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A high-current DC-DC converter can run hot even when its switching devices are efficient because heat comes from losses throughout the power stage—not just the switches. The inductor can still contribute substantially through winding resistance and core losses. A flat-wire inductor may reduce winding resistance by using space more efficiently, but it cannot by itself eliminate core loss, switching loss, or limitations in the PCB and cooling.
Why a high-current converter still generates heat
Every watt of electrical loss becomes heat somewhere in the converter. Switching-device conduction and switching losses are part of the total, alongside inductor, PCB-copper, and other component losses. Improving the switching stage does not remove the losses elsewhere.
TDK identifies winding copper loss and core-material loss as two main sources of inductor temperature rise. In a buck converter, the inductor carries current closely related to the output current, with switching ripple superimposed. That makes the inductor a potentially important heat source even when the converter’s switches perform well. TDK’s power-inductor application guidance explains both loss categories.
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Winding loss: current through resistance
The first-order estimate for winding conduction loss is P = I² × DCR. Here, I should represent the relevant winding current and DCR the winding’s DC resistance. Because current is squared, a small resistance reduction can matter at high current. With ripple, using only average current is an approximation; integrating the current waveform over a switching cycle gives a more accurate estimate. Analog Devices’ efficiency primer discusses this estimate and waveform-aware calculation.
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At switching frequencies, winding loss can also include AC resistance. Skin effect makes current distribution less uniform within a conductor, while proximity effects from nearby conductors and magnetic fields can further change how current flows. Consequently, room-temperature DCR alone does not describe total winding loss in operation.
Core loss: changing magnetic flux
The magnetic core dissipates energy as its flux changes. Hysteresis and eddy-current losses depend on the core material and operating conditions, including frequency and the flux swing. An inductor with low DCR can therefore still run warm if its core loss is significant.
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The balance shifts with load. TDK notes that copper loss often dominates at moderate-to-heavy load, while core loss can dominate at light load in constant-frequency operation. “It gets hot because the current is high” is therefore not a complete diagnosis.
How flat-wire windings can help
Round wire leaves gaps in a winding window. A flat conductor can pack more efficiently, allowing greater conductor cross-section in a similar space or a more compact winding. In a suitable design, that can lower DCR and reduce the winding’s I²R loss. The potential benefit is especially relevant in high-current designs, where resistance is multiplied by current squared.
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Some optimized helical flat-wire geometries can also have lower AC winding resistance than a solid round-wire implementation, but this is design-specific—not a guarantee based on conductor shape alone. Flat wire does not remove core hysteresis or eddy-current losses, and it does not eliminate AC effects. Core material, magnetic-gap design, field distribution, switching frequency, and the current waveform still influence total inductor loss.
For a concrete example, Bourns describes its ER26A family as using a helical flat-wire winding, magnetic shielding, and a high-temperature ferrite core for compact DC-DC converter applications. That is product-family positioning, not evidence of a particular temperature reduction or efficiency gain; no comparative performance figure is established here. The published ER26A description gives the construction details.
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How to choose and compare inductors
Compare candidates under the same converter operating conditions. A flat-wire part is useful only if its electrical, magnetic, thermal, and physical characteristics fit the application.
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- DCR at operating temperature: Note the datasheet’s DCR test temperature, then account for the higher resistance expected as the winding warms. Do not treat room-temperature DCR as total loss.
- Peak current and saturation: Compare peak current with the part’s saturation behavior and read how the manufacturer defines saturation. Inductance may fall as current rises.
- RMS current and temperature rise: Check RMS current against the temperature-rise rating, taking ambient temperature, PCB copper, and cooling into account.
- AC and core loss: Seek loss curves or manufacturer data for the actual switching frequency and ripple waveform, particularly when the datasheet does not provide enough information to estimate them.
- Construction and fit: Consider core material, shielding and stray-field effects, footprint, height, cost, and availability. Shielding may reduce stray-field coupling, but EMI also depends on the full layout, grounding, switching loops, and nearby components.
Do not treat the current ratings as interchangeable. TDK describes rated current generally as the lower of a saturation-related limit and a temperature-rise limit. Coilcraft likewise recommends checking peak current for saturation separately from RMS current for heating, and notes that DCR changes with temperature. See TDK’s selection guidance and Coilcraft’s inductor-selection guidance.
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What published flat-wire family ranges tell you
Eaton lists its HFW high-current flat-wire family for DC-DC converter applications with family-level ranges of 5–100 A current applications, 2–33 μH inductance, 22 mm and 28 mm SMT footprints, and heights of 10–18 mm. These are ranges across the family, not specifications that apply simultaneously to one part. Verify the exact variant’s datasheet, ratings, and test conditions before selection. Eaton’s HFW family page provides the listed ranges.
When changing the inductor will not solve the heat problem
If the converter remains hot after considering the inductor, look at total losses and heat removal rather than assuming the winding is the only cause. Switching-device losses, PCB copper resistance, layout, operating point, airflow, and cooling can all constrain temperature. A lower-DCR inductor cannot fix excessive switching loss, inadequate board copper, poor airflow, or a problematic layout.
Estimate winding loss from the actual current waveform, consider core loss at the operating frequency and flux swing, and evaluate temperatures in the real board and cooling environment. That separates an inductor limitation from a broader power-stage or thermal-design problem.
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