Compensating voltage stabilizer at op-amp. Op-amp based voltage stabilizers

The circuit of a high-quality stabilizer, in which the control transistor is replaced by an operational amplifier, is shown in Fig. 15.7. The op-amp is powered by a unipolar positive voltage U input (in this case, negative voltages are not required at the output of the op-amp), which allows the use of standard operational amplifiers in stabilizer circuits with an output voltage of almost 30 V.

Resistor R 2 and transistor VT 2 form an output current limiting circuit. At rated load currents, the voltage drop is R 2 does not exceed the trigger voltage of the base-emitter junction VT 2, transistor VT 2 is closed and does not affect the operation of the stabilizer circuit. Operational amplifier with additional output current amplifier VT 1 is connected according to the non-inverting UPT circuit, from which follows the relationship for calculating the output voltage

If the voltage drop is R 2 will exceed a value equal to approximately 0.6 V, the transistor VT 2 will open and prevent further increase in transistor base current VT 1. Thus, the output current of the stabilizer is limited by the level
.

Qualitative indicators of the stabilizer according to the diagram in Fig. 15.7 are determined by the following relations:

A) stabilization coefficient (it can be increased if you replace R 1 current source)

;

b) output impedance

,

Where TO– voltage gain of the op-amp;

r out– output resistance of the op-amp;

V) temperature coefficient of voltage

Where
– drift of the op-amp bias voltage;

– drift of the op-amp input current;

TKN st – temperature coefficient of zener diode voltage.

All stabilizers considered effectively suppress instability U input not only due to slow oscillations mains voltage, but also pulsations U input after the rectifier, acting as an electronic smoothing filter. Therefore, a relatively high level of voltage ripple is allowed at the input of the stabilizer.

15.6 DC voltage stabilizer microcircuits

Voltage stabilizers similar to the circuit in Fig. 15.7, are made in the form of integrated circuits. The main characteristics of voltage stabilizer microcircuits of the K142 series are given in Table 15.1. Among them

–voltage instability coefficient;

– current instability coefficient.

Table 15.1 – Characteristics of K142 series DC voltage stabilizer microcircuits

	,	,	,	,	,	,
		35	51% 15

Stabilizers K142EN1 (2, 3, 4) require connection of external components (feedback circuit divider, correction elements, current protection). Microcircuits K142EN5 (6, 8) are functionally complete stabilizers for fixed values U exit The output voltage of the K142EN5 microcircuit is 5 V with a possible change in this value depending on the IC instance by ±0.2 V. The maximum load current is 3 A. The minimum input voltage is 7.5 V. Thermal protection turns off the stabilizer at a crystal temperature of 175 o C ± 10 %, if the permissible current value is exceeded by (20–25)%, current protection is triggered.

A significant disadvantage of parallel and series type stabilizers, called linear, is the large power loss in the control transistor (controlled resistance) and, as a consequence, insufficiently high efficiency. The desire to increase efficiency has led to the creation of stabilizers with pulse regulation, in which the regulating element is a periodically closed switch (usually a transistor in switching mode), connecting the load to a source of input DC voltage U input If during the switching period T the key remains closed for a period of time t on, then the constant component of the voltage across the load U out = U input t on /T.

The regulating transistor in the pulse stabilizer operates in key mode, i.e. most of the time it is either in cutoff or saturation mode. The key operating modes of the transistor and pulse devices will be considered when studying the discipline “Electronic circuits and microcircuitry”.

V. Krylov

CONSTRUCTION OF BIPOLARY VOLTAGE STABILIZERS ON THE OPAMP

Operational amplifiers (OA) are increasingly used in a wide variety of components of amateur radio equipment, including stabilized power supplies. Op amps make it possible to dramatically increase the quality indicators of stabilizers and their operational reliability. The use of op-amps in stabilizers can be read in the magazine “Radio” (1975, No. 12, pp. 51, 52 and 1980, No. 3, pp. 33 - 35). The article below describes the construction of bipolar stabilizers using op-amps.

The simplest is bipolar, a voltage stabilizer can be obtained from two identical unipolar ones, as shown in Fig. 1.

Rice. 1. Scheme of a stabilizer built from two identical unipolar

This bipolar stabilizer can provide a current of up to 0.5 A for each of the arms. The stabilization coefficient when the input voltage changes by ±10% is 4000. When the load resistance changes from zero to maximum, the output voltage of the stabilizer changes by no more than 0.001%, t i.e. its output resistance does not exceed 0.3 MOhm. Output voltage ripple with a frequency of 100 Hz at maximum load current - no more than 1 mV (double amplitude).

The advantage of this method of constructing a bipolar stabilizer is obvious - the possibility of using the same type of elements for both arms. The disadvantage is that the input AC voltage sources in this case should not have a common point, in other words, two secondary windings on the mains transformer, two separate rectifiers and a four-wire stabilizer with rectifiers, are required.

In order to reduce the connecting wires to three, you need a regulating element (transistors V4, V5) move the lower arm of the stabilizer according to the Diagram from its positive to the negative wire (the upper one remains unchanged). This can be done by using transistors of a different structure: n - R -n for transistor V4 And R- n - R For V5 (Fig. 2, a). Op amp output voltage A2 in this case it will be negative relative to the common wire. According to the parameters this . practically no different from that described above.

Note that with the indicated transfer of the regulating element, we can limit ourselves to replacing only one of the transistors, namely V5, if you turn on a compound transistor regulating according to the circuit (Fig. 2, b)- at the same time, powerful regulating transistors in both arms of the stabilizer (VI And V4 according to fig. 2, a) remain the same. The stabilization coefficient with such a modification of the regulating element practically remains the same (about 4000), but the output resistance of the lower arm may increase, since when moving to a composite regulating transistor, the advantage inherent in the combination of two transistors of different structures in the regulating element is lost (more about on this, see “Radio”, 1975, No. 12, p. 51). During experimental testing of the stabilizers under consideration, it was recorded, for example, a threefold increase in the output resistance.

Powerful regulating transistors of the same type in both arms of a bipolar stabilizer can also be used if, according to the circuit of a composite transistor, the regulating element of the upper circuit of the stabilizer arm is included (Fig. 2, c), leaving transistors of different structures in another stabilizer.

Rice. 2. Stabilizer circuit powered by one rectifier

Rice. 3. Stabilizer circuit powering the op-amp from the output voltage

In the considered stabilizers, the op-amp is powered directly by the input unipolar voltage, but this is only possible in cases where the input voltage is approximately equal to the rated supply voltage of the op-amp. If the first of these voltages exceeds the second, then the op-amp can be powered, for example, from the simplest parametric stabilizers that limit the input voltage to the required level.1 Vol. case when the supply voltage of each of the stabilizer arms turns out to be significantly less than that required to power the op-amp. you should switch to feeding it with bipolar voltage. In bipolar stabilizers this is implemented relatively simply.

In Fig. Figure 3 shows a circuit of a stabilizer, the output bipolar voltage of which is equal to the supply voltage, which made it possible to power them directly from the output of the stabilizer. Transistors V3 And V8 provide amplification of the op-amp output voltage to the required level, V4 protects the emitter of the transistor V3 from the reverse voltage, which may appear at the output of the op-amp (with its bipolar power supply), for example, during transient processes. In the case when the maximum permissible reverse voltage between the emitter and the base of the transistor exceeds the supply voltage of the op-amp, the use of such a diode is unnecessary. That is why in the base transistor V8 no diode.

Location of reference voltage sources (zener diodes V5 And V9) in comparison with the previously considered stabilizer (see Fig. 2, a) here it is changed for in order to maintain the negative nature of the feedback in the presence of additional amplifiers on transistors V3 And V8. would also be negative if each of the stabilitrons V5 And V9 connect between the inverting input of the corresponding op-amp and the common wire of the stabilizer, but in the case under consideration such a connection is unacceptable, since this will exceed the maximum common-mode voltage, which for the op-amp K1UT401B (new name K.140UD1B) is equal to ±6 V.

When powering the op-amp with output voltage, special attention should be paid to the reliability of the stabilizer startup. In the case under consideration, such a start is ensured that immediately after applying the input voltage through the load resistors R2 And R9 base transistors are leaking V2 And V7 respectively. At the same time, the regulating elements of the stabilizer arms open, the output voltages increase, introducing the device into operating mode.

An experimental test of this stabilizer gave the following results: stabilization when the input voltage changes by ±10% exceeds 10,000, the output resistance is 3 MOhm.

All bipolar voltage stabilizers discussed above are a combination of two unipolar stabilizers connected by a common wire, the output voltages of which are set independently of one another. With such a construction of a bipolar stabilizer, it is difficult to ensure equality of the voltages of its arms both when setting up the stabilizer and under its operating conditions. In a number of cases, for example, in “-voltage” converters, the bipolar stabilizer is subject to very high requirements regarding the symmetry of its output voltage relative to the common wire. Fulfillment of such requirements is relatively simply ensured in a stabilizer, the diagram of which is shown in Fig. 4.

Rice. 4. stabilizer with symmetrical output voltage

Here, the upper one according to the diagram is no different from the upper arm of the previous stabilizer (see Fig. 3). the shoulder is built differently. The inverting input of the op-amp is connected to a common wire, and, therefore, the voltage at this input is zero. Since the differential input voltage of the op-amp is insignificant (a few millivolts), the voltage at the non-inverting input will be zero. But this op-amp input is connected to the midpoint of the voltage divider R14 R15, connected between the extreme terminals of the stabilizer; Due to this, the absolute value of the voltage UOUT. n at the output of the lower arm of the stabilizer will be determined by the following expression:

where Uout. n - tension of the upper arm.

If the resistances of the resistors are equal R14 And R15 The output of the lower arm is automatically set equal to the voltage of the upper one, and the device constantly “monitors” its value. For example, if we use a trimmer resistor R8 increase the voltage UOut. c, this will lead to an increase in voltage at the non-inverting input of the op-amp A2 and, therefore, at its output. Wherein V8 will begin to close, the voltage on the regulating transistor V6 will decrease. The output voltage of the lower side will increase to a level at which the voltage at the non-inverting input of the op-amp A2 will again become equal to zero, i.e. to the newly established level UВИХ. B.

Thus, in the bipolar stabilizer under consideration, the voltage at the output of both arms is injected with one trimming resistor R8, and the equality of the absolute values ​​of the positive and negative output voltages at R14 = R15 is determined only by the accuracy class of these resistors.

According to their own quality indicators The stabilizer is no different from the previous one.

Stabilizer with op-amp and short circuit protection. In the stabilizer (Fig. 16.41, a) in An op-amp is used as a comparison device. Reference voltage from diode VD2 is supplied to the non-inverting input, and the pulsating output voltage is supplied to the inverting input. Negative feedback via diode VD1 and two transistors perform damping functions. To protect the stabilizer from short circuit, a resistor is included R5. Load characteristics are shown in Fig. 16.41, in (curve 1) and fig. 16.41, G. If you swap the connections of the chains R4, VD2 And R6 - R8, the load characteristic looks like a curve 2 in Fig. 16.41, at. In Fig. 16.41, b The dependence of the deviation of the output voltage on the input voltage of the stabilizer is shown.

Rice. 16.41

Op-amp voltage stabilizers. The stabilizer (Fig. 16.42, a) provides an output voltage of 15 V at a load current of 0.5 A. The stabilizing element in this circuit is an op-amp, with which you can obtain a stabilization coefficient of more than 4-10 4. Reference voltage generated by the diode VD1 and a transistor VT3, is supplied to one input of the op-amp, and the second input is connected to a divider, which ensures that the stabilizer starts when it is turned on. High stability of the reference voltage is ensured by the chain VD1, VT3, in which the transistor acts as a current generator.

To reduce the influence of transistor reverse current VT1 resistor is used R1. Resistor R2 limits the base current of the transistor VT2. Adjustment chain parameters R3 C1 selected taking into account the operation of the op-amp with deep feedback.

To obtain a voltage at the output of the stabilizer that exceeds the supply voltage of the op-amp, you should use the circuit in Fig. 16.42, b. In this circuit, the amplifier is powered from an additional stabilizing stage Rl, VD1, VD2 which provides a voltage of 24 V. Using this circuit, you can obtain a stabilization coefficient of more than 2-10 4 at a load current of 1 A.

Rice. 16.42

Rice. 16.43 Fig. 16.44

Stabilizer with adjustable stabilization coefficient. The stabilizer (Fig. 16.43) has a stabilization coefficient of more than 10 5. Depending on the resistance of the resistor R4 The stabilization coefficient can be positive or negative. To reduce the power dissipated by the transistor VT3, resistor turns on R7. The resistance of this resistor is determined by the constant load current. The current associated with the change in load resistance flows through the transistor VT3.

High voltage stabilizer based on op-amp. A high-voltage voltage stabilizer (Fig. 16.44) has a stabilization coefficient of more than 10 3. It is designed for currents up to 0.1 A. An op-amp is used as an amplifying element, the supply voltage of which is raised to the level of 100 V. To prevent malfunction of the stabilizer, it is desirable to increase the input voltage smoothly to the desired value.

Rice. 16.45

High voltage stabilizer. The high-voltage stabilizer (Fig. 16.45) has £00 V at the output. With a load current of 0.1 A, the input voltage should be 300 V. The circuit has a stabilization coefficient of more than 10 4. This is achieved by three types of pulsation attenuation. Using zener diodes VD1 - VD3 the reference voltage is set to 250 V. To reduce the internal resistance of the zener diodes, a capacitor is included C1, which together with a resistor R1 forms a filter circuit. The main stabilizing circuit is op-amp and control transistors VT1 And VT2. Using zener diodes VD5 And VD6 the voltage at the op-amp input decreases to a few volts. At this level, changes in the output voltage occur. The reference voltage also lies in this range. All changes in the output voltage are multiplied by the gain of the op-amp and are fed to the input of control transistors, which smooth out these changes.

Let's carry out the calculation for the stabilizer channel at 36V and 1A, shown in Figure 4.

Figure 4 - Second channel stabilizer circuit

Let us determine the required stabilization coefficient of the stabilizer:

Let's set the rest point of the control transistor VT1. With a load current of 1 A and an output voltage of 51 V, the average voltage of the collector-emitter junction should be 51-36 = 15 V. Then the power dissipation at the collector of the transistor is about 15 W. We select a transistor with an output characteristic close to that shown in Figure 5, construct a load straight line and mark the rest point A for the average input voltage.

According to graphical calculations, we select the regulating transistor VT1 with a large value of the maximum collector current (since the rated current is large and equal to 1A), for example MT7667. Parameters: maximum collector current I kmax =3 A, maximum collector-emitter voltage U kmax =50 V, maximum power scattering at the collector of the transistor P kmax = 25 W, current gain h 21e = 70..100, cutoff frequency of the current transfer coefficient f g = 30 MHz.

Figure 5 - Output characteristic of the control transistor

Accordingly, on the input characteristic

Figure 6 - Input characteristic of the control transistor

The quiescent base current of the control transistor at an average current gain:

The selected base current, according to Figure 5, 6, is

U outO = U be + U nmax< U выхmaxОУ;

Ube = 1.51 V;

U nmax =36·0.01+36=36.36 V

U output OU = 1.51+36.36=37.9 V

I outputOU = I bmax VT1 = ;

Choose operational amplifier PM155C, with parameters: power supply voltage U IP =40..50 V, gain 450, input resistance Rin =25 MOhm, power consumption 200 mW, input current Iin =80 nA, values ​​of the output voltage and current of the op-amp: U outmaxОА =50 V, I outputmaxОА =40 mA.

We generate the reference voltage using a 2N3623 zener diode, for which: rated stabilization voltage 5 V, stabilization current 20 mA.

U op = U st< U нmin ;

Let's determine the resistance of ballast resistor R1. From the condition I st nom >> I input unit

R1 = = =2300 Ohm.

We accept the standard value of 2.3 kOhm.

Let's determine the resistance of resistor R4 from the expression:

U in =I bVT1 R4+U b,

We accept the standard value of 2.7 kOhm.

The required output parameters of the op-amp can be ensured by introducing feedback. Let's calculate the feedback circuit: R2-R3, with a gain of 10 - at lower values ​​there will be low sensitivity, at larger values ​​the op-amp will quickly go into saturation.

Expressing I, we get:

Also. To prevent resistors from exerting great influence for the operation of the circuit, i.e. The divider current was several milliamps, let's take the value R3=51kOhm, then R2==525kOhm (Nearest standard 510kOhm).

Let's calculate the resistance of the divider R5-R6. Setting the divider current to 1 mA, and forming a feedback voltage close to 5V, but less than it (to obtain a positive signal at the output of the op-amp), we obtain:

R5=(36-5)/0.001=31 kOhm;

R6=5/0.001=5 kOhm.

We accept standard values ​​R5 = 33 kOhm R6 = 5.1 kOhm

Let's check the correctness of the choice of resistances:

The feedback voltage taken from R6 is less than the reference voltage (5V), which means that the selection of resistors was made correctly.

Let's calculate the elements of the short circuit protection circuit. Transistor VT2 with a load current within 1 A is in cutoff mode. When the load current reaches above 1 A, VT2 begins to open and short-circuits the base of VT1, closing it, which causes a limitation of the load current. The voltage applied to the collector-emitter junction VT2 in the open state minus the voltage drop across R4 (36-1.51 = 34.49 V) and the voltage across the diode in the forward direction will be approximately 34 V. The maximum collector current in the open state I to us is about 36 mA (Figure 5).

Let's take resistor R7 with a resistance of 1 Ohm as a current sensor. Then, with a rated current in the load of no more than 1 A, the voltage drop across it will not exceed 1V.

Let us choose transistor 2N2411 as VT2, with the following parameters: maximum collector current I kmax = 160 mA, current gain h 21e = 100, maximum collector-emitter voltage U kmax = 100 V, maximum power dissipation at the transistor collector P kmax = 160 mW. Diode VD4 - DN380: U OBR max =100 V, I max vd =1 A

According to the selected operating mode (Figure 7), the collector current VT2 can be found from the output characteristic (Figure 8).

Figure 7 - Input characteristic of transistor VT2

Figure 8 - Output characteristic of transistor VT2

For cut-off mode U be<1 В и насыщения U бэ >1.2V. The corresponding change in the base current is provided by resistor R8.

R8= U be / I b = 1/1·10 -3 =1 kOhm

Capacitor C1 prevents false operation of the protection circuit when the UPS is turned on and its capacitance is selected accordingly to pass short-duration pulses. Let's take the value C1=3.3 nF.

Let's calculate the ratings of the elements of the overvoltage protection circuit. We select a zener diode 2S514A: stabilization voltage 40V, minimum stabilization voltage 38V, stabilization current 15mA; minimum stabilization current 10mA; transistor optocoupler AOT120EC: input current 3mA, insulation voltage 500V, maximum input voltage 1.6V.

If the load voltage exceeds 38V, equal to the sum the stabilization voltage of the zener diode and the direct voltage drop across the optocoupler (from 0.1 to 0.5V), VD5 opens and current begins to flow (minimum stabilization current). To ensure an input opening voltage of the optocoupler of 1.6V, it is necessary that the resistance R9 be no more than 1.6/0.005=320 Ohm. Let's take the standard value R9=300Ohm.

Figure 9 - Simulation diagram

Figure 10 - Simulation Circuit Output

Supply voltage stability is a necessary condition proper operation many electronic devices. To stabilize the DC voltage across the load when the mains voltage fluctuates and the current consumed by the load changes, DC voltage stabilizers are installed between the rectifier with a filter and the load (consumer).

The output voltage of the stabilizer depends on both the input voltage of the stabilizer and the load current (output current):

Let's find the total differential change in voltage when changing and:

Let's divide the right and left sides by , and also multiply and divide the first term on the right side by , and the second term by .

Introducing the notation and passing to finite increments, we have

Here is the stabilization coefficient equal to the ratio of the increments of input and output voltages in relative units;

Internal (output) resistance of the stabilizer.

Stabilizers are divided into parametric and compensation.

A parametric stabilizer is based on the use of an element with a nonlinear characteristic, for example a semiconductor zener diode (see § 1.3). The voltage on the zener diode in the area of ​​reversible electrical breakdown is almost constant with a significant change in the reverse current through the device.

The diagram of the parametric stabilizer is shown in Fig. 5.10, a.

Rice. 5.10. Parametric stabilizer (a), its equivalent circuit for increments (b) and the external characteristics of the rectifier with a stabilizer (curve 2) and without a stabilizer (curve ) (c)

The input voltage of the stabilizer must be greater than the stabilization voltage of the zener diode. To limit the current through the zener diode, a ballast resistor is installed. The output voltage is removed from the zener diode. Part of the input voltage is lost across the resistor, the rest is applied to the load:

We take into account that , we get

The greatest current flows through the zener diode at

The smallest current flows through the zener diode at

If the conditions are met - zener diode currents limiting the stabilization section, the voltage across the load is stable and equal. From .

As the current increases, the voltage drop increases by . As the load resistance increases, the load current decreases, the current through the zener diode increases by the same value, the voltage drop across and across the load remains unchanged.

To find it, we will build an equivalent circuit for the stabilizer in Fig. 5.10, and for increments. The nonlinear element operates in the stabilization section, where its resistance to alternating current is a parameter of the device. The replacement circuit of the stabilizer is shown in Fig. . From the equivalent circuit we get

Considering that in the stabilizer, we have

To find , just as when calculating the parameters of amplifiers (see § 2.3), we use the equivalent generator theorem and set , then the resistance at the output of the stabilizer

Expressions (5.16), (5.17) show that the parameters of the stabilizer are determined by the parameters of the semiconductor zener diode (or other device) used. Usually for parametric stabilizers it is no more than 20-40, but ranges from several ohms to several hundred ohms.

In some cases, such indicators turn out to be insufficient, then compensatory stabilizers are used. In Fig. Figure 5.11 shows one of the simplest circuits of compensation stabilizers, in which the load is connected to the input voltage source through a regulating nonlinear element, transistor V. An OS signal is supplied to the base of the transistor through the op-amp. The input of the op-amp receives voltages from a high-resistance resistive divider and a reference (reference) voltage.

Rice. 5.11. The simplest scheme compensation stabilizer with op-amp

Let's consider the operation of the stabilizer. Let us assume that the voltage has increased, followed by an increase and In this case, a positive voltage increment is applied to the inverting input of the op-amp, and a negative voltage increment occurs at the output of the op-amp. The difference between the base and emitter voltages is applied to the control emitter junction of transistor V. In the mode we are considering, the transistor current V decreases and the voltage of the output decreases almost to its original value. Similarly, the change in outputs will be worked out when increasing or decreasing: will change, the corresponding sign will appear, and the transistor current will change. is very high, since during operation the operating mode of the zener diode practically does not change and the current through it is stable.

Compensating voltage stabilizers are produced in the form of ICs, which include a regulating nonlinear element, transistor V, an op-amp and circuits connecting the load to its input.

In Fig. 5.10, c shows the external characteristic of a power source with a stabilizer, its working area is limited by current values