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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Compromise (price / quality) switching stabilizer. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Surge Protectors Switching voltage stabilizers (ISN) are very popular among radio amateurs. In recent years, such devices have been built on the basis of specialized microcircuits, field-effect transistors and Schottky diodes. Thanks to this, the technical characteristics of the ISN have improved significantly, especially the efficiency, which "passed" over 90%, while simplifying the circuitry. However, the cost of parts for assembling such an ISN has increased many times over. The ISN described in the article is the result of a search for a compromise between quality indicators, complexity and price. The proposed ISN is built according to the scheme with self-excitation. It has sufficiently high performance and reliability, has protection against overloads and short circuits of the output, as well as against the appearance of input voltage at the output in the event of an emergency breakdown of the regulating transistor. Schematic diagram of the ISN is shown in fig. 1. Its basis is the common OU KR140UD608A. Unlike many devices of this purpose, to monitor the output voltage and overload current, a common OOS circuit formed by the VT4 transistor is used, and the L2 inductor (the active component of its resistance) is used as a current sensor, which is also part of the LC filter (L2C3 ), which reduces the ripple of the output voltage. The output voltage is determined by the zener diode VD2 and the emitter junction of the transistor VT4: Uout \u4d Ube VT2 + UVD2, and the overload current is the normalized active resistance of the inductor L6: lcpa4 \u2d Ube VTXNUMX / RlXNUMX- All this made it possible to somewhat simplify the ISN, reduce the output ripple voltage and increase efficiency by combining the current sensor with an LC filter. The disadvantage of such a circuit solution is a somewhat overestimated output impedance of the device.
The main technical characteristics of the ISN are as follows (obtained using LATR, a step-down transformer ~ 220 / ~ 18 V and a full-wave rectifier with a smoothing capacitor):
In the case of power supply from a stabilized DC source, the device remains operational when the input voltage drops almost to the open state of the transistor VT3. A further decrease in the input voltage leads to a breakdown in generation, but VT3 remains open. If at the same time an overload or short circuit occurs at the output, the generation is restored and the stabilizer starts operating in the current limiting mode. This property allows it to be used as an electronic fuse without a "latch". The stabilizer works as follows. Due to the different ratio of the resistances of the divider resistors R6R7 and R8R9, the voltage at the non-inverting input of the op-amp DA1 at the time of power-up is greater than at the inverting one, so a high level is set at its output. Transistors VT1 -VT3 open and capacitors C2, C3 begin to charge, and the coil L1 - to accumulate energy. After the voltage at the output of the stabilizer reaches a value corresponding to the breakdown of the zener diode VD2 and the opening of the transistor VT4, the voltage at the non-inverting input of the op-amp DA1 becomes less than at the inverting one (due to shunting R9 with resistor R10), and a low level is set at its output. As a result, the transistors VT1-VT3 close, the voltage polarity at the terminals of the coil L1 abruptly changes to the opposite, the switching diode VD1 opens and the energy stored in the coil L1 and capacitors C2, C3 is transferred to the load. In this case, the output voltage decreases, the zener diode VD2 and the transistor VT4 close, a high level appears at the output of the op-amp and the transistor VT3 opens again, thereby starting a new operating cycle of the stabilizer. When the load current increases in excess of the nominal value, the increasing voltage drop across the active resistance of the coil L2 begins to open the transistor VT4 to a greater extent, the current feedback becomes predominant, and the zener diode VD2 closes. Due to the action of the OOS, the output current stabilizes, and the output voltage and input current decrease, thereby ensuring the safe operation of the transistor VT3. After eliminating the overload or short circuit, the device returns to the voltage stabilization mode. The current-voltage characteristics of the stabilizer are shown in fig. 2.
As can be seen from the diagram, transistors VT1 and VT3 form a composite transistor. Such a circuit design is optimal when used as a key element of a bipolar transistor, since in this case a relatively small voltage drop across the open transistor VT3 is provided at relatively low control currents. In this case, the transistor VT1 is saturated, providing optimal static losses of the composite transistor, and VT3 is not saturated, providing optimal dynamic losses. A powerful transistor of the KT4 series is used as a current sensor VT817. In principle, it is also possible to use a cheaper low-power transistor here, however, for powerful ones at low operating currents (as in this case), the opening voltage of the emitter junction is only about 0,4 V, while for low-power ones, for example, KT3102, it is about 0,55 .XNUMX V. Thus, with the same protection actuation current, the resistance of the measuring resistor in the case of using a powerful transistor is less, thereby providing a gain in the efficiency of the stabilizer. In the described ISN, as noted, protection is provided against the appearance of input voltage at the output during a breakdown of the regulating transistor VT3. In this case, the voltage at the zener diode VD3 becomes more than 15 V, the current in the power circuit increases sharply and the fuse FU1 burns out. It is assumed that the latter will burn out before it happens with the zener diode (due to thermal overloads). An accident simulation (short circuit of the VT3 collector and emitter terminals) showed that the KS515A zener diodes (in a metal case) perfectly protect the devices powered by the ISN: when the fuse blows, they, failing, remain "in a deep" short circuit (do not break). The same results were obtained when testing KS515G zener diodes, as well as similar imported ones (in plastic cases). Similar zener diodes in glass cases behaved unsatisfactorily - they managed to burn out simultaneously with the fuse. In the ISN, you can use any transistors of the series indicated in the diagram (except for KT816A as VT1). Oxide capacitors C2, C3 - foreign-made brand SR (approximate analogue of K50-35). In the process of prototyping the stabilizer, the possibility of using op-amps KR140UD708, KR140UD8A-KR140UD8V, KR544UD1 A, KR544UD2A, KR544UD2B, KR574UD1A, KR574UD1 B was checked. At the same time, the conversion frequency, the type of switching processes and efficiency changed somewhat. The most suitable replacement for KR140UD608 is KR140UD708 (it has the same "pinout"), however, attention: in the author's practice, these op-amps were encountered with a "reverse" arrangement of inputs, i.e. the non-inverting input was connected to pin 2, and the inverting input was connected to pin 3 !). The fact that this is the OU KR140UD708 was indicated by the marking on the case. The storage inductor L1 is placed in an armored magnetic core of two cups 422 M2000NM with a gap of about 0,2 mm formed by two layers of self-adhesive paper. This is done in the following way. From a sheet of self-adhesive paper cut out a square slightly larger than the outer diameter of the cup. After removing the protective layer, the paper is placed with the adhesive side up on a hard and even (not smooth) surface. Then one of the cups is placed end-down on the boom and rubbed tightly against the paper. As a result, the paper sticks to the end of the cup to such an extent that it is not difficult to cut off its excess with a sharp scalpel along the contour fragments. In the same way, the gasket is glued to the second cup. The coil is wound with PEL 1,0 wire on a collapsible frame, consisting of a stud 50 ... 100 mm long with an M4 thread at both ends, two restrictive cheek washers with a diameter of 16 and a thickness of 0,5 mm, bushings with an outer diameter of 10, an inner 5 and 7,5 mm long and two M4 nuts. The frame is assembled on a stud (in the sequence: nut, washer, sleeve, washer, nut) and tightly, coil to coil, the coil is wound - 20 turns in three rows (7 + 7 + 6). After winding, its conclusions are twisted by about 90 ° (so that the turns do not "spread") and the frame is carefully disassembled on one side. Then, holding the turns, the coil is carefully removed from the frame and inserted into one of the cups, the leads are untwisted and placed in the corresponding slots in the cup. Due to the springy properties of the wire, the coil is fixed quite well in the cup. So that the coil does not "squeak" at the conversion frequency, the cup with the winding is immersed for some time in a tank with nitro-varnish, then removed and the varnish is allowed to drain. After that, the cup is put on a tightening screw previously inserted into the corresponding hole in the board, a second cup is put on, and the assembly thus obtained is tightened with a screw with a nut and a washer. After the varnish has dried, the coil leads are carefully cleaned, tinned and soldered to the corresponding contacts of the board. Then the rest of the parts are mounted. The coil current sensor L2 is placed in a magnetic circuit of two cups Ch14 made of ferrite of the same brand as the coil L1, and the same dielectric gasket. For winding, a PEL 0,5 wire 700 mm long is used; it is not necessary to impregnate it with varnish. This coil can also be made differently by winding a wire of the specified diameter and length on a standard DPM-0,6 choke, however, the efficiency of suppression of pulses at the conversion frequency will slightly decrease in this case. The stabilizer is assembled on a printed circuit board made of one-sided foil fiberglass, the drawing of which is shown in fig. 3.
If the ISN will be used at the maximum load current, the VT3 transistor must be installed on a heat sink in the form of an aluminum plate with an area of 100 m2 and a thickness of 1,5 ... 2 mm. If, however, long-term operation of the device in the mode of a current source or short circuit is expected, a switching diode VD1 is also fixed on the same heat sink through an insulating gasket (for example, mica). At load currents of less than 1 A, a heat sink for the VT3 transistor and VD1 diode is not required, however, in this case, the protection trip current must be reduced to 1,2 A by replacing the L2 coil with a C5-16 resistor with a resistance of 0,33 Ohm and a power of 1 W. The described ISN practically does not need to be adjusted. However, it may be necessary to clarify the protection trip current, for which the wire of the L2 coil should be taken initially of a greater length. Having soldered it to the corresponding contacts of the board, it is gradually shortened until the required protection trip current is obtained, and then the L2 coil is wound in the manner described above. Do not use a stabilizer at load currents of more than 4 A. The limitation is mainly related to the maximum permissible pulse current of the collector of the KT805 series transistor (8 A at timp < 200 ms at Q=1,5), which, in principle, can take place under unfavorable conditions. Author: A. Moskvin, Yekaterinburg
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