Switching laboratory power supply on the LM2575T-Adj chip, 15-18 / 1,2-15 volts 1 ampere. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The proposed power supply is assembled on the basis of the Motorola LM2575T-Adj chip. This microcircuit is a switching DC voltage regulator with adjustable output voltage. The conversion frequency (52 kHz) is determined by the built-in oscillator.

The microcircuit is operational at an input voltage of up to 40 V. The output voltage adjustment interval is 1,2 ... 35 V at a load current of up to 1 A. The minimum difference between the input and output voltages is about 2 V, there is a built-in protection against overtemperature, short circuit in the load and overcurrent circuit.

The power supply circuit is shown in fig. one.

Rice. 1 (click to enlarge)

The power supply provides current up to 1 A at an output voltage of 1,2 ... 15 V. At a voltage of 15 .... 18 V, the maximum current is reduced to 0,5 A, due to the type of step-down transformer used. If it is necessary that the output current reaches 1 A throughout the entire output voltage range, a step-down transformer with a secondary winding voltage of 22 V should be used. At an output voltage of 5 V and a load current of 1 A, the stabilizer consumes a current of 0,23 A from the rectifier and its efficiency is about 90 %.

The mains voltage of 220 V through the fuse-link FU1 and the closed contacts of the switch SA1 is supplied to the primary winding of the step-down transformer T1. The voltage of the secondary winding through the self-restoring fuse FU2 is fed to a bridge rectifier assembled on Schottky diodes VD3-VD6. The use of such diodes reduces the power loss on the bridge rectifier in comparison with conventional diodes, thereby reducing the heating of the elements, which is important for a power supply in a small package. The HL3 red glow LED indicates the operation of the self-resetting fuse FU2, which is necessary to protect the transformer from overload in case of rectifier or stabilizer microcircuit malfunctions. The varistor RU1, together with the fusible link FU1, protects the transformer, rectifier diodes and the stabilizer from impulse and short-term overvoltages.

The ripple of the rectified voltage is smoothed out by a high-capacity oxide capacitor C1. Ceramic capacitors C3, C8 C9 and tantalum C2 reduce the level of high-frequency ripples. This reduces the noise coming into the network from the switching regulator. The output voltage is regulated by a variable resistor R3, moving its slider down (according to the circuit) leads to an increase in the output voltage. Choke L1 - storage. C6C7L2C10C13 is a low-pass filter that reduces the ripple of the output stabilized voltage. Resistor R6 performs the functions of a load if a real load is not connected to the output of the power supply. The glow of the HL4 LED indicates the presence of an output voltage of more than 2 V. A voltmeter is assembled on the RA1 microammeter and the R5 resistor, which measures the output voltage. Choke L3 reduces the level of common-mode high-frequency noise. High-brightness white LEDs HL1, HL2 illuminate the voltmeter scale and at the same time serve as power-on indicators.

Fixed resistors for surface mounting are used, for example, P1-12, RN1-12. A variable resistor - SDR-4 or a similar small-sized one with a linear characteristic, its metal case must be connected to a common wire, and a handle made of insulating material should be put on the axis - the stabilizer microcircuit is sensitive to interference at the FB input. The MIG10-471 varistor can be replaced with FNR-14K431, FNR-10K471, FNR-14K471 and similar varistors for 430 or 470 V.

Oxide capacitors (except C2 and C14) - imported, domestic K50-35 capacitors are not recommended. Capacitors C2, C14 - tantalum for surface mounting. Capacitor C14 is installed at the end of the output connecting cable to connect the load. In the absence of a capacitor for an operating voltage of 25 V, it can be made up of two connected in series for a voltage of 20 V. All other capacitors are ceramic for surface mounting, sizes 0805, 1206. Capacitor C3 is soldered directly to terminals 1 and 3 of the stabilizer DA1. When using fixed resistors MLT, S2-23 and ceramic capacitors with wire leads, the overall dimensions of the power supply will increase.

Schottky diodes SR360 can be replaced by diodes MBRD350, SK35. MBRS360T3, MBR350, MBR360. KD268B. In the absence of such diodes, diodes of the KD213 series can be used. but the efficiency of the power supply will decrease, and the temperature inside the case will increase. Instead of the 1N4148 diode, you can install any diode from the KD521, KD522 series LEDs HL3 and HL4 - also any of the KIPD66, KIPD21, L-934 series. RL30-WH744D LEDs (white light) can be replaced with 504UWC.

Microammeter RA1 - M68501. M4761 or similar from the recording-playback level indicator of a domestic tape recorder. The power switch is an IRS-101-1 A3 or IRS101-12C key switch with a neon indicator lamp, but any small-sized switch designed for switching 220 V AC can be used. The connecting power cable for connecting the load is two-wire, up to 1 m long with a cross section of each wire 0,75 mm2.

The step-down transformer is used ready from the Icarus electrophone. Instead, you can use any similar one with an overall power of 15 ... 25 W, for example, a unified TP115-8. A self-made transformer can be wound on a W-shaped magnetic circuit with a central core area of ​​​​6,25 cm2. The primary winding contains 1800 turns of PEV-2 wire with a diameter of 0 2 mm, the secondary - 155 turns of the same wire with a diameter of 0,68 mm. In the absence of a tap from the middle of the secondary winding, the resistor R2 is connected to the lower output of the secondary winding according to the circuit. In this case, the resistance and power of this resistor R2 must be doubled.

The inductor L1 is wound on three K20x12-6 magnetic cores glued together from 2000NN ferrite. It contains 12 ... 15 turns of a home-made litz wire, consisting of 20 pieces of PEV-2 wire with a diameter of 0,18 mm. The pieces are put together and twisted using a hand winder, a hand drill, an electric screwdriver or an electric drill operating at low speeds. The use of a litz wire reduces power losses in the L1 choke and facilitates its winding. For the manufacture of L2 and L3 chokes, K16x6x4.5 magnetic cores made of 2000NN ferrite are used, the L2 choke contains 16 turns of wire PEV-2 0.68, L3 - 2 turns of the output two-wire cable. Before winding all the chokes, it is necessary to grind off the edges of the magnetic circuit with a needle file and then wrap them with a layer of varnished cloth.

All elements of the device are placed in a self-made plastic case with overall dimensions of 85x56x106 mm, glued together from boxes for 35 mm film slides (Fig. 2).

Fig. 2

The placement of elements in the case is very dense (Fig. 3), since half of the volume is occupied by a step-down transformer and a microammeter.

Fig. 3.

About a hundred ventilation holes with a diameter of 2,5 mm were drilled in the lower part of the case walls. Most of the parts are placed on a 46x72 mm circuit board. Installation of elements is carried out by a hinged method. Lead elements are located on one side of the board (Fig. 4), and elements for surface mounting are on the second.

Fig. 4

Varistor RU1 is soldered to the transformer terminals. A microammeter is installed on the upper part of the body; a rectangular window of the appropriate size is made for it. LEDs HL1, HL2 are glued to the microammeter on both sides so that they illuminate its scale. It is advisable to install the LM2575T-Adj integral stabilizer on a heat sink with a total area of ​​4...8 cm2, made of aluminum or copper alloy.

Setting up the assembled power supply consists in setting the upper limit of the output voltage by selecting the resistor R4, and by selecting the resistor R5 set the maximum value of the measured voltage. Then, using an exemplary voltmeter, the scale of the built-in voltmeter is calibrated. If it is necessary that the power supply has protection against overcurrent at its various values, which is relevant, first of all, for laboratory use, it is necessary to install several switchable self-restoring fuses in series with the L2 inductor.

Author: Butov A.

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