# What is a flyback?

Trying to find this answer is so difficult that I can't even tell whether What is the difference between a transformer and a coupled inductor? is asking the same thing.

Some reading suggests that a "flyback" element (which may be called a "flyback transformer," may also be called a "coupled inductor?," and which may be (or always is?) part of a "flyback power converter") is at least schematically equivalent to something like a conventional transformer with a parallel inductor. Is that correct, and if so can someone provide the appropriate schematic?

Can someone provide a clear depiction of the physical embodiment of a flyback element? I can't determine what the core must look like. As far as I can decipher it consists of two independent "high-side" windings, and a low-side winding (which with a ZVS driver may actually consist of a split/dual winding).

Finally, what are the relationships between the windings, low-side current, and high-side voltage and current output characteristics?

• "Flyback" is a play on words, the same as "bootstrap" is for amplifiers and software. It is a way of feeding some of the output back to an input stage to boost performance. The term originated in the 1950's with the HV flyback transformer for the picture tube. I know that is not an answer to your list of questions, I was just explaining that the term is used in many different fields, yet it means the same thing in context. Touch of irony there. – user105652 May 24 '16 at 3:25

adding to Sparky, the specific meaning to "flyback" with TV or cathode-ray tubes is that the electron beam that was scanning left-to-right across the screen will quickly "fly back" from right to left to begin the next scan line.

this beam was steered by coils mounted at the electron gun that was shooting electrons at the phosphor screen. the electronics that was connected to these coils had to generate a sawtooth waveform to determine the instantaneous position of the electron beam. this sawtooth waveform was also fed into the high-voltage transformer to get the 25000 volts necessary at the screen of the canthode ray tube to draw the electrons from the electron gun. it was during this "flyback" that the slope (or derivative w.r.t. time) of the sawtooth waveform was much greater and the result was a high voltage in the secondary winding induced by a rapidly changing current in the primary. that's why the transformer was called the "flyback transformer" in these old TVs.

• In the switching converter world, the flyback term comes from the reflection of the output voltage $V_{out}$ scaled by the transformer turns ratio $N$ during $t_{off}$ (in CCM) across the primary inductance $L_p$: "when the primary-side switch opens, its drain or collector voltage rises until the secondary-side diode conducts. At that moment, the output voltage "flies" back to the secondary side and forces primary-side current decay". I never heard about this CRT explanation but it could also be a valid explanation of the term origins. – Verbal Kint Jul 19 '17 at 13:05

There probably isn't much difference in reality, both mean a couple of coils sharing a common core. A flyback is a special kind of transformer as it can store energy like an inductor (an ideal transformer doesn't store energy), maybe the term 'coupled inductor' is used to distinguish between something used like an inductor and something used like a transformer.

Other answers related "flyback" to cathode ray tube but I like to relate flyback in electronics to the flywheel in mechanical. The term is more related to the energy storage.

In the internal combustion engine, without flywheel the rotational output of the engine will be jerky because torque to the crank shaft is supplied by rods by form of pulses during combustion cycle. Engine flywheel which is connected to the one end of the crank shaft accumulates these and smooth it out by "flying". This is more required in single cylinder engine because the torque captured by the flywheel will drive the crank shaft and assist the next compression cycle.

Similarly in electronics when a solenoid is energized, the magnetic energy builds up and when the supply is cut off, collapsing magnetic field will generate voltage which can be used to power something else. Often this voltage will be many magnitude higher than the original input voltage depends on the coil inductance and the rate of change of the magnetic field.

So fly wheeling is energy storage and delivery, its a concept.

Your question is legitimate enough but you fall into the trap of careless use of language. FLYBACK is a product of television and all kinds of visual displays. The requirement to return the trace from "end" to "beginning" results in a waveform which, during the flyback period, must change very quickly. When such a steep change in current is applied to an Inductor it induces a very-large voltage-spike.

The scan circuitries for CRTs consume large "energies" with serious consequences in temperature-rise and so that enormous flyback voltage-spike is used to produce the e.h.t. (extra-high tension) for the final stages of the "electron gun". This is a brilliant solution to "Sod's Law" in that, should the line-scan fail, then so does the e.h.t. The need to call the fire-brigade does not arise.

• My last post ref. FLYBACK has produced comments which need explanation – Ken Green May 28 '16 at 11:05
• My last post ref. FLYBACK has produced comments which need explanation – Ken Green 21 mins ago – Ken Green May 28 '16 at 11:29