Electronic transformer - the name of the ordinary power converter supply voltage of 220 V at 12. It is possible that there will be also other denominations. 12 VAC is widely used for lighting purposes, provided that the device's popularity. Transformer device is called a simple alternative for power transformer 220 V.
You can not get around thanks to Ruben Lee, bothered to collect as much information about the wonderful small transformers in the same book. SV Kulikov has been a great help in explaining the multivibrators device and engineers P. Fichera and R. Skoll from STMicroelectronics GROUP OF COMPANIES explained the current state of the industry, giving advice on the choice of transistors.
Electronic transformer is considerably low and can adjust the output power. The scheme is flexible and easy to implement short-circuit protection. A side effect becomes low noise, no buzz typical power transformer (more precisely, the vibration fixtures above the human hearing).
The name and internal device
Electronic transformer consists essentially of a compact transformer, and a number of transistors. In fact, this is strongly simplified switching power supply. Instead of the generator of an IC multivibrator uncomplicated working of a pair of bipolar transistors. Filtered output voltage no longer needed, the driver is capable of low-voltage discharge lamps independently smooth voltage. No thyristor and power key, power transistors and so are the high frequency voltage generator. Procedure:
- The diode bridge rectifies the voltage filterable partially throttles.
- Pulsating flow feeds transistors that are included under the scheme multivibrator.
- With the high-frequency pulse generator output signal is applied to a small-sized transformer.
The trick is to create transistors that can be fed a high voltage. If the generator is an integrated circuit (a pulse is present in each power supply), the manufacturers do not greatly puzzled by only two power switches. To understand the electronic transformer works need to be of the principles on which is founded the miniaturization of equipment.
The reasons for the small size of the pulse transformer
There is no clear boundary between the power and pulse transformers. Just as the frequency increases greatly reduced and the dimensions of the winding core at the same power skipped. This was first realized Tesla, who wanted to raise the frequency of the power supply equipment to 600-700 Hz, in order to make the current safe for humans. However, with increasing frequency increased core loss, and the wave radiated into space, and the cable needs to be displayed. The first is due to thickening of the hysteresis loop of the magnetization reversal cycle, due, oddly enough, the current in the laminated material by induction currents.
Transformers in its original form came from the electricity network. In the history of the creation of instruments credited Yablochkov but thanking Meyl.ru answers, I want to give a different perspective on the issue:
- In 1831 Michael Faraday invented the first (toroidal) transformer and based on it shows the effect of the law of electromagnetic induction.
- After Michael Faraday transformer design mentioned Joseph Henry, the inventor of the electromagnetic relay. Both did not pay attention to the transformative properties of the device.
- In 1848, Henry Ruhmkorff invented the coil for the arc in the spark gap of the secondary circuit. In fact, it turned out to be a step-up transformer. Such use of Tesla.
- November 30, 1876 Paul created Yablochkov core transformer with concentric windings for the purpose for which the instrument is used to this day.
- John and Edward Hopkins in 1884 created a transformer with a closed core, repeating venture Faraday. A few years later Swinburne taught people to use for winding insulation oil than the increased voltage.
- In 1928 he earned the Moscow Transformer Plant (later - Elektrozavod).
Now uvyazhem described electrical networks. By the early 80s the company already engaged in Edison lighting, Tesla built the first two-phase AC motor. Erupted between them enmity resulted in 90 years to the "war of currents". Voltage networks began to continuously rise until it reached 1.2 MW in 1982 on the line Ekibastuz-Kokshetau. Keeping pace with the above achievements were transformers, increasing in size.
In the "war currents" Tesla discovered that with increasing frequency transformers weight decreases due to the miniaturization of the windings and the core. Which led to the creation of the first designs for the high frequencies. As you know, the events were accompanied by the birth of radio. The introduction of these technologies quickly led to the need to create a relatively small-sized devices. Pulse transformers came from the radio. For example, mobile devices adapters use a simple amplitude detector at formation voltages.
Pulse transformers are usually heavily loaded as opposed to the network. It is estimated that at a voltage of 11 kV power distribution given current 90 kA, and the lamp on the transmitter 70 kW - consumes only 6. power is calculated from the formula that in the first case the resistance is 0.1 Ohm, in the second - 2 ohms. These values define the transformer output impedance. They play a big part weight and dimensions. Because industrial designs transformers are not suitable for electronics: the appointment varies.
Materials of small transformers
These factors led to the search and creation of new materials:
- Steel (cold-rolled) oriented domain structure.
- Polymeric insulation (including lacquer).
- Pure copper radiofrequency.
- Resin devoid of aggressive solvents.
- Electrical steel dopants.
- Permalloy or other ferrite with a high coefficient of magnetic permeability.
Thanks to these achievements of chemistry, physics and technology it became possible to achieve certain objectives:
- Reduce the size of the connected transformers.
- To reduce the volume occupied by a high-voltage part.
- Create filters with sharp rising and falling edges of the amplitude-frequency characteristic.
- The appearance transformers, specifically designed to transmit a pulse signal without loss.
- Raising the transmission spectrum to microwaves.
The last two items show a direct link. Sharp edges of the pulse signal cause the fact - a significant portion of the spectrum lies in the high frequency area. And a conventional transformer would cut portion, distorting the shape smoothing, with simultaneous loss of energy. In the mid-50s people wondered why the pulse transformers are not built in the likeness of power. After all known charts, tables, formulas for calculating the core section, power factor, voltage. Causes:
- The frequency range. The efficiency of the transformer at the lower operating frequency is determined by the inductance of idling, at the top - the distributed self capacitance. These parasitic effects cause leakage power, greatly reducing efficiency. On these parameters depends on: the number of turns of the winding, the size of the core, crossing windings, insulation type and others. High-frequency transformer is made in compliance with the nuances, to transmit the desired range with minimal losses.
- The electronic circuits of the main parameters considered and reactance of the windings resistance. Sometimes going to infringe weight and size characteristics in order to achieve a good transfer rate. The design is highly dependent on the destination and circuit impedance. Predict it, as in the case of power transformers, it is difficult.
In the pulse transformer often armored coaxial core with windings threaded through the window. This allows maximum transfer magnetic flux. Yoke portion closes field lines, energy losses are minimal. Double sidewall thinner rod, the flow is divided here into two parts flowing around the outside of the coil. Periodically rod core is more suitable for a particular purpose. Then the magnetic field circulates in the square, and the windings are put on opposite sides of a ferromagnet. The core is usually an integral, end to end, and to dress up the coil docking halves to simplify the process of the assembly process. Execution and protection casing defined climatic factors (moisture, temperature), the restrictions on size, voltage designation.
Long time could not understand why the laboratory studies of losses in the core magnetization reversal does not coincide with the real data at high frequencies. It turned out, the device for measurement of characteristics produces a constant field (for increase of efficiency) and locking the occurrence of induced currents. The latter becomes the cause of the discrepancies. Induced currents directly affect the width of the hysteresis loop. Today, electrical materials used with low coercivity for the manufacture of cores. Maximal loss observed when the saturation magnetic loop, it is limited to transmit power through a pulse transformer:
- Increase active losses on the windings.
- Small efficiency.
The shape of the hysteresis loop depends on the selected material. Today it is known alloys with a rectangular characteristic. Such unusual qualities allow you to create magnetic amplifiers. The power transmitted to the core, carries a pronounced jet shade for obvious reasons. The active part expresses the losses in laminated material. Reactive component is directly dependent on the magnetic permeability. Cold rolled steel is usually used for high frequencies, and hot rolling steel detects a fair amount of silicon impurity and is used for commercial frequency 50-60 Hz. thickness plates (according to change parameters and induction currents) decreases with increasing frequency.
As a result, core loss is small for small transformers. The main contribution is the ohmic resistance of the windings. In power transformers figures are comparable in magnitude. Ohmic resistance, hence limited the minimum conductor cross-section. It is believed to sustain the specified size, because the size of the core is rigidly defined. These two contradictory factors determine the economic feasibility and suitability of the chosen design.
Brief description of the core alloys
Selection of the core material is determined by the frequency and the inductive part of the load impedance. Cold-rolled steel is used where the reactive component is high, or there is need for a constant current to pass through the winding. In other situations seen relevant nickel alloy with large magnetic permeability but a lower permissible flux density.
Steel, alloyed with silicon, has the worst indicators, but cheap. It has coercive force of 0.5 oersteds at the maximum magnetic permeability of 8500 and a flux density 12 thousands of Gauss. It is used in small-sized low frequency transformers (including the audible range).
Cold-rolled electrical steel shows a much better performance due to the structure-oriented domain. At equal coercivity permeability increases fourfold at the maximum flux density of 17 thousands of Gauss. It serves as a medium power transformer core.
Ferronickel alloy of 50% characterized coercive force close to zero. That minimizes the loss of the hysteresis loop (on the reversal). At low allowable magnetic flux density (10 000 gauss) material characterized stunning permeability (up to 50,000). Good resistance to low frequency induction currents applied to the broadband small-sized transformers.
Ferronickel alloy of 50% with oriented domains structure is used in a saturation mode. In comparison with previous material characterized by increased and a half times the maximum magnetic flux density.
Permalloy (high-grade nickel alloy) is characterized by high magnetic permeability in the hundreds of thousands of units. It operates at a low magnetic flux density, which makes its use in small size transformers.
A composite ferrite steel and are of particular use in transformers and inductors with low loss for RF band. manufacturing features allow to create a solid core of any shape, with low Curie temperature material (magnetic properties). Ferro belt winds well and serves to create a one-piece cores, in particular a toroidal shape. Unusual qualities allow to put into practice the concept of a rectangular hysteresis loop.
Considered acceptable core section 0.645 km. mm to 1 ampere. This allows a first approximation to determine the amount of copper. Lapping is performed on the temperature conditions, the electrical parameters of the transformer, including a capacity (cm. Fig.). Further strongly depends on technological features. For example, a 30 gauge enamelled wire wound manually linearity factor is 97%, automated assembly reduces the parameter to 80%. The same construction has characteristics depending on the product manufacturing site.
Packing density naturally rises with decreasing caliber. From the found section calculates the average winding length to determine its resistance. End of the wire is usually soldered to a conclusion. The main requirement - low ohmic contact resistance. Thick high-power core is difficult to wind, if the end is not attached. As insulators are used:
- Organic materials: silk, resin, cotton, paint, electrical paper. This is the first kind of insulation, entered into the everyday life of Sir Joseph Henry. Upper temperature is considered to be 105 degrees Celsius.
- The second class includes glass, ceramic and resin composition. In general, the materials more expensive predecessors. The upper limit of 130 degrees Celsius.
- Synthetic polymers of various kinds. Advantageously the silicon compound. Their distinctive feature is considered high heat resistance. This includes silicate ceramics. The upper limit of 200 degrees Celsius.
The difference classes mainly limited operating temperatures. And inside - grading is carried out on individual characteristics. For example, the glass obviously takes up less space than asbestos, and equal with silk. Ceramics are often wrapper covers the second layer of another material on top of the resin is a dense stacking.
The essential difference appears when dimensions are of paramount importance. This advantageously power sources 400 and 800 Hz are used in aviation. Then apply the materials of the second class, even if the cost goes higher. Household electronic transformer is often cheaper insulation. This is due to low power requirement and reduce prices. As a result, air manages to reduce power supplies 30-50%.
From this it is now easy to understand why most expensive domestic transformers (from common equipment) specified operating temperature limit of 135 degrees Celsius (allowed a brief rise above the above threshold). It is within a second, an average of the value of the group. Inscription looking at fuse embedded into the winding, inside or VCR player.
in the early fifties options for small transformers have to be measured again. Obtained for the industrial network voltages were not good due to the difference in frequency. Materials of the first group do not allow a qualitatively insulate wire at 50 Hz. The remaining small gap fails to cover the resin, the winding start sparks (corona discharge). To check the insulation resistance test is carried out for a long high voltage.
Pioneers defined test conditions as follows. Assume a sample is taken of the copper wire conductor section 0.5 mm. It is noticed that the first group isolated materials object begins to spark has 1,250 V. Then the test voltage reduced to 20-30% of the threshold is reached. manufacturing accuracy varies among enterprises, in each case, a test for corona discharge.
Full-wave rectifier used in electronic transformers, are discussed in the review by a diode bridge. This part of the circuit converts the AC input voltage to a unipolar. Sometimes the filter is placed to smooth out fluctuations. The difference output potentials of the diode bridge is used to power the push-pull circuit - transistor multivibrator.
Multivibrators - pulse generators
Obviously, a transformer for reducing the weight and placing it in such a small case requires to increase the operating frequency from 50 Hz to ultrasound. A specific value selected by the manufacturer. Flicker of transistors allows you to specify any value, limited only by the available hands on element base. Often electronic transformers with steel hull. This screen, which prevents the emission of high frequency waves in space.
Structurally multivibrators are Class D amplifiers (at least one element is pulsed). Work in a key mode of transistors requires a known speed. In the locked condition between the current collector and emitter is close to zero. Pulse mode in addition increases the efficiency of the multivibrator. The first devices of this class are described Henry Abraham in Annales de Physique magazine for 1919. It is believed that the device was the forerunner of digital technology, a year later came the first trigger Eccles-Jordan.
Multivibrators are managed and unmanaged, but - pulse generators of a given frequency, similar in shape to rectangular. Load it is compact transformer. In the first case it is permissible to change the duty cycle and other settings, but the electronic transformer usually does not offer such complex features, or greatly increased price.
According to the theory-flop allowed to build any type of active elements, but for good reason used transistors. Specifics of operation are achieved through the introduction of feedback capacitive or inductive circuit (for phase shift), both the active element controlled each other in turn.
Larger amplitude vibrations is achieved using composite transistors sequentially enabled by a particular schema. The figure shows a diagram where RC-chain with a given time constant controls a pair of transistors forming pulses of a predetermined frequency. This is a typical electronic transformer 12V for halogen (HID) lamps. Issued denomination 6 and 24, powered by the fieldbus 110 or 220 V. The operating principle of the circuit shown:
- Input voltage 220 is rectified by the diode bridge, producing a charge capacitor. This input string sets the switching frequency Diack. Putting the trimmer capacitor may achieve the effect of dimming lamps.
- Deacon opens and charges the RC-chain of the second transistor, causing start hesitation.
- The diode prevents the voltage drop finally to the transistor T2 is closed at the end of the period.
- At the saturation point of the feedback transistor switches off core choke.
Outer switching frequency is limited only by the design of the pulse transformer core and transient characteristics of transistors. A typical switching frequency is 35 kHz. duty cycle is given by RC-chains on the bases of the transistors. The second diagram shows an embodiment of the short circuit protection. Defective halogen bulbs, consume too much current, the transistors become a cause overheating and failure. Semi-conductor p-n-transitions irreversibly lose properties.
At too high consumption of switched transistor circuit protection, RC elements which delay the triggering transistor T1. The situation observed in the arc ignition. Cold cathode finds little resistance and easily conducting. As the metal electrode warm current is reduced, a transformer and the transistors are located at a normal mode. This extended product life. After the delay time (set by Rs and Cs) device tries to start again, and if the current does not exceed a predetermined value, the circuit enters the normal mode.
Requirements for transistors
Due to the high operating voltage and the requirements for low cost bipolar transistors are selected. To reduce the indicators used half-bridge switching circuit. The peak voltage is 350 V, and is turned off when the input filter, the energy stored throttle produces pulse amplitude of up to 500 V.
The peculiarity of a half-bridge circuit: the voltage is divided between the two transistors. Therefore, the maximum operating current is via output power. For the device to be 50 W 0.64 A As stated above, when first powered lamps, this value sometimes greatly exceeded (up to 10 times the rated value). Consequently, the transistors through a current can flow temporarily to 6.5 A.
From these considerations it is recommended for the electronic transformer 50W select transistors with a maximum voltage of 450 V or higher at a current up to 7 A. About frequency mentioned above. It depends on the parameters of the pulse transformer and the time constant determined by the RC-charge chain. A typical value - 35 kHz. Too slow transistors can lead to failure frequency and the input of the pulse transformer core into saturation at the end of each cycle. The stored energy will be returned to collectors in the form of a significant height of the peak, which hypothetically lead to product failure.