Low ripple rectifier. Encyclopedia of radio electronics and electrical engineering

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Power supplies are usually assembled according to the scheme: AC voltage rectifier, filter and stabilizer. However, in some cases, you can do without a filter, often the most cumbersome unit.

It is known that a capacitor connected to an alternating voltage circuit shifts the phase of the current by 90 degrees. A phase-shifting capacitor is often used, for example, when connecting three-phase electric motors to a single-phase network [1].

On fig. 1 shows the shape of a rectified three-phase voltage. It can be seen that there are no "drops" of voltage to zero level, which are typical for a single-phase rectifier (Fig. 2).

The capacitive filter smoothes the output voltage ripple of single-phase rectifiers, creating an acceptable value of its constant component, and the larger the capacitance of the filter capacitors, the less the ripple and, accordingly, the greater the constant component. In three-phase rectifiers, due to the mutual "overlapping" of half-waves of the voltage, the constant component is larger, which in many cases makes it possible to dispense with a capacitive filter.

If a phase-shifting capacitor is used in a single-phase rectifier, which provides mutual "overlapping" of the half-waves of the rectified voltage, in many cases, at a constant load, a bulky capacitive filter can be dispensed with or its capacitance can be significantly reduced [2]. A diagram of such a stabilized rectifier is shown in fig. 3.

A three-phase rectifier VD1-VD6 is connected to a single-phase AC voltage source through active (resistor R1) and capacitive (capacitor C1) resistance. The output voltage of the rectifier stabilizes the Zener diode VD7.

The calculation of the resistance of the resistor and the capacitance of the capacitor is similar to the usual calculation of the ballast resistance of zener diodes. It is calculated by the formula R \u0,75d (0,75 Upit-Ust) / lst. The coefficient 1 takes into account the minimum level of the rectified voltage at the points of "overlap" of the half-waves of the current flowing through the active (resistor R1 - solid line) and capacitive (capacitor C2 - dashed line) resistance (see Fig. 27). The capacitance of the phase-shifting capacitor (in farads) is calculated by the formula C \uXNUMXd (XNUMXπfR)^-1 where f is the frequency of the alternating current, Hz; R - previously calculated active resistance, Ohm. The diagram shows the specific values ​​​​of the resistance of the resistor and the capacitance of the capacitor for a stabilized rectifier with a supply voltage of 220 V. The capacitor (or several connected in parallel to obtain the required capacitance) must be designed for operation in AC circuits. Here, for example, two K73-17 capacitors connected in parallel with a capacity of 0,1 uF are suitable.

The described rectifier can be used where it is necessary to reduce the dimensions of the electronic device, since the dimensions of the oxide capacitors of the capacitive filter are usually larger than the phase-shifting capacitor of a relatively small capacity. The gain in dimensions is especially noticeable in mains rectifiers, when the mains voltage is directly rectified without the use of a step-down transformer.

Another advantage of the proposed option is that the consumed current is almost constant (in the case of a constant load), while in rectifiers with a capacitive filter at the moment of switching on, the starting current significantly exceeds the steady-state value (due to the charging of the filter capacitors), which in some cases is undesirable.

The described device can also be used with series voltage stabilizers with a constant load, as well as with a load that does not require voltage stabilization.

Literature

1. Shatalov N. Connecting a three-phase asynchronous electric motor to a single-phase network. - Radio, 2000, No. 7. p. 39.
2. RF patent No. 2159460 (author L. B. Mashkinov) - Bulletin "Discoveries, inventions, ...", 2000, No. 32.

Author: L.Mashkinov, Chernogolovka, Moscow region

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