How to Specify a Custom SMPS Transformer Without Costly Prototype Rework

Short answer: A usable SMPS transformer specification needs more than input voltage, output voltage, and power. Before a winding supplier can build a meaningful sample, the RFQ should define the converter topology, switching frequency, operating mode, turns ratio or controller reference design, magnetizing inductance, isolation requirement, winding arrangement, mechanical envelope, temperature limit, and the test conditions used to approve the part.

Leave those points open and the first sample may still look correct on an LCR meter—but fail once it is installed in the converter.

Why SMPS Transformer Samples Fail Even When the Drawing Looks Complete

Most prototype problems start before the first turn of copper goes onto the bobbin.

A buyer sends a drawing with a core size, pinout, primary turns, secondary turns, and a target inductance. The transformer supplier follows it. The sample arrives, but the MOSFET drain spike is higher than expected, the snubber runs hot, regulation changes under load, or the winding temperature rises too quickly.

The usual problem is not simply “bad winding.” The drawing did not capture the electrical conditions that control the magnetic design.

An SMPS transformer is part of a switching circuit. Its behavior depends on the controller, duty cycle, waveform, switching frequency, operating mode, clamp circuit, rectifier arrangement, insulation system, PCB layout, and production tolerances. A specification that ignores those conditions forces the transformer manufacturer to guess.

The purpose of a good RFQ is to remove those guesses before tooling and sampling begin.

1. Start With the Converter Topology, Not the Core Size

The first line of the specification should identify the power topology:

• Flyback
• Forward
• Push-pull
• Half-bridge
• Full-bridge
• LLC resonant
• Auxiliary bias supply
• Isolated gate-drive supply

This matters because the magnetic component does not perform the same job in every topology.

In a flyback converter, energy is stored in the magnetic field during the primary switch on-time and delivered to the secondary during the off-time. The component behaves more like a coupled inductor than an ideal transformer. Magnetizing inductance, air gap, peak current, and leakage inductance are therefore central design parameters.

In forward, bridge, and resonant converters, energy transfer and core reset behave differently. Reusing a flyback specification template for an LLC or forward transformer is a common way to create an unusable sample.

If the design follows a controller vendor’s reference circuit, include the exact controller part number and a link or document revision for the reference design. That gives the transformer engineer the operating assumptions behind the winding data.

2. Define the Real Input and Output Conditions

“Input: 230 VAC; Output: 24 V” is not enough.

The design should state:

For multi-output designs, identify which output is regulated by feedback. Also state whether the other outputs may be unloaded. Cross-regulation cannot be judged from output voltages alone.

A useful specification describes the worst case, not just the nominal operating point.

3. State the Switching Frequency and Operating Mode

Switching frequency affects core loss, copper loss, winding capacitance, and the number of turns needed to avoid excessive flux density.

Provide:

• Nominal switching frequency
• Minimum and maximum switching frequency
• Fixed-frequency, valley-switching, burst, or variable-frequency operation
• Continuous conduction mode (CCM), discontinuous conduction mode (DCM), critical conduction mode (CrM), or mixed operation
• Maximum duty cycle
• Frequency during startup, standby, and overload if different

The same transformer may behave well at the nominal frequency and run hot during a low-frequency startup or overload condition. If the controller folds back frequency under fault conditions, include that behavior in the design review.

For flyback converters, controller design guides commonly derive magnetizing inductance and turns ratio from minimum input voltage, peak current, duty cycle, output requirement, and switching frequency. Texas Instruments discusses these relationships in its flyback design references, including its Magnetics Design 5 guide and LM3481 flyback design note.

4. Do Not Specify Magnetizing Inductance Without Test Conditions

A line such as “Primary inductance: 450 µH” is incomplete unless the measurement conditions are defined.

Include:

• Measurement frequency
• Measurement voltage or current
• Series or parallel measurement mode
• Which windings remain open or shorted
• Allowed tolerance
• Ambient temperature, if critical
• Whether the value applies before or after assembly and varnish

For a gapped flyback transformer, inductance also has to remain compatible with the required saturation margin at peak primary current. A supplier may hit the room-temperature LCR value while the finished part still has inadequate margin under low-line, full-load, or elevated-temperature operation.

If the design was calculated from a controller spreadsheet, send the calculation sheet. It is more useful than copying only the final inductance number into a drawing.

5. Treat Leakage Inductance as a Circuit Parameter

Leakage inductance is not just a transformer datasheet line. In a flyback converter, the energy stored in leakage inductance contributes to the drain-voltage spike when the primary switch turns off. That affects:

• MOSFET voltage margin
• RCD or active-clamp design
• Snubber loss
• EMI performance
• Efficiency
• Long-term component stress

Lower leakage inductance usually requires tighter magnetic coupling, but winding arrangements that improve coupling can increase primary-to-secondary capacitance. That may worsen common-mode noise. The correct winding structure is therefore a trade-off, not a contest to achieve the smallest possible leakage value.

Specify leakage inductance together with:

• Measurement frequency
• Which winding is measured
• Which winding is shorted
• Maximum allowed value
• Maximum primary-to-secondary capacitance, if EMI is sensitive
• Any required shield winding
• The actual clamp or snubber method

Coilcraft’s transformer guidance notes that flyback designs generally aim to control leakage inductance because it contributes to switching spikes. Its Getting Started with Transformers guide provides a useful overview of transformer selection and flyback applications.

6. Specify the Insulation System Before Winding Layout

Isolation requirements change the bobbin, tape system, wire type, winding margins, creepage distance, clearance, and sometimes the achievable coupling.

State:

• Functional, basic, supplementary, or reinforced insulation
• Working voltage
• Required dielectric withstand test and duration
• Pollution degree and overvoltage category, where applicable
• Maximum operating altitude
• Applicable end-product safety standard
• Creepage and clearance targets
• Whether triple-insulated wire is acceptable
• Whether margin tape, sectional bobbin, or shielding is required

For general switch-mode power supply units and their transformers, IEC 61558-2-16:2021 addresses particular safety requirements and tests. The transformer specification should still follow the requirements of the final product category; medical, industrial, automotive, household, and information-technology equipment may impose different insulation and test conditions.

Do not tell a supplier only that the transformer must be “3 kV isolated.” A hipot number by itself does not define the complete insulation system.

7. Give the Manufacturer a Mechanical Envelope That Matches the PCB

A core code is not a complete mechanical specification.

Include:

• Maximum length, width, and height
• Pin count, pin pitch, row spacing, and pin diameter
• Pin assignment and winding polarity dots
• PCB keep-out area
• Maximum seating height above the PCB
• Orientation requirements
• Bobbin and core restrictions
• Lead-free soldering profile, if relevant
• Vibration or shock requirements
• Potting, varnish, tape, and marking requirements

If height is the limiting dimension, say so. A supplier may otherwise choose a larger core that improves temperature rise but cannot fit the enclosure.

Send both a dimensioned PDF and a STEP file when possible. The PDF controls the drawing; the STEP file helps check mechanical interference.

8. Define Temperature Limits at the Correct Location

“Temperature rise below 40°C” raises several questions:

• Measured on the core, winding, bobbin, or ambient air?
• At what input voltage and load?
• In open air or inside the final enclosure?
• With what airflow?
• After how much stabilization time?
• At what ambient temperature?

A bench test on an open PCB can hide a thermal problem that appears inside a sealed product.

For the first sample review, define at least three operating points:

1. Minimum input, full continuous load
2. Nominal input, full continuous load
3. Maximum input, the specified load condition

If standby, burst mode, overload, or peak power matters, add those points separately.

9. Ask for a Sample Report, Not Just Samples

A transformer sample without measurement data slows the review process. Ask the supplier to report the agreed parameters for each sample or lot.

A practical first-article report may include:

• Turns ratio and polarity
• Primary magnetizing inductance
• Leakage inductance
• DC resistance for each winding
• Interwinding capacitance, when required
• Hipot or insulation-resistance result
• Core and winding temperature at agreed test points
• Physical dimensions
• Sample identification and winding revision

For production control, agree which parameters require 100% testing and which can be sampled. Do not automatically demand every possible test on every unit; match the inspection plan to the real failure risks of the converter.

10. Use This SMPS Transformer RFQ Checklist

The following checklist gives an engineering team enough context to review a custom transformer request without a long chain of clarification emails.

Converter information

• Topology:
• Controller IC and reference design:
• Operating mode:
• Nominal/minimum/maximum switching frequency:
• Maximum duty cycle:

Electrical input

• Input type:
• Minimum input:
• Nominal input:
• Maximum input:

Outputs

• Main output voltage/current:
• Peak output power and duration:
• Auxiliary output voltages/currents:
• Feedback-regulated output:
• Allowed cross-regulation:

Magnetic targets

• Preferred core and bobbin, if fixed:
• Primary turns:
• Secondary turns:
• Auxiliary turns:
• Magnetizing inductance and measurement condition:
• Maximum peak primary current:
• Maximum leakage inductance and measurement condition:
• Maximum interwinding capacitance, if specified:

Safety and insulation

• Working voltage:
• Insulation class:
• Hipot requirement:
• Creepage and clearance requirement:
• Applicable end-product standard:
• Maximum operating altitude:

Mechanical and environmental

• Maximum dimensions:
• Pinout and polarity:
• Ambient temperature range:
• Maximum permitted temperature rise:
• Cooling condition:
• Vibration, potting, varnish, and soldering requirements:

Commercial information

• Prototype quantity:
• Forecast volume:
• Target sample date:
• Drawing and BOM revision:
• Required first-article data:

When requesting an engineering review from Chengpin Technology, attach this checklist to the schematic, transformer drawing, and controller reference. It makes the discussion specific and gives both sides a clear revision baseline.

11. What Should You Send If the Design Is Not Finished?

A complete transformer drawing is not always available during early development. That is fine. Send the information that is already fixed and clearly mark what is still open.

At minimum, provide:

• Topology and controller
• Input range
• Output requirements
• Switching-frequency range
• Isolation requirement
• Mechanical envelope
• Peak current or power target
• The design stage: concept, EVT, DVT, or production transfer

Do not invent target values simply to complete the RFQ. A good engineering review should separate customer-controlled requirements from values the transformer designer is expected to calculate.

Frequently Asked Questions

Is an SMPS transformer the same as a 50/60 Hz power transformer?

No. An SMPS transformer operates with high-frequency switched waveforms, so core material, winding loss, leakage inductance, parasitic capacitance, insulation layout, and switching behavior are central to the design. A mains-frequency transformer specification cannot simply be scaled down for high-frequency use.

Why does a flyback transformer need an air gap?

In a flyback converter, energy is stored in the magnetic field during the switch on-time. The air gap helps set the required magnetizing inductance and increases the energy-storage capability before saturation. The correct gap depends on the target inductance, core geometry, turns, and peak current.

What causes a high MOSFET drain spike after changing transformer suppliers?

One common cause is a change in leakage inductance or winding capacitance caused by a different winding arrangement. Lead length, PCB layout, clamp components, and measurement setup also matter. Compare the transformer and circuit together before assigning the fault to one component.

Should leakage inductance always be as low as possible?

Not without considering the rest of the converter. Lower leakage generally reduces switching spikes and clamp loss, but tighter coupling can increase interwinding capacitance and common-mode noise. The specification should define both performance limits when EMI is important.

Which values should appear on an SMPS transformer drawing?

At minimum: turns ratio, polarity, magnetizing inductance with test conditions, leakage inductance with test conditions, winding resistance limits, pinout, dimensions, insulation construction, hipot requirement, materials or approved equivalents, and revision control.

Can a transformer supplier design from only input voltage, output voltage, and power?

Those values are enough for a preliminary discussion, not a production design. The supplier still needs the topology, switching behavior, controller, current limits, isolation requirements, mechanical envelope, thermal conditions, and approval criteria.

When should the transformer be reviewed again?

Review it whenever the controller, switching frequency, maximum duty cycle, MOSFET voltage rating, output configuration, insulation requirement, PCB layout, enclosure, or cooling condition changes. A transformer approved for one converter revision may not remain suitable after those changes.

Final Takeaway

The best custom SMPS transformer RFQ is not the longest drawing. It is the one that defines the converter’s real operating conditions and the measurements used to accept the sample.

Start with topology and worst-case electrical conditions. Define inductance and leakage with test conditions. Set the insulation system before finalizing the winding layout. State the mechanical and thermal limits. Then require a sample report that uses the same revision and test definitions.

That process prevents most avoidable prototype loops—and gives the transformer supplier enough information to solve the actual converter problem instead of guessing what the drawing left out. If you want Chengpin Technology to review a new SMPS transformer project, send the completed checklist together with the schematic and the current drawing revision.

Technical References

1. Texas Instruments, Magnetics Design 5 — Inductor and Flyback Transformer Design.
2. Texas Instruments, How to Design a Flyback Converter With the LM3481.
3. Coilcraft, Getting Started with Transformers.
4. IEC, IEC 61558-2-16:2021 — Safety of Switch Mode Power Supply Units and Transformers.