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). More precisely, the flyback converter is a buck-boost converter with the inductor split to form a transformer, so that the voltage ratios are multiplied with an additional advantage of isolation. When driving for example a plasma lamp or a voltage multiplier the rectifying diode of the Buck-Boost converter is left out and the device is called a flyback transformer. 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 buck-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).

The flyback converter is an isolated power converter, therefore the isolation of the control circuit is also needed. In order to control the output voltage, a proper voltage signal related to it is needed. A primary side feedback control can be used for DCM operation, because in this case, the output voltage is in proportion to the turn-on and reset time of the transformer. This voltage can be picked up from a separate sense winding. More often, and for the CCM operation, however, the secondary voltage is sensed and 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

Similar to a buck-boost converter the switch in the primary circuit must withstand higher voltages than originally applied to the primary.The amount of voltage it has to withstand is

Where V_p=voltage applied to primary
N₁=number of turns in primary
N₂=number of turns in secondary
d=duty ratio of switch

In contrast to the buck-boost converter and to the autotransformer leakage inductance just increases this voltages without increasing the secondary voltage. In contrast to push-pull converters a core with an air gap is needed.

[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.