How to Write a Transformer RFQ That Gets Accurate Quotes

A Vague RFQ Costs Weeks, Not Dollars

Custom transformer quotes come back fast when the request states the electrical, mechanical, and approval envelope in a form the supplier can price without guessing. They come back slow — or wrong — when the RFQ says “100 W transformer, 12 V output, please quote.” Every missing parameter turns into either a clarification round trip or a silent assumption baked into the price, and both surface late in the program.

This is a specification checklist you can paste into your next RFQ. It is written from the supplier’s side: each item is one the factory must have before it can size a core, cost a winding, or commit a tooling date.

The Nine Blocks of a Complete RFQ

RFQ blocks, the fields inside each, and why the factory needs them
Block	Fields to state	What it changes in the quote
Electrical input	Voltage range, frequency, waveform, inrush condition	Core flux density, wire gauge, insulation class
Electrical output	Voltage(s), current(s), regulation, ripple allowed	Secondary turns, winding losses, rectifier assumption
Power and duty	Continuous VA, peak VA, duty cycle, ambient	Core size and temperature rise margin
Topology	Isolation, autotransformer, LLC, flyback, forward, PFC choke	Winding structure and leakage inductance target
Safety	Standard, insulation class, working voltage, pollution degree	Barrier stack, TIW vs tape, creepage, hipot level
Mechanical	Footprint, height, pin layout, mounting, potting	Bobbin tooling, fill factor feasibility
Connection	Lead wire length and gauge, terminals, pin pitch	Handling cost and assembly time
Quality	AQL, test coverage, PPAP level, traceability	Test time per unit, documentation cost
Commercial	Annual volume, release schedule, target price, Incoterm	Tiered pricing, tooling amortization, lead time

The Fields Buyers Leave Out Most Often

Ambient and duty cycle. A 100 VA transformer quoted at 25 °C ambient and 20 % duty will be a different, cheaper build than the same part specified for 55 °C continuous. The factory cannot know which you mean.

Leakage inductance and inter-winding capacitance. In switch-mode magnetics these are design targets, not side effects. State a number with a tolerance and a test frequency, or expect whichever the supplier’s standard split winding produces.

Inrush. Low-frequency transformers with low DC resistance and a large core draw a high first-cycle current. If the upstream breaker or fuse is sized tightly, say so; it may drive core selection.

Mechanical hard limits. Height is the binding constraint in most redesigns. Give a maximum height with the keep-out envelope drawn, not a “similar to existing” reference.

Approval evidence. If the part must arrive with a UL 1446 system file number, insulation-system documentation, or a specific test report format, state it before quoting. It affects material choice, not just paperwork.

A Minimum Viable RFQ, in Order

One paragraph of application context: what the transformer sits in, what it must survive, and the expected service life.
A parameter table with nominal, minimum, and maximum values for every electrical quantity.
An insulation and safety statement with the standard, class, and working voltage.
A mechanical drawing or an envelope with dimensions, pin positions, and height limit.
Volume forecast by year, plus first-year ramp, plus the target landed price and Incoterm.
Required documents: first article report, RoHS/REACH declarations, insulation system file, test data.
The decision timeline: when you need the quote, when you need samples, when production release is planned.

What a Good Quote Looks Like in Return

A competent supplier replies with more than a price. Expect: the proposed core material and size with the flux density and loss calculation, the winding structure with wire gauges and fill factor, the insulation barrier stack, the expected temperature rise, the tooling cost and lead time separately from unit price, the MOQ with the tier break, and the assumptions the supplier made where your RFQ was silent. Those assumptions are where cost surprises hide, so read them before comparing two quotes side by side.

Comparing Quotes Fairly

Normalize before comparing: same Incoterm, same annual quantity, tooling included or excluded consistently, test coverage identical, and both quotes referencing the same safety standard. A quote that is 12 % cheaper usually differs in one of four places — thinner barrier, lower insulation class, reduced test coverage, or a core material downgrade. Ask which one, and the comparison becomes a decision instead of a gamble.