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Flyback transformer
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Flyback transformer
A flyback transformer (FBT), also called a line output transformer (LOPT), is a special type of electrical transformer. It was initially designed to generate high-voltage sawtooth signals at a relatively high frequency. In modern applications, it is used extensively in switched-mode power supplies for both low (3 V) and high voltage (over 10 kV) supplies.
The flyback transformer circuit was invented as a means of controlling the horizontal movement of the electron beam in a cathode-ray tube (CRT). Unlike conventional transformers, a flyback transformer is not fed with a signal of the same waveshape as the intended output current. A convenient side effect of such a transformer is the considerable energy that is available in its magnetic circuit. This can be exploited using extra windings to provide power to operate other parts of the equipment. In particular, very high voltages are easily obtained using relatively few turns of windings which, after rectification, can provide a very high accelerating voltage for a CRT. Many more recent applications of such a transformer dispense with the need to produce high voltages and use the device as a relatively efficient means of producing a wide range of lower voltages using a transformer that is much smaller than a conventional mains transformer.[citation needed]
The primary winding of the flyback transformer is driven by a switch from a DC supply (usually a transistor). When the switch is closed, the primary inductance causes the current to build up in a ramp. An integral diode connected in series with the secondary winding prevents the development of a secondary current that would eventually oppose the primary current ramp.
When the switch is opened, the current in the primary falls to zero. The energy stored in the magnetic core is released to the secondary as the magnetic field in the core collapses. The voltage in the output winding rises very quickly (usually in less than a microsecond) until the load conditions limit it. Once the voltage reaches such a level as to allow a secondary current, the charge flow is like a descending ramp.
The cycle can then be repeated. If the secondary current is allowed to drop completely to zero (no energy stored in the core), then it is said that the transformer works in discontinuous mode. When the secondary current is always non-zero (some energy is always stored in the core), then this is continuous mode. This terminology is used especially in power supply transformers.
The low voltage output winding mirrors the sawtooth of the primary current and, e.g. for television purposes, has fewer turns than the primary, thus providing a higher current. This is a ramped and pulsed waveform that repeats at the horizontal (line) frequency of the display. The flyback (the vertical portion of the sawtooth wave) can be a potential problem for the flyback transformer if the energy has nowhere to go: the faster a magnetic field collapses, the greater the induced voltage, which, if not controlled, can flash over the transformer terminals. The high frequency used permits the use of a much smaller transformer. In television sets, this high frequency is about 15 kilohertz (15.625 kHz for PAL, 15.734 kHz for NTSC), and vibrations from the transformer core caused by magnetostriction can often be heard as a high-pitched whine. In CRT-based computer displays, the frequency can vary over a wide range, from about 30 kHz to 150 kHz.
The transformer can be equipped with extra windings whose sole purpose is to induce a relatively large voltage pulse when the magnetic field collapses as the input switch is turned off. There is considerable energy stored in the magnetic field, and coupling it out via extra windings helps it to collapse quickly, and avoids the voltage flash over that might otherwise occur. The pulse train coming from the flyback transformer windings is converted to direct current by a simple half-wave rectifier. There is no point in using a full wave design as there are no corresponding pulses of opposite polarity. One turn of a winding often produces pulses of several volts. In older television designs, the transformer produced the required high voltage for the CRT accelerating voltage directly with the output rectified by a simple rectifier. In more modern designs, the rectifier is replaced by a voltage multiplier. Color television sets must also use a regulator to control the high voltage. The earliest sets used a shunt vacuum tube regulator, but the introduction of solid-state sets employed a simpler voltage-dependent resistor. The rectified voltage is then used to supply the final anode of the cathode-ray tube.
There are often auxiliary windings that produce lower voltages for driving other parts of the television circuitry. The voltage used to bias the varactor diodes in modern tuners is often derived from the flyback transformer ("Line OutPut Transformer" LOPT). In tube sets, a one or two-turn filament winding is located on the opposite side of the core as the HV secondary, used to drive the HV rectifier tube's heater.
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Flyback transformer
A flyback transformer (FBT), also called a line output transformer (LOPT), is a special type of electrical transformer. It was initially designed to generate high-voltage sawtooth signals at a relatively high frequency. In modern applications, it is used extensively in switched-mode power supplies for both low (3 V) and high voltage (over 10 kV) supplies.
The flyback transformer circuit was invented as a means of controlling the horizontal movement of the electron beam in a cathode-ray tube (CRT). Unlike conventional transformers, a flyback transformer is not fed with a signal of the same waveshape as the intended output current. A convenient side effect of such a transformer is the considerable energy that is available in its magnetic circuit. This can be exploited using extra windings to provide power to operate other parts of the equipment. In particular, very high voltages are easily obtained using relatively few turns of windings which, after rectification, can provide a very high accelerating voltage for a CRT. Many more recent applications of such a transformer dispense with the need to produce high voltages and use the device as a relatively efficient means of producing a wide range of lower voltages using a transformer that is much smaller than a conventional mains transformer.[citation needed]
The primary winding of the flyback transformer is driven by a switch from a DC supply (usually a transistor). When the switch is closed, the primary inductance causes the current to build up in a ramp. An integral diode connected in series with the secondary winding prevents the development of a secondary current that would eventually oppose the primary current ramp.
When the switch is opened, the current in the primary falls to zero. The energy stored in the magnetic core is released to the secondary as the magnetic field in the core collapses. The voltage in the output winding rises very quickly (usually in less than a microsecond) until the load conditions limit it. Once the voltage reaches such a level as to allow a secondary current, the charge flow is like a descending ramp.
The cycle can then be repeated. If the secondary current is allowed to drop completely to zero (no energy stored in the core), then it is said that the transformer works in discontinuous mode. When the secondary current is always non-zero (some energy is always stored in the core), then this is continuous mode. This terminology is used especially in power supply transformers.
The low voltage output winding mirrors the sawtooth of the primary current and, e.g. for television purposes, has fewer turns than the primary, thus providing a higher current. This is a ramped and pulsed waveform that repeats at the horizontal (line) frequency of the display. The flyback (the vertical portion of the sawtooth wave) can be a potential problem for the flyback transformer if the energy has nowhere to go: the faster a magnetic field collapses, the greater the induced voltage, which, if not controlled, can flash over the transformer terminals. The high frequency used permits the use of a much smaller transformer. In television sets, this high frequency is about 15 kilohertz (15.625 kHz for PAL, 15.734 kHz for NTSC), and vibrations from the transformer core caused by magnetostriction can often be heard as a high-pitched whine. In CRT-based computer displays, the frequency can vary over a wide range, from about 30 kHz to 150 kHz.
The transformer can be equipped with extra windings whose sole purpose is to induce a relatively large voltage pulse when the magnetic field collapses as the input switch is turned off. There is considerable energy stored in the magnetic field, and coupling it out via extra windings helps it to collapse quickly, and avoids the voltage flash over that might otherwise occur. The pulse train coming from the flyback transformer windings is converted to direct current by a simple half-wave rectifier. There is no point in using a full wave design as there are no corresponding pulses of opposite polarity. One turn of a winding often produces pulses of several volts. In older television designs, the transformer produced the required high voltage for the CRT accelerating voltage directly with the output rectified by a simple rectifier. In more modern designs, the rectifier is replaced by a voltage multiplier. Color television sets must also use a regulator to control the high voltage. The earliest sets used a shunt vacuum tube regulator, but the introduction of solid-state sets employed a simpler voltage-dependent resistor. The rectified voltage is then used to supply the final anode of the cathode-ray tube.
There are often auxiliary windings that produce lower voltages for driving other parts of the television circuitry. The voltage used to bias the varactor diodes in modern tuners is often derived from the flyback transformer ("Line OutPut Transformer" LOPT). In tube sets, a one or two-turn filament winding is located on the opposite side of the core as the HV secondary, used to drive the HV rectifier tube's heater.
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