Flyback transformer
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The flyback transformer (FBT), also called a 'line output transformer' (LOPT), is a special transformer which is used to generate high voltage (HV) signals at a relatively high frequency. It was invented as a means to control the horizontal movement of the electron beam in a cathode ray tube (CRT). As with all step-up transformers, it receives low voltages and transforms them into high voltages; in this case, it does so at a relatively high frequency--much faster than the vertical movement of the electron beam (known as the vertical scan rate).
The flyback transformer is used in the operation of CRT-display devices such as the oscilloscope and television, and in other HV display devices such as the DIY plasma lamp. Like many other transformers, the flyback transformer is typically used to generate a single output voltage at a single frequency. The voltage and frequency can each range over a wide scale depending on the device. For example, a large color TV CRT may require 20 to 50 kV with a horizontal scan rate of 15.734 kHz for NTSC devices. However, a much smaller oscilloscope tube may require only a few kilovolts (kV). Unlike a power (or 'mains') transformer which uses an alternating current of 50 or 60 Hertz, a flyback transformer typically operates with switched currents at much higher frequencies in the range of 15 kHz to 50 kHz.
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[edit] 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 used permits the use of a much smaller transformer. In television sets, this high frequency is about 15 kilohertz (15.734 kHz 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.
[edit] 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.
[edit] Construction
Around a ferrite rod the primary is wound first and then the secondary around that. This is to reduce the leakage inductance of the primary. Finally, a ferrite frame is wrapped around the whole mess, closing the magnetic field lines. Between the rod and the frame is an air gap, which reduces the remanence. The secondary is wound layer by layer with enameled wire, and 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 will be needed to protect the surrounding. In a variant, to avoid some stray capacitance, 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[citation needed].
[edit] Failure
Flyback transformers are a frequent source of failure in CRT displays. Frequently, the CRT tube itself is blamed when the display has actually experienced a flyback transformer 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, can result in leakage between the windings. As the leakage heats the insulation it carbonizes, increases conduction; in turn heat and carbonization continues a downward spiral until the leaked current is high enough for the high voltage to arc between the windings, and destroy the transformer (and sometimes other components in the display). 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. It is debatable among technicians if displays installed in dirty, dusty locations experience more failures than those in cleaner locations, but many do say that dirty conditions contribute to malfunctions.
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. A common injury that occurs when one is shocked is actually to be injured not as much by the shock itself, but when the victim's hand or arm is thrown back against other internal components in the display device. 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 attached to the flyback 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 internally within the flyback transformer to ensure the charge is safely grounded when not in use, but many sets lack this, especially older models.
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 picture reproduction. In older televisions and monitors, these needed adjustments were performed by turning potentiometers inside, or on the back of the set (sometimes called "tweaking" by those in the electronics trade) to achieve optimal picture quality. Also, when flyback transformers fail, they frequently will also take out the horizontal output transistor that drives the flyback transformer, and sometimes even blow fuses in the low voltage power supply circuits.
Unless the owner of the display device is savvy enough to repair it themselves, the failure of a flyback transformer frequently condemns the device as unrepairable, because the cost of repair can be higher than the replacement cost. Although the cost of the flyback transformer, and other damaged parts is relatively inexpensive, the labor time needed to disassemble, replace the parts, and then re-adjust the display can make the repair job expensive.
[edit] References
- Dixon, Lloyd H, Magnetics Design Handbook, Section 1, Introduction and Basic Magnetics, Texas Instruments, 2001
- Dixon, Lloyd H, Magnetics Design Handbook, Section 5, Inductor and Flyback Transformer Design, Texas Instruments, 2001
[edit] Patents
- U.S. Patent 3,665,288 - "Television sweep transformer" - Theodore J. Godawski
