Flyback transformers are used in various low-power switching power supplies, typically for applications below 150W.  

• Advantages of Flyback Transformers :  
Simple topology : Requires fewer switching components.  
Wide input voltage range : Operates within 85 to 265Vac without the need for switching input voltage settings to achieve stable output.  
Compact design : Energy is released from the inductor when the switch is off, allowing the core’s magnetic field to realign, resulting in a smaller size.  
Cost-effective : Multiple secondary windings with different turn counts enable multi-output capabilities, reducing overall power supply costs.  


• Limitations of Flyback Transformers :  
Output ripple : Output voltage may have significant ripple.  
Efficiency : Load efficiency typically ranges between 75 and 90 percent, with less precision compared to other topologies.  

2. Operating Principle
The operating principle of a flyback converter is as follows: U1 is a DC voltage obtained by rectifying and filtering an AC voltage. When the switch transistor Q1 is turned on, U1 is applied to the primary winding of the flyback transformer. At this time, the polarity of the secondary winding causes the diode to remain in reverse bias, blocking current flow through the secondary winding. Energy is stored in the primary winding during this phase. 

When Q1 is turned off, the voltage polarity across the secondary winding reverses, placing the diode in forward bias and allowing current to flow through the secondary winding. The energy stored in the primary winding during Q1’s conduction phase is released through the secondary winding and the diode to the load.  
 

Flyback Power Supply Core Operation:
• During Q1 conduction (Ton) : Energy is stored in the transformer’s primary winding.  
• During Q1 cutoff (Toff) : Energy is discharged from the transformer’s secondary winding to the output.  
 

3. Operating Modes
At low input voltage, as the load varies from light to heavy, the transformer shifts from DCM  to BCM to CCM .
At high input voltage, regardless of the load, the transformer operates in CCM mode.

Notes:
DCM: Discontinuous Conduction Mode
BCM: Boundary Conduction Mode
CCM: Continuous Conduction Mode
 

4. Key Design Considerations
The selection of flyback transformer core size and design should be based on calculations of transmitted power and switching frequency.

Aw: Winding window area.
Ae: Effective core cross-sectional area.
Po: Output power.
Bm (ΔB): Magnetic flux density swing.
F: Operating frequency (Hz).
J: Current density.
Ku: Window utilization factor.



Note : When selecting a magnetic core using the core area product, the adjustment of the core cross-sectional area (Ae) and the core window area (Aw) must meet the following requirements:  

Ae : The core cross-sectional area of the transformer should ensure the core does not saturate, expressed as Bmax = Bm = △B. 
 
△B = (Bs - Br) × 0.6  
(Bs: Saturation flux density, Br: Residual flux density, under 100°C conditions).  

Aw : The core window area should ensure all windings can fit. A larger Aw allows for more turns to be wound.  


• Selection of Magnetic Materials
Transformer cores are typically made of manganese-zinc ferrite due to the working frequency of transformers usually being in the range of 100–300 kHz. Higher-frequency nickel-zinc ferrite is unnecessary. Manganese-zinc ferrite is preferred for its high permeability and excellent saturation flux density characteristics.  



• Considerations for Wire Gauge in Flyback Transformers
When high-frequency current flows through a conductor, the combined effects of the primary current and eddy currents become concentrated at the conductor’s surface, weakening towards the center. This phenomenon is known as the skin effect.  


To minimize losses caused by the skin effect, the wire gauge should be selected so that the wire diameter is smaller than the skin depth. In this case, the AC resistance can be approximated by the DC resistance. If the wire diameter exceeds twice the skin depth, a litz wire (multi-stranded wire) should be used as an alternative to reduce losses.  
 

△ : Collector depth
F : Operating frequency (Hz)




 

5. Introduction to GOTREND Flyback Transformers
GOTREND flyback transformers offer high reliability, exceptional consistency, and customizable designs tailored to customer needs. These transformers are typically of a winding type construction, featuring a universal structure with excellent usability and thermal management. They are widely used in various applications such as Ethernet PoE power supply, household appliance power supplies, industrial power supplies, and communication power supplies.  


To meet PoE power supply standards, GOTREND has launched the GPOE series, covering the three primary PoE system standards:  
• 802.3af (PoE)  
• 802.3at (PoE+)  
• 802.3bt (UPoE+)  
 

GPOE series supports a power range of 3W to 100W, voltage ranges from 3.3V to 48V, and operating frequencies between 100 kHz and 500 kHz. These transformers feature both forward and flyback topologies, providing flexible solutions for various PoE power supply systems.  

GOTREND PoE Selection Guide【GPOE Series】

Related articles  :
•  Research on hot air stranded wire materials and processes
•  Advantages and Disadvantages of Adding Nitrogen in SMT Reflow Ovens
•  Ferrite Wire-Wound Inductors【GNLD Series】Enhance Power Conversion Efficiency