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Flyback Converter


What is a Flyback Converter?


The flyback converter is a type of power converter which have galvanic isolation between the applied input and generated output. Output may be one or more than one.

If we see the flyback converter in perspective of buck-boost converter, we can see that it matches to buck-boost converter except of the inductor. Inductor is split to form a transformer in the flyback converter.

Flyback Converter
Flyback Converter based SMPS
This transformer gives an advantage of voltage control at secondary and also we can drive many different output voltages.

Flyback converter is an isolated power converter and provide isolation between primary and secondary side.

Output voltage in the flyback topology can be match with each other by transformer winding turns ratio.

Setting transformer turns ratio will be helpful for storing energy in primary side and to generate multiple secondary outputs.

Working principle of flyback converter


Below is the schematic of flyback converter, here we can see that it is same like buck-boost converter, only difference we can see that inductor is replaced with transformer. This makes it’s working principle same like buck-boost converter.

Flyback Converter
Flyback Converter Working Principal Schematic and Waveforms
Below are it’s working principle in both switch open and switch close cases;

Switch close

Flyback Converter
Working of Flyback Converter when switch is close
Above figure shows that, when in flyback converter switch is closed we can see that the primary winding of the transformer connected to the input voltage supply source.

This will lead to store energy in the transformer, which will cause by flow of primary current and magnetic flux across it.

In this case there is no voltage is available in the secondary winding of transformer, so the negative voltage will be induced in the secondary winding, hence the schottky diode connected at secondary side is reverse-biased.

Switch open

Flyback Converter
Working of Flyback Converter when switch is open
When the switch is open as shown in the above figure, the current in the primary winding drops and there will be no magnetic flux across it.

The voltage in the secondary winding is now positive which makes connected schottky diode forward-bias and hence current flows from the transformer.

The current flowing from the transformer charges the capacitor at secondary side and supplies the energy to the connected load.

Again Switch close

In this case all the steps will be same as mentioned in the “Switch close” case.

Now the capacitor which is charged because of “Switch open” case will supply energy to the load.

Special Characteristics of Flyback Converter


Flyback Converter working modes

There are two working modes for flyback converter, these are Critical Conduction Mode (CCM) or Discontinuous Conduction Mode (DCM). These modes are helpful to understand the flyback converter transfer function.

Flyback Converter
Flyback Converter Waveforms
As shown in below figure, if the switching device like MOSFET is turned ON before all of the energy from primary side is transferred to the secondary side, the secondary side current never reaches to zero. This condition is called as continuous conduction mode (CCM).

Flyback Converter
Flyback Converter in Continuous Current Mode
As shown in below figure, if the stored energy in primary side is completely transferred to the secondary before the switching device like MOSFET is turned ON, the secondary side current reaches to zero before the end of the time period. This creates an idle time – “Tidle” during the work cycle. This is called as discontinuous conduction mode (DCM).
Flyback Converter
Flyback Converter in Discontinuous Current Mode
Generally, flyback transformers designed for both the types; CCM and DCM. It is decided by input voltage to apply and output side load situations.

For working of CCM and DCM, the MOSFET driver IC requires a signal associated to the output voltage. This signal can be generated by three common ways as mentioned below;

The first way is to use an optocoupler on the secondary side of circuitry to direct or send a signal to the driver IC. Optocoupler gives tight voltage and tight current regulation.

The second way is to place an another winding on the coil and sense the cross regulation of the design. Here output is not tightly controlled.

The third way is to sample the amplitude of voltage on the primary side of circuitry during the primary side discharge, which is referenced according to primary DC voltage. Primary side sensing is accurate and cheap, but needs a minimum load to be connect so primary side discharge always occurs, by which secondary voltage at the primary winding side can be sample.

Snubber Circuit in Flyback Converter

Transformer plays important role in flyback converter topology.

A gap is provided in between the pair of ferrite cores of the transformer to increase reluctance in ferrite core to control magnetic flux lines, but this method involves a rise in magnetic leakage flux, which further causes the generation of leakage inductance.

Along with the leakage inductance a switching current also flows which results in an energy build-up in the primary winding of transformer. As this transformer is not coupled or attached to the other windings, the generated power remains in primary winding and didn’t transfers to other winding.

This results in the generation of a surge voltage at the MOSFET’s drain terminal.

If this generated surge voltage crosses the Vds i.e. drain to source voltage range of the MOSFET then it will destroy it.

To prevent this risk or damage to the MOSFET, a snubber circuit is provided in flyback converter circuit which suppresses the generated surge voltage.

We can see in the above “Flyback Converter Working Principal Schematic” that this circuit contains a resistor, a diode, and a capacitor on the primary side. This circuit is called as a snubber circuit.

Applications of Flyback Converter

  • Mostly used as SMPS for cell phone charger.
  • Multiple output power supplies.
  • Communication Devices.
  • Industrial equipment.
  • High voltage supply generation for CRT in TV’s.

Conclusion

A Flyback Converter is an important power converter topology to meet our high and low voltage requirements. This topology is widely used in switched-mode power supplies. Most of DC – DC converters are designed using this topology because of provision of isolation between primary and secondary winding. Which means a perfect safety. Also, it provides good output efficiency, which means less loss.

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