How to Specify a Custom Transformer: A Step-by-Step Spec Guide for Design & Procurement Engineers

Specifying a custom transformer means handing your magnetics supplier a complete, unambiguous set of requirements — application and topology, electrical parameters, isolation and safety, parasitics, mechanical envelope, and compliance — so the first sample is buildable instead of a guess. This guide turns that into a numbered procedure for design and procurement engineers, ending with a spec-intake checklist that maps directly to a quote request, so nothing critical is missing when you reach out to a manufacturer like febetek.

What "specifying" a custom transformer means

Specifying a custom transformer means giving the magnetics manufacturer a complete, unambiguous requirement set — application and topology, electrical parameters, isolation and safety, parasitics, mechanical envelope, and compliance — so they can build a correct first sample instead of guessing at your intent. This page is written for the design and procurement engineers who own that spec at OEMs, PCBA shops, and design houses, and it delivers two things: a numbered procedure that walks through every spec dimension, and a single spec-intake checklist you can fill in before requesting a quote. febetek (febe Inc.), a Taiwan-based magnetics manufacturer founded in 2016, supplies custom transformers to OEM and design-house customers, so the checklist below is structured the way a supplier actually reads an incoming spec.

Step 1 — Define the application and topology

The transformer type follows from the circuit role, so the first thing your supplier needs is what the part does and where it sits. Pin this down before any numbers, because the rest of the spec changes depending on the family:

• Power transformer — line-frequency voltage step-up/step-down and isolation; the spec is driven by voltage ratio, VA rating, and frequency (50/60 Hz).
• Switch-mode (SMPS) transformer — energy transfer in DC-DC and AC-DC converters; the spec is driven by switching frequency, converter topology (flyback, forward, push-pull, half/full-bridge, LLC), and the primary/magnetizing inductance the topology assumes.
• Signal transformer — coupling and impedance matching in data or audio paths; driven by bandwidth, impedance ratio, and insertion loss.
• Current-sense transformer — measuring current via a known turns ratio; driven by the turns ratio, burden resistor, and the current range to be sensed.
• Gate-drive transformer — isolated gate signals to high-side switches; driven by volt-second product, pulse duty cycle, and isolation.
• Planar transformer — windings realized in PCB copper or lead-frame rather than wound wire; chosen for low profile, repeatable parasitics, and high-frequency SMPS use.

This taxonomy mirrors how the industry segments transformers (e.g. Triad Magnetics and Cadence both split by application/topology); the families themselves are industry-general. What is febetek-specific is the product line: febetek manufactures custom transformers across these roles, with L2 series for Current Sense, Gate Drive, DC-DC Converter, AC-DC Converter, Planar, and Server Power transformers. If your design needs low height or tightly controlled parasitics, see the planar transformer pillar for how PCB/lead-frame winding changes the spec. Browse the full range under Transformers.

Step 2 — Specify the electrical parameters

With the topology fixed, the next block is the core electrical envelope. These are the parameters every transformer datasheet leads with, and the ones a supplier needs before quoting a winding design. The example values below are illustrative, industry-typical figures for orientation only — they are not febetek measured data.

| Parameter | Unit | Why it matters | Example value (illustrative) |
|---|---|---|---|
| Turns ratio (Np:Ns) | ratio | Sets output-to-input voltage and reflected impedance | 4:1 |
| Primary / secondary voltage | V (RMS + peak) | Defines insulation stress and core flux | 400 V / 100 V |
| Primary / secondary current | A (RMS + peak) | Sizes wire gauge and copper loss | 2 A RMS / 8 A pk |
| Magnetizing / primary inductance | µH or mH | Sets ripple current and topology behaviour | 500 µH ±10% |
| Operating frequency | kHz | Drives core material and loss | 100 kHz |
| Duty cycle / waveform | % / shape | Affects flux swing and RMS currents | 45%, square |
| Power rating | W or VA | Overall thermal and core sizing | 75 W |

Give both RMS and peak for voltage and current, and state the inductance tolerance — a tolerance band is part of the spec, not an afterthought. The single most common gap we see is under-specifying turns ratio versus inductance: a ratio alone does not constrain the inductance the converter needs, and an inductance alone does not fix the ratio. State both (see the FAQ on ratio vs inductance).

Step 3 — Define isolation and safety requirements

Isolation is where safety lives, and it is the dimension most often left vague in an incoming spec. Be explicit about each of the following, because they drive bobbin geometry, winding separation, and insulation materials — none of which can be retrofitted cheaply after the first sample:

| Parameter | Unit / class | Why it matters | Example (illustrative) |
|---|---|---|---|
| Working voltage | V | Continuous voltage across the barrier | 250 V AC |
| Isolation / hi-pot withstand | kV (1 min) | Dielectric proof test the part must survive | 3 kV AC |
| Creepage | mm | Surface path between conductors (pollution + CTI dependent) | per IEC 60664 |
| Clearance | mm | Through-air gap between conductors | per IEC 60664 |
| Insulation class / system | class (e.g. B/F/H) | Thermal endurance of the insulation | class F |
| Isolation grade | functional / basic / reinforced | Sets number of insulation layers | reinforced |

Creepage, clearance, and insulation-class concepts are defined by safety standards such as IEC 60664 and IEC 62368-1 — cite the standard your end product is certified to so the supplier designs to the right pollution degree and material group.

On the manufacturer side, the relevant trust signal is the insulation system itself. febetek manufactures within a UL-recognized insulation system (UL E533808), whose scope is limited to the transformer insulation system. This is a product-level recognition for the insulation system — not a company-wide certification — and no specific withstand-voltage number is implied by the recognition; the hi-pot rating of any given part is set by your spec and verified by test. Stating the recognition accurately, and separately from the company-level ISO 9001 (see Step 6), is what keeps the claim honest.

Step 4 — Account for parasitics and performance targets

Steps 2 and 3 make a transformer that is theoretically right; the parasitics decide whether it works in your actual circuit. These second-order parameters are where a "to-ratio" part quietly fails at speed, so set targets where they matter:

| Parameter | Unit | Why it matters | How to specify |
|---|---|---|---|
| Leakage inductance | µH or % of L_pri | Drives voltage spikes, snubber loss, ZVS timing | Customer target, e.g. ≤ TBD % of primary L |
| Interwinding / winding capacitance | pF | Couples common-mode noise, affects EMI | Customer target or "minimize", value TBD |
| DC resistance (DCR) | mΩ | Conduction loss and temperature rise | Customer target per winding, value TBD |
| Temperature rise | °C | Reliability and insulation life | Customer target above ambient, value TBD |
| Efficiency | % | System loss budget | Customer/system target, value TBD |

This is the same parameter set buyers research when they look up "leakage inductance" or "winding capacitance," and the rules here are strict: febetek has no published measured leakage, winding-capacitance, DCR, temperature-rise, or efficiency figures, so every number in this table is written as a customer-supplied target or an industry-general concept, never as a febetek specification or test result. Where a real measured value would eventually live on a febetek datasheet, this guide marks it TBD rather than inventing one. State each target as a maximum (e.g. "leakage ≤ X% of primary L") and the manufacturer designs and tests to it; the actual achieved values are confirmed on the sample report you request in Step 6.

Step 5 — Specify the mechanical and thermal envelope

A transformer that meets every electrical target is still wrong if it does not fit. Give the supplier the physical box and the thermal context up front:

| Constraint | What to give | Notes |
|---|---|---|
| Footprint / max dimensions | L × W max (mm) | Board area budget; mark hard limits |
| Height | max Z (mm) | Critical for low-profile / planar designs |
| Mounting | through-hole / SMD / chassis | Affects termination and assembly |
| Termination / pin-out | pin map, pitch | Match your PCB land pattern |
| Potting / encapsulation | yes/no, material | For vibration, moisture, isolation |
| Operating / ambient temp range | °C min…max | Sets insulation class and derating |
| Cooling assumptions | natural / forced air | Affects allowed temperature rise |

If height is the binding constraint, planar construction is the lever — windings in PCB copper or lead-frame collapse the Z-height that wound wire cannot. febetek's manufacturing capability here is real and can be stated: PCB/lead-frame planar winding, with custom turns ratio, custom insulation build, and custom mechanical outline. What this guide does not state is any specific febetek height, footprint, or temperature-rise figure — those are part-specific and marked TBD until a quoted design exists. See the planar transformer pillar for how planar construction trades against the parasitics in Step 4.

Step 6 — Pin down standards, compliance and testing

The last spec dimension is how the part is proven and documented. Two sub-parts: which standards apply, and which deliverables you want back.

Environmental and safety standards. State the environmental compliance you need (RoHS, REACH) and the safety/performance standards your end product is certified to — for example IEC 62368-1 or IEC 61558 for safety isolation, IEC 60664 for creepage/clearance, or relevant IPC standards for planar PCB construction. Cite a standard by number only when it genuinely applies to your product; an over-broad list just slows the quote.

Test and documentation deliverables. Ask explicitly for what you need to qualify the part: hi-pot (dielectric withstand) test, inductance and turns-ratio verification, DCR measurement, a first-article/FAT report, and a per-lot sample report. Specifying the deliverables in the RFQ avoids a second round later.

febetek's quality basis is two recognitions and no more: ISO 9001 at the company level (quality management system) and UL E533808 at the product level (scope limited to the transformer insulation system, as in Step 3). No other certifications are claimed and no audit data is fabricated here. Any RoHS / REACH / Conflict Minerals documentation a part requires is handled as part of the per-project deliverables you list in the RFQ, not asserted as a standalone marketing claim.

The spec-intake checklist

This is the part to keep. Fill it in before you ask for a quote, and hand the completed list to febetek — it maps field-for-field onto the questions a supplier asks back. Mandatory fields are the minimum to start a quote; nice-to-have fields tighten the design and shorten the back-and-forth.

| Step | Field | Mandatory? |
|---|---|---|
| 1 | Application / circuit role | ✅ Mandatory |
| 1 | Transformer family (power / SMPS / signal / current-sense / gate-drive / planar) | ✅ Mandatory |
| 1 | Converter topology (if SMPS) | ✅ Mandatory |
| 2 | Turns ratio and primary/magnetizing inductance (+ tolerance) | ✅ Mandatory |
| 2 | Primary / secondary voltage (RMS + peak) | ✅ Mandatory |
| 2 | Primary / secondary current (RMS + peak) | ✅ Mandatory |
| 2 | Operating frequency | ✅ Mandatory |
| 2 | Duty cycle / waveform | ☐ Nice-to-have |
| 2 | Power rating (W/VA) | ✅ Mandatory |
| 3 | Working voltage | ✅ Mandatory |
| 3 | Isolation grade (functional / basic / reinforced) | ✅ Mandatory |
| 3 | Hi-pot withstand requirement | ✅ Mandatory |
| 3 | Creepage / clearance (or the standard you certify to) | ☐ Nice-to-have |
| 3 | Insulation class | ☐ Nice-to-have |
| 4 | Leakage inductance target | ☐ Nice-to-have |
| 4 | Winding capacitance target | ☐ Nice-to-have |
| 4 | DCR target | ☐ Nice-to-have |
| 4 | Temperature-rise / efficiency target | ☐ Nice-to-have |
| 5 | Max footprint and height | ✅ Mandatory |
| 5 | Mounting type (TH / SMD / chassis) | ✅ Mandatory |
| 5 | Termination / pin-out | ☐ Nice-to-have |
| 5 | Operating / ambient temperature range | ✅ Mandatory |
| 5 | Potting / cooling assumptions | ☐ Nice-to-have |
| 6 | Environmental compliance (RoHS / REACH) | ☐ Nice-to-have |
| 6 | Applicable safety standard(s) | ☐ Nice-to-have |
| 6 | Required test reports (hi-pot / inductance / FAT / sample) | ☐ Nice-to-have |

The absolute minimum to start a quote is: the application and topology (Step 1), the key electricals — ratio + inductance, voltages, currents, frequency, power (Step 2), the isolation requirement — working voltage, grade, hi-pot (Step 3), and the mechanical envelope — footprint, height, mounting, temperature range (Step 5). Parasitic targets (Step 4) and documentation requests (Step 6) refine the design but do not block a first conversation. Fields here map directly onto the RFQ categories Magnetics - Transformers (catalog-adjacent custom) and Magnetics - Custom (fully bespoke).

Request a quote — submit your spec

Once the checklist is filled in, send it to febetek to start a custom transformer quote. Use the RFQ form with the category prefilled to Magnetics - Transformers; for a fully bespoke part with no catalog starting point, select Magnetics - Custom instead. Paste the completed spec-intake table into the description field — that single block is usually enough for a first response.

febetek serves OEM and design-house engineers and manufactures custom magnetics under a UL-recognized insulation system (UL E533808, scope limited to the transformer insulation system), with company-level ISO 9001 quality management. The more of the checklist you complete, the closer the first sample lands to your circuit.