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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Pulse charger. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Automobile. Batteries, chargers To charge starter batteries, motorists use a wide variety of devices, most of which are built using a step-down network transformer. Such devices are characterized by relatively low efficiency, large dimensions and weight. And if the efficiency can be somehow raised, then it is practically impossible to improve the other indicators of such devices. It is possible to significantly improve the performance of the charger if it is built on the principle of a pulsed voltage inverter. Impulse charging stations manufactured abroad (Bochsch, Telwin, etc.) have excellent technical performance, but the cost is beyond the reach of most of our motorists. At the same time, the independent manufacture of such devices is not within the power of every radio amateur, especially those who do not have the necessary experience in the field of pulse circuitry and the establishment of such devices. However, pulse chargers should not be considered overwhelmingly complex. So, in [1] an amateur radio device built on the basis of a flyback converter is described. The undoubted advantage of such converters is their relative simplicity and small size. However, they also have disadvantages. One of the most serious of them is the magnetization of the transformer magnetic circuit, because of which it is necessary to use a magnetic circuit with a cross section 2 ... 2,5 times larger than for push-pull converters. In addition, voltage surges on the switching element of flyback converters, as a rule, significantly exceed the supply voltage, which requires the introduction of additional suppression and regenerative circuits. Energy losses in them are most noticeable at high output power, so single-cycle converters are used in power units with a power not exceeding hundreds of watts. A lead-acid battery is usually charged in one of three ways: constant voltage, constant current, and the so-called amp-hour rule. Constant voltage charging is quite simple to implement, but it does not guarantee XNUMX% use of battery capacity. Charging according to the amp-hour rule (according to Woodbridge) can be considered an ideal way, but it is not widely used due to circuit complexity. The method of charging with a stable charging current is recognized as the most optimal. Devices that implement this method can be easily equipped with nodes that allow you to automate the charging process. This group of chargers also includes what is described below. The device (see diagram) is based on a push-pull half-bridge pulse converter (inverter) on powerful transistors VT4 and VT5, controlled by a pulse-width controller DA1 on the low-voltage side. Such converters, which are resistant to an increase in the supply voltage and a change in the load resistance, have proven themselves well in the power supplies of modern computers. Since there are two error amplifiers in the K1114EU4 [2] SHI controller, no additional microcircuits are required to control the charging current and output voltage.
High-speed diodes VD14, VD15 protect the collector junction of transistors VT4, VT5 from reverse voltage on winding I of transformer T2 and divert the emission energy back to the power source. Diodes must have a minimum turn-on time. Thermistor R1 limits the charging current of capacitors C4, C5 when the device is connected to the network. The mains filter C1C2C3L1 is used to suppress interference from the converter. Circuits R19R21C12VD8 and R20R22C13VD9 are used to force the process of closing switching transistors by applying negative voltage to their base circuit. This reduces switching losses and increases the efficiency of the converter. Capacitor C8 prevents the magnetization of the magnetic circuit of transformer T2 due to the unequal capacitance of capacitors C4 and C5. The R17C11 circuit helps to reduce the amplitude of voltage surges on winding I of transformer T2. Transformer T1 galvanically decouples the secondary circuits from the network and transmits control pulses to the base circuit of switching transistors. Winding III provides proportional current control. The use of transformer isolation made it possible to make the operation of the device safe. The charging current rectifier is made on diodes KD2997A (VD10, VD11), capable of operating at a relatively high operating frequency of the converter. Resistor R25 - current sensor. The voltage from this resistor, applied to the non-inverting input of the first controller error amplifier DA1, is compared with the voltage at its inverting input, set by resistor R2 "Charging current". When the error signal changes, the duty cycle of the control pulses, the open time of the switching transistors of the inverter and, therefore, the power transferred to the load change. The voltage from the divider R23R24, proportional to the voltage on the battery being charged, is fed to the non-inverting input of the second error amplifier and compared with the voltage across the resistor R5 applied to the inverting input of this amplifier. Thus, the output voltage is regulated. This avoids intense boiling of the electrolyte at the end of charging by reducing the charging current. The SHI controller has a built-in stable voltage source of 5 V, which feeds all voltage dividers that set the required voltage at the output of the device and the charging current. Since power is supplied to the DA1 chip from the output of the device, it is unacceptable to reduce the output voltage of the device to 8 V - in this case, the stabilization of the charging current stops and it may exceed the maximum allowable value. Such situations are excluded by a node assembled on a VT3 transistor and a VD12 zener diode - it blocks the charger from turning on if it is loaded with a faulty or highly discharged battery (with an EMF of less than 9 V). The zener diode, and hence the node transistor, remain closed, and the DTC input (pin 4) of the DA1 chip is connected through resistor R7 to the Uref output of the built-in reference voltage source (pin 14). At the same time, the voltage at the DTC input is at least 3 V, and the formation of pulses is prohibited. When a healthy battery is connected to the output of the device, the VD12 zener diode opens, followed by the VT3 transistor, closing the DTC input of the controller to a common wire and thereby allowing the formation of pulses at the outputs C1, C2 (open collector). The pulse repetition rate is about 60 kHz. After current amplification by transistors VT1, VT2, they are transmitted through the transformer T1 to the base of switching transistors VT4 and VT5. The pulse repetition frequency is determined by the elements R10 and C9. It is calculated by the formula F=1,1/R10·C9. Diodes KD257B can be replaced with RL205, KD2997A - with others, including Schottky diodes with a reverse voltage of more than 50 V and a rectified current of more than 20 A, FR155 - with high-speed pulse diodes FR205, FR305, and also UF4005. The SHI controller K1114EU4 has many foreign analogues - TL494IN [3], DBL494, GLRS494, IR2M02, KA7500. Instead of KT886A-1, transistors KT858A, KT858B or KT886B-1 are suitable. Transformers are the most critical and labor-intensive elements of any pulse converter. Not only the characteristics of the device, but also its overall performance depend on the quality of their manufacture. Transformer T1 is wound on an annular magnetic circuit of size K20x12x6 made of M2000NM ferrite. Winding I is wound with wire PEV-2 0,4 evenly over the entire ring and contains 2x28 turns; windings II and IV - 9 turns of wire PEV-2 0,5 each. Winding III - two turns of MGTF-0,8 wire. The windings are isolated from one another and from the magnetic circuit by two layers of thin PTFE tape. The T2 transformer is wound on an armored magnetic circuit Ш10х10 made of ferrite M2000NM (or, even better, M2500HMC); an annular magnetic circuit of a similar cross section is also suitable. Winding I contains 35 turns of PEV-2 0,8 wire, and winding II - 2x4 turns of a bundle with a cross section of at least 4 mm2 from several PEV-2 or PEL wires. If the transformer is forced to cool, the cross section of the bundle can be reduced. It should be noted that not only the reliability of the device, but also the safety of its operation depends on the quality of the interwinding insulation of transformers, since it is this insulation that isolates the secondary circuits from the mains voltage. Therefore, you should not make it from improvised materials - wrapping paper, stationery tape, etc. - and even more so neglect it, as inexperienced radio amateurs sometimes do. It is best to use thin fluoroplastic tape or capacitor paper from high-voltage capacitors, laying it in 2-3 layers. Assemble the device in a metal box of suitable dimensions. Transistors VT4 and VT5 are installed on heat sinks with a surface area of at least 100 cm2. Diodes VD10, VD11 also provide a common heat sink with a surface area of at least 200 cm2. The walls of the device box as a heat sink, as well as the common heat sink for diodes and transistors, should not be used for reasons of safe operation of the charger. Heatsinks can be drastically reduced in size by forcibly cooling them with a fan. To establish a converter, you will need a LATR, an oscilloscope, a working battery and two meters - a voltmeter and an ammeter (up to 20 A). If the radio amateur has an isolation transformer 220 V x 220 V with a power of at least 300 W, the device should be turned on through it - it will be safer to work. First, through a temporary current-limiting resistor with a resistance of 1 Ohm with a power of at least 75 W (or a car lamp with a power of 40-60 W), a battery is connected to the output of the device and make sure that there is a positive voltage of 5 V at the Uret output (pin 14) of the SHI controller. Connect the oscilloscope to the outputs C1 and C2 (pins 8 and 11) of the controller and observe the control pulses. The engine of the resistor R2 is set to the lowest position according to the scheme (minimum charging current) and a voltage of 36.. .48 V is supplied from the LATR to the network input of the device. Transistors VT4 and VT5 should not get very hot. The oscilloscope controls the voltage between the emitter and collector of these transistors. If there are surges at the front of the pulses, you should use faster diodes VD14, VD15, or more accurately select the elements R17 and C11 of the damping circuit. It must be borne in mind that not all oscilloscopes allow measurements in circuits galvanically connected to the network. In addition, remember that some of the elements of the device are under mains voltage - this is not safe! If everything is in order, the voltage at the mains input is smoothly increased by LATR to 220 V and the operation of transistors VT4, VT5 is monitored by an oscilloscope. The output current in this case should not exceed 3 A. Rotating the slider of the resistor R2, make sure that the current at the output of the device changes smoothly. Next, a temporary current-limiting resistor (or lamp) is removed from the output circuit and the battery is connected directly to the output of the device. Resistors R4, R6 are selected so that the limits for changing the charging current by regulator R2 are 0,5 and 25 A. Set the maximum output voltage to 15V by selecting resistor R5. The regulator knob R2 is provided with a scale calibrated in charging current values. You can equip the device with an ammeter. The box and all metal non-current-carrying parts of the charger must be reliably grounded during its operation. It is not recommended to leave a working charger unattended for a long time. Literature
Author: V.Sorokoumov, Sergiev Posad
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Comments on the article: Victor Thanks, nice diagram.
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