Power supply with galvanic isolation. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Power supply with galvanic isolation. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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There are circuits for low-frequency amplifiers, transmitters, and other devices that require power not only from a bipolar source, but also from two galvanically isolated sources that do not have a connection to ground or common connected circuits. It is very simple to organize the power supply of such a device in stationary conditions, since the power supply is the mains, which means there will be a power or pulse transformer. It is enough to make two secondary windings that are not connected to other circuits, and apply alternating voltages from them to separate independent rectifiers.

It is more difficult to organize power supply from two galvanically isolated sources if the power source must be a direct current source, for example, the on-board network of a car or boat.

The figure shows a diagram of a pulsed flyback source, the output of which has two galvanically isolated voltages of 15 V, with a maximum allowable current of 1 A each. zero wire.

(click to enlarge)

The scheme has already become standard - a direct current source, a pulse generator, a pulse transformer, rectifiers in the secondary circuits.

The voltage of 12 V from the car battery is supplied to the A1 chip of the LT1070 type (flyback DC-DC converter). The C1-C2-L1-C3-C4 circuit blocks the path of interference that may enter from vehicle or boat systems.

Chip A1 generates pulses with a frequency of about 40 kHz. At the output of the microcircuit there is a key output to pin 4. It is loaded with the primary winding of the T1 pulse transformer. Circuit C6-R3-VD1 limits negative emissions in the winding.

In the secondary windings is induced. EMF. Winding 4 is the control. The alternating voltage on it is rectified by the VD2 diode and, through the trimmer resistor R2, is fed to the control input of the A1 microcircuit (pin 2). The LN1070 output voltage stabilization system works in such a way that the microcircuit controller changes the duty cycle of the pulses at pin 4 so that the voltage at pin 2 is 1,24 V. That is, in order to obtain voltage stabilization, you need to remove the voltage from the secondary circuit and through the divider to resistors, apply it to pin 2.

The ratio of the divider arms should be such that at a normal voltage at the output, pin 2 was 1,24 V. In this circuit, it is undesirable to remove voltage for the stabilization system from the output, since the initial goal was to create a source with galvanically isolated output voltages as from each other , and from primary circuits. Therefore, there is a third secondary source here, consisting of winding 4 and a VD2-C7 rectifier. It serves only to obtain a control voltage. Since winding 4 is part of the transformer, the voltage on it is in the same dependence on the duty cycle of the pulses as the voltage on the other windings.

Transformer T1 is wound on a ferrite ring with a diameter of 28 mm. The primary winding contains 40 turns of PEV 0,47 wire. She winds up first. Then, on it, in the same direction, you need to wind the secondary windings 2 and 3. They take the same wire and fold it in half and wind 50 turns. Winding 4 - in the same direction as the rest, - 10 turns of PEV 0,12. The adjustment comes down to setting the output voltage by adjusting R2.

Author: Kuzyansky. P.

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