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The high-frequency transformer is the heart of every switching power supply. It provides galvanic isolation, steps the voltage up or down, and transfers energy across the isolation barrier at frequencies from tens of kilohertz to several megahertz. Unlike a mains-frequency transformer, its design is a trade-off between core material, winding structure, parasitics and thermal performance — and small decisions in each area decide whether the converter is efficient, quiet and reliable or hot, noisy and failure-prone.
This guide covers the key design considerations for high-frequency transformers in switching power supplies, from core selection to insulation and encapsulation.
Why High-Frequency Transformers Are Different
At 50/60 Hz, transformer losses are dominated by the winding resistance. At 100 kHz and above, the picture changes completely:
- Core loss dominates at the edges of the design space — hysteresis and eddy current losses grow quickly with frequency and flux density.
- Skin effect and proximity effect force current into thin conductor layers, raising effective AC resistance far above the DC value.
- Parasitic capacitance and leakage inductance become part of the resonant and switching behaviour and cannot be ignored.
- Smaller core sizes become possible, because the required core cross-section shrinks as frequency rises — the whole point of switching at high frequency.
Core Material Selection
The core must provide high flux capability at the operating frequency with acceptable loss and temperature rise.
- Mn-Zn power ferrite (PC40, PC44, PC47 class): the default choice for most switching power supplies from roughly 20 kHz to 500 kHz. Different grades trade off saturation flux density, loss at temperature and cost. PC44/PC47-class materials offer lower loss at higher frequency and at elevated temperature.
- Ni-Zn ferrite: higher resistivity, used where very high frequency or high-voltage isolation is required, at the cost of lower permeability.
- Nanocrystalline cores: very high saturation flux density and low loss, used in high-power and high-performance applications where ferrite would need a much larger core.
- Iron powder and alloy powder cores: used where high DC bias tolerance matters, though their higher core loss usually limits them to filter and choke duties rather than power transfer.
A good starting rule: match the material’s optimum frequency range to your switching frequency, then check core loss at your actual flux swing and temperature.
Turns Ratio and Winding Design
The turns ratio sets the voltage conversion relationship, but the winding design determines efficiency and EMI.
- Turns ratio: derived from the input/output voltage, duty cycle (for forward converters) or resonant gain (for LLC), and the expected voltage drops.
- Skin effect: at 100 kHz the skin depth in copper is about 0.2 mm. Use thin wire, foil, or multiple strands so that most of the conductor cross-section carries current.
- Proximity effect: adjacent layers with opposing currents push current to the edges of each layer, multiplying AC resistance. Interleaving primary and secondary windings reduces this effect and also lowers leakage inductance.
- Litz wire: braided thin strands, each insulated, significantly reduce AC resistance in high-frequency windings. Use it when the AC current is high relative to the DC component.
Leakage Inductance and Parasitic Capacitance
Parasitics are unavoidable; the goal is to design them, not fight them.
- Leakage inductance stores energy that is not transferred to the secondary. In flyback converters it causes voltage spikes across the switch and requires snubbers; in LLC converters it can be absorbed into the resonant tank. Leakage is reduced by interleaving windings and by tight coupling geometry.
- Inter-winding capacitance couples high dv/dt noise across the isolation barrier, causing common-mode EMI. A Faraday shield (a grounded copper layer between primary and secondary) redirects this displacement current to ground and is standard practice in many designs.
- Document the measured leakage inductance and capacitance on the transformer datasheet — they are as important as the turns ratio to the power electronics engineer.
Insulation and Safety Compliance
Isolation transformers must satisfy the safety standards of the target market, typically IEC 61558 or IEC 62368-1 (and their UL equivalents). Design in the following from the start:
- Reinforced insulation between primary and secondary where the standard requires it, with adequate creepage and clearance distances based on the working voltage and pollution degree.
- Triple-insulated wire (TIW) for secondary windings, which allows creepage-reducing constructions while meeting reinforced insulation requirements.
- Sleeving, tape layers and bobbins with appropriate voltage ratings; document the build so the insulation system can be certified.
- Thermal class of wire and insulation materials matched to the maximum operating temperature.
Potting and Encapsulation
Encapsulated transformers use epoxy or other resins to seal the winding. Potting provides:
- Moisture and dust protection for harsh or outdoor environments
- Mechanical stability against vibration and shock
- Higher dielectric strength and better voltage withstand
- Improved thermal path to the core and terminals
The trade-off is reduced repairability and, in some resin systems, added thermal resistance. For industrial, automotive and outdoor power supplies, encapsulation is often the deciding factor between a short-lived and a long-lived transformer.
Design Checklist for Your Next Project
1. Define the electrical specification: input/output voltages, power, frequency, isolation class, ambient temperature.
2. Select the core material family from frequency, flux swing and loss budget.
3. Calculate turns ratio, primary and secondary turns, and the required core cross-section (including saturation margin at worst-case input and temperature).
4. Design the winding: conductor size for skin and proximity effects, interleaving, Litz wire where needed.
5. Minimise and characterise leakage inductance and inter-winding capacitance; add a Faraday shield where common-mode EMI is critical.
6. Apply the insulation system required by the safety standard, with documented creepage and clearance.
7. Build prototypes and measure: temperature rise, efficiency, leakage inductance, capacitance, and conducted/radiated EMI.
8. Qualify the final build with pre-production samples and 100% inspection.
Custom High-Frequency Transformers from Sirui-Tech
Shaanxi SiRui Technology Co., Ltd manufactures custom high-frequency transformers — flyback, forward, push-pull, half-bridge, full-bridge and LLC types — with ferrite cores and full epoxy encapsulation. We offer OEM & ODM service, free samples, 100% pre-shipment inspection and RoHS/REACH-compliant materials, so your converter can ship with a transformer built to your exact specification.
Send us your schematic or specification and we will propose a transformer design, prototype and production plan.
About Shaanxi SiRui Technology Co., Ltd
Shaanxi SiRui Technology Co., Ltd (Sirui-Tech) is a professional magnetic components manufacturer specializing in electrical transformers, power inductors, current sensors and magnetic cores. We offer OEM & ODM services with free samples, fast lead times and 100% pre-shipment inspection. Browse our products or contact us for a quote.