Widespread pulse DC-DC power supplies, due to their higher efficiency compared to linear. Although they concede less linear stability of the output voltage, but with a significant change of the input voltage on them dissipates less power. Produced quite a wide range of different chips, based on which we can perform similar tasks. This article discusses the practical schemes of two power supply units on the basis of chip LM2577T-AJI.
Brief characteristics of the chip
The voltage................................................3,5...30 In
The frequency of the internal oscillator..............................52 kHz
The structure of this IC includes 3 amp output transistor structure n-p-n with boundary voltage emitter-collector V. 6S
The chip has internal current and thermal protection.
Fig. 1
Schematic diagram shown in Fig.1, is a simple inductor Converter. Mains voltage of 220 V is transformed by a transformer T1 in the lower 8... 16 V, then rectified by a diode bridge on VD1-VD4 and smoothed by the capacitor C1. The rectified voltage is fed to pin 5 of D1, resulting in the internal circuit starts and the generator. Internal transistor closes conclusions 3 and 4, resulting in the accumulation of energy in the external inductor L1, diode VD5 is closed. Once the transistor is closed, energy from the inductor will go through an open diode VD5 on the capacitor C4, and he is charged to a certain voltage. The output voltage, i.e. the voltage on the capacitance is compared with a reference 1,23 In, and if it is below, the procedure is repeated in the next cycle. If it is larger, the duration of the opening time of the transistor is reduced. Thus, the capacitor is observed sawtooth voltage amplitude of a few millivolts and a frequency generator. The output voltage can be adjusted within wide limits, but it cannot be lower than the input, as in this case appears diode VD5, and the voltage will go to the output, the transistor chip is always closed. If you need to create a Converter with a lower voltage than the input, it is better to use a flyback Converter (Fig.2).
Fig. 2
The main part of the work of the scheme is not different from the previous one with the only difference that the energy is not stored in the inductor L1 and transformer T2, and after the closure of the transistor chip is transmitted to the secondary winding. Due to the transformation coefficient, and to a greater extent because the DC voltage cannot pass through the transformer, it is possible to achieve a smaller output voltage than the input. It should be afraid to be coupled to the input circuit a voltage higher than 32 V, as emissions in the output transistor can be higher than the maximum permissible voltage emitter-collector V. 65 Chain VD6VD7 serves to limit the reverse voltage across the primary winding of transformer T2.
Details. Chip with index "-AJI" is designed for adjustable by the consumer of the output voltage. With index "-12" and "-15" - fixed output voltage, respectively, 1 2 and 15 V. In this case, the pin 2 of the chip must be resistive divider connected directly to the output. The transformers T1 in both schemes the capacity of 8...10 W, if the output voltage, as in the schemes, 24 V and a current of 200 mA. If you want to receive a wide range of input voltages, for example from 8 to 16 In for the first scheme and 8...32 In the other, so the wire must select a minimum diameter of 1 mm (the higher the input voltage, the less current is consumed from the winding).
As the diodes VD1-VD4 in both schemes applied to the diodes CD, at higher input voltage, and lower power consumption in the load you can use less powerful diodes, for example CD. All type resistors MLT 0,125, electrolytic capacitors type K50-35 or similar. Ceramic capacitors type KD. They are preferably set as close as possible to the chip. The inductor L1 is wound on a ferrite 4-18x11 and contains 53 of the coil wire pelsho diameter of 0.4 mm Between the cups install the gasket with a thickness of 0.2 mm. Transformer T2 is wound on the ferrite E20/10/6 with a gap in the Central core of 0.25 mm and contains the primary winding coil 33 of PEV-2 with a diameter of 0.45 mm, the secondary - 45 turns of wire sew-2 diameter 0,31 mm.
Author: S. M. Abramov, Orenburg; Publication: www.radioradar.net