Flyback converter

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The Flyback converter is a DC to DC converter with a galvanic isolation between the input and the output(s). It has the same structure as the boost converter, with a transformer instead of an inductor. Thanks to this transformer, it can achieve very high conversion ratios. Due to intrinsic limitations, this converter is only used in low power applications (up to about 250 W).

[edit] Structure and principle

Fig. 1: Schematic of a flyback converter.

The schematic of a flyback converter can be seen in figure 1. It is equivalent to that of a boost converter, with the inductor split to form a transformer. Therefore the operating principle of both converters is very close:

• When the switch is on (see figure 2), the primary of the transformer is directly connected to the input voltage source. This results in an increase of magnetic flux in the transformer. The voltage across the secondary winding is negative, so the diode is reverse-biased (i.e blocked). The output capacitor supplies energy to the output load.
• When the switch is off, the energy stored in the transformer is transferred to the output of the converter.

[edit] Operation

The two configurations of a flyback converter in operation: In the on-state, the energy is transferred from the input voltage source to the transformer (the output capacitor supplies energy to the output load). In the off-state, the energy is transferred from the transformer to the output load (and the output capacitor).

For control of the output voltage, a proportional voltage picked up from a separate sense winding can be used. More often, however, the secondary voltage is compared to a reference, signalling the controller through an optocoupler to maintain isolation.

Fig. 3:Waveforms of current and voltage in a flyback converter operating in continuous mode.

Fig. 4:Waveforms of current and voltage in a flyback converter operating in discontinuous mode.

[edit] Limitations

• Energy transfer through stored magnetic energy requires a larger transformer core than forward converters
  • This is similar to a boost converter
  • If isolation is not needed an autotransformer is a smaller alternative
• High voltages are present in the primary winding at the end of the primary flyback pulse
  • a part is wanted to create also a higher voltage at the secondary winding like in a boost converter
  • a part is due to the self-inductance of the first winding
    • leads to charging the primary stray capacities and leading to high voltages at the switch
    • or requires a diode (complicating the circuit (if you need DC isolation you probably already have a complicated circuit))
      • to dump the energy ( Zener diode )
      • to recover the energy into a capacitor and use the winding ratio as known from (50,60) Hz AC transformers
      • or use a full bridge and a bipolar S-shaped pulse is used to additionally fight hysteresis

[edit] Applications

• Low-power switch-mode power supplies (cell phone charger, standby power supply in PCs, main PC supplies < 250 W)
• High voltage supply for the CRT in TVs and monitors (the flyback converter is often combined with the horizontal deflection drive).
• High voltage generation, e.g. for Xenon flash lamps, lasers, copiers etc.
• The ignition system in Spark-Ignition engines is also a flyback converter, the ignition coil being the transformer and the contact breaker forming the switch element.
• Isolated gate driver.