flyback?

what is a flyback on a monitor for a cab?

This description of what a flyback transformer is and does was written by Sam Riner. Its well explained
Good on you Sam.


What does the flyback (LOPT) transformer do?


The typical flyback or Line OutPut Transformer (LOPT) consists of two parts:

1. A special transformer which in conjunction with the horizontal output
transistor/deflection circuits boosts the B+ (120 V typical for a TV) of the
low voltage power supply to the 20 to 30 KV for the CRT as well as provide
various secondary lower voltages for other circuits.

A HV rectifier turns the high voltage pulses into DC and the CRT capacitance
smooths it. The HV may be developed from a single winding with many many
turns of wire or a lower voltage winding and a diode-capacitor voltage
multiplier.

The various secondary voltages power the logic, tuner, video signal,
vertical deflection circuits, and CRT filaments. In fact, with many TV
designs, the only power not derived from the flyback is for the keep-alive
circuitry needed to maintain channel memory and provide startup drive to
the horizontal deflection/high voltage system.

2. A voltage divider that provides the focus and screen supplies. The pots
are in this divider network - and these things fail resulting poor focus,
uncontrolled brightness, or fluctuating focus and/or brightness. A total
short could also result in failure of other components like the horizontal
output transistor. The focus and screen are generally the top and bottom
knobs, respectively. In some TVs, the focus and screen divider and/or
controls are external to the flyback and susceptible to dust and problems
particularly on damp days.


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2) How is a flyback transformer different than a regular transformer?


While the following is not always strictly true for TV and monitor flyback
transformers, it is a nice overview:

(From: Sivasankar Chander (siva@bond.bocaraton.ibm.com)).

The main difference between a flyback transformer and a regular transformer
is that a flyback transformer is designed to store energy in its magnetic
circuit, i.e., it functions like a pure inductor, whereas a regular transformer
is designed to transfer energy from its primary to secondary and to minimize
stored energy.

Secondly, a flyback transformer in its simplest form has current flowing
either in its primary, or in its secondary (but not both at the same time).
(This is more complicated in practice because of finite turn-off times for
transistors and diodes, need for snubber circuits, etc).

Thirdly, the reluctance of the magnetic circuit of a flyback transformer
is usually much higher than that of a regular transformer. This is because
of a carefully calculated air-gap for storing energy (it's an inductor).

Fourthly, the voltages applied to a flyback transformer on the primary
side are almost always rectangular (pulsed) whereas regular transformers
usually have sinusoidal voltages applied to them.

Fifthly, the currents flowing through either side of a flyback transformer
are either increasing or decreasing linear sawtooths, whereas a regular
transformer usually has sinusoidal currents.

Finally, due to the properties of core materials, flyback transformers
are most conveniently operated in the range from 10^3 to 10^6 Hz, whereas
regular transformers have a much wider range, from a few Hz to 10^12 Hz.

I may have succeeded in confusing you beyond redemption, so the best
recourse for you would be to read any introductory textbook on switching
power supplies for a more comprehensive picture.


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3) The origin of the term, 'flyback'


In the U.S. (possibly all of North America), the transformer that generates
the high voltage in a TV, monitor, or other CRT based equipment, is called
the 'flyback' or 'flyback transformer'. Most everywhere else in the world, it
is either LOPT (Line OutPut Transformer) or simply LOT.

The term 'flyback' probably originated because the high voltage pulse that
charges the CRT capacitance is generated by the collapse of the magnetic
field in the core of the transformer during the short retrace period - when
the electron beam in the CRT 'flies back' to the start of a new scan line.
The flux in the core changes slowly during scan and is abruptly switched
in polarity by the HOT turning off and damper diode turning on during this
flyback or retrace period.

Many off-line switchmode power supplies and DC-DC converters are also of
the 'flyback' type with energy transferred to their output circuits mainly
during the same time in the cycle - but there is no CRT involved and their high
frequency transformers are not generally called flyback transformers.

LOPT and LOT derive from the fact that it is the line scan circuit that is
involved and the transformer is in the output stage.

I still think flyback is much more quaint! :-).

Of course, others have their own definition:

(From: Sam Riner (riner@inet2000.com)).

When I was about 12 I touched the wire coming from the FBT on the picture
tube, this was a BIG floor model TV, and I flew about five feet backwards. I
know this isn't the real history for the name but for many years I believed it
was.