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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Half-bridge inverter in charger. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters The development of switching power supplies based on inverters makes it possible to create low-cost chargers with low weight and dimensions. Push-pull pulse converters are critical to asymmetric magnetization of the magnetic circuit and the occurrence of through currents. In a half-bridge inverter with a saturable transformer, there is no direct current component of the primary winding, and the voltage across closed transistors does not exceed the mains voltage. In the inverter circuit, a triple conversion occurs:
The proposed device (Fig. 1) is designed to charge car and other powerful batteries.
The generator of rectangular pulses is made on the analog integral timer DA1 of the 555 series. The internal structure of the timer contains two comparators, the inputs of which are connected to pins 2 and 6, an RS flip-flop with an input (pin 4) reset to zero, an output amplifier to increase the load capacity, a key transistor with a collector connected to pin 7, control input (pin 5 from the supply voltage divider). To operate the microcircuit in the oscillator mode, the inputs 2 and 6 of the internal comparators DA1 are connected together. The charge of the external capacitor C1 continues when the voltage on it rises to the level of 2/3 Upit, and the high level at output 3 DA1 is replaced by a low one. When the voltage across the capacitor C1 drops to the level of 1/3 Upit due to the discharge through the internal transistor of the microcircuit, a high level is again set at output 3 DA1. The processes of charge and discharge of the time-setting capacitor C1 occur cyclically. The charge of C1 occurs through the diode VD1, R2 and the part of the variable resistor R1 turned on (left according to the diagram), the discharge - through VD2, R2, R4 and the right side of R1. This scheme allows using R1 to adjust the duty cycle of the pulses (the ratio of duration to period). In this case, the generator frequency remains constant, but the width (duration) of the pulses changes. This sets the desired output voltage at the terminals. XT1, XT2. The HL1 LED indicator allows you to visually monitor the presence of a high level at output 3 DA1. A pulse of positive polarity from output 3 DA1 through the limiting resistor R4 enters the base of the transistor VT1 and opens it. As a result, transistors VT2 and VT3 switch to opposite conduction states (VT2 closes and VT3 opens). At the end of the pulse and a change in the high level at pin 3 of DA1 to zero, VT1 closes, respectively, VT3 closes and VT2 opens. At the connection point of the emitter VT2 and collector VT3 (on the primary winding of the pulse transformer T1), a rectangular pulse is formed. Resistors R11, R12 and boost capacitors C4, C5 in the base circuits of transistors VT2, VT3 reduce the through current and bring the transistors out of saturation at the moment of switching, reducing losses in the control circuits and heating of the transistors. To open the transistor VT1 with some delay and quickly close, which has a positive effect on switching the output transistors, the bit transistor of the timer (pin 7) DA1 is connected to the base VT1. Damping diodes VD5, VD6, connected in parallel with transistors VT2, VT3, protect them from reverse voltage pulses. In some transistors, they are already installed in the case, but this is not always reflected in the passport data. During the closed state of the keys, the energy accumulated in the transformer T1 is transferred to the load and partially returned to the power source through damper diodes. Isolating capacitor C8 eliminates the flow through the primary winding of the transformer T1 direct current component with different characteristics of transistors VT2, VT3 and filter capacitors C9, C10. The snubber chain C7-R16 eliminates the reverse voltage surges that occur at the moment of switching the current in the windings T1. Inductor L1 reduces dynamic losses in switching transistors, narrowing the spectrum of generated oscillations. Filter capacitors C9, C10 with equalizing resistors R18, R19 create an artificial midpoint for the inverter transformer. The pulse generator is powered by a transformerless circuit through a parametric stabilizer R6-R10-VD3. Mains voltage passes through the filter C12-T2-C11. Limiting the charge current of the filter capacitors C9, C10 when the device is turned on produces the thermistor RT1. Its high resistance in the "cold" state turns into a low one as it is heated by the charge currents of the filter capacitors. The varistor RU1 shunts the voltage surges coming into the network during the operation of the converter. High-frequency diodes VD7, VD8 rectify the voltage from the secondary winding T1, and a constant voltage is obtained on the filter capacitor C6, supplied to the load through the ammeter PA1 with an internal shunt of 10 A. Using the HL2 LED, visual control of the presence of voltage is carried out. Inverter short circuit protection is provided by fuse FU1. The rechargeable battery is connected to the terminals XT1 and XT2 in the appropriate polarity with a wire with a cross section of 2 ... 4 mm2. To maintain a given output voltage, a feedback circuit is introduced into the circuit. The voltage from the divider R14-R15, proportional to the output, is supplied through the limiting resistor R13 to the LED of the optocoupler VU1. Zener diode VD4 limits the excess voltage on the LED. The phototransistor of the optocoupler is connected to the control input (pin 5) of the DA1 timer. With an increase in the output voltage, for example, due to an increase in the load resistance, the current through the VU1 LED increases, the phototransistor of the optocoupler opens more and shunts the timer control input. The voltage at the input of the upper comparator DA1 drops, it switches the internal trigger at a lower voltage on the capacitor C1, i.e. the duration of the DA1 pulse decreases. Accordingly, the output voltage decreases, and vice versa. The temperature dependence of the output voltage of the device can be compensated by replacing R15 with a thermistor and fixing it through the gasket on the transistor heatsink. Details and design. The high-frequency transformer T1 of the ERL-35R320 or AR-450-1T1 type was used without modification from the AT / ATX computer power supply. The approximate number of turns of the primary winding is 38 ... 46, wire 0,8 mm. The secondary winding has 2x7,5 turns and is made with a 4x0,31 mm bundle. Inductor L1 is used from the secondary voltage filter of the computer power supply. Core - ferrite, dimensions 10x26x10 mm. Number of turns - 15...25, wire 0,6...0,8 mm. Inductor T2 - two-winding, type 15-E000-0148 or filter HP1-P16 for a current of 1,6 A (inductance - 2x6 mH). As a timer DA1, you can use the domestic chip KR1006VI1 or imported analog chips, the main parameters of which are given in Table 1. To replace power transistors VT2, VT3, the types indicated in Table 2 are suitable.
The elements of the device are placed on two printed circuit boards, the drawings of which are shown in Fig. 2 and 3.
Transistors VT2, VT3 must be installed on the radiator through gaskets and insulated studs. The assembled printed circuit boards are mounted in a suitable housing on racks, the ammeter is installed in the cut hole, LEDs HL1, HL2 are glued nearby and the current regulator R1, switch SA1 and fuses FU1, FU2 are fixed. Before turning on the device for the first time, a refrigerator light (220 Vx15 W) is connected instead of the mains fuse, and a car light (12 Vx55 W) instead of the load. A weak glow of the refrigerator bulb indicates the working condition of the circuit. After a few seconds of operation after disconnecting from the network, the heating of the transistors is checked. If the temperature is normal, the output voltage (under load) of 14 V is set by resistor R1 at the middle position of the R13,8 slider. When the R1 slider is turned, the brightness of the car light should change. In case of insufficient cooling of transistors and rectifier diodes, a fan is additionally installed on the charger case. But it is better to use a case from an outdated computer power supply with a standard fan. Authors: V.Konovalov, E.Tsurkan, A.Vanteev, Creative laboratory "Automation and telemechanics", Irkutsk
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Comments on the article: Victor In the diagram, the first leg of the optocoupler goes to the resistance r13 and on the board to minus where it is correct. Yshan R10,8 ratings confuse me. It seems to me that they should be an order of magnitude larger, otherwise the dissipated power is ~ 15W.
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