other engineers using the
term 'soft' to indicate more or less the way the core goes into saturation...
in other words, 'hard' would be when a small change in dc bias would cause
a huge decrease in inductance, where when a large change in dc bias is needed
to cause a large decrease in inductance that would be called 'soft'.
Another way of saying it is that adding a small gap 'softens' the sat characteristic
because we can then make a large change in dc bias and enter saturation very
gradually, the same we we can compress a pillow quite a bit before we reach the
point where it is as flat as paper and can no longer be compressed...ie 'soft'.
That's the way we always talked about it.

 

[RCinFLA]

You don't get something for nothing. The ferrite material has not changed because of gap.

Starting out with a permeability of 4000 for an ungapped core that goes to a very low permeability close to hard saturation is a lot bigger inductance change then putting in a gap that drops the initial system permeability to 120 that goes to low value close to hard saturated.

Sort of like putting a really bad tolerance high value resistor in parallel with a tight tolerance low value resistor.

In the case of gapped core you pay with a lot more turns of wire and its resistance required to get same inductance value. It does give you more useful range out of the ferrite as SMPS stability is limited to a given range of inductance change. The gap also gives more field leakage that can muck up near by circuits with EMI coupling.

 

[MrAl]

You don't get something for nothing. The ferrite material has not changed because of gap.

Starting out with a permeability of 4000 for an ungapped core that goes to a very low permeability close to hard saturation is a lot bigger inductance change then putting in a gap that drops the initial system permeability to 120 that goes to low value close to hard saturated.

Sort of like putting a really bad tolerance high value resistor in parallel with a tight tolerance low value resistor.

In the case of gapped core you pay with a lot more turns of wire and its resistance required to get same inductance value. It does give you more useful range out of the ferrite as SMPS stability is limited to a given range of inductance change. The gap also gives more field leakage that can muck up near by circuits with EMI coupling.

Hi,

I didnt say that the material itself changed, i said that we described the
gapped effect as 'softening' the magnetic sat characteristic.