Micropower voltage stabilizers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Surge Protectors

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One of the most important indicators of self-powered electronic equipment is the efficiency of its components. In the micropower voltage regulators described below, the exemplary voltage source is not made on a zener diode, the minimum operating current of which is several milliamps, but on a field-effect transistor with a pn junction. In this case, the cutoff voltage of the transistor will be exemplary. Such a circuit solution made it possible to reduce the current consumed by the stabilizer to about 100 μA. By taking additional measures to ensure the thermal stability of the output voltage, such stabilizers can be used as sources of exemplary voltage (ION) of very high accuracy.

The first version of the voltage stabilizer is assembled on a frequency-corrected operational amplifier K154UD1B (Fig. 1), which has a high voltage gain (Ku>=2*105) and low current consumption (Iп<= 1,2*10-4). Despite the simplicity of the circuit, the stabilizer has high technical characteristics:

• Current consumption, A ....... 10-4
• Stabilization coefficient, not less than ....... 4 * 104
• Output impedance, Ohm, no more.......10-3
• Maximum load current, A.......10-2
• Temperature coefficient of output voltage, 1/°С, not more than.......5*10-4

The mixing voltage of the field-effect transistor VT1, which is exemplary in the stabilizer, is formed on the resistor R1. Op-amp DA1 is connected according to the scheme of a non-inverting amplifier, the gain of which is set by the divider R2R3, included in the negative feedback circuit. Since an exemplary voltage Uobr is applied to the inverting input of the op-amp DA1, then its output will be Uout \u3d (R2 / R1 + XNUMX) * Uobr.

The drain of the field effect transistor VT1 is connected to the output of the stabilizer, so the reference voltage is maintained with very high accuracy. Tests have shown that with an increase in the supply voltage from 6,7 V to 32 V, a change in the output voltage cannot be registered by a five-digit digital voltmeter Shch68002 (with a resolution of 0,1 mV at a limit of 10 V). Thus, the instability of the output voltage in the considered stabilizer is mainly due to the quality of its passive elements (resistors) and the temperature dependence of the reference voltage.

This dependence can be reduced to almost zero at the cost of a small increase in current consumption. The fact is that for field-effect transistors of various types, there is a drain current value at which the gate-source voltage does not depend on temperature.

By the way, it is known that this value for transistors with a p-channel and a cutoff voltage of 1 ... 2 V lies in the range from 25 to 250 μA. In fact, these limits seem to be wider than commonly believed. So, for one of the copies of the field-effect transistor, tested in the considered stabilizer, it turned out to be 650 μA.

Fig. 1

Due to the high technical characteristics, it is advisable to use the described voltage stabilizer in equipment with mains power supply. The input voltage should not exceed 32 V. To increase the permissible load current, it must be connected to the output of the op-amp DA1 through an emitter follower on a transistor of the appropriate power. At a current greater than 1 A, a composite follower on two transistors is most likely required. The required value of the output voltage is set by selecting resistors R2, R3. To ensure the normal operation of the op-amp DA1, the reference voltage should not be less than 2 V, and the output voltage (at pin 6) should not exceed (Upit - 2) V.

A schematic diagram of the second version of the stabilizer is shown in Fig.2. It is assembled on widely used elements and has the following technical characteristics:

• Consumed current, A, no more....... 9*10-5
• Stabilization coefficient ..... 8*102
• Output impedance, Ohm.......2*10-2
• Maximum load current, A ....... 5 * 10-2
• Temperature coefficient of output voltage, 1/°С, not more than.......5*10-5

An interesting feature of this stabilizer is the use of a current stabilizer based on field-effect transistors VT1, VT2 as a temperature-compensating element, which, in addition, performs its main function of a dynamic load with high internal resistance. Unlike the first option, here it is possible to set the current mode of operation of the transistors, and hence the power consumption. For example, if you increase the resistance of all resistors several times, then the current consumption will decrease accordingly.

The stabilizer is built according to the compensation scheme. The control element is made on the transistor VT3, connected according to the OE scheme. This element is covered by deep negative feedback through a composite voltage follower on transistors VT4, VT5. The load of the transistor VT3 is the current stabilizer VT1, VT2, R1. Thanks to the cascode connection, it was possible to obtain a very large internal resistance of the current stabilizer - about 150 MΩ, which significantly improved the technical characteristics of the entire device as a whole.

In order for the voltage follower VT4, VT5 not to affect the current flowing through the transistors VT1-VT3, the first repeater transistor is selected as a field transistor. The second transistor of the follower must be bipolar, since, due to the greater steepness of the characteristic compared to the field one, this can significantly reduce the output impedance of the voltage follower and the stabilizer as a whole.

Fig. 2

The idea of ​​temperature stabilization of the output voltage is as follows. The voltage Ube between the base and emitter of a bipolar transistor at a fixed collector current has a negative temperature coefficient of -2 mV/°C. In turn, the drain current of the FET is in the microcurrent region due to the temperature drift of the cutoff voltage. equal to about +2 mV/°C, depends on the temperature with a coefficient of about +10-3/°C. This current, flowing through the resistor R2 of the stabilizer, creates a voltage drop, which, at a certain value of resistance R2, will have a temperature coefficient of +2 mV / ° C. Thus, the output voltage equal to Uout \u3d (UBE2 + UR4) (R5 / R1 + 3) will almost not depend on temperature (UBE3 is the voltage at the emitter junction of the transistor VT2). The smallest value of the temperature coefficient can be achieved if the resistor RXNUMX is carefully selected.

For reliable operation of the thermal compensation unit, it is necessary to maintain the temperature difference between the pn junctions of transistors VT1 and VT3 at the minimum level (no more than 0,05 ° C). This problem can be solved most simply by providing thermal contact between the cases of these transistors. But this measure is not always justified and may be unnecessary. If there are no factors that can cause a thermal gradient (closely located heating parts, for example, heat sinks of powerful transistors), then the cases of transistors VT1 and VT3, even installed separately, will have the same temperature to within a few hundredths of a degree. Their own thermal power released in them does not exceed 30 μW, and this leads to an increase in the temperature of the semiconductor crystal by no more than 0,03 ° C (a typical value of the thermal resistance of the junction - the environment, for low-power transistors is 0,5 .. .1 S/mW). This shows that high thermal stability of the output voltage can be ensured in some cases even without thermal contact between the cases of transistors VT1 and VT3.

When choosing parts for stabilizers, special attention should be paid to the selection of field-effect transistors by cutoff voltage. For the first version of the stabilizer (Fig. 1), it must be more than 2 V. The transistor VT1 in the second version (Fig. 2) must have a cut-off voltage within 0,6 ... 1 V, VT2 - 1,8 ... 2,2 ,3 V. VT1 - 3..303 V. There are no other special requirements for transistors, therefore, instead of KP302E, you can use transistors of the KP307 and KP315 series, instead of KT3102G - KT3102G - KT342E, KT342B, KTXNUMXV.

Since the current stabilizer VT1VT2R1 (Fig. 2) is a two-terminal device, instead of field-effect transistors with a p-channel, transistors with an n-channel can be used, while observing the desired switching polarity.

As a replacement for the K154UD1B OU, K140UD12 and KR1407UD2 can be recommended, but they have a different pinout and a permissible load current of less than 1 mA. Correction capacitor C1 - any ceramic series KM-5, KM-6, etc.

With low requirements for temporal and temperature stability of the output voltage in stabilizers, it is better to use MLT-0,125 or MLT-0,25 resistors with a tolerance of 5%, otherwise all resistors (except R3 in Fig. 2) must be precision, for example, C2 -13-0,25 with a tolerance of 0,1%.

Establishing stabilizers consists in setting the desired value of the output voltage by choosing the ratio of the resistance of the feedback circuit resistors. In each stabilizer, measures have been taken to eliminate self-excitation at high frequency by including correction capacitors C1 of small capacity in the negative feedback circuit. Nevertheless, the possibility of the appearance of parasitic generation cannot be ruled out. This is possible if there are load stabilizers with a capacitance of 500 pF ... 0,1 microfarads at the output. To eliminate parasitic generation, it is enough to turn on an oxide capacitor with a capacity of 1 ... 10 microfarads in parallel with the load of the stabilizer.

Author: S. Fedichin

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