The flyback converter is the most popular isolated topology for low-to-medium power power supplies—chargers, adapters, LED drivers, auxiliary power supplies. The flyback transformer is the key component that provides isolation, energy storage and voltage transformation.
This guide walks through flyback transformer design step by step.
Shaanxi SiRui Technology Co., Ltd. designs and manufactures flyback transformers for SMPS applications. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. Flyback Basics
A flyback transformer is not a true transformer—it’s a coupled inductor. Energy is stored in the core during switch-on and released to the output during switch-off.
Key parameters: input voltage range (e.g., 90–265 VAC), output voltage/current, switching frequency (typically 65–132 kHz), CCM or DCM operation.
2. Turns Ratio Calculation
Reflected voltage V_or = V_out × (N_pri/N_sec) + V_diode. Typical V_or = 80–120 V for universal input.
Example: 90 VAC input, 12 V output, V_or = 100 V: N_pri/N_sec ≈ 7.9:1
3. Primary Inductance
L_pri = (V_in_min × D_max)² / (2 × f_sw × P_out × (1 – η))
For a 24W adapter at 100 kHz: L_pri ≈ 1–3 mH (typical range).
4. Core Selection
| Power | Core Size |
|---|---|
| <10 W | EE10–EE16, EER28 |
| 10–30 W | EE16–EE25, EER28 |
| 30–60 W | EE25–EE30, EER30 |
| 60–120 W | EE30–EE40, ETD34 |
Core material: PC40, PC44, or 3C95 ferrite for 65–132 kHz.
5. Winding Layout for Low EMI
- Sandwich winding: primary-secondary-primary (reduces leakage)
- Auxiliary winding: closest to secondary for cross-regulation
- Leakage inductance target: 2–5% of L_pri
6. Safety and Isolation
- Reinforced insulation between primary and secondary
- Minimum creepage: 8 mm for 230 VAC (IEC 60950)
- Hi-pot test: 3 kV AC for 1 minute
- Triple-insulated wire option for tight space
7. Common Design Mistakes
- Core too small: saturates at peak current
- Leakage too high: snubber dissipates too much power
- Poor winding layout: high EMI, bad cross-regulation
- No margin for V_in_min: browns out at low line
- Inadequate insulation: fails hi-pot test
8. Sirui-Tech Flyback Capability
- Universal input flybacks: 5–120 W, 65–132 kHz
- QR (quasi-resonant) flybacks for high efficiency
- Multiple outputs: main + auxiliary windings
- Safety certified: reinforced insulation, hi-pot tested
Conclusion
Flyback transformer design requires careful calculation of turns ratio, primary inductance, core size and winding layout. The most common mistakes are undersized core (saturation), high leakage inductance (EMI and snubber loss), and inadequate safety insulation. Always verify with prototype testing.
Need a custom flyback transformer? Contact Sirui-Tech at sales@sirui-tech.com with your power level, input voltage range and switching frequency.