What is a Flyback Transformer : Working & Its Applications

The flyback transformer is a special class of transformers‘ family. Fundamentally it’s a step-up transformer, but with a huge potential of stepping up the voltage. As compared to power transformers, it is compact in size and mobile. One of the common applications of flyback transformers is in CRT tube televisions, where a very high voltage is required in the picture tube. For an input of 230 V, a flyback transformer can obtain an output of up to 20,000 V. Such is the potential of flyback transformers. It can even operate with a low voltage such as 12 V or 5V. Constructional aspects are different from a normal transformer. The early application of the flyback transformer started with controlling the horizontal movement of the electron beam in a cathode ray tube. With the advent of technology and devices, at present flyback transformer can even be energized with a DC pulse with the help of a rectifying circuit consisting of electronic devices like MOSFET.

What is a Flyback Transformer?

Definition: A flyback transformer can be defined as an energy conversion device that transfers energy from one part of the circuit to the other part at constant power. In a flyback transformer, the voltage is stepped up to a very high value based on the application. It is also called a line output transformer, as the output line voltage is fed to the other part of the circuit. With the help of the rectifying circuit, the primary winding of the transformer can be driven by a DC circuit.


Flyback Transformer
Flyback Transformer

Design

Like a conventional transformer, a flyback transformer differs in design and application. In a conventional transformer, the primary has to be fed with an AC voltage, which is stepped up or down based on the number of turns. The output voltage of the conventional transformer is limited but can be used for various applications.

Flyback Transformer Design
Flyback Transformer Design

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

But in the case of the flyback transformer, it is of the formed arc, which is of very high voltage. This output voltage cannot be transmitted to long distances, but can only use for specific applications like SMPS or CRT tube. The core of the flyback transformer is similar to the conventional transformer but is compact in size.

Why it is Called a Flyback Transformer?

The name flyback was coined due to the application of flyback transformers in the CRT tube. A flyback transformer can be energized with a very low voltage. When the primary winding of the transformer is excited with a sawtooth voltage, of low value, due to the nature of the sawtooth waveform, it gets energized and de-energized quickly. Due to this, the beam on the CRT is fly back from right to left. With this peculiar property which is obtained due to the functioning of the transformer, the name was coined as flyback transformer.

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Flyback Transformer Circuit

The circuit diagram for the flyback transformer is shown below. As shown, L1 and L2 are the turns of the windings. In general, for flyback transformer L2 is very high than L1, as basically it’s a step-up transformer. The capacitor at the input side is provided to maintain the voltage constant. The switch SW is used to rectify the input voltage.

Flyback Transformer Circuit
Flyback Transformer Circuit

The diode D is used to maintain the unidirectional flow of the secondary current. The capacitor at the secondary side is provided to maintain the constant output voltage. Vin is the input voltage and Vout is the output voltage. The dot convention shown in the circuit implies its series additive equivalent inductance for the overall core of the transformer.

Flyback Transformer Arc

The output voltage of the transformer is of a high value even up to 10 to 20 kV. The high voltage is not sinusoidal in nature but is of the form of an arc. An arc is formed in the air when two highly conducting bodies are placed nearby. The air in between is ionized and the arc the formed. The concept is the same whenever a breaker is energized, the isolator is operated, or the phenomenon of the corona.

Flyback Transformer Winding

In order to obtain a very high voltage on the secondary side, the secondary turns are very large as compared to primary turns. The windings are generally made of copper. And like in a conventional transformer, the windings are properly insulated with each other. Mica insulation is generally used to provide the insulation. In some applications like SMPS and converters, paper insulation is also used. Unlike a conventional transformer, no oil is used for insulation or colling purpose. The windings are generally thin in size, and hence the winding loss and efficiency improve.

How to Test a Flyback Transformer?

This transformer can be tested in various aspects. To check whether there is any fault in the winding, a line operated potential transformer tester is used to check for the faults. In the case of open winding, the tester will indicate very high impedance in the winding side, and in the case of a short circuit, the impedance would be relatively low.

This one indication of winding faults. In recent testers, a graphical display will also indicate the healthiness of the winding. For faults in the capacitor, it will be a noisy operation. A noise like a tic-tac will appear on the monitor side. This happens for the opening of the capacitor. In the case of the shorting of the capacitor, the display will be blank. It will show a power blink. In such cases, the capacitor needs to be replaced.

Other common problems in the transformer are shorting of windings, cracking in the core, external arcing to ground, etc. All these problems can be tested through line operated tester. A common multimeter can also be used to test the continuity of the circuit, and measure the voltage at each point.

Flyback Transformer Working

The working principle of the flyback transformer is the same as the conventional transformer except for its design aspects. As shown in the circuit diagram, when the primary winding of the transformer is excited with a low voltage sawtooth waveform, the primary winding is energized.

As shown in the waveforms, when the primary winding is energized, the primary inductance develops a ramp current as shown in the diagram. When the ramp current reaches its peak value, the flyback waveform develops a high potential. Which is induced on the secondary side. The diode on the secondary side prevents the ramp to be flown on the reverse side.

The secondary current follows a ramp down, the time at which the voltage reaches int knee point. At this point, high voltage is obtained on the secondary side. But since it can not be of the AC in nature, it follows an arc-like structure of very high potential which all directs the electron beam in one particular direction. In applications like SPMS, the second potential is less, but converting principle to convert the secondary AC in switched-mode. Based on the nature of the waveform, the operation can even be classified as the continuous or discontinuous mode of operation.

Circuit Waveforms
Circuit Waveforms

The flyback transformer construction involves a primary winding, secondary winding, and core. In case it is excited from a DC supply, it also consists of a rectifying unit. In general, the primary winding turns are less than secondary winding turns. The windings are made of copper and insulated from each other. The winding techniques are the same as the conventional transformer.

The windings are placed over the core forming a series of magnetic circuits. This allows the transformer to withstand more voltage at low power specifications. The core leg is of equal dimensions on both sides and the winding is encircled over the core. It forms the magnetic circuit to be additive in nature.

Applications

The flyback transformer applications include the following.

  • CRT Tube
  • SPMS
  • DC-DC Power technologies
  • Battery charging
  • Telecom
  • Solar applications

Thus, this is all about an overview of the flyback transformer. We have seen the operating principle and properties of the flyback transformer. Due to the advent of technology, it has gained huge applications, most notably in the renewable energy sector. One interesting aspect would study the secondary voltage of flyback transformer, which is of huge potential and storing in for charging of battery units with a low time constant. The capacitor on the secondary winding can be modified to achieve this.

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