[Lowfer] Ferrite Loading Coil ?

[Pete Cranwell]

-----Original Message-----
From: lowfer-bounces at mailman.qth.net
[mailto:lowfer-bounces at mailman.qth.net]On Behalf Of Ed Phillips
Sent: Monday, February 11, 2008 7:49 PM
To: Discussion of the Lowfer (US, European, & UK) and MedFer bands
Subject: Re: [Lowfer] Ferrite Loading Coil ?


In receiving you don't have high currents therefore you won't saturate a
core in receiving applications"

	The flux density in the core doesn't depend on the current.  It's
proportional to the voltage across the coil and inversely proportional to
core area and number of turns.

==========================================================

POWER CONSIDERATIONS
(Iron Powder & Ferrite)


How large a core is needed to handle a certain amount of power? This is a
question often asked, but unfortunately there is no simple answer.

There are several factors involved such as: cross sectional area of the
core, core material, turns count, and of course the variables of applied
voltage and operating frequency.

Overheating of the coil will usually take place long before saturation in
most applications above 100 KHz. Now the question becomes "How large a core
must I have to prevent overheating at a given frequency and power level? "

Overheating can be caused by both wire and core material losses. Wire
heating is affected by both DC and AC currents, while core heating is
affected only by the AC content of the signal. With a normal sine wave
signal above 100 KHz, both the Iron Powder and Ferrite type cores will first
be affected by overheating caused by core losses, rather than by saturation.

The extrapolated AC flux density limits (see table below) can be used for
BOTH Iron Powder and Ferrite type cores as a guideline to avoid excessive
heating. These figures may vary slightly according to material being used.

Operating frequency is one of the most important factors concerning power
capability above 100 KHz. A core that works well at 2 MHz. may very well
burn up at 30 MHz. with the same amount of drive.

Core saturation, a secondary cause of coil failure, is affected by both AC
and DC signals. Saturation will decrease the permeability of the core
causing it to have impaired performance or to become inoperative. The safe
operating total flux density level for most Ferrite materials is typically
2000 gauss, while Iron Powder materials can tolerate up to 5000 gauss
without significant saturation effects.

Iron Powder cores (low permeability) are superior to the Ferrite material
cores for high power inductors for this reason: Fewer turns will be required
by the Ferrite type core for a given inductance. When the same voltage drop
is applied across a decreased number of turns, the flux density will
increase accordingly. In order to prevent the flux density from increasing
when fewer turns are used, the flux drive will have to be decreased.

Either core material can be used for transformer applications but both will
have 'trade-offs'. Ferrite type cores will require fewer turns, will give
more impedance per turn and will couple better, whereas the Iron Powder
cores will require more turns, will give less impedance per turn, will not
couple as well but will tolerate more power and are more stable.
Frequency: 	100 KHz 	1 MHz 	7 MHz 	14 MHz 	21 MHz 	28 MHz
AC Flux Den.: 	500 Gauss 	150 Gauss 	57 Gauss 	42 Gauss 	36 Gauss 	30 Gauss

The equation for determining the maximum flux density of a given toroidal
core is as follows:


      Epk = applied RMS volts
      Ae = cross-sect. area (cm2)
      N = number of wire turns
      F = frequency

The safety factor may be increased by using the peak AC voltage in the
equation. This is a standard practice among many RF engineers who design
broadband RF power transformers.

The above equation may be changed as shown below to make is more convenient
during calculations of Bmax at radio frequencies.


      Epk = applied RMS volts
      Ae = cross-sect. area (cm2)
      N = number of wire turns
      F = frequency

The sample calculation below is based on a frequency of 7 MHz, a peak
voltage of 25 volts and a primary winding of 15 turns. The cross-sectional
area of the sample core is 0.1333 cm2. From previous guidelines we know that
the maximum flux density at 7 MHz should be not more than 57 gauss.

This hypothetical toroid core will have a flux density of 40 gauss according
to the above formula and when operated under the above conditions. This is
well within the guidelines as suggested above.

Temperature can be the result of using an undersized wire gage for the
amount of current involved as well as magnetic action within the core. Both
will contribute to the overall temperature of the transformer. This can be
calculated with the following equation:

If the operating temperature (ambient temperature + temperature rise) is
more than 100° C when used intermittently, or more than 75° C if used
continuously, a larger size core and/or a heavier gauge wire should be
selected.

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