VIPER-100A and a pocket charger based on it

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

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The new line of chips has all the advantages of its predecessor - the UC384X series PWM controllers - and, in addition, has several significant advantages.

First of all, this is the approximately halved number of discrete elements of the "strapping" of the microcircuit.

An important circumstance is the high reliability of the thermal protection of the VIPer-switched SMPS. In the event of poor thermal contact between the switching transistor and the heat sink, a separately located PWM controller will only respond to overheating of the microcircuit case. The heavy duty operation of the transistor can lead to its thermal breakdown, and during the avalanche-like increase in the drain current, the transistor becomes virtually uncontrollable. The rectified mains voltage through a faulty transistor can destroy the PWM controller even before the fuse blows. For a VIPer-switched SMPS, this situation is excluded.

And the most important advantage is the possibility of automated design of SMPS.

The VIPer-110A microcircuit is made in a TO-220-5 five-pin metal-plastic package with a zigzag pin arrangement. Consider the operation algorithm and a simplified functional diagram of the product shown in Fig. eleven].

Comparing Fig. 1 and the functional diagram of the UC384X PWM controller [2], it is easy to see their similarity. The purpose of many nodes either coincides absolutely, or differs slightly. In particular, the comparator of the input supply voltage of the A1 microcircuit provides a threshold level when the VIPer switch goes into the "on" state of approximately 11 V, "off" - 8 V. Thermal protection works similarly. When the crystal temperature rises to 140...170°C, the safe mode trigger D1 blocks the operation of PWM D2 at input R1. Operation will be resumed automatically as soon as the chip temperature drops by 40°C compared to the thermal protection trip level.

The current consumed by the microcircuit does not exceed 1 mA in the "Off" state and 15 mA - "On". One of the features of the VIPer product is that during start-up, pins 3 (DRAIN) and 2 (Vdd) inside the microcircuit are connected by a current-limiting circuit. The limit level is 3 mA. This current is shared between the input voltage comparator A1 (1 mA) and the oxide filter capacitor connected to pin 2 (capacitor charging current is about 2 mA). After a relatively slow increase, the voltage on the oxide capacitor reaches the threshold level of switching on the microcircuit (11 V), then the capacitor is discharged with an operating current of the microcircuit of 15 mA. If the microcircuit for some reason (large capacitance of the filter discharged before turning on the capacitor or a short circuit in the load) fails to switch from starting to operating mode, the voltage on the capacitor quickly decreases to the shutdown threshold level, after which the process is repeated cyclically.

When trying to switch to the operating mode, the microcircuit generates "packages" of triggering pulses. The "packs" filling factor is determined by the ratio of the charging current of the capacitor to the discharge current and is only 2/15 "13%, which prevents damage to the input and output rectifiers in the starting mode or during short circuits in the load. The formation of several "packs" in the starting mode contributes to a smooth increase in the output voltage of the SMPS and characterizes its "soft" inclusion.

The process of regulating the output voltage of the SMPS is similar to that considered for the prototype. The internal circuits provide stabilization of the supply voltage of the microcircuit at the level of 13 V using two control loops: internal and external. The internal circuit is a conventional stabilizer for powering all components of the microcircuit. The external control loop is formed by the auxiliary winding of the transformer, connected to pin 2 through an external resistor, and the error signal amplifier A3 connected to this pin. Double stabilization of the supply voltage of the microcircuit provides a minimum deviation in the frequency of switching pulses. In [1], it is indicated that when the supply voltage changes in the range of 9 ... 15 V, as well as the discrepancy between the values ​​​​of the frequency-setting resistor and capacitor and the calculated values ​​​​within ± 1% and ± 5%, respectively, the deviation of the pulse repetition rate will not exceed ± 10%. The temperature instability of the frequency will not exceed -4% if the crystal temperature increases from 25 to 125°C.

Just like in the UC384X PWM controller, the same-named and functionally equivalent output 5 (COMP) of the VIPer microcircuit with a voltage on it in the operating mode of about 4,5 V can be used to force the SMPS to turn off.

Inside the microcircuit, this pin can be connected to a common wire by a field-effect transistor V2 under the influence of a safe mode trigger D1, which reacts to the blocking signals of the thermal protection unit A2, and an input voltage comparator A1. If the forced connection of output 5 to the common wire occurred during the action of the switching pulse, the next pulse is possible no earlier than in 1,7 ... 5 μs, although the generator continues to work all this time. The capacitor connected to pin 5 will delay the voltage rise to the threshold level of 0,5 V for some time, and at least one switching pulse will be missed. By changing the number of transmitted pulses, it is also possible to regulate the output voltage of the SMPS. The time delay of the switching pulses is carried out by the element A5 connected to the output of the current control comparator A4.

Of particular interest in the VIReg product is the current control method used, for which all the necessary elements are formed on the crystal. A signal proportional to the current is supplied from the additional output of the switching transistor V3 to the current-voltage converter U1, and then amplified in the current sensor amplifier A9. The voltage level at the input R3 PWM D2 is proportional to the drain current, and when the specified threshold level is reached, the duration of the switching pulse will be limited. A special quenching unit, within 0,25 µs after the start of the switching pulse, suppresses surges at the front due to the reverse recovery current of the rectifier diode in the secondary winding and the distributed capacitance of the storage winding. These spikes can cause premature clipping of the pulse width. During normal operation of the SMPS, the duration of the switching pulses is limited by the PWM input R2.

In the event of a short circuit in the load after switching on the SMPS, the output current will initially slowly increase in accordance with the dynamic characteristics of the control loop, and when the VIPer-100A limit value of 3 A is reached, the current will be limited in each switching pulse. Attention should be paid to the fact that the limiting current 4 A given in the reference books is the minimum of the range possible for individual samples. The typical current value for most is 5,4 A, and individual microcircuits are operational even at a limit level of 5 A. It is possible to limit the current through the switching transistor at a lower level if you use an external current-voltage converter, the output of which is connected to pin XNUMX (COMP ). All this guarantees the prevention of damage to the SMPS in extreme situations.

The appearance of the VIPer-100A chip allows a completely new approach to the problem of creating a simple and reliable charger (charger) for car batteries (AB).

Most chargers charge batteries with a stable current. However, in all vehicles, including cars, charging takes place at a constant voltage. In the on-board network, the relay-regulators maintain the voltage at a level of 14 ± 0,5 V. Therefore, the discharge of the battery in the starting mode with a current of several tens of amperes is followed by a subsequent short period of time when the charging current can reach 30 or more amperes, and then it quickly decreases to units and fractions of an ampere.

A similar charging mode can be used by motorists to solve a different kind of problem. If you urgently need to leave, and the car has not been used for a long time, then, most likely, due to the self-discharge of the battery, attempts to start the engine, especially in winter, will be unsuccessful. Some motorists in such cases use long-term (for half a day or more) recharging of the battery with a low current, thereby accelerating the corrosion of the positive electrode grids [3] and bringing the battery to failure. It is more rational in this case to use a charger for 15 ... 30 minutes, charging the battery at a constant voltage. A resistor with a small (fraction of an ohm) resistance connected in series with the battery will limit the charging current at the initial moment, and as the battery is charged, the voltage on the battery will increase and the current will decrease.

Thanks to its small dimensions and weight, the VIPer-switched charger can be transported to the garage without any hassle even in your pocket. On the other hand, it can be used not only as a full-fledged charger, but also as a power source for other purposes.

Since such an SMPS is circuit-protected from short circuits, it can be connected to both a partially and completely discharged battery. Depending on the degree of discharge, the SMPS will "pump" into the AB energy, limited by a power of about 100 W, i.e., the charging current will be adjusted automatically, without going beyond the SMPS safe operation mode.

The charger allows you to charge the battery with a current of at least 6 A at the beginning and bring the voltage on it to 15 V at the end of charging. The operating conversion frequency of the used SMPS is 100 kHz. The efficiency of the device is not less than 87%. Dimensions of SMPS without housing - 55x80x42,5 mm.

The service functions of the memory are determined by the properties of the used VIPer-100A chip. They have already been mentioned: protection against short circuits and breaks in the load, the implementation of safe operating modes, thermal protection, automatic regulation of the charging current depending on the degree of discharge of the battery.

The only drawback of the memory, which must be taken very seriously, is the vulnerability to polarity reversal. If the battery is connected incorrectly, the transformer and other elements of the charger may be damaged, so you need to connect it very carefully.

The memory circuit, developed with the help of DESIGNE SOFTWARE ("Evolution of flyback pulsed IP" in "Radio", 2002, No. 8), is shown in fig. 2. The design methodology has been described in detail previously. The mains voltage parameters did not change, the conversion frequency was chosen equal to 100 kHz, the output parameters correspond to a voltage of 15 V at a current of 6 A. The transformer magnetic circuit was selected RM10 (domestic analogue of KB 10) from N67 material (analog - M2500NMS1).

Thanks to a detailed analysis of the functioning algorithm of the VIPer-100A product used in the memory, it makes no sense to re-describe the purpose of individual elements of the device. The PCB drawing is shown in fig. 3.

Despite the minimum number of elements used, the installation turned out to be very dense, which is explained by the author’s desire to use a faulty high-voltage capacitor K41-1a with a capacity of 0,1 μF for a voltage of 10 kV as a finished device case.

Chip VIPer-100And it is installed on a pin heat sink with an effective area of ​​about 60 cm2 through a mica plate using a heat-conducting paste, connected to a common wire.

The diode bridge is imported, designed for a forward current of 1,5 A and a reverse voltage of 1000 V. The VD4-VD7 diode assembly consists of three duralumin plates connected by two screws (the thickness of the extreme ones is 1,5 mm, the middle one is 2 mm) with dimensions of 30x40 mm, between which, in pairs, on each side of the central plate, four KD213B diodes are clamped with the cathode to the center without an insulator using a heat-conducting paste. During installation, attention should be paid to the insulation of all anode terminals.

The current-limiting resistor R6 - C5-16MV with a power of 5 W is installed perpendicular to the board. Microammeter PA1 - M4283 or any other used in portable tape recorders to indicate the recording level. When establishing it, it is connected to a stabilized voltage source of 0,6 V and, by selecting a resistor R5, an arrow is set on the edge of the green sector.

Oxide capacitors are imported, since domestic ones will not "fit" into the indicated dimensions of the SMPS. Capacitor C7 is soldered in parallel to resistor R3, and then the latter is soldered with one output perpendicular to the board, and the second is connected in a hinged way with a free output of a similarly installed diode VD2.

Particular attention should be paid to the manufacture and installation of a pulse transformer. Its magnetic circuit must be with a non-magnetic gap of 0,7 mm.

The windings of the transformer are wound on a homemade frame. A small fiberglass plate is stratified with a scalpel or a sharp knife and one layer 0,1 ... 0,15 mm thick is separated from it. Having cut a strip of the required dimensions, using nitro-glue without distortions, it is wound in 2-3 layers on a rod of a suitable diameter, and after the glue has dried, it is removed.

On the frame thus obtained, the first layer is wound - 11 turns of wire PEV-2 0,41 in two conductors, then interlayer insulation from lavsan film or varnished cloth and the second layer - 9 turns. Then the interwinding insulation is wound.

Winding III, consisting of 7 turns of PEV-2 1,5 wire, is wound on a rod of slightly larger diameter so that it fits on winding I. Leads 8 ... 10 mm long are left on each side of the coil. The resulting winding III is carefully put on the first section of the winding I so that their conclusions are diametrically opposed, centered and a layer of interwinding insulation is fixed with glue. After that, it is useful to check the placement of the coil in the magnetic circuit, and if both plates are freely connected, the coil is removed and its ends are filled with glue to fix and seal the windings.

After the glue has dried on the coil, it is wound in two layers of 8 and 7 turns each of the second section of the winding I. The winding is completed with winding II of 6 turns of the PEV-2 0,15 wire, and after a trial placement of the coil in the magnetic circuit, the ends of the coil are again sealed with glue.

The measured inductance of the winding I of the transformer coincided with that calculated in DESIGNE SOFTWARE and amounted to 225 μH. The finished transformer is closed along the side surface with an electrostatic screen - one layer of copper foil and fixed on the board with a bracket. A strip of rubber 1 mm thick is laid between the transformer and the bracket. It is not necessary to glue the plates of the magnetic circuit during assembly. All transformer leads, except for 7, 2 and 3, are soldered into the corresponding holes on the board. Conclusions 2 and 3 are connected in a hinged way, isolated, and then "hidden" under an electrostatic shield. Pin 7 is connected to the board with a short piece of coaxial cable with a stranded center conductor.

A power switch, a 2 A fuse holder, a microammeter and two terminals for connecting the battery are placed on the cover of the device. In addition, to facilitate the thermal regime of the SMPS, a small-sized fan is fixed on the housing cover, used to blow the microprocessors, preferably with the highest possible performance, and air intake holes are provided for it. The terminals of the fan, designed for a voltage of 12 V, are connected to the capacitor C9 through a current-limiting resistor MLT-0,125 with a resistance of 8,2 ohms. Depending on the model and performance, the current consumed by the fan will be from 40...50 mA at 12 V to 55...65 mA at 15 V.

If the memory is assembled from serviceable parts without errors and the deviation of the operating frequency from the calculated value is not more than 10%, the adjustment of the device is not required.

On fig. 4 shows the dependence of the output voltage (solid line) and output power (dashed line) on the load current. The measurements were carried out with a closed resistor R6.

To reduce the ripple at the output, an oxide capacitor with a capacity of 22000 uF was connected.

Literature

1. VIPer Switch Mode Flyback Power Supply DESIGNE SOFTWARE/Documentation/Data-sheet/Viper100 - Viper100A: p. 1-20, February 2001.
2. Microcircuits for switching power supplies and their application. Directory. - M.: DODEKA, 1997.
3. Dasoyan M. A., Kurzukov N. I., Tyutryumov O. S., Yagnyatinsky V. M. Starter batteries. Device, operation, repair. - M.: Transport, 1994.

Author: S. Kosenko, Voronezh

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