Tooling Is the Point of No Return
In a custom magnetics project, the expensive mistakes are almost always made in the two weeks before tooling is approved. After that, a bobbin change means a new mould, a core change means a new supplier lead time, and a winding change means a new machine programme. Ten questions, asked in this order, catch most of them while they are still cheap.
Design and Electrical
- Is the inductance specified at the operating point? Frequency, DC bias and temperature belong in the specification. “10 mH” alone will be built by whoever quotes it, and their interpretation will not be yours.
- Are the tolerance and the consequence both stated?
- Has the leakage inductance been given a target, not just a maximum? In flyback and LLC designs the leakage value shapes the resonant behaviour; a part 30 % under the maximum can still ruin the waveform.
- Is the winding construction fixed? Layer, interleaving, margin tape and termination method decide parasitics and creepage. If the drawing leaves them open, two suppliers will deliver two different parts under one part number.
Mechanical and Thermal
- Does the part fit the enclosure with the tolerances stacked worst-case? Bobbin moulding tolerance, core dimension tolerance and potting thickness all add. A nominal fit is not a fit.
- Is the thermal path defined? Bolted to a chassis, potted to a heatsink, or free air: the same design gives radically different rises, and the mounting is usually decided by the mechanical team without telling the magnetics team.
- What is the permitted hot-spot rise and at what ambient? If this is not written down, the supplier will design to a comfortable rise at 25 °C and the field will disagree.
Compliance and Manufacturing
- Which insulation system is the part qualified under, and does the proposed construction match it? A UL-recognised system covers a specific combination of wire, tape, bobbin and varnish. Substituting any one of them voids the qualification.
- What is the test plan, and who pays for the first article? The first article should include dimensional, electrical, hipot, temperature rise and material declarations. If the test plan is not agreed before tooling, it will be negotiated after, when the supplier has leverage.
- What changes require a new approval? Write the change-control rule into the PO: core grade, wire supplier, varnish, bobbin resin, winding machine and test method. Each of these is invisible in a delivered part and visible in the field.
Red Flags in the Quotation Stage
- A price without a drawing, or a drawing without tolerances.
- A lead time that does not mention the tooling or the first-article cycle separately.
- “Same as your current supplier” with no drawing reference.
- A temperature rise quoted without stating the ambient and the mounting.
- A certificate list offered instead of a test plan.
Related reading
- Custom Magnetic Components: The OEM Development Process
- MOQ, Lead Time and Tooling
- Supplier Scorecard
- Bobbin Design Guide
Before you approve tooling, send us the drawing and the application conditions; we will return a marked-up design review with the ten questions answered in writing. Magnetic Core team: sales@sirui-tech.com.