Flyback transformer

Flyback transformer

A flyback or line output transformer (FBT or LOPT) is a type of transformer used in the power supply of a cathode ray tube that generates the high voltage needed to drive a "CRT" type monitor, and can also be used to power a plasma globe. It generates a voltage ranging from a few kilovolts for an oscilloscope tube to 20 to 30 kilovolts for a color TV tube. Unlike a mains (line) transformer, which works with sinusoidal alternating currents at 50 or 60 hertz, a flyback transformer operates with switched currents at much higher frequencies, usually in the range of 17kHz to 50kHz.

How it works

Unlike mains transformers and audio transformers, an LOPT is designed not just to transfer energy, but also to store it for a significant fraction of the switching period. This is achieved by winding the coils on a ferrite core with an air gap. The air gap increases the reluctance of the magnetic circuit and therefore its capacity to store energy.

The primary winding of the LOPT is driven by a relatively low voltage sawtooth wave, which is ramped up (and sweeping the beam across the screen to draw a line) and then abruptly switched off (and causing the beam to quickly "fly back" from the right to the left of the display) by the horizontal output stage. This is a ramped and pulsed waveform that repeats at the horizontal (line) frequency of the display. The flyback (vertical portion of the sawtooth wave) is extremely useful to the flyback transformer: the faster a magnetic field collapses, the greater the induced voltage. Furthermore, the high frequency reduces the size of the transformer. In television sets, this high frequency is about 15 kilohertz (15,734Hz for NTSC), and vibrations from the related circuitry can often be heard as a high-pitched whine. In modern computer displays the frequency can vary over a wide range, from about 30 kHz to 150 kHz.

The alternating current coming from the flyback transformer is converted to direct current by a high-voltage rectifier. If the output voltage of the LOPT is not high enough by itself, the rectifier is replaced by a voltage multiplier. Early color television sets (like the 1954 RCA CT-100) also used a regulator to control the high voltage. The rectified voltage is then used to supply the anode of the cathode ray tube.

There are often auxiliary secondary windings that produce lower voltages for driving other parts of the display's circuitry — often the CRT's filament will be driven from the flyback. In tube sets, a two-turn filament winding is located on the opposite side of the core as the HV secondary, used to drive the rectifier tube's heater.

Practical considerations

In modern displays, the LOPT, voltage multiplier and rectifier are often integrated into a single package on the main circuit board. There is usually a thickly insulated wire from the LOPT to the anode terminal (covered by a rubber cap) on the side of the picture tube.

One advantage of operating the transformer at the flyback frequency is that it can be much smaller and lighter than a comparable transformer operating at mains (line) frequency. Another advantage is that it provides a failsafe mechanism — should the horizontal deflection circuitry fail, the flyback transformer will cease operating and shut down the rest of the display, preventing the screen burn that would otherwise result from a stationary electron beam.

Construction

Around a ferrite rod the primary is wound first and around this the secondary. This is to reduce the leakage inductance of the primary. And around this a ferrite frame closing the magnetic field lines. The gap between the rod and the frame is an air gap which reduces the remanence. The secondary is wound layer by layer with enameled wire with mylar film between the layers. In this way parts of the wire with higher voltage between them have more dielectric material between them. The outside of the winding is on the highest voltage so insulation and screening may be needed to protect the surrounding. In a variant, to avoid some stray capacity, every layer of the windings is connected by a rectifying diode to the next layer. Windings go up the rod and the diodes go down. In this way the AC voltage increases along the rod (axial) and the DC voltage increases radial from inside to outside. When applied to tape wound coils this would mean each coil goes from inside to outside and the diode goes back to the inside Fact|date=March 2007.

Failure

Flyback transformers are a frequent source of TV tube failure. The high voltage present in the many turns of wire, with the somewhat thin insulation required for the transformer to be of reasonable size is likely to result in leakage. As the leakage heats the insulation, it carbonizes increasing conduction; in turn heat and carbonization until the leaked current is high enough for the high voltage to cease to function. As a result, replacement flyback transformers for almost every set on the market are available through dealers in electronic parts, typically for under $50. The problem is exacerbated by the tendency of the flyback to accumulate a coating of dust due to electrostatic attraction, which serves as a path to ground for leaks which might otherwise not be of sufficient magnitude to initiate the chain of events leading to destructive failure, as described.

As a result, occasional cleaning of the accumulated dust from the high voltage circuitry inside a television can be beneficial if proper precautions are taken -- however the small amount of additional life that is gained for the flyback transformer rarely justifies the time and effort necessary.

A flyback transformer and its associated circuitry operate at very high voltages at low currents (>1mA-15mA), far beyond mains voltage. While most flybacks do not supply enough power to kill directly, the voltage they employ can cause violent muscle spasms if touched; and such spasms usually cause injury. Therefore, only trained persons should touch or modify these devices, after first ensuring that the transformer is switched off and any stored energy has been safely discharged. The CRT has an inherent capacitance which can hold a high voltage charge for up to a week or more after the power is switched off. Often, a high-resistance bleeder resistor is connected in parallel to ensure the charge is safely grounded when not in use, but many sets lack this.

In many recent televisions, after replacing the flyback transformer, the control firmware must be recalibrated to account for slight differences in performance between transformers in order to maintain accurate color reproduction.

References

* [http://focus.ti.com/lit/ml/slup123/slup123.pdf Dixon, Lloyd H, "Magnetics Design Handbook, Section 1, Introduction and Basic Magnetics", Texas Instruments, 2001]
* [http://www-s.ti.com/sc/techlit/slup127.pdf Dixon, Lloyd H, "Magnetics Design Handbook, Section 5, Inductor and Flyback Transformer Design", Texas Instruments, 2001]

Patents

* - "Television sweep transformer" - Theodore J. Godawski

ee also

Flyback converter


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