The resistor value should be 1 or less (depending on the inductor current), and the resistor must be sized to dissipate the power.
#Ferrite core inductor saturation. effect on sound series#
If a current probe is not available, a less direct but still effective method is to measure the voltage across a small sense resistor in series with the inductor. Notice that when inductor literature does not have a “DC current” rating, or shows only an “AC amps” rating, such inductors are often prone to saturation.įigure 9-52: Effects of saturation on inductor currentįrom a simplified design standpoint, the effects or presence of inductor saturation can best be observed with a scope and a current probe. This worst case peak current must be less than the peak current rating of the inductor. It is therefore important in all switching regulator designs to determine the peak inductor current expected under the worst case conditions of input voltage, load current, duty cycle, etc. If saturation occurs, however, the inductor current increases exponentially, corresponding to the drop in effective inductance. Normal conditions the slope is linear for both the charge and discharge cycle. Figure 9-52 shows a normal inductor current waveform in a switching regulator as well as a superimposed waveform showing the effects of core saturation. The effects of inductor core saturation in a switcher can be disastrous to the switching elements as well as lowering efficiency and increasing noise. Proper design and manufacturing techniques, however, can be used to minimize this EMI problem, so air-gapped cores are popular in many applications. Of them is the tendency of the air-gapped inductor to radiate high frequency energy more than a nongapped inductor. Air gaps do have their problems, however, and one The air gap reduces the slope of the curve, but provides a wider linear operating range of inductor current. This effect can be reduced somewhat while still maintaining higher inductance than an air core by the addition of an air gap in the ferromagnetic core. It is obvious that inductor saturation can wreak havoc in a switching regulator, and can even burn out the switch if it is not current limited. The addition of a ferromagnetic core increases the slope of the curve and increases the effective inductance, but at some current level, the inductor core will saturate (i.e., the inductance is drastically reduced). Inductor current, magnetic field strength (H)įigure 9-51: Magnetic ß ux density versus inductor current Note that B is linear with respect to H for the air core inductor, i.e., the inductance remains constant regardless of current. A relative permeability of 1,000, for instance, will increase inductance by 1,000:1 above that of an equivalent air core.įigure 9-51 shows magnetic flux density, B, versus inductor current for the air core and also ferromagnetic cores. Core permeability is often specified as a relative permeability which is basically the increase in inductance which is obtained when the inductor is wound on a core instead of just air. Therefore, in order to make a reasonable inductor, the wire is wound around some type of ferromagnetic core having a high permeability. This simple air core inductor is not very practical for the values of inductance required in switching regulators because of wiring resistance, interwinding capacitance, sheer physical size, and other factors. The effective inductance of the coil is proportional to the ratio of the magnetic flux density to the field strength. Using a number of turns of wire to form a coil increases the magnetic flux density for a given current. The magnetic field strength produces a magnetic flux density (B, measured in webers per square meter, or gauss). The strength of this field is measured in ampere-turns per meter, or oersteds and is proportional to the current flowing in the wire. Choosing the proper type of inductor is much more complicated as the following discussions will indicate.įundamental magnetic theory says that if a current passes through a wire, a magnetic field will be generated around the wire (right-hand rule). Note that for the boost PWM, even though the input current is continuous, while the output current pulsates, we still base the inductance calculation on the peak-to-peak inductor ripple current.Īs was previously suggested, the actual selection of the inductor value in a switching regulator is probably the easiest part of the design process.
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