Working Principle of DC Linear Power Supply

If there is a circuit as shown in Figure 1, the input voltage Uin is applied to both ends of the series circuit of the variable resistor R and the load resistor RL, then the voltage can be stabilized by changing the voltage drop across R. For example, when the input voltage increases, the resistance of the variable resistor R can be increased so that the increase in the input voltage is completely dropped at both ends of it, so that the output voltage can be maintained unchanged; when the input voltage decreases, the resistance of R can be reduced so that the voltage drop across its two ends decreases with the decrease in the input voltage to maintain the output voltage unchanged; if the input voltage remains unchanged and the load current changes, the resistance of the variable resistor R can also be changed to keep the voltage drop across its two ends unchanged, that is, the output voltage = input voltage - the voltage drop across the variable resistor. This power supply with an adjustment element (variable resistor) connected in series with the load is called a series-type voltage-stabilized power supply.

Figure 1 Voltage stabilization using variable resistors

In actual power supply circuits, the negative feedback principle is usually used to control the resistance value between the collector and emitter of the transistor with the change in output voltage. The principle circuit is shown in Figure 2. Figure 2 shows the simplest single-tube series transistor voltage regulator. The adjustment element is transistor VT1. After the 220V AC voltage is stepped down by the transformer, it is converted into DC Uin by the rectifier and filter circuit and added to the adjustment tube and the load. R'1, R'2 and R3 (R3=R31+R32) form a voltage divider to measure the change in output voltage Uout. VD is a silicon voltage regulator tube that generates a reference voltage, and R4 is its current limiting resistor. The amplifier composed of VT2 plays a role of comparison and amplification, and Rc is its collector resistor. The output of the VT2 collector is directly added to the base of the adjustment tube VT1 to change the resistance between the c and e poles of VT1.

Figure 2 Principle circuit diagram of series transistor regulated power supply

If the output voltage Uout decreases due to a decrease in grid voltage or an increase in load current, the base voltage of VT2 will decrease through the voltage divider composed of R'1, R'2 and R3. Since the emitter of VT2 is connected to the voltage regulator tube VD, the potential at this point is basically unchanged, so VT2's Ube2 decreases, the collector current Ic2 decreases, and thus Uc2 increases. The increase in Uc2 causes an increase in Ib1 and Ic1, and the resistance between c and e of VT1 tube decreases, thereby restoring the output voltage to near the original value. The greater the amplification factor, the smaller the change in output voltage.

When the output voltage increases, it can also be reduced through negative feedback to maintain the output voltage basically unchanged.

By changing the voltage divider ratio of the sampling circuit, the output voltage can be adjusted. The approximate expression of the output voltage Uout is:

In actual voltage-stabilized power supplies, the adjustment tube is often composed of several transistors to expand the output current; in order to reduce the temperature drift of the amplifier and increase its amplification factor, a two-stage differential amplifier circuit is often used. In order to eliminate the impact of unstable input voltage on the amplifier and output voltage, a set of auxiliary power supplies is often added to the voltage-stabilized power supply, which is connected in series with the output voltage to supply power to the amplifier. In precision power supplies, in order to reduce drift and noise, a series of measures will be taken in circuit design, component selection and manufacturing process. For example, a low-pass filter is added between the power grid and the power transformer; the differential tube is placed in a constant temperature bath; noise suppression measures are taken in the sampling circuit; the DC circuit is floated to the ground, etc.