1. Separate average and ripple loss contributions
An inductor in a switching converter carries a waveform, rather than a single current with one universal resistance attached. Coilcraft distinguishes the average component from the alternating ripple superimposed on it. That distinction determines which winding-loss evidence addresses which part of the current. A load-current label can identify an application requirement while still leaving the ripple contribution unspecified.
The note's small-ripple, continuous-current example explains a common comparison error. Applying the resistance read at switching frequency to the entire average load current assigns the high-frequency treatment to current that has a different loss basis. Its explanation instead separates the low-frequency winding contribution from the contribution associated with ripple. The example is useful for its allocation of mechanisms, without transferring its numerical assumptions to another converter.
Read a proposed loss statement by asking which current component it represents. A DC resistance entry addresses the winding's DC basis; a frequency-dependent resistance description adds information about alternating excitation. Neither should silently absorb the other. Before comparing inductors, establish whether the available evidence preserves that separation or compresses the waveform into an unexplained headline current.
2. Read core loss as an excitation-dependent property
Core loss is a magnetic contribution, distinct from heating in the winding conductor. Coilcraft identifies ripple-current excursion, ripple frequency, core material, core size and turn count as relevant factors. Some belong to the application and some to the inductor. The interaction explains why a component can have a clearly stated DC resistance while the core-loss question remains open.
Frequency alone is therefore an incomplete description of the excitation. A comparison at the intended switching frequency must still retain the ripple context that drives magnetic excursion. Conversely, changing the component's construction can change the magnetic response without changing the application's nominal frequency. The useful question is how the documented loss evidence connects the excitation to the particular construction.
Keep the conductor and magnetic questions separate when interpreting a low-DCR description. Lower winding resistance addresses one contribution; it does not establish what happens in the core under alternating excitation. This distinction supports a more precise requirement than asking for a generally efficient inductor. It identifies why the intended waveform and the manufacturer's applicable magnetic-loss evidence need to describe the same operating context.
3. Understand what a small-signal ESR curve omits
A frequency-dependent ESR or ACR characteristic can contain useful information about winding behaviour. Coilcraft discusses skin and proximity effects as reasons that winding resistance increases at higher frequencies. Such evidence adds a frequency coordinate that DC resistance lacks. It still needs its measurement basis before it can be interpreted as part of the application's loss description.
The note explains that these curves are commonly produced at very low current. That measurement does not capture all current-dependent core loss. Reading a high-frequency resistance value from the curve and calling it complete inductor loss therefore creates two separate problems: the waveform contribution may be misassigned, and a magnetic contribution may be absent. Correcting one does not automatically correct the other.
Distinguish evidence that characterizes small-signal resistance from evidence intended to describe total loss at application current. The former can remain relevant without being sufficient for the latter. This is a question about what the measurement includes, rather than whether the curve is useful or trustworthy. Preserving that scope avoids treating a familiar plot heading as a complete model of a different excitation.
4. Compare total loss together with magnetic and size constraints
Total loss connects the winding and core contributions, but selecting only the lowest reported loss can discard other requirements. Coilcraft relates loss to wire and core size, and notes compromises involving package size and magnetic construction. The comparison therefore needs both the intended excitation and the physical constraints under which the component must operate.
Inductance at peak current supplies another independent coordinate. A construction that addresses the loss objective still has to maintain the inductance the application requires at its relevant current condition. Likewise, an acceptable saturation-current description does not identify the lowest total loss. These are complementary magnetic and thermal questions, rather than alternative names for one preferred part.
A useful comparison can therefore align three kinds of information: the loss basis under the intended waveform, the inductance behaviour at peak current, and the available size. Tradeoffs become visible when those coordinates remain attached to each candidate. The result is a technically defined comparison, without ranking a named series, calculating converter efficiency or assuming that one manufacturer's illustrative result transfers to another component.
Sources and further reading
Coilcraft Document 1400, revised July 2020, physical pages 1–4: qualitative loss allocation, measurement scope and size/inductance tradeoffs. Numerical examples, curves, tools and product rankings are not used.