Flyback transformer

A flyback transformer (FBT), also called a line output transformer (LOPT), is a special type of electrical transformer. It was initially designed to generate high voltage sawtooth signals at a relatively high frequency. In modern applications, it is used extensively in switched-mode power supplies for both low (3 V) and high voltage (over 10 kV) supplies.

The flyback transformer circuit was invented as a means of controlling the horizontal movement of the electron beam in a cathode ray tube (CRT). Unlike conventional transformers, a flyback transformer is not fed with a signal of the same waveshape as the intended output current. A convenient side effect of such a transformer is the considerable energy that is available in its magnetic circuit. This can be exploited using extra windings that can be used to provide power to operate other parts of the equipment. In particular, very high voltages are easily obtained using relatively few turns of winding which, once rectified, can provide the very high accelerating voltage for a CRT. Many more recent applications of such a transformer dispense with the need to produce high voltages and just use the device as a relatively efficient means of producing a wide range of lower voltages using a transformer much smaller than a conventional mains transformer would be.

The primary winding of the flyback transformer is driven by a switch from a DC supply (usually a transistor). When the switch is switched on, the primary inductance causes the current to build up in a ramp. An integral diode connected in series with the secondary winding prevents the formation of secondary current that would eventually oppose the primary current ramp.

When the switch is turned off, the current in the primary falls to zero. The energy stored in the magnetic core is released to the secondary as the magnetic field in the core collapses. The voltage in the output winding rises very quickly (usually less than a microsecond) until it is limited by the load conditions. Once the voltage reaches such level as to allow the secondary current to flow, then the current in the secondary winding begins to flow in the form of a descending ramp.

The cycle can then be repeated. If the secondary current is allowed to discharge completely to zero (no energy stored in the core), then it is said that the transformer works in discontinuous mode (DCM). When some energy is always stored in the core (and the current waveforms look trapezoidal rather than triangular), then this is continuous mode (CCM). This terminology is used especially in power supply transformers.

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