Custom Magnetic Components: The OEM Development Process from RFQ to Mass Production

A custom transformer or inductor is not a catalogue item with a different label — it is a joint development project with its own gates, deliverables and failure modes. The process below is the one that consistently reaches mass production without a redesign loop, and it is the sequence a magnetic component manufacturer should be able to describe before you send them a drawing.

1. RFQ and technical intake

The intake package decides the quality of everything downstream. A workable RFQ contains: the operating envelope (input and output voltage, frequency, duty cycle, load profile), the electrical targets (inductance, DCR, turns ratio, isolation), the mechanical constraints (footprint, height, pinout, mounting), thermal and environmental limits, the qualification standard, and annual volume with forecast shape.

Missing any one of these produces a first sample that meets some requirements and misses others — the most common cause of a two-round prototype cycle.

2. Design review and DFM feedback

The supplier should return a design proposal, not a quotation: core shape and material grade, turns and wire gauge, winding sequence, gap method, insulation scheme, potting or varnish choice, and a thermal estimate. This is where manufacturability is settled. Typical DFM questions a good supplier raises at this gate:

Can the required creepage and clearance be met in this height, or does the bobbin need a wall?
Is the winding automated or hand-wound, and does the volume justify the fixture cost?
Does the target DCR force a larger window, which changes the core size and cost?
Is the insulation class required by the ambient actually reachable without a larger core?

3. Prototype build and electrical verification

Gate	Deliverable	Verification
Design freeze	Drawing, BOM, core grade, process flow	Signed specification, agreed measurement conditions
Prototype (3–10 pcs)	Samples plus measured data	Inductance vs bias, DCR, turns ratio, leakage, hipot
DVT	Design verification test report	Thermal rise at max load, efficiency, EMI pre-scan
PVT	Pilot lot on production tooling	Cpk on key dimensions, yield, first-pass rate
Qualification	Reliability report	Thermal cycling, vibration, damp heat, hipot, life test
PPAP / approval	Full documentation package	FAI, material certs, control plan, MSA
Ramp	Production lots	Lot data review for first three shipments

4. Where custom magnetics projects usually slip

Thermal budget. A design that passes at 25 °C ambient with natural convection can fail at 55 °C inside a sealed enclosure. Temperature rise must be tested in the actual enclosure, at maximum load, with the actual airflow.

Bias-dependent inductance. Datasheet inductance is measured at zero bias. A converter running at 60% of saturation sees a much lower value, and the ripple current that results is the difference between a compliant design and a failing one.

Creepage in a small package. Miniaturisation and isolation requirements pull in opposite directions. Resolving it requires a margin design or a taller package, and it must be decided before tooling.

Core grade substitution. A “drop-in” core with the same AL value but a different loss characteristic changes temperature rise and EMI. Lock the grade contractually and require notification of any change.

5. Documentation to require at PPAP

Drawing with revision control, bill of materials with approved sub-suppliers, core material certificate per lot, FAI report with measured values, process flow diagram, control plan, measurement system analysis for the inductance and hipot stations, and the qualification test report. Without these, the first production deviation has no baseline to compare against.

6. Timeline that holds

For a wound part on an existing core family: 7–10 working days to first samples, 3–4 weeks to DVT, 6–8 weeks to PPAP. For a new core geometry or a new bobbin tool: add 3–4 weeks for tooling and 2–3 weeks for qualification. Programs that compress the qualification stage usually recover the schedule later in field returns.

Shaanxi SiRui Technology runs this sequence in one plant: the Magnetic Core series (Mn-Zn and Ni-Zn ferrite, iron powder, sendust, high flux, MPP, nanocrystalline and amorphous) is produced and gapped in-house, then wound, potted and tested in the same facility — so a design change in gap or material does not restart a supplier negotiation.

Send your RFQ to sales@sirui-tech.com for a design proposal and costed BOM.