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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Electronic transformers for 12 V halogen lamps. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies The article describes the so-called electronic transformers, which in fact are pulse step-down converters for powering 12 V halogen lamps. Two versions of the transformers are proposed - on discrete elements and using a specialized microcircuit. Halogen lamps are, in fact, a more advanced modification of a conventional incandescent lamp. The fundamental difference lies in the addition of halogen compound vapors to the lamp bulb, which block the active evaporation of the metal from the surface of the filament during lamp operation. This allows the filament to be heated to higher temperatures, resulting in higher light output and a more uniform emission spectrum. In addition, the lamp life is extended. These and other features make the halogen lamp very attractive for home lighting, and more. A wide range of halogen lamps of various powers for 230 and 12 V is commercially produced. Lamps with a supply voltage of 12 V have better technical characteristics and a longer life compared to 230 V lamps, not to mention electrical safety. To power such lamps from a 230 V network, it is necessary to reduce the voltage. You can, of course, use a conventional network step-down transformer, but this is expensive and impractical. The best solution is to use a 230V/12V step-down converter, often referred to in such cases as an electronic transformer or a halogen converter. Two variants of such devices will be discussed in this article, both are designed for a load power of 20 ... 105 watts. One of the simplest and most common circuit solutions for step-down electronic transformers is a half-bridge converter with positive current feedback, the circuit of which is shown in fig. 1. When the device is connected to the network, capacitors C3 and C4 are quickly charged to the amplitude voltage of the network, forming half the voltage at the connection point. The R5C2VS1 circuit generates a trigger pulse. As soon as the voltage across the capacitor C2 reaches the opening threshold of the dinistor VS1 (24.32 V), it will open and a forward bias voltage will be applied to the base of the transistor VT2. This transistor will open and the current will flow through the circuit: the common point of the capacitors C3 and C4, the primary winding of the transformer T2, the winding III of the transformer T1, the collector-emitter section of the transistor VT2, the negative terminal of the diode bridge VD1. On the winding II of the transformer T1, a voltage will appear that maintains the transistor VT2 in the open state, while the reverse voltage from the winding I will be applied to the base of the transistor VT1 (windings I and II are turned on in antiphase). The current flowing through the winding III of the transformer T1 will quickly bring it into saturation. As a result, the voltage on the windings I and II T1 will tend to zero. Transistor VT2 will start to close. When it is almost completely closed, the transformer will begin to go out of saturation.
Closing the transistor VT2 and exiting the saturation of the transformer T1 will lead to a change in the direction of the EMF and an increase in the voltage on the windings I and II. Now, a forward voltage will be applied to the base of the transistor VT1, and the reverse voltage will be applied to the base of VT2. Transistor VT1 will start to open. The current will flow through the circuit: the positive terminal of the diode bridge VD1, the collector-emitter section VT1, the winding III T1, the primary winding of the transformer T2, the common point of the capacitors C3 and C4. Further, the process is repeated, and the second half-wave of voltage is formed in the load. After starting, the VD4 diode maintains the capacitor C2 in a discharged state. Since the converter does not use a smoothing oxide capacitor (it is not necessary when working on an incandescent lamp, on the contrary, its presence worsens the power factor of the device), then at the end of the half-cycle of the rectified mains voltage, the generation will stop. With the advent of the next half-cycle, the generator will start again. As a result of the operation of an electronic transformer, oscillations close in shape to sinusoidal ones with a frequency of 30 ... 35 kHz (Fig. 2) are formed at its output, following in bursts with a frequency of 100 Hz (Fig. 3).
An important feature of such a converter is that it will not start without load, since in this case the current through the III T1 winding will be too small, and the transformer will not enter saturation, the self-generation process will fail. This feature makes idle protection unnecessary. The device with indicated in fig. 1 rating stably starts at a load power of 20 watts or more. On fig. 4 shows a diagram of an improved electronic transformer, in which a noise suppression filter and a short-circuit protection unit in the load are added. The protection unit is assembled on a transistor VT3, a diode VD6, a zener diode VD7, a capacitor C8 and resistors R7-R12. A sharp increase in load current will increase the voltage on the windings I and II of the transformer T1 from 3...5 V in nominal mode to 9...10 V in short circuit mode. As a result, a bias voltage of 3 V will appear on the basis of the transistor VT0,6. The transistor will open and shunt the start circuit capacitor C6. As a result, with the next half-cycle of the rectified voltage, the generator will not start. Capacitor C8 provides a protection shutdown delay of about 0,5 s.
The second version of the electronic step-down transformer is shown in fig. 5. It is easier to repeat because it does not have one transformer, while being more functional. This is also a half-bridge converter, but controlled by a specialized IR2161S chip. All the necessary protective functions are built into the microcircuit: from low and high mains voltage, from idle mode and short circuit in the load, from overheating. The IR2161S also has a soft start function, which consists in a smooth increase in the output voltage when turned on from 0 to 11,8 V for 1 s. This eliminates a sharp surge of current through the cold filament of the lamp, which significantly, sometimes several times, increases its service life.
At the first moment, and also with the arrival of each subsequent half-cycle of the rectified voltage, the microcircuit is powered through the VD3 diode from the parametric stabilizer on the VD2 zener diode. If the power is supplied directly from the 230 V network without using a phase power regulator (dimmer), then the R1-R3C5 circuit is not needed. After entering the operating mode, the microcircuit is additionally powered from the output of the half-bridge through the d2VD4VD5 circuit. Immediately after starting, the frequency of the internal clock generator of the microcircuit is about 125 kHz, which is much higher than the frequency of the output circuit C13C14T1, as a result, the voltage on the secondary winding of the transformer T1 will be small. The internal oscillator of the microcircuit is controlled by voltage, its frequency is inversely proportional to the voltage across the capacitor C8. Immediately after switching on, this capacitor begins to charge from the internal current source of the microcircuit. In proportion to the increase in voltage on it, the frequency of the microcircuit generator will decrease. When the voltage on the capacitor reaches 5 V (approximately 1 s after switching on), the frequency will decrease to an operating value of about 35 kHz, and the voltage at the output of the transformer will reach a nominal value of 11,8 V. This is how a soft start is implemented, after it is completed, the DA1 microcircuit goes into operating mode in which pin 3 of DA1 can be used to control the output power. If you connect a variable resistor with a resistance of 8 kOhm in parallel with the capacitor C100, you can, by changing the voltage at pin 3 of DA1, control the output voltage and adjust the brightness of the lamp. When the voltage at pin 3 of the DA1 chip changes from 0 to 5 V, the generation frequency will change from 60 to 30 kHz (60 kHz at 0 V is the minimum output voltage and 30 kHz at 5 V is the maximum). The CS input (pin 4) of the DA1 chip is the input of the internal error signal amplifier and is used to control the load current and voltage at the half-bridge output. In the event of a sharp increase in load current, for example, during a short circuit, the voltage drop across the current sensor - resistors R12 and R13, and therefore at pin 4 of DA1, will exceed 0,56 V, the internal comparator will switch and stop the clock generator. In the event of a load break, the voltage at the output of the half-bridge may exceed the maximum allowable voltage of transistors VT1 and VT2. To avoid this, a resistive-capacitive divider C7R10 is connected to the CS input through the VD9 diode. When the threshold value of the voltage across the resistor R9 is exceeded, the generation also stops. The operating modes of the IR2161S chip are discussed in more detail in [1]. You can calculate the number of turns of the windings of the output transformer for both options, for example, using a simple calculation method [2], you can choose a suitable magnetic circuit for overall power using the catalog [3]. According to [2], the number of turns of the primary winding is NI = (Uc maxt0 max) / (2 S BMax), where Uc max - maximum mains voltage, V; t0 max - maximum time of the open state of transistors, µs; S - cross-sectional area of the magnetic core, mm2; B.Max- maximum induction, Tl. Number of turns of the secondary winding NII =NI / k where k is the transformation ratio, in our case we can take k = 10. A drawing of the printed circuit board of the first version of the electronic transformer (see Fig. 4) is shown in fig. 6, the location of the elements - in fig. 7. The appearance of the assembled board is shown in fig. 8. covers. The electronic transformer is assembled on a board made of fiberglass laminated on one side with a thickness of 1,5 mm. All elements for surface mounting are installed on the side of printed conductors, output elements are on the opposite side of the board. Most of the parts (transistors VT1, VT2, transformer T1, dynistor VS1, capacitors C1-C5, C9, C10) will come from cheap mass electronic ballasts for T8 type fluorescent lamps, for example, Tridonic PC4x18 T8, Fintar 236/418, Cimex CSVT 418P, Komtex EFBL236 / 418, TDM Electric EB-T8-236 / 418, etc., since they have similar circuitry and element base. Capacitors C9 and C10 are metal-film polypropylene, designed for high pulsed current and alternating voltage of at least 400 V. Diode VD4 - any high-speed diode with a permissible reverse voltage of at least 11 V in Fig. 150.
Transformer T1 is wound on an annular magnetic circuit with a magnetic permeability of 2300 ± 15%, its outer diameter is 10,2 mm, its inner diameter is 5,6 mm, and its thickness is 5,3 mm. Winding III (5-6) contains one turn, windings I (1-2) and II (3-4) - three turns of wire with a diameter of 0,3 mm. The inductance of windings 1-2 and 3-4 should be 10...15 µH. The output transformer T2 is wound on an EV25/13/13 (Epcos) magnetic circuit without a non-magnetic gap, N27 material. Its primary winding contains 76 turns of 5x0,2 mm wire. The secondary winding contains eight turns of 100x0,08 mm litz wire. The inductance of the primary winding is 12 ±10% mH. The inductor of the interference suppression filter L1 is wound on an E19/8/5 magnetic core, material N30, each winding contains 130 turns of wire with a diameter of 0,25 mm. You can use a standard two-winding choke with an inductance of 30 ... 40 mH that is suitable in size. Capacitors C1, C2, it is desirable to use the X-class. A drawing of the printed circuit board of the second version of the electronic transformer (see Fig. 5) is shown in fig. 9, the location of the elements - in fig. 10. The board is also made of fiberglass laminated on one side, the elements for surface mounting are located on the side of the printed conductors, the output elements are on the opposite side. The appearance of the finished device is shown in fig. 11 and fig. 12. The output transformer T1 is wound on a ring magnetic circuit R29.5 (Epcos), material N87. The primary winding contains 81 turns of wire with a diameter of 0,6 mm, the secondary - 8 turns of wire 3x1 mm. The inductance of the primary winding is 18 ±10% mH, the secondary is 200 ±10% mH. Transformer T1 was calculated for a maximum power of up to 150 W, to connect such a load, transistors VT1 and VT2 must be installed on a heat sink - an aluminum plate with an area of 16 ... 18 mm2, 1,5 ... 2 mm thick. In this case, however, a corresponding alteration of the printed circuit board will be required. Also, the output transformer can be used from the first version of the device (you will need to add holes on the board for a different pin arrangement). Transistors STD10NM60N (VT1, VT2) can be replaced with IRF740AS or similar. The zener diode VD2 must have a power of at least 1 W, the stabilization voltage is 15,6 ... 18 V. Capacitor C12 is preferably disk ceramic for a rated DC voltage of 1000 V. Capacitors C13, C14 are metal-film polypropylene, designed for high pulse current and AC voltage not less than 400 V. Each of the resistance circuits R4-R7, R14-R17, R18-R21 can be replaced with one output resistor of the appropriate resistance and power, but this will require changing the printed circuit board.
Literature
Author: V. Lazarev
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