Flyback transformer

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common abbreviations: "LOPT" ("Line Output Transformer") and FBT ("Flyback Transformer")

DC Flyback transformer: also known as a "modern" FBT


Contents

General Specifications

  • Rated voltage output 10-30 kV
  • Rated output between 4-15 mA
  • Driven near 15 kHz (if television component)
  • Driven between 30-150 kHz (if monitor component)

Overview

There are two main types of flybacks: DC and AC. AC flybacks are simply ferrite transformers, AC in equals AC out, but DC flybacks include a rectifying diode and often a multiplier stage inside. AC flybacks are found in older televisions and the rectifier and multiplying circuits will be found separately inside the television. It may be important to note that a plasma globe can only be effectively driven by an AC flyback. An AC flyback can be made from a modern DC flyback by either disabiling the rectifier diode via high energy pulse or overvolt, or by cutting into the epoxy and removing the diode. These methods do not work very well, and the flyback tends to fail soon after conversion because the insulation is not designed to handle AC currents.

Function

Flybacks supply high voltage to the CRT in a television or monitor. They are driven by a sawtooth waveform at the frequency of the video signal. It is an integral part of the horizontal deflection circuit that scans the beam from left to right, quickly returning the beam to the left. In a television, NTSC is 15 734 Hz, PAL is equal to 15 625 Hz, PAL M is equal to 15 750 Hz. A computer monitor's frequency varies generally between 30 kHz and 150 kHz. See the specifications for refresh rate to determine the horizontal frequency. Knowing the frequency at which the flyback is designed to run is beneficial in the design of a driver circuit.

Driving a Flyback

Arc from a DC Flyback Transformer using ZVS driver.

Once you have the transformer, a few common components are enough to complete the circuit and generate high voltages. Both AC and DC FBT's can be driven with any of these drivers. The original flyback primary can be used, provided one can identify the pins, or if more power is required, a new primary is wound on the ferrite core that is exposed.

Some purists like to point out that the following drivers do not drive the FBT's in the actual "flyback" topology. To read about driver topologies: SMPS topologies For specifically the flyback topology: Flyback Converter

Simple Circuits

The simplest schematic for driving a flyback is the classic "'3055" circuit, so called because it is driven by the 2n3055 NPN bipolar transistor. This circuit is extremely popular because of it's ease of assembly, and widespread availability of parts (all parts can be obtained at the local Radio Shack or Maplin). Characterized by it's mesmorizing high pitched squeal that varies with electrode spacing, it is generally known as one's gateway into the hobby of high voltage. It is also condsidered a "safe" project. One can recieve painful shocks and small burns, but only enough to learn to respect high voltage. The amateur considering high voltage as a hobby is strongly encouraged to build this circuit to become familiar with the "suprises" of high voltage before moving on to more powerful drivers.

Simple 2n3055 driver: Although popular this circuit is somewhat unstable. The 2n3055 tends to die easily, especially when driven over 12V. A common solution is to use the transistor that came with the FBT. The experimenter soon tires with it and is left hungry for more power!


A common variant of this circuit is seen here:

Push-Pull 2n3055 driver: This schematic offers more power than the previous one. But, like its relative, it too is prone to failure. Due to the transistor, not design.

Fixed Frequency/PWM Circuits

The next wave of drivers came in the form of 555 and TL494 chips driving the flyback transformer at resonance. When properly customized to the particular flyback, they can be more powerful than any other driver.

  • Insert schematics to VCore's 555 driver, Michi's TL494, and Steve Conner's driver. All mentioned can be seen here The 555 and TL494 drivers are the only "published" schematics with audio modulation.

High Power Drivers

A very powerful and recently very popular driver was designed by Vladmiro Mazilli. It uses resonant zero-voltage switching to drive the flyback, this topology is known as "ZVS." This means that the transistors are designed to switch when the voltage across them becomes zero. The 5 turns on the primary is no hard and fast rule, but works well with pretty much any flyback. More performance can be obtained by adding or subtracting the windings. The voltage input to the driver may affect the number of turns required as well. The "47-200 µH" inductor is customized to the desired output of the FBT. In general, if one requires more voltage output, he should use a larger inductor. For more current, use a smaller value inductor. The 0.68 µF capacitor can vary a bit, depending on the FBT, increasing the capacitance may increase performance. This capacitor must be bipolar.

Original Mazilli Driver:

4HV debut of driver found here. Vladmiro Mazilli's website: Link


There is a revision of the ZVS by Andrinerii, He adds 2 separate 3 turn coils on top of the existing 10 turn primary, each with a series 100 Ohm resistor. This circuit supposedly adds a 35% increase in performance. Important: Make sure your 3 turn winds are the same direction as the primary, otherwise your driver will be dead!

Revised Schematic

Driving Multiple Flybacks

Multiple flybacks can be driven in series (for more voltage) or parallel (for more amperage). The primaries are driven in parallel using either syncronized multiple drivers, or a single driver with extra primaries. The HV output is then put in series or parallel depending on intended results. If a combination of series/parallel is required, more than one driver may be required. AC flybacks require phase matching or their outputs may cancel out. Simply reverse the primary wires on one or more flybacks. DC flybacks, because they are internally recified, do not require phase matching. However, the DC flybacks, if to be placed in series, require that the HV+ of one FBT is connected to the HV- of the next FBT in the series. Voltages in excess of 100 kV can easily be had using this method, however, the stresses on the flybacks are increased and this may require them to be put under oil to prevent arc-over.


Pin Identification

Flyback transformers have complex internal construction with many separate windings. The high voltage secondary has a high resistance due to the winding length, some flybacks also contain internal diodes in the secondary. Due to the high resistance (and voltage drop due to diodes), the high voltage ground pin cannot be identified by testing resistance/continuity.

To identify the HV ground pin, a low voltage (~30V) power supply can be connected in series with the HV output lead, and a voltmeter can be used to test each pin individually to determine the ground. Note that due to the internal diodes in most modern flybacks, polarity does matter and the power supply negative output should be connected to the HV output lead, with voltmeter in series testing for voltage at each pin.

For primary windings, many separate windings exist with high impedance, and are normally driven by voltages between 100-200V by TVs or monitors. It is recommended to wind a custom primary for all low voltage driven methods (single transistor, dual, ZVS, etc) with the appropriate wire gauge for the current expected to be drawn.

A simpler method for HV ground pin identification is to draw an arc to each ground pin, with the longest/strongest arc being the HV ground.


Troubleshooting

  • What do I do if the bottom pins start arcing?

First, try turning down the "screen" or "focus" pots on the side of the flyback. If that doesn't help, some cut the pins off and cover with silicone. Some report that hot glue works well to insulate them, others report that hot glue only makes it worse. If all these methods were tried, then the FBT must be put under oil.

  • I broke the ferrite core!!!

Superglue it back together. If you do a good job, you shouldn't notice any difference in performance.


Flyback construction

Further Reading

Flyback Driven Tesla Coils

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