Len Crane, Senior Technologist, Coilcraft
Inductors function as circuit elements by reaction to current flow in the windings. Inductors are fundamentally current-driven devices, and anyone who has designed a dc-dc converter knows power inductor datasheets are covered with Irms and Isat current ratings. Picking a power inductor is all about current ratings.
Published current ratings are fairly standardized and have been presented the same way for a long time. While these ratings serve as valuable guideposts when designing your first or your hundredth converter, they are really only a starting point. Achieving the best converter performance depends on knowing what the ratings really mean and how they are derived. This article will further your understanding of current ratings and provide practical advice to overcome limitations of the traditional datasheet.
Irms as shown on inductor datasheets is a measure of how much the inductor temperature will rise above room temperature due to self-heating from current in the inductor winding. This is a measure of how the inductor impacts circuit efficiency and of course a measure of inductor temperature. This is fairly straightforward, as figure 1 shows 39.9 A of current produces 40°C temperature rise in Coilcraft XGL1060-102ME inductor (and 29.2 A produces 20°C rise). The power loss that generates the self-heating is easy enough to calculate by multiplying the rms current squared times the dc resistance. This is helpful information for the circuit designer, and allows easy comparison between inductors.
Irms “ratings” like this on inductor datasheets are used all the time for comparing inductors and making choices, but the user should be clear to use this as information only, not an absolute max rating. The application might require a limit on the inductor power loss or a max hotspot, but for the inductor itself, the Irms alone rarely is an absolute max rating.
The inductor in this example, Coilcraft’s XGL1060-102ME is rated for a Max Temperature = 165°C (ambient + self-heating), based on the integrity of the insulation. Then why are inductors so commonly characterized on datasheets for a 40°C self-heating temperature rise? The first answer is because of legacy ratings. Even though Irms may not be an absolute max rating, it does provide information to easily compare across a wide range of inductors. It is likely that at one time this particular data point represented a real physical limitation of insulating materials. The Irms rating may simply be a holdover from when many electronic materials simply were not reliable at temperatures over 65°C or so. With modern materials, a room temperature ambient of 25°C + 40°C self-heating rise = 65°C does not usually represent a physical or fundamental limit of any kind. In this example, an inductor temperature of 65°C is fully 100°C below the max rated temperature of the inductor. Considering this number as an absolute limit would severely limit the choice of smaller inductors and leave a design larger than necessary and undesirable to customers. To make the best choice that balances size, performance, and reliability, the Irms rating must be considered along with the expected ambient environmental temperature and the max inductor temperature rating. The takeaway here should be that the Irms rating on a datasheet is helpful, true information, but should not necessarily be a limit when making today’s design choices.
An even bigger limit of the Irms rating on a datasheet is that rms current is not the only loss mechanism driving temperature rise in power inductors. Simply looking at Irms current rating cannot predict skin effect, proximity effect or core loss. For these it is necessary to know the rms current, the peak-peak current, switching frequency, and waveshape.
It is not feasible for a datasheet to provide curves with all possible current wave shape and frequency combinations. For this, it is necessary to move beyond the traditional Irms datasheet ratings and use sophisticated loss prediction tools like Coilcraft MAGProTM, which can predict inductor efficiency based on these inputs. For example, the MAGProTM Power Inductor Finder & Analyzer confirms the inductor temperature 65°C for a “mostly dc” waveform with 39.9 A rms current. In this case as shown in figure 2a, the datasheet Irms has correctly calculated the inductor loss and temperature and directed us to a good inductor choice.
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Figure 2a: Temp Rise Confirmed 65C for Irms = 39.9 Arms if Peak-peak minimal
However, when the inductor current has a significant peak-peak aspect as in figure 2B, the Irms rating cannot predict all the losses. In this case the MAGProTM Power Inductor Finder & Analyzer confirms a higher inductor temperature with 12 A pk-pk, even though the rms current is still the same 39.9 A.
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Figure 2b: Calculation Including AC Loss Mechanisms show higher temp rise
Irms ratings are still very useful and will remain a part of our everyday discussions. Nevertheless, they have limitations and cannot predict all the important aspects of inductor performance in modern converter circuits.
It should be clear from this discussion that Irms does not directly correlate to circuit function. Irms is all about one thing and that is the efficiency of the inductor in the application.
In contrast to Irms, Isat can have a direct impact on circuit performance. The Isat rating on a datasheet describes how the inductance value falls due to core saturation as current in the inductor increases, but it is very dependent on the application whether this represents a problem. Datasheets typically show curves of inductance vs current (figure 4) and the Isat is simply a point chosen from that curve. Whereas Irms relates to time-averaged power loss, Isat is an indicator of how the inductance value can change nearly instantaneously and impact the circuit in real time.
Figure 3 gives an idea of the possible circuit impact. Constant inductance results in a smoothly ramping current (A) whereas inductance decrease from core saturation (B) produces a higher peak current as the inductance decreases.
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Figure 3: inductance drop from core saturation changes di/dt
is the fundamental relationship designers use to choose an inductance value to set peak-peak current to a desirable value. For constant voltage and on-time, the peak-peak current and the peak current depend only on the inductance value. As long as the inductance remains constant with voltage and current, the current wave shape is “well behaved”, but decreased inductance causes current to rise faster.
An inductor datasheet might list an Isat “rating” for how much current causes the inductance to drop 10%, 20% or 30% from the nominal value. This way of describing Isat was developed so a number could be used to signal the transition between a constant inductance region and a saturation region in which inductance drops rapidly with current as shown in figure 4A. This saturation curve shape is typical of ferrite materials and a single Isat point does a fairly good job of describing the saturation if the transition is fairly distinct.
A comparison between the saturation curves in figures 4a and 4b quickly shows a limitation of the Isat number for today’s popular soft-saturation inductors and demonstrates the one size “does not fit all”.
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Figure 4: Difference between hard and soft saturation
Figure 4b shows a modern soft-saturation inductor for which the curve is essentially a straight line. There is no particular physical significance to any chosen point on the curve and the Isat rating is fairly arbitrary. Nevertheless the Isat rating is still useful. Focusing on how much the inductance has dropped with current is a little bit of an outdated concept. For most applications it is now more appropriate to consider the actual inductance value at the peak current. In other words, for saturation it is more useful to consider how much inductance remains rather than how much was lost. In early controller schemes, it was preferred to maintain a constant inductance for stability of the converter operation. Nowadays, most modern control schemes are quite capable of utilizing a changing inductance. This allows the converter design to optimize size and performance with compact, soft-saturating inductors.
Curves, curves, curves…it is quite clear from figure 4 that an Isat rating number cannot fully describe inductor performance as it once might have. The two inductors might perform very similarly at low current, but at higher current there is a big difference. Understanding the full picture is necessary to avoid selecting an inductor that is either under or overdesigned for the intended application conditions. And of course maybe the most important point is whether some amount of inductance decrease is important to the application.
Relationship Between Isat and Irms
A rule-of-thumb for many experienced designers is to expect the Isat rating for an inductor to be higher than Irms. This makes sense since Isat relates to peak current and Irms relates to average. New inductors with greatly reduced dcr, however, can flip that relationship and give us cause to examine that “rule”. Consider the example shown in figure 5 where Coilcraft’s XGL5030-102 has lower dcr than the previous generation inductor. One might look at Irms > Isat and conclude the higher Irms is somehow wasted or something is simply wrong with the design. The real issue, though, is the limitation of trying to describe inductor performance with a simple number. The Irms and Isat ratings simply don’t provide all the information. So what does it mean when Irms is greater than Isat? The high Irms is telling us that this inductor will be very efficient at all current levels compared to a similar inductor with a lower Irms rating, and that of course makes the newer inductor extremely valuable. The graph in figure 5 from the Coilcraft MAGProTM Power Inductor Finder & Analyzer shows the tangible benefit of a high Irms rating is a lower inductor temperature rise at all current levels for the inductor with higher Irms. This specific case shows that opportunities for design optimization can easily be lost without a good understanding of datasheet ratings and without the proper tools to see the bigger picture.
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Figure 5: Visualizing the efficiency of inductors with higher irms rating
Datasheet current ratings that were developed for early generations of inductors must be applied differently today to insure appropriate inductor use for optimizing dc-dc converter performance. It is important to keep in mind that Isat and Irms ratings as shown on datasheets are not so much fixed limits, but rather should be considered as informative reference points. Graphs and sophisticated tools can go well beyond datasheet ratings to provide the full picture needed to make the best design decisions.