Subminiature switching power supply in the dimensions of a thimble. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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To provide energy to master generators of switching power supplies, measuring instruments, as well as some other low-power loads, you can use the device proposed in the article.

Main technical characteristics:

• AC supply voltage, V......220 +20% -45%
• Mains voltage frequency, Hz......48...380
• Stabilized constant output voltage, V......20
• Maximum allowable load current, mA......50
• Instability of constant output voltage, %......2
• Output voltage ripple amplitude, mV......25

The device is made on only seven components, and the main role is played by a specialized HV-2405E chip from Harris Semiconductor [1, p. 25-31]. Functionally, the HV-2405E microcircuit consists of two systems - a preliminary switching regulator and an end linear voltage regulator. A schematic diagram of the power supply, positioned as a typical one, is shown in fig. 1. This single-chip power supply does not have a galvanic voltage isolation of the input and output circuits, but has a protection circuit against a short circuit in the load by the current limiting method.

Purpose and possible replacement of components

Thermistor RK1 prevents the breakdown of the DA1 chip by the charge current of the capacitor C1. A small-sized thermistor of the MZ21-N151RM brand was used in the power source, which can be exchanged for devices of the MZ21-N101RM, MZ21P121RM, MZ21P181RM or similar brands.

The constructive fuse FU1 protects the supply network from overload in the event of an accident. The fuse is made of a piece of copper wire with a diameter of approximately 0,05 mm.

The electrolytic capacitor C1 accumulates energy during one part of the period of the mains voltage, and during the other part of the period the accumulated energy feeds the final linear voltage stabilizer. The maximum current that the power supply is capable of delivering to the load depends on the capacitance of this capacitor. The capacitor can be taken of any brand and manufacturer, but always small-sized.

Ceramic capacitor C2 provides a delay in turning on the DA1 chip for the duration of the transients. For a mains frequency of 50 Hz or 60 Hz, a capacitor capacitance of 150 pF is recommended. Capacitor C2 can be used of any type with a nominal voltage of 40 V. The eight-pin DA1 chip, enclosed in a typical DIP-8 package, converts the AC input voltage to a DC output voltage and stabilizes the latter. The operating temperature range of the HV3-2405E-5 chip is 0°C ... +75°C, and the HV3-2405E-9 chip reaches -40°C ... +85°C. The microchip crystal can be heated up to +150°C.

The purpose of the pins of the microcircuit is as follows:

1. - output for connecting the neutral wire of the supply network;
2. - external capacitor of the pulse stabilizer;
3. - common wire;
4. - an external delay capacitor for turning on the microcircuit;
5. - output adjustment of the constant output voltage;
6. - output of positive direct voltage for connecting the load;
7. - the output is not involved;
8. - output for connecting the phase wire of the supply network.

The electrolytic capacitor C3 acts as a capacitive filter, smoothing out the ripple of the output voltage of the power supply. The larger its capacitance, the less the ripple of the output voltage will become.

The resistance of the resistor R1 included in the feedback circuit determines the value of the constant output voltage. The resistor can be used brand MLT, S2-22, S2-23 or similar. The amount of current flowing through the resistor is approximately 1 mA.

Design

Mounting of the power supply can be carried out by hinged method. At least 10 cm long leads from a flexible wire in high-quality insulation should be soldered to the radio components, which should be oriented in one direction. The brand of wire can be, for example, MGTF. The wires should be immediately tinned and marked so that there are no further questions regarding their connection to the components of the power source. After assembly and confirmation of performance, the product is dipped in an epoxy compound and wrapped in several layers of fiberglass. Then the blank is placed inside a specially prepared metal tailor's thimble in such a way that the flexible leads come out of it. In this case, the thimble performs the functions of the body and the electromagnetic shield. The contents of the thimble are finally filled with epoxy compound so that the fiberglass is completely covered with polymer. In order for air bubbles to come out, the thimble is gently shaken.

After a day, the compound will harden, and the power source can be used for its intended purpose. It should be noted that after filling the thimble, the power source will become unrepairable. That is, all adjustment operations must be carried out before assembling the product into a single whole.

Setting and adjusting

The power supply does not require adjustment if it is made in strict accordance with the circuit diagram and is made from serviceable parts. If it is necessary to adjust the output voltage, change the resistance of the resistor R1. So, to obtain an output voltage of 18 V, the resistance of the resistor R1 should be 13 kOhm, for 15 V - 10 kOhm, for 12 V - 6,8 kOhm, and for 9 V - 3,9 kOhm.

Literature

1. Integrated circuits: Microcircuits for switching power supplies and their application. Edition 2nd. - M.: DODEKA, 2000. - 608 p.

Author: Evgeny Moskatov, Taganrog, moskatov.narod.ru

See other articles Section Power Supplies.

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