The flyback converter is the most widely used isolated topology in low-to-medium power switch-mode power supplies (SMPS), from 5 W phone adapters to 200 W industrial supplies. The flyback transformer is the heart of this topology—it stores energy in its core during the ON phase and releases it to the output during the OFF phase.
This guide covers flyback transformer operation, core selection, key design parameters, typical calculations, and common design mistakes.
Shaanxi SiRui Technology Co., Ltd. manufactures flyback transformers in EE, EER, EI and planar configurations for adapter, LED driver and industrial power applications. Contact: sales@sirui-tech.com | Web: www.sirui-tech.com
1. How a Flyback Transformer Works
Unlike a conventional transformer that transfers energy continuously, the flyback transformer is actually an energy storage inductor with isolated windings.
- Switch ON: primary switch conducts, energy builds in the core. Output diode is reverse-biased.
- Switch OFF: primary current stops, magnetic field collapses, inducing voltage in the secondary. Output diode conducts, delivering stored energy.
The core must be gapped (or use distributed-gap material) to prevent saturation and store the required energy.
2. Core Selection
Core shapes: EE/EI (most common, low cost), EER/ETD (better winding distribution), RM (shielded, low EMI for adapters), planar (low profile, high density).
Material: Mn-Zn ferrite (PC40, N87 grades). Lower loss grades for >100 kHz designs. Gap is required.
| Output Power | Frequency | Typical Core |
|---|---|---|
| 5–20 W | 65 kHz | EE16–EE20 |
| 20–50 W | 65–100 kHz | EE25–EE30 |
| 50–100 W | 100 kHz | EE30–EE40 |
| 100–200 W | 100–200 kHz | EER40–EER49 |
3. Key Design Parameters
- Max duty cycle: 45–50% at nominal input
- Turns ratio: determined by Vin_min, Vout, Vf and desired duty
- Primary inductance (Lp): sets peak current and ripple
- Peak current: core must not saturate at peak plus current limit
- Auxiliary winding: powers the controller IC, usually 3–5 turns
4. Common Design Mistakes
- Insufficient gap—core saturates at peak current, destroying the switch
- Too much gap—increased fringing loss and EMI
- Poor winding technique—high leakage inductance causes voltage spikes
- Skipping saturation testing—bench test required, not just calculation
5. Sirui-Tech Flyback Capabilities
- EE/EI series: 5 W to 200 W adapters, chargers, LED drivers
- EER/ETD series: industrial power supplies
- Planar flyback: low-profile, high-density applications
- Custom designs: OEM/ODM with specific pinout and inductance
Conclusion
The flyback transformer is critical in isolated SMPS. Correct core size, proper gapping and verified saturation are essential. EE core with Mn-Zn ferrite remains the cost-effective standard.
Need a flyback transformer? Contact Sirui-Tech at sales@sirui-tech.com with your output power, input voltage range and switching frequency.