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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Theory: switching power supplies. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur Traditional power supplies with a low-frequency transformer, a rectifier and a stabilizer are simple, reliable, practically do not interfere, but with high output power they have significant dimensions, weight and low efficiency. These shortcomings are especially noticeable at high powers. The dimensions and weight of the transformer, as well as the capacitance of the smoothing capacitors, decrease with increasing frequency of the supply network. In this regard, some local power networks, especially military ones, have an increased frequency (400 Hz). In household equipment, in particular in modern televisions and computers, switching power supplies are used (Fig. 70), the principle of operation of which is as follows. Mains voltage 220 V is rectified by a diode bridge VD1-VD4. The resulting DC voltage of approximately 300 V is applied to a generator that generates a sequence of pulses that feed a small-sized pulse transformer T1 on a ferrite magnetic circuit. It also provides galvanic isolation of powered equipment from the mains.
To reduce the penetration of impulse noise into the supply network, be sure to install a filter containing chokes Lf and capacitors Cf. Resistor R1 is needed to limit the current through the rectifier diodes at the moment of switching on, when the oxide capacitor C1 (with a capacity of up to 100 microfarads or more) is not yet charged. A ceramic capacitor C2 of a much smaller capacity reduces the high-frequency ripple of the rectified voltage during generator operation. A powerful high-voltage transistor VT1 operates in a key mode with high efficiency. It is opened by the pulses of the generator and creates a current in the primary winding of the transformer. The impulse voltage from the secondary windings (III and IV) is rectified and smoothed. Another winding (II) feeds the stabilization circuit, which controls the duration and / or frequency of the pulses so that the voltages U1 and U2 are stabilized. A more detailed description of switching power supplies can be found in journal articles and specialized literature. In powerful switching power supplies, both push-pull generators and rectifiers are used. Pulse generators and stabilization circuits (indicated by rectangles in Fig. 70) are now often made in the form of ready-made integrated circuits. Switching voltage stabilizers are performed according to similar schemes, but instead of a transformer, they use chokes on ferrite magnetic cores. Consider the circuit of a buck-stabilizer converter (Fig. 71), which generates, for example, a stabilized voltage of 5 V from an unstabilized 12 ... 18 V. It operates with high efficiency (only at a stable load current), reaching 90% or more. This means that the current in the load is more than consumed!
The key transistor VT1 is turned on by short pulses from the master oscillator. The current in the inductor L1 increases during the pulse to a relatively large value (of the order of the load current). When, at the end of the pulse, the transistor closes, the current in the inductor continues to flow through the opened diode VD1 until the next pulse begins. In this case, the energy stored in the magnetic field of the inductor is consumed. The stabilization circuit regulates the duration or repetition rate of the pulses so that the output voltage remains unchanged. For example, as the output voltage increases, the pulse duration decreases. Given that switching regulators create noise, they require good voltage filtering at the input and output. Author: V.Polyakov
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