What Is a Flyback Converter?

A flyback converter is a type of power converter topology commonly used in DC-DC conversion and isolated power applications. It stores energy in the magnetic core of a transformer during the switch-on phase and then releases this energy to the output when the switch is turned off. Flyback converters are widely utilized in low-power power supply devices such as adapters and chargers. Many distributors offer a wide range of components of flyback converter to cater to diverse application needs, like LTC4365IDDB

Flyback Converter Components

The core components of a flyback converter include a switch, transformer, diode, and capacitor. The switch is controlled by pulse-width modulation (PWM), generating a high-frequency square wave signal across the transformer through its on-and-off operation. The transformer transfers this square wave signal to the secondary coil via magnetic induction. Through the filtering and rectification functions of the diode and capacitor, a stable DC output is obtained at the output side.

Flyback Converter Working Principle

The operation of a flyback converter consists of two stages: the switch-on stage and the switch-off stage.

During the switch-on stage, the transformer’s primary coil is directly connected to the input voltage. Current flows through the primary coil, increasing the magnetic field in the transformer core, where energy is stored. A reverse voltage is induced in the secondary coil, putting the diode in a reverse-biased state, preventing it from conducting. At this point, the capacitor supplies voltage and current to the load.

In the switch-off stage, the current in the primary coil drops to zero, and the magnetic field in the core begins to collapse. A forward voltage is induced in the secondary coil, putting the diode in a forward-biased state. The current flows through the diode to the capacitor and load, transferring the stored energy from the core to the capacitor and load.

Flyback Converter Related Formula

The operation of a flyback converter is governed by several key formulas. These equations relate to various parameters such as duty cycle, output voltage, and transformer turns ratio. Here are some of the important formulas associated with a flyback converter:

1. Output Voltage (Vout)

The output voltage in a flyback converter is determined by the input voltage (Vin), the duty cycle (D), and the turns ratio (N) between the primary and secondary windings of the transformer.

• D = Duty cycle (ratio of the on-time to the total period)

• N = Transformer turns ratio (N=Nsecondary/Nprimary)

2. Duty Cycle (D)

The duty cycle D of the switch can be calculated by considering the output voltage, input voltage, and transformer turns ratio.

• N is the transformer turns ratio

• Vin is the input voltage

• Vout is the output voltage

3. Energy Stored in the Transformer (W)

The energy stored in the transformer core during the on-time of the switch is given by:

• Lp = Primary inductance

• Ip = Peak current through the primary winding

4. Peak Primary Current (Ip)

The peak current in the primary winding during the on-time of the switch is:

• Ts = Switching period (inverse of switching frequency)

• Lp = Primary inductance

5. Output Power (Pout)

The output power delivered by the flyback converter is:

• fs = Switching frequency

Flyback Converter Applications

The flyback converter is widely used in low-power power supply devices, primarily for applications requiring isolation in DC-DC and AC-DC power conversion. Its simple structure, low cost, and support for multiple outputs make the flyback converter common in mobile phone chargers, laptop power adapters, LED driver power supplies, and small household appliance power supplies. Additionally, it is suitable for low-power applications in power management modules and industrial control devices.

Flyback Converter Limitations

1. The presence of zeros in the transfer function, so only low bandwidth can be used for voltage feedback circuits.

2. When using current feedback mode at duty cycles greater than 50%, slope compensation is required to meet the system stability requirements.

3. There are large pulse currents in the primary and secondary coils.

Conclusion

The flyback converter is a widely used power converter for low-power power supply devices. Its simple structure, low cost, and ability to support multiple outputs make it popular in applications such as mobile phone chargers and laptop power adapters. Although it performs exceptionally well in low-power applications, it may have lower efficiency in high-power conversion, necessitating careful consideration of specific application requirements in design choices.