Three voltages from one Krona. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The use of operational amplifiers (op-amps) in portable equipment immediately poses the problem of how to power them with a bipolar voltage of +15 V. A similar question arises because in the reference materials the parameters of most op-amps are given precisely for these supply voltages, and many radio amateurs get the impression that op amps can work well only in this mode. In most amateur radio devices, a bipolar voltage of ±15 V is also supplied to the op-amp. But if you carefully study the technical data on the op-amp, you will find that the lower limit of operating voltages for most op-amps is ± 5..6 V. So, for widely used microcircuits K140UD6 and K140UD7 the minimum supply voltage is ± 5 V, and for a low-power op-amp K140UD12 this limit is ± 1,5 V (see Kudryashov B.P. Analog integrated circuits: A Handbook. - M .: Radio and communication, 1981).

With a decrease in the supply voltage level, the current consumed by the op-amp decreases - this also simplifies the problem of a power source for portable equipment. For most op-amps, when supplied with a voltage of ±5 V, the current consumption decreases by about 3 times compared with a supply voltage of ±15 V. Of course, a decrease in the supply voltage leads to a change in other parameters of the op-amp, but these deviations usually do not affect the operation of the circuit. It is convenient to use a Krona-VTs or Korund battery with a voltage of 9 V as a power source for portable devices, and obtain a bipolar supply of +5,5 and -4,8 V using the device described below. The +5,5 V voltage is stabilized, it is intended not only to power the op-amp, but can be used for digital microcircuits of the K134, K176, K561 series. The power node also produces a voltage of -10 V, which, if necessary, is used to control electronic switches based on field-effect transistors of the K168 and K190 series.

The asymmetry of the supply voltages for the op-amp practically does not affect the operation of the microcircuit, since the coefficient of influence of the instability of the power supplies for the op-amp does not exceed -60 dB. The power node is distinguished by the presence of a stabilized voltage and low current consumption without load. The efficiency depends on the input voltage and is 0,4 ... 0,5.

The diagram of the power unit is shown in fig. 1. It consists of a positive polarity voltage stabilizer and a pulse converter.

Fig.1 (click to enlarge)

The positive voltage stabilizer contains a two-stage DC amplifier (transistors VT2 and VT3), in which the reference zener diode is connected to the base circuit of the transistor VT3. Feeding the reference element with an output stabilized voltage makes it possible to obtain a high voltage stabilization factor (more than 500) with a low output resistance (no more than 0,2 Ohm). The regulating element of the stabilizer is p-n-p transistor VT1, so the stabilization mode at load currents up to 20 mA occurs when the voltage at the input of the stabilizer is only 0,05 ... 0,1 V more than the output. When the power is turned on, the stabilizer enters the operating mode due to the chain of elements C1, R1, VD2, R3. In this case, the charging current of the capacitor C1 passes through the start circuit: VD2, R3, the base-emitter transition of the transistor VT2 and brings the transistors VT1 and VT3 to the operating mode. The stabilizer has short circuit protection.

The pulse converter contains a generator, an output transistor stage and a capacitive voltage multiplier. For reasons of economy, the generator is assembled on a CMOS-type DD1 chip. The output voltage of the generator is a pulse square wave with a frequency of about 10 kHz. It is fed to the bases of transistors VT4 and VT5 of the output stage and alternately switches them to the open state. When the VT4 transistor is open, the capacitor C6 is charged through this transistor and the VD6 diode. In the next half-cycle of the pulsed voltage of the generator, the transistor VT5 opens and the capacitor C6, discharging through it and the diode VD7, transfers energy to the capacitor C7. As a result, the capacitor C7 is charged approximately to the output voltage of the stabilizer. When VT4 is open, the capacitor C8 is charged along the circuit: + Ustab, VT4, C8, VD8, C7, common bus. There are two voltage sources connected in series in this circuit: Ustab.

Consequently, the capacitor C8 will be charged to approximately the voltage Uc8 = Ustab + Uc7 = 10V. This voltage, when the transistor VT5 is opened, is transmitted through the diode VD9 to the output capacitor C9. With each recharge cycle of the capacitors of the voltage multiplier, there is a loss of voltage across the diodes and open transistors VT4 and VT5, so the output voltage decreases with increasing load current. This dependence for a negative voltage of -4,5 V is shown in fig. 2.

Ris.2

In idle mode, when the load current is zero, the negative polarity voltage for the two outputs is -5,3 and -10,2 V. In this mode, the converter consumes a current equal to 0,3...0,4 mA. Due to the fact that the converter is powered by a stabilized voltage, the voltage at its outputs depends only on the load resistance, i.e. at a constant load, the negative output voltage will be unchanged. The efficiency of the described pulse converter at a load current In equal to 3 mA reaches a value of 0,7, but with a deviation from this value by ±2 mA it decreases to 0,6. The amplitude of the output voltage ripple under load does not exceed 10 mV.

Structurally, the power node is best done on the printed circuit board of the circuit that it feeds, so the wiring of the printed circuit board of the power node is not given. The area occupied by the circuit elements does not exceed 12 cm2. It uses MLT-0,125 resistors and small capacitors C1, C8, C9 - K53-1; C3 - C5 - KM; C2, C6, C7 - K52-1B.

The described power unit is simple, which eliminates any adjustment work after installation. If the output stabilized positive voltage differs from the nominal value by more than 5%, it is set by selecting the Zener diode VD3. The criteria for the health of the power unit are the presence of output voltages and no-load current not exceeding 2,5 mA.

References:

1. Kudryashov B.P. et al. Analog integrated circuits: A Handbook.-M .: Radio and communication, 1981.
2. Khodakovsky E. Voltage polarity converter.- Radio, 1984, No. 7, p. 48-49.

Authors: V.Efremov, V.Fedko

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