FLYBACK TRANSFORMERS Customisable Coupled Inductors

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Introduction

A flyback transformer is a coupled inductor with a gaped core. The working can be explained as: During each cycle when input voltage is applied to the primary winding energy is stored in the gap of the core and then transferred to the secondary winding to provide energy to the load.

In flyback topology energy is stored in the magnetic field of the transformer during first half of the switching cycle and then released to the secondary winding connected to the load in the second half.

Flyback transformers or converters also known as Coupled Inductors because they have a gaped core construction and store energy in the core.


Working of Flyback converter



When switch is ON current is conducted through the transformer primary coil that setsup a magnetic field in which energy is stored(Faradays First law). 

The combination of the winding polarities reverse biases the output diode to ensure that no energy is transferred to the secondary coil.

During this portion of the cycle current in primary is ramping up over time to store energy(½ LI2).

When switch os OFF the magnetic field collapses transferring the stored energy to the secondary winding and ultimately the load.

During this portion of the cycle current in secondary is at its peak and ramps downwards as the stored energy is transferred to the load.

If the switch is turned back ON before all the flyback energy is transferred to the secondary coil, the secondary current never goes reaches zero(Figure 2) hence resulting in CCM continuous conduction mode.


If stored flyback energy is completely emptied to the secondary coil before switch is turned ON, the secondary current reaches zero before the end of period creating an idle time(tidle) (figure 3)during the cycle hence resulting in DCM Discontinuous Conducting Mode.

Applications of Flyback converter

You can use Flyback transformers in many applications, including:

  • DC-DC power supplies

  • Telecom

  • LED Lighting

  • Power over Ethernet (PoE)

  • Capacitor charging

  • Battery charging

  • Solar Microinverters

  • AC-DC power supplies

Off-the shelf flyback transformers are available for many applications where low cost, small size, and high-efficiency are required. They are typically used in DC-DC controllers in the telecommunications (telecom) voltage range of 37 – 72Vdc, sometimes at extended voltages ranging from 2 – 400 Vdc, and also within the universal AC line input voltage range (85 – 265 Vac ).

Flyback transformers are commonly used for output current below about 10 Amps and output power below about 100 Watts. Coilcraft offers standard, off-the-shelf flyback transformers with power capabilities ranging from a few Watts to up to around 120 Watts. When higher current and power is required, forward-mode, push-pull, or half-bridge / full-bridge topologies become more efficient alternatives.


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