Switching power supply on the LM2577 chip, 220 / 3,5-30 volts 0,2 amperes. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Switching DC / DC power supplies are widely used due to their higher efficiency compared to linear ones. Although they are inferior to linear ones in less stable output voltage, with a significant change in the input voltage, less power is dissipated on them. A fairly wide range of various microcircuits is produced, on the basis of which it is possible to perform a similar task. This article discusses practical diagrams of two power supplies based on the LM2577T-AJI chip.

Brief characteristics of the microcircuit

Supply voltage................................................ 3,5...30 V
Internal oscillator frequency...................................52 kHz

The structure of this microcircuit includes a 3-amp output transistor of the npn structure with a boundary voltage emitter-collector of 6S V.

The microcircuit has internal current and temperature protection.

Fig. 1

The circuit diagram shown in Figure 1 is a simple inductor converter. The mains voltage of 220 V is converted by the transformer T1 to a lower 8 ... 16 V, then it is rectified by a diode bridge on VD1-VD4 and smoothed by capacitor C1. The rectified voltage is applied to pin 5 of D1, which starts the internal circuit and the generator. The internal transistor closes terminals 3 and 4, as a result of which energy is accumulated in the external inductance L1, while the diode VD5 is closed. After the transistor closes, the energy from the inductance will flow through the open diode VD5 to the capacitor C4, and it will charge up to a certain voltage. In this case, the output voltage, i.e. the voltage on the capacitance is compared with the reference 1,23 V, and if it is lower, the procedure will be repeated in the next cycle. If it is larger, then the duration of the transistor opening time will decrease. Thus, a sawtooth voltage with an amplitude of several millivolts and the frequency of the generator is observed on the capacitor. The output voltage can be adjusted over a wide range, but it cannot be lower than the input voltage, since in this case the VD5 diode will open and the voltage will go to the output, while the transistor of the microcircuit is always closed. If it is necessary to create a converter with a lower voltage than the input, then it is better to use a flyback converter (Fig. 2).

Fig. 2

The operation of the main part of the circuit does not differ from the previous one, with the only difference being that the energy is stored not in the inductor L1, but in the transformer T2, and after the transistor of the microcircuit is closed, it is transferred to the secondary winding. Due to the transformation ratio, and to a greater extent because the DC voltage cannot pass through the transformer windings, it is possible to achieve a lower output voltage than the input. In this case, one should be careful not to apply a voltage higher than 32 V to the input of the microcircuit, since emissions at the output transistor may be higher than the maximum allowable emitter-collector voltage of 65 V. The VD6VD7 circuit serves to limit the reverse voltage on the primary winding of the T2 transformer.

Details. The microcircuit with the "-AJI" index is designed for a user-adjustable output voltage. With the index "-12" and "-15" - for a fixed output voltage of 1 2 and 15 V, respectively. In this case, output 2 of the microcircuit must be connected directly to the output without a resistive divider. Transformers T1 in both circuits for a power of 8 ... 10 W, if the output voltage, as in the diagrams, is 24 V and a current of 200 mA. If it is necessary to obtain a wide input voltage range, for example, from 8 to 16 V according to the first circuit and 8 ... 32 V according to the second, then the wire must be selected with a diameter of at least 1 mm (the higher the input voltage, the less current is consumed from the winding).

Diodes KD1 are used as diodes VD4-VD213 in both circuits, with a higher input voltage of the converter, as well as lower power consumption in the load, it is possible to use less powerful diodes, for example KD209. All resistors of the MLT 0,125 type, electrolytic capacitors of the K50-35 type or similar. Ceramic capacitors type KD. It is desirable to install them as close as possible to the microcircuit. Choke L1 is wound on ferrite 4-18x11 and contains 53 turns of PELSHO wire with a diameter of 0,4 mm. A spacer 0,2 mm thick is installed between the cups. Transformer T2 is wound on E20 / 10/6 ferrite with a gap in the central core of 0,25 mm and contains a primary winding of 33 turns of PEV-2 wire with a diameter of 0,45 mm, a secondary winding of 45 turns of PEV-2 wire with a diameter of 0,31 mm.

Author: S.M. Abramov, Orenburg; Publication: radioradar.net

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