Editing Flyback transformer (section)

==Operation and usage==
[[File:Flyback converter PSS kneepoint trace.png|thumb|400px|Example of current waveforms in a flyback transformer]]
The primary winding of the flyback transformer is driven by a switch from a DC supply (usually a [[transistor]]). When the switch is closed, 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 a secondary current that would eventually oppose the primary current ramp.  <ref>{{cite magazine | url = http://www.overunityresearch.com/index.php?action=dlattach;topic=2578.0;attach=15525 | title = Designing Flyback Transformers |last1=Billings |first1=Keith | date = April 2003 | magazine = Power Electronics Technology  }}</ref>

When the switch is opened, 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 the load conditions limit it. Once the voltage reaches such a level as to allow a secondary current, the charge flow is like a descending ramp.

The cycle can then be repeated. If the secondary current is allowed to drop completely to zero (no energy stored in the core), then it is said that the transformer works in ''discontinuous mode''.<ref>{{cite magazine | url = https://www.powerelectronics.com/passive-components/designing-flyback-transformer-discontinuous-mode | title = Designing Flyback Transformer for Discontinuous Mode |last1=Billings |first1=Keith | date = April 1, 2003 | magazine = Power Electronics Technology  }}</ref> When the secondary current is always non-zero (some energy is always stored in the core), then this is ''continuous mode''.<ref>{{cite magazine | url = https://www.powerelectronics.com/content/flyback-design-continuous-mode-operation | title = Flyback Design for Continuous Mode of Operation |last1=Billings |first1=Keith | date = May 1, 2003 | magazine = Power Electronics Technology  }}</ref> This terminology is used especially in power supply transformers.

The low voltage output winding mirrors the sawtooth of the primary current and, e.g. for television purposes, has fewer turns than the primary, thus providing a higher current. This is a ramped and pulsed waveform that repeats at the horizontal (line) [[frequency]] of the display. The flyback (the vertical portion of the sawtooth wave) can be a potential problem for the flyback transformer if the energy has nowhere to go: the faster a magnetic field collapses, the greater the [[electromagnetic induction|induced]] voltage, which, if not controlled, can flash over the transformer terminals.  The high frequency used permits the use of a much smaller transformer. In [[television]] sets, this high frequency is about 15 [[kilohertz]] (15.625 kHz for PAL, 15.734 kHz for [[NTSC]]), and vibrations from the transformer core caused by [[magnetostriction]] can often be heard as a high-pitched whine. In CRT-based [[computer display]]s, the frequency can vary over a wide range, from about 30 kHz to 150 kHz.

The transformer can be equipped with extra windings whose sole purpose is to induce a relatively large voltage pulse when the magnetic field collapses as the input switch is turned off.  There is considerable energy stored in the magnetic field, and coupling it out via extra windings helps it to collapse quickly, and avoids the voltage flash over that might otherwise occur.  The pulse train coming from the flyback transformer windings is converted to [[direct current]] by a simple half-wave [[rectifier]].  There is no point in using a full wave design as there are no corresponding pulses of opposite polarity.  One turn of a winding often produces pulses of several volts.   In older television designs, the transformer produced the required high voltage for the CRT accelerating voltage directly with the output rectified by a simple rectifier.  In more modern designs, the rectifier is replaced by a [[voltage multiplier]]. Color television sets must also use a regulator to control the high voltage.  The earliest sets used a shunt vacuum tube regulator, but the introduction of solid-state sets employed a simpler voltage-dependent resistor.  The rectified voltage is then used to supply the final [[anode]] of the cathode-ray tube.

There are often auxiliary windings that produce lower voltages for driving other parts of the television circuitry.  The voltage used to bias the varactor diodes in modern tuners is often derived from the flyback transformer ("Line OutPut Transformer" LOPT).  In tube sets, a one or two-turn filament winding is located on the opposite side of the core as the HV secondary, used to drive the HV rectifier tube's heater.