How to understand the circuit diagram - voltage regulator circuit
Regulator circuit
Fluctuations in the AC grid voltage and changes in the load current cause the output voltage and current of the rectified power supply to fluctuate accordingly. Therefore, a higher power electronic circuit must use a regulated power supply.
(1) Zener diode shunt regulator circuit
A circuit with a Zener and load in parallel is the simplest regulator circuit, see Figure 4 (a). In the figure, R is a current limiting resistor. The output current of this circuit is very small, and its output voltage is equal to the stable voltage value VZ of the Zener diode.
(2) series regulator circuit
A series regulator circuit with amplification and negative feedback is the most commonly used regulator circuit. Its circuit and block diagram are shown in Figures 4 (b), (c). It detects the change of the output voltage from the sampling circuit (R3, R4), compares it with the reference voltage (VZ) and amplifies it by the amplifier (VT2) and applies it to the adjusting tube (VT1) so that the voltage across the adjusting tube Variety. If the output voltage drops, the voltage drop of the adjustment tube is also lowered, and the output voltage is raised. If the output voltage rises, the voltage drop of the adjustment tube is also increased, so that the output voltage is depressed, and the output voltage is substantially unchanged. . On the basis of this circuit, it has developed into many variant circuits or added some auxiliary circuits, such as a composite tube as an adjustment tube, an output voltage adjustable circuit, an operational amplifier for comparative amplification, and an auxiliary power supply and overcurrent protection circuit. Wait.
(3) Switching regulator circuit
The new regulated power supply widely used in recent years is a switching regulator power supply. Its adjustment tube works in the switching state, its own power consumption is very small, so it has the advantages of high efficiency, small size, etc., but the circuit is more complicated.
There are many different types of switching regulators. Its basic block diagram is shown in Figure 4 (d). In the figure, inductor L and capacitor C are energy storage and filter components, and diode VD is a freewheeling diode that provides a current path for the L and C filters when the regulator is in the off state. The switching frequency of the switching power supply is very high, generally several to several tens of kilohertz, so the volume of the inductor is not large, and there are not many high harmonics in the output voltage.
Its basic working principle is: From the sampling circuit (R3, R4), the sampling voltage is detected and amplified to control a rectangular wave generator. The output pulse of the rectangular wave generator controls the conduction and cut-off time of the regulating tube (VT). If the output voltage U 0 decreases due to fluctuations in the grid voltage or load current, the output pulse of the rectangular wave generator is widened, so that the conduction tube conduction time is increased, so that the L and C tank circuits obtain more energy. As a result, the output voltage U 0 is boosted to achieve a stable output voltage.
(4) Integrated voltage regulator circuit
In recent years, a large number of integrated voltage regulator products have been introduced, with many varieties and different structures. At present, there are three-terminal integrated voltage regulators, such as the CW7800 series that output positive voltage and the CW7900 series that output negative voltage. The output current is from 0.1A to 3A, and the output voltage is 5V, 6V, 9V, 12V, 15V, 18V, 24V, etc.
This integrated regulator has only three terminals, and all parts of the regulator circuit, including the high-power adjustment tube and protection circuit, are integrated into the chip. When you use it, just add the heat sink and connect it to the rectifier filter circuit. There are few external components, high voltage regulation accuracy, reliable operation, and generally no debugging is required.
Figure 4 (e) is a three-terminal regulator circuit. In the figure, C is the main filter capacitor, C1 and C2 are the capacitors for eliminating parasitic oscillations, and VD is the protection diode used to prevent the input short circuit from burning out the integrated block.
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