18 W low-voltage soldering iron power supply. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The article brought to the attention of readers describes a pulse unit with a nominal output voltage of 6 V to power loads up to 18 W. It is possible to quickly switch to an output voltage of 5 V. In the author's version, the unit is used to power a low-voltage soldering iron, but it can be used for any load of the appropriate power, designed for a voltage of 5 or 6 V.

At present, microelectronics has become so widespread in household and industrial appliances that soldering irons for a voltage of 220 V are no longer suitable not only for its repair, but also for amateur radio creativity. You have to use "mini-soldering irons" of low power with low supply voltage. As a rule, to work with them, classic transformer power supplies are used, which have solid dimensions and weight. But the modern trend towards using flyback switching power supplies (SMPS) for power supply in household (and not only) equipment and the appearance of a wide set of chips for this allow you to assemble a lightweight compact unit.

The proposed version of the power supply is designed to work with soldering irons with a nominal voltage of 6 V with a power of up to 18 W. The device provides for a stepwise decrease in the soldering iron supply voltage to 5 V, which corresponds to a decrease in soldering iron power by up to 70%. The small throughput capacity of the SMPS allows it to be used to work with elements that are exposed to static electricity.

Main Specifications

• Input voltage interval, V .......180...250
• Rated conversion frequency, kHz ....... 100
• Rated output voltage, V ....... 6
• Load current, A ....... 0 ... 3

On fig. 1 shows a diagram of a power converter for a soldering iron. The main element of the device is a specialized TOP223Y chip. The design of such SMPS is described in detail in the article [1].

Rice. 1 (click to enlarge)

The device is assembled on a printed circuit board made of fiberglass foiled on one side with a thickness of 1,5 ... 2 mm. Its drawing is shown in Fig. 2. Imported oxide capacitors are used in the device to reduce the size. Capacitors C1, C5 are ceramic or film capacitors for a nominal DC voltage of at least 400 V or AC at least 250 V, the rest are ceramic for a voltage of at least 50 V. Resistors R1, R2, R4, R8, diodes VD3, VD4 are installed perpendicular to the board. To increase the reliability, the printed conductors of the output circuits (from winding III of the T1 transformer to the output - on the printed circuit board drawing they are slightly wider than the others) I recommend to "strengthen" with a layer of solder increased during tinning.

Fig. 2

Elements R4 and C8 were reserved according to the manufacturer's recommendations for the case of an unstable start of the converter, but they were not needed. The output rectifier uses a dual Schottky diode in the TO-220 package. The output filter inductor L2 is wound on a dumbbell-shaped ferrite core with dimensions of 9x12 mm from a faulty power supply unit of a personal computer with a PEV-2 wire of 0,5 mm until it is filled. Recommendations on the possible replacement of used parts can also be found in the article [1].

The DA1 converter chip and the VD5 diode are installed on heat sinks made of 1 mm thick copper sheet. Due to the flexibility of the material, it was relatively easy to manufacture heat sinks with a maximum cooling surface. The shapes and sizes of heat sinks can be judged by the appearance of the device board shown in Fig. 3. The finished product is shown in fig. 4.

Fig. 3

Fig. 4

The power switch is located on the top cover, the LEDs are mounted on a separate small board and glued to the cover. LED HL2 - green glow, HL1 - red. The HL2 LED indicates the presence of the output voltage, and HL1 is turned on by the SA2 switch when the latter is set to the low output voltage mode.

Finished products are used in the device: inductor L1 - surge protector PMCU-0330 0,4 A 300 V or self-made, as suggested in the article [1]. Switch SA2 - B1550 (SS8) sliding 50 V imported for two positions of horizontal design. Power connector (not shown in the diagram) - RF-180S plug on the block, angled two-pin 250 V / 2,5 A, output connector (not shown in the diagram) - DS-210. Power switch SA1 - SC719 (SMRS-101), 250V/1A or equivalent. The TOP223Y microcircuit can be replaced by increasing power with the TOP224-6U without changes in the circuit, the difference is only in the increased cost of the design.

The converter transformer is assembled on a Ш-shaped magnetic circuit Ш6х6 with dimensions of 24x24x6 mm with a low-profile frame made of ferrite, presumably with a permeability of 1500...2000. A set of a frame and a magnetic circuit was purchased at a store, where, apart from the price, nothing could be found out.

The TOP22X line of microcircuits has internal overcurrent protection due to the built-in current-limiting resistor, so the parameters of the manufactured transformer (primarily the inductance of the primary winding) are of paramount importance.

Winding the transformer "blindly" did not give the desired results. I had to acquire instruments for measuring inductance, after which the problem with determining the number of turns of the primary winding disappeared.

Using the recommendations in the article [1] for TOP223Y and the specified magnetic circuit, I decided on the value of the inductance - 1300 μH. As you know, the inductance of a coil with a magnetic circuit (in microhenries) is calculated by the formula

L = (N/K)²,

where N is the number of turns; K is the parameter of the magnetic circuit.

Next, we experimentally determine the parameters of a suitable magnetic circuit. To calculate K, we wind a test winding on the frame, for example, 50 turns, and assemble the transformer with spacers in the extreme cores 0,2 mm thick from a non-magnetic material, such as textolite. Sometimes the magnetic cores already have a ready-made gap, then an additional gap is not needed.

After assembling the transformer, we measure the inductance of the winding and determine the coefficient K of the existing magnetic circuit. Then, according to the formula N = K√L we calculate the required number of turns of the primary winding.

In my version, the primary winding contains 92 turns of PEV-2 wire with a diameter of 0,3 mm. Winding II - 13 turns of the same wire. The output winding contains seven turns of PEV-2 wire with a diameter of 0,5 mm, wound into three cores. Compliance with the phasing of the windings is mandatory. The beginning of the winding in the diagram is indicated by a dot.

All windings are insulated with each other by a double layer of TEA 5K5 polyester insulating tape, which can be replaced with varnished cloth or other material with a total thickness of 0,1 mm. After final assembly, be sure to measure the inductance of the primary winding.

The power supply is assembled in a BOX-KA12 case with dimensions of 90x65x35 mm. Holes are drilled in the case for cooling.

If the parts are in good condition and there are no errors in the installation, the establishment of an SMPS is not required. When you turn it on for the first time, it is imperative to use a 1-40 W incandescent lamp instead of the FU60 fusible insert. This will save you from possible troubles. From my own experience, it turned out that non-observance of the phasing of the primary winding and winding II is guaranteed to disable the TOP223Y microcircuit.

If necessary, for the replacement and selection of the magnetic circuit, you can refer to the article [5].

When wiring the board yourself, it is necessary to take into account the recommendations of the manufacturer. The topology of the printed circuit board of modern SMPS at high conversion frequencies has its own characteristics. They, as well as the parameters of the TOP22X series microcircuits, can be found in [6].

Literature

1. Kosenko S. Designing flyback SMPS on TOPSwitch-II using the VDS program. - Radio, 2006, No. 3, p. 30-32.
2. Terentiev E. Capacitance and inductance meter. - Radio, 1995, No. 4, p. 37.
3. Potachin I. Attachment-meter LC to a digital voltmeter. - Radio, 1998, No. 12, p. 31.
4. Belenetsky S. Prefix for measuring inductance in the practice of a radio amateur. - Radio, 2005, No. 5, p. 26-28.
5. Kosenko S. Selection of domestic analogues of imported transformers in a flyback converter. - Radio, 2006, No. 5, p. 31.
6. TOP221-TOP227. TOPSwitch II. Three-terminal Off-line PWM Switch. - URL: powerint.com/sites/default/files/product-docs/top221-227.pdf.

Author: S. Chernov

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