Protection of switching power supplies. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies
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Switching power supplies operating in a non-self-oscillating mode have certain advantages over self-oscillating ones:
- more rigid load characteristic;
- the ability to control discrete digital signals;
- improved maintainability.
The launch of such power supplies is carried out by a master oscillator (MG), usually in a microcircuit version. For the operation of the CG itself, it is necessary to provide its initial power supply from some external source. Sometimes for this purpose they use mains power with a separating capacitor connected in series, then a rectifier, a smoothing capacitor and a zener diode (Fig. 1).
Ris.1
However, with a significant power consumed by the master oscillator, this option is unacceptable, since the circuit seems to "freeze", increasing the voltage drop across the capacitor C1 and not reaching the supply voltage of the ZG, determined by the zener diode VD5. Increasing the capacity of C1 is not effective. The supply of the ZG from an additional network transformer reduces the advantages of the circuitry solution of the pulsed source.
We suggest using a transformerless circuit with a storage capacitor and a diode-thyristor optocoupler for the initial start-up (Fig. 2).
In this variant, in comparison with the scheme of Fig. 1, there is no "hanging" of the circuit with a significant current consumption of the CG. The storage capacitor is capacitance C2. It is charged through C1 and the rectifier VD1 ... VD4 to a value determined by the zener diode VD5. The efficiency of the storage capacitor, even with a small capacitance C1, is ensured by the absence of the supply current of the CG, because the dinistor of the optocoupler is closed.
By selecting a resistor R1, the optoelectronic pair VU1 is tuned to a response voltage slightly lower than Ust VD5. At the moment of opening VU1, a current pulse powerful enough for the initial start-up of the CG circuit is formed, depending on the energy of the storage capacitor. Further, the circuit is powered from the output voltage of the working converter.
Diode VD6 is decoupling, preventing shunting of the start-up circuit by the output resistance of the supply circuit of the converter itself.
Authors: P. Redko, I. Rusetsky, Novopolotsk; Publication: N. Bolshakov, rf.atnn.ru
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