Converter 12/220 volts. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Voltage converters, rectifiers, inverters

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The use of powerful field-effect transistors makes it possible to significantly simplify the circuit and increase the efficiency of the converter. The upgraded circuit is shown in Fig.1.

A master oscillator with a frequency of 1.1 Hz is assembled on the elements DD1.2, DD500. The divider on DD2 generates two pulse sequences with a frequency of 50 Hz with phases shifted by 180 ° to control the power switches VT1 and VT2 of a push-pull converter. To avoid through switching currents, there is a "dead zone" between turning off one key and turning on the other - 10% of the period duration.

(click to enlarge)

When a high level (logical "1") is applied to the "Lock" input, both output keys are locked.

The output power of the converter is limited by the power of the power transformer T1 and the maximum allowable current of the output transistors. The transformation ratio of the power transformer Kt=20. IRFZ034 (15 A), IRFZ044 and RG723A (30A), IRFZ046 (50 A), IRFP064 (100 A) are suitable as output transistors. For reliability, you need to have a double current margin and a triple voltage margin. Power circuits should be as short as possible and made with wires of the appropriate section.

It is desirable to supplement the proposed converter circuit with protection and service circuits, including:

- protection against stopping the master generator, it is also a blocking (Fig. 2, a);

- protection of output transistors from excess voltage on the battery over 15 V (Fig. 2,b);

- protection of the battery from deep discharge (Fig. 2, c). The same circuit serves as an indicator of battery voltage. At 10 V, the VD9 LED goes out, at 15 V it shines at full strength;

Fig.2 (click to enlarge)

- protection against incorrect connection, i.e. battery polarity reversal (Fig. 2, d);

- Automatic transfer to backup power supply in case of power failure in the network, and return to power supply from the network when mains voltage appears (Fig. 2,e).

The converter circuit consumes about 7 mA in standby mode.

To obtain an uninterruptible power supply scheme, the proposed converter must be supplemented with an automatic charger (Fig. 3), which provides charging and maintenance of the battery (AB). The converter and charger must be connected to the battery with separate wires.

The K561IE8 distribution meter (Fig. 1) has a reset input (pin 13), a high level at which resets the microcircuit. At the same time, the counting stops, and all outputs, except for zero (pin 3), are reset to zero. Both output transistors VT1 and VT2 are closed, i.e. the converter is blocked.

The emergency blocking scheme is shown in Fig. 2a. Capacitor C4 is charged through R13 to the supply voltage in the absence of pulses from the DD1.2 output and supplies a logical "1" to the blocking input (pin 13 DD2) through VD13. During normal operation of the converter, a logical "1" appears every 2 ms at the "Lock Reset" output (pin 20 DD1), which opens the VT11 transistor through R5 and discharges C4, thereby preventing the lock from working.

Protection against overvoltage on the battery (Fig. 2, b). When Ua>15 V is exceeded, the zener diode VD10 opens, VT9 opens with current through R4 and supplies a logical "1" through VD12 to the blocking input. This blocking is needed to prevent failure of the power transistors. To protect the entire circuit in parallel with C5, you need to turn on the zener diode KS515. This situation will not occur unless the charger is connected to the inverter without a battery. It is better to connect the converter and the charger to the battery with different wires.

DB protection from deep discharge (Fig. 2, c). The value of R7 is selected in such a way that when Ua<10,5 V, the transistor VT3 is already closed, the VD9 LED goes out, and a logical "8" is applied to the blocking input through R11 and VD1. C2 prevents blocking in the event of a momentary drop in Ua.

Protection against incorrect inclusion (polarity reversal) of AB (Fig. 2d). In case of emergency blocking, there is a logical "9" at pin 1.4 of DD1, and "1.4" at the output of DD0. Transistor VT6 closes, relay K1 releases and disconnects the battery from the power part of the converter. In the event of a polarity reversal, when the battery is connected, relay K1 does not work at all.

Automatic switching to backup power (Fig. 2e). In the event of the presence of voltage in the network, relay K2 is turned on, and with its contacts connects the load directly to the network. The optocoupler transistor VU1 is open, and through R14 it supplies a logical "1" to the blocking input. In this case, the converter is blocked.

When the mains voltage fails, relay K2 releases, switching the load to the output of the converter. The optocoupler transistor closes, and a logical "0" appears at pin 5 of DD1.3. Leaning at the output DD1.3-"1", a positive pulse opens the transistor VT5, C5 is discharged, "1" disappears from the blocking input, and the converter starts up.

Switch S1 "On" allows you to turn off the converter in the event that, in the absence of voltage in the network, backup power is not required; "+" power is supplied through switch S1 and R14 to the blocking input. When the contacts of the switch S1 are opened, the converter starts up - just as after a power failure in the network.

Author: O.Lokseev, Bakhchisarai; Publication: N. Bolshakov, rf.atnn.ru

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