29-05-2012, 05:37 PM
FUNDAMENTALS OF MAGNETICS DESIGN
[attachment=23424]
This presentation is introductory and does not present all of
the different magnetic materials, their properties, vendors,
core geometries, sizes, mounting hardware etc. However
an extensive list of references is provided at end of the
presentation for further exploration.
There are over 100 slides so I will go quickly. Please
reserve all questions for the end. This presentation will be
posted on the IEEE Website in the next few days.
SOME BASIC DEFINITIONS
B is the magnetic flux density and H is the magnetizing force that
generated the flux.
• In the CGS system B is measured in Gauss which is Lines of
Magnetic Flux per Square Centimeter.
• In the SI system the unit of measurement of B is the Tesla which
is Webers per Square Meter.
• A Tesla is equal to 10,000 Gauss.
THE BH CURVE
A plot of B vs. H (BH Curve) appears as follows. Let us start at point A
and increase the magnetizing force to obtain point B. Decreasing the
magnetizing force will pass through point C and then D and then E as
the magnetizing force is becomes negative. An increasing magnetizing
force, again in the positive direction, will travel to B through point F.
The enclosed area formed by the curve is called a hysteresis loop and
results from the energy required to reverse the magnetic molecules of
the core.
DC Resistance also known as DCR or copper loss consists strictly of the
DC winding resistance and is determined by the wire size and total length
of wire required as well as the specific resistivity of copper.
• The Core Loss is composed mostly of losses due to eddy currents and
hysteresis. Eddy currents are induced in the core material by changing
magnetic fields. These circulating currents produce losses that are
proportional to the square of the inducing frequency.
• Hysteresis is represented by the enclosed area within a BH curve and
results from the energy required to reverse the magnetic domains in the
core material. These core losses increase in direct proportion to frequency,
since each cycle traverses the hysteresis loop.
• Skin Effect increases the wire resistance above approximately 50 kHz
because the current tends to travel on the surface of a conductor rather
than through the cross section. This reduces the current-carrying crosssectional
area.
DISTIBUTED CAPACITANCE
•The equivalent circuit of an inductor is shown above. Rdc is the DC resistance,
Rac is the core loss and skin effect and Rd is the dielectric loss. The dielectric
losses occur mainly due to the losses in the wire insulation. All these factors
affect inductor Q.
•The capacitance Cd is the turn-to-turn and turn-to-core distributed capacitance.
The result is that the inductor has a self-resonant frequency (SRF) where it
appears like a parallel resonant circuit. Above this frequency the inductor
behaves like a capacitor.
•The effect of the SRF is to increase the effective inductance so it appears
higher than the true (low frequency) inductance. For example if a 10mH
inductor has 100pF of distributed capacitance, the actual resonating
capacitance needed for a particular frequency would be 100pF lower due to
the distributed capacitance. Therefore the effective inductance appears as if it
were higher than 10mH.
[attachment=23424]
This presentation is introductory and does not present all of
the different magnetic materials, their properties, vendors,
core geometries, sizes, mounting hardware etc. However
an extensive list of references is provided at end of the
presentation for further exploration.
There are over 100 slides so I will go quickly. Please
reserve all questions for the end. This presentation will be
posted on the IEEE Website in the next few days.
SOME BASIC DEFINITIONS
B is the magnetic flux density and H is the magnetizing force that
generated the flux.
• In the CGS system B is measured in Gauss which is Lines of
Magnetic Flux per Square Centimeter.
• In the SI system the unit of measurement of B is the Tesla which
is Webers per Square Meter.
• A Tesla is equal to 10,000 Gauss.
THE BH CURVE
A plot of B vs. H (BH Curve) appears as follows. Let us start at point A
and increase the magnetizing force to obtain point B. Decreasing the
magnetizing force will pass through point C and then D and then E as
the magnetizing force is becomes negative. An increasing magnetizing
force, again in the positive direction, will travel to B through point F.
The enclosed area formed by the curve is called a hysteresis loop and
results from the energy required to reverse the magnetic molecules of
the core.
DC Resistance also known as DCR or copper loss consists strictly of the
DC winding resistance and is determined by the wire size and total length
of wire required as well as the specific resistivity of copper.
• The Core Loss is composed mostly of losses due to eddy currents and
hysteresis. Eddy currents are induced in the core material by changing
magnetic fields. These circulating currents produce losses that are
proportional to the square of the inducing frequency.
• Hysteresis is represented by the enclosed area within a BH curve and
results from the energy required to reverse the magnetic domains in the
core material. These core losses increase in direct proportion to frequency,
since each cycle traverses the hysteresis loop.
• Skin Effect increases the wire resistance above approximately 50 kHz
because the current tends to travel on the surface of a conductor rather
than through the cross section. This reduces the current-carrying crosssectional
area.
DISTIBUTED CAPACITANCE
•The equivalent circuit of an inductor is shown above. Rdc is the DC resistance,
Rac is the core loss and skin effect and Rd is the dielectric loss. The dielectric
losses occur mainly due to the losses in the wire insulation. All these factors
affect inductor Q.
•The capacitance Cd is the turn-to-turn and turn-to-core distributed capacitance.
The result is that the inductor has a self-resonant frequency (SRF) where it
appears like a parallel resonant circuit. Above this frequency the inductor
behaves like a capacitor.
•The effect of the SRF is to increase the effective inductance so it appears
higher than the true (low frequency) inductance. For example if a 10mH
inductor has 100pF of distributed capacitance, the actual resonating
capacitance needed for a particular frequency would be 100pF lower due to
the distributed capacitance. Therefore the effective inductance appears as if it
were higher than 10mH.