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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 / Chargers, batteries, galvanic cells The device 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 the K1114EU4 SHI controller contains two error amplifiers, 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 the I winding of the transformer T2 and divert the emission energy back to the power source. Diodes must have a minimum turn-on time. Thermistor R9 limits the charging current of capacitors C7, C8 when the device is connected to the network. The mains filter C1, C2, C5, L1 is used to suppress interference from the converter. Circuits R19, R21, C12, VD9 and R20, R22, C13, VD10 serve to speed up 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 C9 prevents magnetization of the magnetic circuit of transformer T2 due to the unequal capacitance of capacitors C7 and C8. Circuit R17, C11 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 (VD11, VD12), capable of operating at a relatively high operating frequency of the converter. Resistor R26 acts as a current sensor. The voltage from this resistor applied to the non-inverting input of the first error amplifier of the DAI controller is compared with the voltage at its inverting input, set by resistor R1 "CURRENT CHARGE". 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 R23, R24, proportional to the voltage on the battery being charged, is fed to the inverting input of the second error amplifier and compared with the voltage across the resistor R4 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. SHI - controller has a built-in stable voltage source of 5 V, which feeds all voltage dividers that set the required values of the 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 VD13 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 R6 to the Uref output of the built-in reference voltage source (pin 14) (the voltage at the DTC input is at least 3 V, and impulse generation is prohibited. When a healthy battery is connected to the output of the device, the VD13 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 pins 8 and 11 (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 rate is determined by the elements R10 and C6. It is calculated by the formula: F=1,1/R10-C6 Device setup To establish the converter will be required. 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 Uref output (pin 14) of the SHI controller. An oscilloscope is connected to terminals 8 and 11 (outputs C1 and C2) of the controller and control pulses are observed. The engine of the resistor R1 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. SP 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 gradually increased. LATRom up to 220 V and control the operation of transistors VT4, VT5 on an oscilloscope. In this case, the output current should not exceed 3 A. Rotating the slider of the resistor RI, 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 R2, R5 are selected so that the limits for changing the charging current by the regulator R2 are 0,5 and 25 A. Set the maximum output voltage to 15 V by selecting resistor R4. 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. Details Diodes KD257B can be replaced with RL205, and 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 UF400S. Diodes VD11, VD12 also provide a total heat sink with a surface area of at least 200 cm2. The K1114EU4 SHI controller has many foreign analogues - TL494IN, DBL494, mPC494, IR2M02, KA7500. Instead of KT886A-1, transistors KT858A, KT858B or KT886B-1 are suitable. Transistors VT4 and VT5 are installed on heat sinks with an area of at least 100 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. 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 K20x 12x6 made of M2000NM ferrite. Winding I is wound with PEV-2 0,4 wire 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 wire. MGTF-0,8. The windings are isolated from one another and from the magnetic circuit by two layers of thin PTFE tape. Transformer T2 is wound on an armored magnetic core. SH10x10 from M2000HM ferrite (or, even better, M2500NMS), an annular magnetic circuit of the same cross section is also suitable. Winding I contains 35 turns of wire PEV-2 0,8. Winding II - 2x4 turns of a bundle with a cross section of at least 4 mm1 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. Author: Shelestov I.P.
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Comments on the article: Alexander, shurik_eryoma@rambler.ru A very interesting circuit, but can it be converted to 2 output voltages of 12 and 24 V? I can’t find a pulse memory circuit for 2 voltages of 12 and 24 V. Maybe the author knows one. I hope for your help.
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