Inductors/Transformers for Switching Power
3.1 Inductor Design Basics
3.1.8 Core Material Trade-offs
Table 3-1 summarizes the trade-offs for various inductor types used in low- power switching supplies. Ferrites are attractive because of the combined low cost and high resistivity, which minimizes eddy-current losses. (Ferrites are the only choice for switching frequencies of 500 kHz and up.) On the other hand, the high permeability of ferrites usually requires an air gap with all the associated complica
tions (high EMI for bobbin types, and extra assembly steps for pot-core types).
Table 3 - 1. Trade-offs for various inductor types (Maxim Engineering Journal, Vol. 4, p. 7)
TYPE Ferrite bobbin Ferrite bobbin with ferrite shield Ferrite pot core
Molded (low cost) EMI High
Low
Low
High
Silicon steel toroid ! Low
I
Ferrite toroid j Low Molypermalloy
powder (MPP) toroid
1 Iron-powder toroid Low
COMMENT
1
Makes compact, low cost, ι axial-lead (cylindrical) in
ductors. Low core losses support high efficiency.
Efficient but prone to i saturation
Efficient. Easily gapped to i the correct value. Best for high-current or high-fre- ; quency applications.
OK for light loads. Prone to saturation and often in
efficient. Observe current ratings carefully.
Tape wound; similar to i iron powder. Use thinner tape for higher frequen
cies.
Prone to saturation Best available for fre
quencies less than 400kHz. Low EMI, low losses, compact, and ex
pensive. Use high-flux type.
| Specify core material '■ carefully to achieve low j losses. Best is "Kool- j mu" (Magnetics, Inc.).
Powder MPP combines good saturation characteristics with low hysteresis losses. However, MPP is expensive (it contains nickel) and requires many process
ing steps. Iron powder and silicon-steel tape, in spite of a tendency to sustain eddy- current and hysteresis losses, are inexpensive, and well suited to general-purpose applications.
3.1.9 High-Flux MPP Cores
High-flux MPP cores combine good EMI performance with small size. Ordi
nary MPP cores (for radio frequency [RF] applications) contain 80% nickel, plus iron and molybdenum. High-flux MPP contains 50% nickel, which is not good for
RF but is good for switching supplies. Although high-flux MPP is expensive, it is sometimes more cost-effective than ferrite because of the precision air-gap require
ments.
3.Ί.Ί0 DC Winding Resistance and l2R Losses
Core material affects the power level available for a given inductor size. How
ever, DC winding resistance can also limit available output current and circuit effi
ciency. As shown in Fig. 3-3, a high DC winding resistance alters the inductor-current waveform (causes a slope rather than the desired linear rise). The resulting I2R losses (power losses) affect overall efficiency and cause the core tem
perature to rise. DC winding resistance is significant for battery-powered and low- voltage applications of about 3 V or less, in which the inductance values must be low to get an acceptable slope for the inductor-charging current.
3.1. J I Temperature Rise
Inductor specifications often include two current ratings: continuous or RMS and DC saturation (sometimes called peak or incremental current). A continuous rating accounts for the temperature rise caused by winding resistance as well as the operating-temperature range and properties of insulation or potting material. The continuous rating is usually higher than the DC-saturation rating (but not always, es
pecially for high-value inductors). As a guideline for simplified design, make sure the average current of the inductor is less than the continuous rating. If high fre
quencies are involved, include a safety margin (at least 10%) for additional temper
ature rise because of core losses. The approximate average current for an inductor in a switching supply is
V + V - V
T T v OUT ^ v D v IN 1 AAVE ~~ xL0AD
( MN * S W '
3.1.12 High-Frequency Losses and Inductor Q
High-frequency losses in a switching-supply inductor include three major components: those from hysteresis, those from eddy currents in the core, and those from eddy currents in the wire. (Residual loss, a fourth component, is usually in
significant.)
Magnetic hysteresis, which occurs as flux density nears the saturation point, becomes a problem in iron-powder cores at switching frequencies of 100 kHz or less. One cure is to reduce the peak flux density at high currents by enlarging the core. However, larger cores make the eddy-current problem worse by providing more low-resistance paths for the eddy currents. Core eddy current is a function of frequency squared and quickly becomes excessive as the frequency approaches 300 to 400 kHz. As a guideline, to offset the core eddy-current problem, use another core material rather than change the core size.
Table 3-2. Frequency limits for some common core materials (Maxim Engineering Jour
nal, Vol. 4, p. 8)
tolOOkHz to 200kHz to 400kHz to 500kHz toi MHz to 10MHz
Standard iron powders and steel tape Low-permeability, high-frequency iron powders
Hieh-flux MPP , Standard MPP
Manganese-zinc ferrite Nickel-zinc ferrite
Table 3-2 shows the frequency limits for some common core materials. No
tice that high switching frequencies (about 100 kHz and up) applied to iron-powder and steel-tape cores can cause significant eddy currents that contribute to a rise of core temperature and efficiency loss. Because the regulator may appear to be operat
ing properly, the eddy-current problem can be difficult to detect, except as an unex
plained efficiency loss or core-heating effect.
Winding eddy currents (circulating currents within the wire) can also be a problem at frequencies of 500 kHz and higher. The practical solution is to use a minimum wire thickness. Litz wire (ultra-thin, multi-stranded wire) or windings made from PC traces, are common approaches to the winding eddy-current problem.
It also helps to position the windings within the core as far as possible from the air gap.
One practical approach to the inductor-loss problem is to measure the inductor Q with a sine-wave inductance bridge (Q meter), at the switching frequency. As a guideline, if the Q value is 25 or more, the efficiency loss produced by the inductor is 5% or less. If you are not familiar with inductor Q measurements, read the au
thor's McGraw-Hill Electronic Testing Handbook, 1994 (he needs the money).
3.1.13 EMI Problems
The solutions to EMI problems (interference to adjacent circuits or to nearby equipment) usually depend on application trade-offs. For example, use shielded in
ductors if EMI must be kept to a minimum. However, shielded inductors tend to be larger, more expensive, and more difficult to mount than unshielded types. If mod
erate amounts of EMI can be tolerated, use an unshielded bobbin-type inductor.
Usually, such cores are about half the price and size of an electrically equivalent pot core or toroid. Notice that bobbin inductors generate the highest magnetic fields near the ends along the axis, so point the bobbins away from sensitive circuits, and mount them at 90° angles to other magnetic components.
Whenever material must be cut to provide an air gap (such as with ferrite cores), EMl-pÎoâucing fringe fields are set up at the cut. Fringe fields also occur in cores with large inherent gaps, such as between the ends of a bobbin inductor wound on a cylindrical core. Pot cores or similar designs allow air gaps in the ferrite mater
ial while keeping the EMI from radiating. The gap spacings in powdered-material
cores are so small that EMI is seldom a problem, provided the core is a toroid or similar design with a closed magnetic path.
3.Ί.14 Self-Resonant Frequency
All inductors have some distributed capacitance that combines with the in
ductance to form a resonant circuit. The frequency of this self-resonance should be between 5 and 10 times the switching frequency (but not at an exact multiple of the switching frequency!). As the inductance value is set by circuit requirements, the SFR is determined by the distributed capacitance (a higher capacitance produces a lower SRF).
When the SRF is low, the normal linear ramp of the inductor current (see Fig.
3-3) is preceded by a sudden jump of current when the switching transistor turns on.
This results in so-called switching losses that lower the regulator's overall effi
ciency. As a result, distributed capacitance should be kept at a minimum so that the SRF will be high and will not seriously affect the inductor current. Distributed ca
pacitance can be lowered when the toroid is wound, either by overlapping the ends of the winding somewhat or by leaving a gap between the winding ends (rather than ending the winding at one full layer).