Intelligent charger for Ni-Cd batteries. Scheme, description

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Intelligent charger for Ni-Cd batteries. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Chargers, batteries, galvanic cells

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The article brought to the attention of readers describes a pulsed stabilized network remote power supply (in everyday life and, often, in the technical literature they are called adapters) based on a VIPer series microcircuit and an "intelligent" charger fed by it on a specialized MAX713CPE microcircuit.

"Intelligent" chargers (memory) on the pages of "Radio" paid a lot of attention. Of course, one can speak of intelligence only conditionally: usually, it means the ability of a device to analyze the state of a battery being charged and, based on some mandatory features, choose one or another charging mode. Moreover, the charging algorithm is determined by the type of battery. For lithium-ion (Li-Ion) it must correspond to that described in the article [1], and nickel-cadmium and nickel-metal hydride (Ni-Cd, Ni-MH) - [2].

In publications [1, 3], specific memory options are proposed. Despite the "intelligence" of these devices and contrary to the recommended method of charging batteries at the initial moment with the maximum possible current (more than 1 A), they use a current of only 250 ... 300 mA! Why? The answer, it seems to the author, is simple. If widely used stabilized and non-stabilized network remote power supplies (PSUs) are used as a source of charging current - they are often called adapters (according to foreign terminology - Wall Cube), it is very difficult to find an instance with a maximum current of 1 A or more. In addition, the market is extremely filled with fakes. The author's attempt to use the BPS 12-0,5 PSU, manufactured by the "mysterious" MAX Company, was unsuccessful: the adapter with a guaranteed output current of 0,5 A overheated even at a load current of 300 mA. But the body of the device is made quite ergonomically, so it was used for our own development of a pulsed stabilized network power supply unit.

Main Specifications

Input voltage interval, V......150...250
Rated conversion frequency, kHz......60
Output voltage, V ...... 6
Rated load current, A ...... 1
Root-mean-square value of output voltage ripples, mV, no more......15
Efficiency, %...... 82
Dimensions (without mains plug), mm......70x48x36

The PSU is protected from short circuits in the load. It can be used to power other equipment (portable radios and tape recorders, players, telephone answering machines, digital devices, etc.), the battery compartment of which is designed for four AA batteries. If necessary, the output stabilized voltage can be changed in the range of 3...9 V without rewinding the pulse transformer.

The power supply circuit is shown in fig. 1. The main element of the device is a specialized chip VIPer12A, produced in DIP-8 and SO-8 packages (for surface mounting). The design of such switching power supplies is described in detail in the article [4].

(click to enlarge)

Information about the microcircuit can be found in the VIPer Designe Software/Documentation/Datasheet/VIPerl 2A recommended there.

Features of the microcircuit used are a built-in generator of a fixed conversion frequency of 60 kHz, which allows minimizing the number of "strapping" elements, as well as a control unit for the limit value of the drain current in the microcircuit by an external positive voltage. In the absence of this voltage, VIPer12A provides a current limit of 0,4 A. In the device, pin 3 FB (FeedBack - feedback) is supplied through the zener diode VD2 with the supply voltage of the DA1 microcircuit (approximately 24 V). The input current at the FB input must not exceed 3 mA. An increase in the input current leads to a decrease in the amplitude value of the drain current (and vice versa) with a gain of about 320. As a result of comparing the voltage on the coupling winding II of the transformer T1 with the stabilization voltage of the zener diode VD2, the duty cycle of the switching pulses changes so that the output voltage remains stable. When the mains voltage changes in the range of 150 ... 250 V, the deviation of the output voltage from the nominal does not exceed 0,1 V.

The purpose of the remaining elements of the PSU is no different from similar ones in the previously described similar devices.

All parts are mounted on a printed circuit board made of one-sided foil fiberglass, the drawing of which is shown in fig. 2. To reduce the interference created by the PSU, an electrostatic shield made of tin with the dimensions of a printed circuit board is attached from the side of the printed conductors through a reliable insulator, electrically connected to a common wire (with the negative terminal of the VD1 diode bridge). To do this, you can use the same one-sided foil fiberglass from which the printed circuit board is made.

(click to enlarge)

In order to reduce the dimensions, the device uses imported oxide capacitors. Capacitors C1-C3, 07, C8 - ceramic or film for a nominal voltage of at least 630 V, the rest - ceramic for a voltage of at least 50 V. Resistors - MLT or the like. Choke L2 - high-frequency compact DPM-2,4. The diode bridge S1WB40 (VD1) with a current limit of 1 A and a permissible reverse voltage of 400 V can be replaced by any other with similar parameters, while it will be necessary to change the configuration of the printed conductors or to mold the conclusions of the bridge accordingly. Diode FR207 (VD3) can be replaced by domestic KD257D. When choosing an analogue of the recommended diode KD212AM (VD4), it should be taken into account that for it the reverse voltage in the device significantly exceeds 100 V.

The output rectifier uses a Schottky diode 1 N5822 (VD5) with a maximum current of 3 A and a permissible reverse voltage of 40 V. It can be completely replaced by a domestic one with similar parameters. The efficiency of stabilization of the output voltage is provided by the parameters of the zener diode. Instead of that indicated in the diagram, you can use the KS224Zh zener diode. If you use a composite zener diode from the domestic D814 series and the like, the voltage stability will be underestimated. You can change the output voltage of the PSU by simply selecting a zener diode or switching it.

The device uses the VIPer12A chip in the SO-8 package. According to the specifications, all four drain pins 5-8 must be soldered to the copper foil of the printed circuit board with an area of at least 200 mm2. At an ambient temperature of 25 °C, the design temperature of the microcircuit case will not exceed 72 °C. To reduce the thermal load on the microcircuit in conditions of dense mounting, the author used the copper flange of a faulty transistor in the TO-220 package, which is installed on a pin heat sink with dimensions of 13,5x16x23 mm. The stock leads are soldered to the flange. The case of the microcircuit, lubricated with heat-conducting paste, is pressed against the flange with a spring plate. Segments of MGTF conductors are soldered to the remaining pins of the microcircuit, which are then soldered into the board. The electrical connection of the drain leads to the printed conductors is provided by one of the M1 mounting screws that attach the flange to the board. It has an appropriate contact pad. The second screw is installed through the insulating washer. During installation, it should be noted that the heat sink of the microcircuit should not come into contact with the closely spaced magnetic circuit of the inductor LXNUMX, which is electrically connected to a common power wire.

Line filter inductor L1 is made on the basis of B14 armored magnetic core with magnetic permeability 1500...2000. The inductor windings have the same number of turns. They are wound with PEV-2 0,41 wire in a two-section frame (each in its own section) until filled.

The pulse transformer was calculated using VIPer Designe Software [4]. For it, a KV8 magnetic core made of M2500NMS1 ferrite with a standard frame and mounting clips is used. A cheek free of leads and half of the leads are removed from the frame. Winding III, containing five turns of wire PEV-2 with a diameter of 1 mm, is wound separately on a mandrel of a suitable diameter, and then put on winding 1.1, consisting of 31 turns of wire PEV-2 0,41. Winding I.2 of 27 turns of wire PEV-2 0,41 is wound over winding III, and winding II of 19 turns of wire PEV-2 0,12 is wound on top. The layers of turns of the half-windings 1.1 and I.2 are insulated with one layer, and the windings with two or three layers of a film used in high-voltage capacitors, or another, preferably heat-resistant insulating material.

The transformer is assembled with a gap of 0,02 mm on the side walls, which is provided with a gasket from the same film. The calculated value of the inductance of the winding I of the transformer T1 is 3210 μH, the measured value is about 3530 μH. Winding III with pin 8 is soldered into the board, and free pin 7 is connected in a hinged way to the anode of the VD5 diode installed perpendicular to the board (like most other elements). Conclusions 2 and 3 of the windings 1.1 and I.2 of the transformer T1 are soldered to one of the terminals of the frame. Then this conclusion of the frame is shortened by 1,5 ... 2 mm and insulated with nitro paint. It is not soldered to the board.

The device does not require adjustment, but before turning it on for the first time, it is advisable to make sure that the pulse transformer is of high quality (this operation is performed before the DA1 chip is installed in the PSU), as well as that the elements used are installed correctly and in good condition. To do this, you can use a universal device for testing switching power supplies [5]. To ensure a switching pulse frequency of 60 kHz, another one with a capacity of 4 ... 160 pF is soldered in parallel with the capacitor C180 in the device. An oscilloscope is connected in parallel with resistor R9 (Fig. 1 in [5]). The device is connected to a pulse transformer. A load equivalent is connected to the PSU output. By smoothly increasing the mains voltage at the input of the device using a laboratory autotransformer, an oscillogram is observed. With a mains voltage of 220 V, the equivalent load should be approximately 6 V, and the amplitude of the sawtooth current pulses observed on the oscilloscope screen should not exceed 0,25 A. By increasing the mains voltage to 250 V, make sure that the magnetic circuit is not saturated.

In addition, they check the phasing of the winding II, for which they measure the voltage across the capacitor C6 PSU, which should correspond to approximately 25 V. By controlling the shape of the pulses at the drain of the transistor VT2 in the device, they make sure that the damping circuit VD3C7R1 PSU is functioning effectively, after which the device is turned off, and on PSU board install chip DA1. The device is ready for use.

(click to enlarge)

A stabilized voltage of 6 V is supplied through the XS1 connector to the input of the memory, the circuit of which is shown in fig. 3. Since only one specific type of battery is usually used, it does not make much sense to make the device universal. The described version of the "intelligent" charger is designed to charge Ni-Cd batteries with a capacity of 1000 mAh. The device is based on a specialized MAX713CPE chip from Maxim. The functional purpose of its conclusions is given in the table.

(click to enlarge)

As noted above, such a device is described in article [3]. However, it is designed to charge six batteries with a current of 0,25 A. In addition, it is completely incomprehensible why the author of the design connected pins 1 and 15 of the microcircuit, thereby violating the developer's recommendations and excluding one of the "intelligent" properties of the charger - to stop fast charging the battery, when the voltage at its terminals reaches a certain predetermined value. And such a phenomenon is very possible if you use a battery that has been in operation for several years, in which case its further fast charging is unsafe.

In the proposed device, you can quickly charge one or two batteries (depending on the position of the SA1 switch) with a current of 1,1 A, which is approximately numerically equal to its capacity. The device's timer limits the fast charging time to 66 minutes. The timer setting error is ±15%, it is determined by the design features of the microcircuit.

According to the author, the simultaneous charging of two batteries is advisable only in emergency cases, when it is important to at least partially charge them, without achieving full charge. This is due to the method used in the microcircuit for detecting the end of charging by reducing the voltage on the battery by 2,5 mV relative to its maximum value (the so-called AV method). Obviously, even by special selection it is very difficult to achieve an absolutely equal capacity of the elements in the battery. If the capacity of the rechargeable batteries differs significantly, a decrease in voltage on one of them, with a smaller capacity, can be perceived by the microcircuit as the moment when fast charging ends. In this case, to achieve a truly full charge, the battery must be recharged for several hours with a small current.

In addition, the chip allows for 22 minutes to carry out the so-called ultra-fast charging current, 4 times the battery capacity. But here one should take into account the fact that no manufacturer guarantees the long-term preservation of the technical characteristics of the batteries with such charging. Therefore, the objectively justified maximum can be considered a charging current numerically equal to the battery capacity.

The algorithm for the operation of the charger is very simple. After the rechargeable battery is connected and the supply voltage is turned on, the HL1 "Power" LED lights up. The DA1 chip includes a charging timer and measures the voltage applied to one battery cell. If it is less than 0,4 V, the charging mode is activated with a small current, approximately equal to 30 mA. As soon as the measured voltage exceeds the specified threshold, the fast charging mode is automatically switched on with a current of 1,1 A (this value is determined by the resistance of the resistor R5), the field-effect transistor in the microcircuit, the drain of which is connected to pin 8, opens, and the HL2 LED "Fast charging" lights up. Both during recharging and in the case of fast charging, the microcircuit measures the voltage drop across the sensor - resistor R5 and opens the regulating transistor VT1 exactly as much as is required to create the necessary voltage drop (with fast charging - 0,25 V) on the current sensor. Current stabilization thus allows for some instability in the supply voltage of the device, but "dips" of voltage below an acceptable level must be excluded, since this can disrupt the normal functioning of the microcircuit.

During the charging process, every 42 s, the charging current is turned off for 5 ms and the microcircuit measures the voltage on the battery being charged, "remembering" the dynamics of its change over time. When approaching the moment corresponding to a full charge, the voltage on the battery stops increasing, and then begins to decrease. As soon as the voltage applied to one battery decreases by 2,5 mV, fast charging is replaced by the boost charging mode. The same thing will happen if the time set by the timer expires or the voltage on the battery exceeds 2 V. This value is set by the voltage at pin 1 of the DA1 microcircuit, in our case it is supplied with an exemplary voltage from pin 16, equal to 2 V. In the recharging mode, the battery can be for as long as you like.

The described charger can be modified. For example, introduce thermal monitoring of the case of the battery being charged, which is highly recommended by the manufacturer for ultra-fast charging. Instead of a linear one, it is permissible to use a pulsed operation of a transistor that regulates the battery charging current. If necessary, with the help of additional elements it is possible to reduce the charging current to less than 30 mA. These and some other improvements are easy to make if you use information about the MAX713CPE chip.

The chip must be handled with care. Despite the absence of any warnings about the dangers of exposure to static electricity in the company's documentation, practice has shown that it is very susceptible to it. Moreover, some radio amateurs who previously used CMOS chips with protective diodes at the inputs could get used to the fact that they can be soldered with a soldering iron with an operating voltage of 220 V. However, it should be remembered that the MAX71CPE chip is, in fact, a microcontroller and touching the terminals with a soldering iron with an operating voltage of 220 V, due to interference from the mains voltage, it can be fatal for her! Therefore, it is advisable to install the microcircuit on the board through the adapter panel after the final completion of all installation work. If it is necessary to change the connection of the programming pins or the position of the SA1 switch, this should be done only with the power supply turned off.

The memory does not require adjustment, therefore, we will characterize its design features in more detail. It is mounted on a printed circuit board made of one-sided foil fiberglass, the drawing of which is shown in Fig. 4. Wire jumpers are soldered before mounting the DA1 chip or adapter panel for it. The finished case was used from the XM-508 charger. Green (HL1) and red LEDs - HL2 are taken from it (possible domestic analogues are indicated on the diagram), as well as switch SA1.

(click to enlarge)

Resistor R5 - imported, the rest - MLT-0,125 or the like. Oxide capacitors - any domestic or imported ceramic capacitors C2, C3 for a nominal voltage of 50 V or more. In addition to that indicated in the diagram, you can use any other transistor with a current transfer coefficient of at least 50, a permissible collector current of at least 3 A and a saturation voltage of not more than 1,5 V at a current of 1 A. It is installed on a heat sink with dimensions of 40x32x8 mm, made from a piece cooling radiator from Rep-tium-100 processor. When one battery is charged, about 4 W of power is dissipated on the transistor, therefore, to facilitate its thermal conditions, a small-sized fan for blowing the Pentium-100 processor model DF1204SM is built into the device case, which rotates silently at a supply voltage of 6 V, but very efficiently. If the device is always used to charge two batteries, the fan can be omitted. Of course, it is permissible to do without a fan at all, but in this case the dimensions of the heat sink and, accordingly, the device case will have to be increased.

When charging one battery, a closing plug is installed in the compartment instead of another, or an ammeter of 2 ... 3 A is connected to the free charging terminals.

Literature

Lithium-ion batteries. - Radio, 2001, No. 7. S. 44, 45.
Grigoriev B. Algorithm for fast charging of batteries. - Radio, 2001, No. 8, p. 38.
"Intelligent" charger for Ni-Cd batteries. - Radio, 2001, No. 1.S.72.
Kosenko S. Evolution of flyback pulsed IP. - Radio, 2002, No. 6, p. 43,44; No. 7, p. 47,48; No. 8, p. 32-35; No. 9, p. 24-26.
Kosenko S. Universal device for testing SMPS. - Radio, 2003, No. 8, p. 38-41.

Author: S. Kosenko, Voronezh

See other articles Section Chargers, batteries, galvanic cells.

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