Introduction to Flyback Transformer

Catalog

A special class in the family of transformers is the flyback transformer. It's essentially a step-up transformer, but with a massive ability to raise the voltage. It is small in size and mobile, as opposed to power transformers. In CRT tube televisions, where a very high voltage in the image tube is necessary, one of the common applications of flyback transformers is. A flyback transformer can achieve an output of up to 20,000 V for an input of 230 V. The capacity of flyback transformers is such. With a low voltage such as 12 V or 5V, it can also run. Building components are distinct from a standard transformer. The early use of the flyback transformer began with the regulation of the electron beam's horizontal movement in a cathode ray tube. With the advent of technology and equipment, with the help of a rectifying circuit consisting of electronic devices such as MOSFET, the new flyback transformer can also be energized with a DC pulse.

Because of the application of flyback transformers in the CRT tunnel, the term flyback was coined. With a very low voltage, a flyback transformer can be energized. When the transformer's primary winding is excited with a low-value sawtooth voltage, due to the existence of the sawtooth waveform, it gets easily energized and de-energized. The beam on the CRT flies back from right to left because of this. The name was coined as a flyback transformer with this unusual property obtained because of the working of the transformer.

I. Features

1. Design

A flyback transformer varies in nature and operation, much like a traditional transformer. The primary has to be fed with an AC voltage in a traditional transformer, which is stepped up or down depending on the number of turns. The traditional transformer's output voltage is small but can be used for different applications.

The primary winding in a flyback transformer does not need to be excited by AC voltage, but can be excited even with a DC pulse input. The low rating of the DC pulse input can be 5 V or 12 V, which can be obtained also from a function generator. With a rectifying circuit, the DC voltage is converted to DC pulses. In a typical transformer, the output voltage is pure AC voltage.

But in the case of a flyback transformer, the arc created is a very high voltage arc. This output voltage can not be transmitted over long distances, but can only be used for SMPS or CRT tube specific applications. The heart of the flyback transformer is similar but compact in size to the standard transformer.

2. Circuit

Flyback Transformer Design

The circuit diagram is shown below for the flyback transformer. As demonstrated, the turns of the windings are L1 and L2. In general, L2 is very broad for flyback transformer than L1, as it is essentially a step-up transformer. To hold the voltage constant, the capacitor on the input side is supplied. For rectifying the input voltage, the switch SW is used.

The diode D is used to maintain the secondary current's unidirectional flow. To maintain the constant output voltage, the capacitor on the secondary side is supplied. Vin is the voltage of the input and Vout is the voltage of the output. For the overall center of the transformer, the dot convention shown in the circuit indicates its series additive equivalent inductance.

3. Arc

The transformer's output voltage is high, even up to 10 to 20 kV. In nature, the high voltage is not sinusoidal but is in the shape of an arc. When two highly conductive bodies are positioned together, an arc is created in the air. The air in between is ionized and created by the arc. Whenever a breaker is energized, the isolator is worked, or the corona phenomenon, the notion is the same.

4. Winding

As compared to primary turns, the secondary turns are very wide in order to achieve a very high voltage on the secondary side. Generally, the windings are made out of copper. And the windings are properly separated from each other, as in a traditional transformer. To provide the insulation, mica insulation is commonly used. Paper insulation is also used in some applications, such as SMPS and converters. No oil is used for insulation or collating purposes, unlike a traditional transformer. Generally, the windings are thin in size, thereby improving the winding loss and efficiency.

II. Working Principle

Flyback Transformer Waveform

Except for its architecture elements, the operating concept of the flyback transformer is the same as the traditional transformer. The primary winding is energized, as seen in the circuit diagram, when the transformer's primary winding is excited with a low voltage sawtooth waveform. 

As seen in the waveforms, as shown in the diagram, when the primary winding is energized, the primary inductance produces a ramp current. The flyback waveform produces a high potential when the ramp current hits its peak value. Which is caused on the peripheral side. The diode on the reverse side stops the reverse side of the ramp from flying.

A ramp down follows the secondary current, the moment at which the voltage hits the point of the internal knee. High voltage on the secondary side is obtained at this point. But because it cannot be of the AC in nature, it follows a very high-potential arc-like structure that all guides the electron beam in one specific direction. The second potential is less in applications such as SPMS, but the concept of conversion to transform the secondary AC into switched mode is converted. The operation may also be defined as a continuous or discontinuous mode of operation, depending on the nature of the waveform.

The main winding, secondary winding, and core are involved in the flyback transformer construction. It also consists of a rectifying unit if it is stimulated from a DC supply. In general, there are fewer main winding turns than secondary winding turns. The windings are made of and insulated from each other by copper. As with the traditional transformer, the winding methods are the same. 

The windings are positioned over a series of magnetic circuits forming the heart. This helps, at low power specifications, the transformer to tolerate more voltage. On both sides, the core leg has equal dimensions and the winding is encircled around the core. The magnetic circuit is shaped in nature to be additive.

III. Applications

The flyback transformer applications include the following.

• CRT Tube

• SPMS

• DC-DC Power technologies

• Battery charging

• Telecom

• Solar applications