Jochen's High Voltage Page

Flyback transformers

In contrast to OBITs and MOTs, flybacks cannot be connected to the mains directly. They work at a frequency of about 20kHz, whereas the mains has only 50Hz. The higher frequency has many advantages, such as smaller, lighter cores, smaller caps for rectifiers etc., but it makes an electronic control circuit necessary.

Flybacks can be found in all types of monitors and screens that use a cathode ray tube (CRT), e.g. TV sets, computer monitors etc. A flyback serves several purposes in a TV set, mainly the generation of the acceleration voltage for the CRT (typically 20-30kV), and of several auxiliary voltages. Very roughly, three types can be distinguished:

Different types of flybacks.
(1) from color TV 8kVpp
(2) and (3) from b&w TV 15-20kVpp
(4) diode split transformer 25kV DC
(5) flyback with integrated cascade 25kV DC

One major difficulty, to my experience, is to find the useful ones out of the vast number of connector pins a flyback usually has. The high voltage output is in most cases obvious (the single lead coming from the secondary). With normal flybacks, i.e. not internally rectified, the other end of the high voltage winding can be found using an Ohmmeter or circuit tester. The rest of the pins can be grouped as belonging to several different windings by the same method. Which winding and of this, which taps are best suited for input must be tried.

The original primary winding of a flyback is often designed for about 150V, i.e. the circuit which produces the necessary 20kHz signal works at 150V DC. Voltage pulses produced thereby across the primary measure easily 700V and more. The simplest (though not best, for safety reasons) way of producing such high working voltages is directly from the mains. If you don't want to take the risks of working with mains voltage, or if the device must be independent of the mains, you will have to add a new primary winding. One winds about 10 turns (try out optimum number) 1mm diameter enamelled copper wire around the core, preferably directly underneath the secondary winding. If there is no room, it often helps to rip off some of the unused original windings. However, such a new primary winding with a low number of turns will be less efficient than the original winding.

Simple driver circuit

Simple flyback driver.
KW: 2 x 5 turns enamelled copper wire 1mm
BW: 2 x 2 turns enamelled copper wire 0.5mm
Transistor 2N3055 must be mounted on heat sink.

The circuit above requires a new primary winding (KW) and an additional feedback winding (BW). Both windings are center-tapped. Instead of enamelled copper, any sufficiently thick, insulated wire may be used. The circuit runs on 12-24V DC, e.g. a car battery. The current draw is rather high (several Amps). The supply voltage should be well filtered - this can be achieved by connecting an additional electrolytic capacitor (10000uF) across the supply points (marked "+" and "-"). Both transistors (2N3055) must be mounted on large heatsinks. The output high voltage depends on the type of flyback and the supply voltage, but should be on the order of 10-30kV.

The circuit can be optimized by varying the number of turns and the resistor values. If the oscillator doesn't start, i.e. it draws current but produces no output, the polarity of the feedback winding is wrong and must me inverted. In any case, the transistors will run quite warm (a sign of inefficiency). Also, there is no over-voltage protection present - some additional MOVs across the primary winding might prolong transistor life considerably.

Efficient driver circuit

Efficient flyback driver working on mains voltage (230V).

This is a much more efficient circuit, which make use of the original primary winding of a flyback. The necessary working voltage of 150V DC is rectified from the center tap of the mains transformer Tr1 (this is for 230V mains voltage - if mains voltage is already 115V, the rectifier can be directly connected to the mains). The special high voltage transistor BU208 switches this working voltage with a frequency of 20kHz to the flyback primary, inducing pulses of about 700V peak to peak) there. The hair-drier in the supply line limits current draw in case of secondary short-circuit, protecting the BU208 from overload. The VDR (Voltage Dependent Resistor, also MOV, Metal Oxide Varistor) absorbs high voltage pulses and to protect the transistor. The driving signal for the BU208 is generated by a standard circuit involving a NE555 timer IC. The complementary transistor pair BC161/BC141 acts as a current amplifier to provide the necessary base current for the BU208, which is adjusted to about 1A by the subsequent resistor.

Tr1 can be any decently powerful (100VA) mains transformer suitable for both 230V and 115V. Well suited are so-called mains voltage adaptors, which make it possible to run US devices on the European 230V line, or vice-versa. Of course, Tr1 need not be a autotransformer. From a safety point of view, an isolation transformer with 115V output is highly recommend (but expensive)! For the BU208, any high voltage switching transistor with similar data (700V, 8A, 7MHz) may be used. The hair drier may be substituted by a fan-cooled high power resistor etc.

Understanding how this circuit works requires some knowledge of physics or electronics. I'll try an explanation anyway. You might want to look at the schematic oscilloscope pictures while reading (this link will open another window).

While the BU208 is "on", there are 150V DC across the primary winding, thus the current rises linearly from zero according to the laws of self-inductance. With increasing current, a magnetic field is built up, in which energy is accumulated. With switch-off, current and magentic field suddenly fall to zero, releasing the energy stored in the field. This gives rise to a large negative voltage pulse, which is only limited through the charging of C*. The LC oscillator formed by C* and the flyback winding it is attached to performs one half-cycle. During this half-cycle, the peak voltage across the primary is up to -700V. The oscillation is interrupted after a half-cycle, when the voltage goes above +150V. The diode SKE4F2/08 becomes conducting and the remaining energy is spent charging the electrolytic caps of the DC supply. Thus, energy drawn from the supply during in the "on" phase and not used (e.g. for a power arc) in the "off" phase is given back to the supply, making the circuit quite efficient.

C* is connected to a winding with a relatively high number of turns (relatively high output voltage, around 1000V) and must be a high-voltage type. WIMA FKP caps (PP dielectric) work well, polyester caps get hot due to the high frequency switching. The capacitance value must be tried out in conjunction with the frequency adjustment described below. 1nF is a good choice to start with. A higher value of C* decreases the pulse peak voltage and increases the half-cycle (time of oscillation). Flybacks from black-and-white TVs often have already a relatively high self-capacitance, so that C* can be reduced or even left out. A secondary load such as a cascade (voltage multiplier) acts like an increased value of C*, so that C* must be reduced accordingly.

The switching frequency is adjusted with the potentiometer (near NE555). The lower the frequency, the longer the "on" phase and the higher the pulse peak voltage. For safe and reliable operation the peak voltage should be kept below about 600V, otherwise the transistor BU208 is likely to be destroyed. On the other hand, the "off" phase must be longer than the half-cycle (1/2 oscillation time) defined by C* and the winding it is attached to, but not so long that a second half-cycle can start (i.e. 3/2 oscillation time). Whether one of these is the case can be seen on the scope (see schematic scope pictures). Such a condition is unwanted, because it acts like a transient short circuit between supply voltage and induced primary voltage pulse, causing the BU208 to get hot and substantially increasing supply current (without secondary load). If a frequency meeting all conditions cannot be found, the supply voltage must be reduced.

The small extra circuit on the left implements a rudimentary output voltage regulation. If the output voltage crosses a threshold set by the potentiometer, the LED does what it was made for, reducing the resistance value of the LDR. This causes an increase in frequency,resulting in reduction of the pulse peak voltage. This is a useful add-on, but not necessary for basic operation.

A note on safety: without an isolation transformer, every part of the circuit is on mains potential and must not be touched. Grounding of any point of the primary circuit (hopefully) causes a short circuit and blows the fuse. However, on most flybacks, the secondary winding (high voltage output) is separate, i.e. not connected to any of the other windings. In this case, the "lower" end of the secondary (usually one of the pins) should be grounded, e.g. on the earth lead of the mains socket. Then, if the insulation between secondary and other windings is damaged, any mains voltage then present on the secondary is short-circuited to ground, again blowing the fuse and thus eliminating the danger of a mains voltage shock.

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