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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Small-sized powerful voltage converter
Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters Some electronic devices require a DC voltage of more than 12 V to power them. Therefore, when operating such equipment, for example, in a car or from a car battery, an appropriate voltage converter is required. Based on modern microcircuits and field-effect transistors, it is possible to assemble an economical voltage converter, the dimensions of which will be determined mainly by the transformer. We offer readers one of the options for such a converter. The circuit of the DC voltage converter to a larger DC voltage is shown in fig. 1. It is assembled on a KR1211EU1 chip [1] and field-effect transistors IRLR2905 [2]. These transistors have very low on-resistance (approximately 0,027 ohms), provide high current flow (at least 26 A) and are controlled by signals with logic levels of digital microcircuits. In most cases, they can be used without heat sinks, thereby reducing the dimensions of the converter.
The DA2 chip generates control pulse signals for field-effect transistors, their frequency is determined by the parameters of the R3C12 frequency-setting circuit. Control pulses are formed in such a way that there is a pause between them. As a result, the flow of through current through the transistors is excluded and the efficiency of the converter is increased. Transistors switch the primary winding of the step-up transformer T1. The voltage of the secondary winding rectifies the diode bridge VD1-VD4 and smoothes the filter C13C14L2C15. Here, the inductor mainly provides suppression of high-frequency harmonics in the output voltage. The supply voltage of the control chip DA2 is pre-smoothed by the L1C9 filter and stabilized by the integrated voltage regulator DA1. The R2C11 circuit ensures that the microcircuit starts when the power is turned on. On relay K1, a converter overload protection device is assembled. When the current consumption increases above the set level, the contacts of relay K1.1 will close, a high logic level will go to the FC input of the DA2 microcircuit and a low logic level will be set at its outputs - the transistors will close and the converter will stop working. To restart it, turn the power off and on again. If necessary, you can enter the LED indication of the operation of the converter. To do this, the circuits of the LED and the current-limiting resistor are connected in parallel with the capacitors C1 (input voltage control) and C15 (output voltage control). In the device, it is permissible to replace the 78L05 (DA1) chip with KR1157EN502A, 78M05, KR142EN5A, it is advisable to use oxide capacitors for surface mounting or K52, K53 series tantalum, however, in this case, the dimensions of the board may have to be increased, non-polar capacitors - K10-17V or K10 -17a with pins of minimum length. Resistors - MLT, S2-33, inductor L1 - DM-0,1 with an inductance of 50 ... 100 μH. The inductor L2 is wound on a K20x12x6 ring magnetic circuit made of 2000NM ferrite, its winding contains 5 turns of MGTF 0,75 wire, and the inductance is about 50 μH. Any LEDs can be used, and the resistance and power of the current-limiting resistors are chosen based on the current flowing through them. Switch SA1 - P2T. The K1 current relay is self-made, the winding is made of a copper insulated wire with a diameter of 2 mm, wound on a mandrel with a diameter of 3 ... 4 mm, inside which a KEM2 reed switch is inserted (such are used, for example, in the RES44 relay). The approximate number of turns for a current of 7 A is 4, and for 10 A - 3. The sensitivity of the relay can be smoothly adjusted by changing the position of the reed switch in the coil; after the final adjustment, the reed switch is fixed with glue. The T1 transformer is made on two glued ring magnetic cores K45x28x12 made of ferrite 2000NM-17, the sharp edges of the rings must be rounded off. Both windings are wound with MGTF 0,75 wire. The primary contains 5 turns of eight conductors folded together, it is divided into two parts and the beginning of one is connected to the end of the second. The secondary winding for an output voltage of 32 V contains 15 turns in two wires. For other values of the output voltage, the number of turns of the secondary winding should be proportionally changed. Most of the parts are placed on a printed circuit board made of double-sided foil fiberglass, the drawing of which is shown in Fig. 2. The areas of the foil that need to be removed are highlighted in black. All elements are mounted on the side of printed conductors. The second side is left metallized and connected to the common wire of the first side. To do this, pieces of tinned wire are inserted into the through holes shown in the drawing and soldered on both sides of the board. The primary terminals of the transformer should be soldered closer to the drain terminal of the transistor, as they will provide additional heat dissipation.
Establishment begins with setting the frequency of the converter, it can be controlled at one of the outputs of the DA2 chip with an oscilloscope or frequency meter. The frequency recommended for the used ferrite magnetic circuits is 80 ... 100 kHz, it is set by selecting the capacitance of the capacitor C12 or the resistance of the resistor R3 (it is desirable to change its value upwards). To reduce interference, the converter is placed in a metal case. Tests of the device showed that at a load current of 3 A (output power - about 100 W), the efficiency of the converter is approximately 91 ... 92%. Field-effect transistors heat up slightly, rectifier diodes - noticeably warmer. Therefore, the efficiency can be further increased if high-speed Schottky rectifier diodes are used instead of KD213A. If the transistors are equipped with heat sinks and the dimensions of the transformer are increased, the power of the converter can be increased several times. Literature
Author: I. Nechaev, Kursk
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