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.

Structure and principle

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.

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. The two prevailing control schemes are voltage mode control and current mode control. Both require a signal related to the output voltage. There are two common ways to generate this voltage. The first is to use an optocoupler on the secondary circuitry to send a signal to the controller. The second is to wind a separate winding on the coil and rely on the cross regulation of the design.

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 voltage without increasing the secondary voltage. In contrast to push-pull converters, a core with an air gap is needed. In addition, the output storage capacitor required is larger than in forward converter topologies and has to be able to withstand a substantial amount of ripple current.

Discontinuous mode has the following disadvantages:

• High RMS and peak currents in the design
• High flux excursions in the inductor

These limit the efficiency of the converter.

Continuous mode has the following disadvantages:

• The voltage feedback loop requires a lower bandwidth due to a zero in the response of the converter.
• The current feedback loop used in current mode control needs slope compensation in many cases.
• The power switches are now turning on with positive current flow.

These complicate the control of the converter.

Applications

• Low-power switch-mode power supplies (cell phone charger, standby power supply in PCs)
• Low cost multiple-output power supplies (e.g. 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.

References