Reduce output voltage ripple. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Reduce output voltage ripple. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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Stabilized power supplies are an important part of electronic devices. The main parameters of the device as a whole often depend on the quality indicators of the power supply. The most important characteristics of the stabilizer are the stabilization coefficients Kst and pulsation suppression Kpulse, and the second parameter is often subject to more stringent requirements than the first.

It is relatively easy to make a stabilizer with a large stabilization factor. It is much more difficult to obtain a low ripple level, since the implementation of a high gain in the feedback circuit of a compensation stabilizer is associated with a deterioration in the stability of its operation. With the advent of op-amps with high gain and low bias voltage, the quality indicators of power supplies have increased dramatically.

Reducing output voltage ripple

However, along with the implementation of high gain, a fairly simple and effective way to increase the ripple suppression factor in almost any compensating voltage regulator is known. This method demonstrates the well-known stabilizer circuit. In it, on the op-amp DA1 and the transistor VT2, a control element (an error signal amplifier) is made, and on VT1 - a control element. If, for example, the change in the output voltage dUout of the stabilizer should not exceed 1 mV when the input dUin changes by 1 V, then theoretically the control element should provide a gain of K=1000.

However, this does not include the attenuation of the signal dUout in the measuring element (measuring voltage divider), consisting of resistors R4, R7. With a low input impedance of the applied op amp, Rin op amp also enters the measuring element. Therefore, in reality, K should be 10 ... 30 times greater than indicated.

The presence of the measuring element reduces the amplitude of the ripple coming through the negative feedback circuit to the input of the control element op amp by the same factor as the signal dUout. With an increase in Uout of the stabilizer, the attenuation of the mismatch signal also increases, therefore, for high-voltage stabilizers, the ripple suppression coefficient is usually lower than for low-voltage ones.

It follows from the foregoing that the stabilizer ripple suppression coefficient can be significantly increased by introducing an additional negative AC feedback through the C4R5 circuit. Output voltage ripples are transmitted to the input of the op-amp either directly (through capacitor C4), if Uout <= 15 V, or through a capacitor and resistor r5 with relatively low resistance. In this case, the division factor of the measuring element for the variable voltage component increases and the Kpulse coefficient increases approximately as many times as the total resistance of the resistor r5 and capacitor C4 at the ripple frequency is less than the resistance of the resistor r4. Resistor r5 is necessary to protect the input circuits of the op-amp at a high output voltage uout (it limits the charging and discharging current of the capacitor C4 when the stabilizer is turned on and off). Diodes vu1 and vd2 serve the same purpose.

The capacitance of the capacitor C4 is selected depending on the resistance of the resistor R4 and the load current. With a decrease in the resistance of the resistor R4 and an increase in the load current of the stabilizer, the capacitance must be increased. Due to the fact that the capacitor C4 is included in the negative feedback circuit, it can greatly affect the time and temperature stability of the output voltage. Therefore capacitor C4 must have low leakage at nominal temperature. With a large capacitance of this capacitor, the voltage at the output of the stabilizer is set more slowly. It should also be noted that the inclusion of capacitor C4 in the OS circuit significantly increases the stability of the stabilizer to low-frequency pickups.

An analysis of the stabilizer operation showed that when working with the same source of reference voltage Uobr, the amplitude of the output voltage ripple without feedback for alternating current reaches 40 ... 50 mV, and with the connection of the feedback circuit it decreases to 1 mV.

In conclusion, it should be noted that the considered method for improving ripple suppression can be applied not only in op-amp stabilizers, but in almost any compensating voltage regulator.

See other articles Section Power Supplies.

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