How it works
A copper winding is surrounded by a compacted magnetic composite. The winding stores energy in a magnetic field; the magnetic path and distributed gap help it carry DC current. Inductance usually rolls off progressively with current. “Soft saturation” gives a less abrupt knee, but it does not stop ripple and peak current from growing.
Current in a winding creates a magnetic field. Increasing current can change core permeability and reduce inductance.
Parameters that matter
- Effective inductance
- L is the small-signal inductance at the stated frequency and bias. Use the minimum effective L at operating current and temperature for ripple calculations.
- Two current limits
- Isat is tied to an allowed drop in L; 10%, 20% and 30% definitions cannot be compared directly. Irms or heat-rating current is a separate temperature-rise test.
- Loss and temperature
- DCR drives copper loss, approximately Irms² × DCR. Copper resistance rises with temperature. Core and AC winding losses also grow with the actual ripple waveform and switching frequency.
- Leakage field
- Shielding helps contain leakage flux; it is not zero magnetic coupling. Check clearance from current sensing and RF traces.
A worked selection example
Illustrative calculation · not a product guarantee
Size the inductor for a 12 V to 5 V buck
Ideal buck: 12 V input, 5 V output, 2 A load and 500 kHz switching. Target ripple is 0.6 A peak to peak. A 10 µH candidate has ±20% initial tolerance. DC-bias reduction and temperature effects are not yet included.
- Calculate the starting inductance: L = Vout × (1 − Vout/Vin) / (f × ΔI) = 9.72 µH.
- Apply the candidate’s negative tolerance: 10 µH × 0.8 = 8 µH. Use this value to recalculate ripple: ΔI = 0.729 A.
- Calculate current stress: peak = 2 + 0.729/2 = 2.365 A; RMS = √(2² + 0.729²/12) ≈ 2.01 A.
- Request the hot L-versus-current curve and recalculate with the minimum effective inductance. Include startup and current-limit overshoot in the peak-current check; assess temperature rise separately at the RMS current.
A nominal 10 µH marking is only a starting point. The candidate still needs magnetic and thermal verification; this example does not approve a specific MWSA part.
Where it fits
Suitable starting points
- Compact buck/boost rails; point-of-load conversion; battery-powered equipment with peak-load steps.
Where to take extra care
- Do not assume the smallest molded part is lowest loss. High ripple or unusual frequency can favor a different core. Not a signal-line bead or an RF matching part.
Series directions to investigate
Screen Sunlord MWSA/MWTC and CODACA molded families by footprint, maximum height and current curves. The family prefix is a search direction; suffixes can change materials, dimensions and qualification.
What Chinese suppliers can offer
Where the opportunity lies
Mainland catalogs offer multiple molded sizes alongside custom magnetics. This can make a height-constrained redesign practical: request two catalog sizes and a custom option against the same hot-loss target. CODACA explicitly describes custom core/material capability. Whether customization pays off depends on tooling, MOQ, validation cost and annual demand.
What still needs evidence
Confirm the exact manufacturing source, current datasheet, sample results and commercial terms before placing an order. Series availability does not establish qualification for your application.
Questions for the supplier
- 01
L-I curves at room and hot temperatures, with the Isat drop criterion.
- 02
Temperature-rise fixture, copper area and allowable component temperature including self-heating.
- 03
Loss estimate at your frequency/ripple; reflow, mechanical and lot-traceability evidence for the exact code.
Technical references
These references support the principles and catalogue directions discussed here. Public documentation is a different evidence level from a supplier reply, a lot document or an independent test. This guide does not claim those later stages have been completed.
- 1. Ferrite vs. Pressed Powder-core Inductors, Doc 1733
- 2. Getting Started: Inductor Specifications, Doc 1287
- 3. Selecting the Best Inductor, Doc 469
- 4. Basic Calculation of a Buck Converter Power Stage, SLVA477B
- 5. MWSA-S molded power inductor datasheet
- 6. Declaration of conformity: product platforms and series
- 7. VSEG0660 molded power inductor
- 8. Customized Products
These guides explain selection principles. Final decisions require the current datasheet for the exact ordering code, operating conditions and appropriate application testing.