How to Design a Custom Transformer: Step-by-Step Process

When standard off-the-shelf transformers don’t fit your application, a custom design is the answer. But designing a custom transformer from scratch can seem daunting—what information do you need? How does the process work? What can you expect from a magnetics supplier?

This guide walks through the complete custom transformer design process, from specification to production.

Shaanxi SiRui Technology Co., Ltd. designs and manufactures custom transformers and inductors for power electronics applications. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com

1. What Information to Provide

Topology: buck, boost, flyback, LLC, forward, line frequency
Input voltage range: e.g., 90–265 VAC or 400 VDC bus
Output voltage and current: e.g., 12 V, 5 A
Power level and switching frequency
Isolation requirement: e.g., 3 kV, 4 kV reinforced
Mechanical constraints: max dimensions, mounting style
Ambient temperature and target efficiency
Safety standards: IEC 60601, UL, IEC 60950

2. Design Process Steps

Specification review: manufacturer reviews and flags constraints
Core selection: material and shape based on frequency, power, size
Turns calculation: based on voltage ratio and target flux density
Wire sizing: round wire, litz, or foil based on frequency and current
Loss calculation: core loss + copper loss, verify efficiency target
Thermal check: verify temperature rise within insulation limits
Prototype build: 5–20 pieces for your testing
Testing and iteration: you test in application, request changes
Production qualification: tooling, test procedures, qualification batch
Mass production: 100% tested units

3. Typical Timeline

Phase	Duration
Design and quotation	1–3 days
Prototype build	1–2 weeks
Testing and iteration	2–4 weeks
Production qualification	2–4 weeks
Total to mass production	4–8 weeks

4. Common Mistakes

Incomplete specification: missing frequency or isolation data delays design
Unrealistic size requirements: 100 W in a 20 mm cube
No prototype testing: assuming calculations are perfect
Ignoring EMI: transformer design heavily affects conducted EMI
No efficiency margin: target 97%, design for 98%

5. Sirui-Tech Custom Design

Free design review and quotation within 1–3 days
Prototypes in 1–2 weeks
Full test report with every prototype batch
100% production testing: L, DCR, hi-pot
Experience: transformers, inductors, CMCs, current sensors

Conclusion

Designing a custom transformer is a collaborative process. Provide a complete specification, test prototypes thoroughly, and allow enough time for iteration. A well-designed custom transformer outperforms any off-the-shelf solution.

Need a custom magnetics design? Contact Sirui-Tech at sales@sirui-tech.com with your application requirements—free design review and quotation.