Flyback Construction

While details can vary somewhat, all flybacks consist of a set of windings on a gapped ferrite core. High voltage diodes and resistive dividers (often with adjustment pots) for focus and screen (G2) may also be present.

A typical flyback includes the following components:

• Drive winding - for a line powered TV, there will be perhaps a hundred turns of medium gauge (e.g., AWG #26) wire. For low voltage powered, it may only be a dozen turns of thicker wire. This is what is connected in series with the B+ to the horizontal output transistor in a TV or monitor.

• High voltage winding - several thousand turns. This winding may be split into several series sections with a high voltage rectifier for each or could be a single winding. An alternative is provide a lower voltage winding and use a voltage multiplier (diode-capacitor ladder) to boost this to that required by the CRT. Very fine wire (e.g., AWG #40) will be used for the high voltage winding. The high voltage lead to the CRT is fed from the highest voltage output of the rectifier or multiplier.

  Some TV and monitor designs use a physically separate (external - not part of the flyback transformer) voltage multiplier. In this case, the flyback high voltage winding will generate 6 to 10 kVAC and the multiplier will boost this typically 3X or 4X to 20 to 30 kVDC. The focus and screen (G2) network will generally be part of the multiplier module in this case.

• Resistive divider network for focus and screen (G2). This will probably be fed from only one of the series connected windings (if used). Often, there are adjustments for focus and screen right on the flyback. The outputs from this divider may be connected to pins in the base of the flyback or have their own separate leads which connect to the CRT socket/board.

• Auxiliary windings - anywhere from a couple of turns (for the CRT filament) to several hundred turns (for a boost source). These supply various voltages for the typical TV or monitor - CRT filament, logic power, analog power, boost source (where the flyback does not include its own screen supply), etc. The gauge of these windings will depend on the current requirements of each output. They are connected to solder pins at the base of the flyback.

• Ferrite core - consisting of two C shaped pieces clamped together with either a spring arrangement or studs and nuts. There will be a gap of a fraction of a mm provided by a set of spacers between the two C sections.

Most modern flybacks have all the windings on the same leg of the core. The drive winding and auxiliary windings will be wound and separately insulated under the high voltage winding. The high voltage winding will consist of many layers which have insulating material (i.e., mylar) between them.

The other components will be mounted in a separate part of the assembly and the entire unit is then potted in an Epoxy type filler. Part of the core is generally accessible - often one entire leg.

A flyback is not an ordinary transformer. The ferrite core contains a gap. Energy is stored in the magnetic field of the core during scan as the current is ramping up. Energy is also coupled to certain secondary outputs during scan. However, energy for the high voltage (HV) is coupled to the its secondary windings almost entirely when the primary current is shut off at the end of the scan (probably the source of the name flyback because it is during the retrace of the electron beam).

Which type of coupling is in effect depends on the direction of the rectifiers on the secondary side of the flyback:

Here, V1 is just a typical example of an auxiliary supply derived from a scan rectifier and HV is the best known example of the use of a flyback rectifier.

Note that the ratio of the number of turns for each winding *cannot* be used to calculate expected output voltages since the rate of collapse of the magnetic field (determined by the design of the horizontal output circuit) affects this.

The gap is critical to the proper operation and is usually determined by some plastic spacers. CAUTION: mark each one and replace them in exactly the same position if you disassemble the core for any reason.