What Is a Flyback Transformer?
A flyback transformer is a transformer used in a flyback converter.
It is one of the circuit methods used in isolated switching power supplies such as isolated DC-DC converters and AC-DC converters, and other types include the forward method, LLC resonance method, and push-pull method.
In the flyback method, energy is repeatedly accumulated and released into the transformer during power conversion. Therefore, unlike other types of transformers, flyback transformers must have the characteristics to store large amounts of energy.
Uses of Flyback Transformers
Flyback transformers are used in switching power supply circuits used in a variety of consumer and industrial applications. The power that can be handled by this power supply is only a few tens of watts, making it suitable for relatively low-power applications compared to isolated switching power supplies of other methods.
On the other hand, it features a wide input voltage range. It has the advantage of simple power supply circuit configuration, fewer components, and lower cost. However, the peak current is higher, so care must be taken with specifications when selecting switching elements, rectifier diodes, and other components.
Also, the ripple current is higher than in other methods, so the smoothing capacitor capacity must be larger.
Principle of Flyback Transformers
Flyback transformers are based on the principle of charging when current flows through the primary side of the transformer and discharging through the secondary side of the transformer after the current is interrupted. Due to the principle of operation, it is very important for the transformer to accumulate electromagnetic energy in terms of its characteristics.
A switching element (power transistor, power MOSFET, etc.) is connected to the primary side of the flyback transformer, and a smoothing capacitor is connected to the secondary side through a rectifying diode. The polarity of the windings is reversed so that when an excitation current is applied to the primary side of the transformer, a voltage is generated in the direction that the rectifier diode on the secondary side turns off.
When the switching element is turned on, the excitation current flows in the primary side of the transformer and electromagnetic energy is stored in the transformer. When the switching element is turned off in this state, the excitation current on the primary side is interrupted, the direction of the voltage on the secondary side is reversed, and the electromagnetic energy stored in the transformer is released through the rectifier diode. This is smoothed by a capacitor to output a DC voltage.
The flyback transformer must store a large amount of electromagnetic energy. Transformers intended for energy transmission, like other methods, use core materials with high magnetic permeability to increase the coupling coefficient. However, such materials usually cannot store large amounts of electromagnetic energy. Therefore, in flyback transformers, an air gap is created in a part of the core where the magnetic permeability is low, and energy is stored in this gap.
Other Information on Flyback Transformers
1. Difference Between Flyback and Forward Power Supplies
Flyback power supplies require a relatively large transformer because electromagnetic energy must be stored in the transformer. A minute gap is created in part of the core to lower the magnetic permeability and store electromagnetic energy. On the other hand, forward power supplies do not require such a minute gap because the transformer does not need to accumulate electromagnetic energy, and the transformer can be made smaller.
In addition, flyback power supplies do not require isolation separation of the primary and secondary feedback circuits, making the circuit configuration very simple, whereas forward power supplies require isolation and a voltage feedback circuit from the output.
2. Ferrite Material Used in Flyback Transformers
Flyback transformers are widely used in AC-DC switching power supplies due to their configuration. However, the semiconductor devices for switching and the transformer are the two areas of high loss in AC-DC switching power supplies. During high-speed switching operations, rapid magnetic flux changes occur in the transformer, and if an iron core is used for the transformer core, heat is generated due to eddy current losses, rendering the transformer unusable.
For this reason, flyback transformers for high-frequency AC-DC switching power supplies often use ferrite as their core material. Since the characteristics of ferrite are frequency and temperature dependent, it is important to select a flyback transformer with a core made of ferrite material suitable for the desired operating speed.