Analysis of the working principle of linear regulated power supply

The principle of voltage regulation by linear regulated power supply

As shown in the figure below, the variable resistor RW and the load resistor RL form a voltage divider circuit, and the output voltage is:

Uo=Ui×RL/（RW+RL）, so by adjusting the value of RW, the output voltage can be changed. Please note that in this formula, if we only look at the value change of the adjustable resistor RW, the output of Uo is not linear, but if we look at RW and RL together, it is linear. Also note that our figure does not draw the lead end of RW connected to the left, but to the right. Although this does not make any difference from the formula, drawing it on the right just reflects the concepts of "sampling" and "feedback"----in practice, most of the power supplies work in the sampling and feedback mode, and the feedforward method is rarely used, or even if it is used, it is only an auxiliary method.

Let's continue: If we use a transistor or field effect tube to replace the variable resistor in the figure, and control the resistance of this "variable resistor" by detecting the output voltage, so that the output voltage remains constant, we can achieve the purpose of voltage regulation. This transistor or field effect tube is used to adjust the voltage output, so it is called an adjustment tube.

As shown in Figure 1, since the adjustment tube is connected in series between the power supply and the load, it is called a series voltage regulator. Correspondingly, there is also a parallel voltage regulator, which is to adjust the output voltage by connecting the adjustment tube in parallel with the load. The typical reference voltage regulator TL431 is a parallel voltage regulator. The so-called parallel means that the voltage regulator in Figure 2 is used to ensure the "stability" of the emitter voltage of the attenuation amplifier tube by shunting. Maybe this figure cannot let you see it is "parallel" at once, but it is indeed the case if you look carefully. However, everyone should also pay attention to this: the voltage regulator here works in its nonlinear region. Therefore, if it is considered as a power supply, it is also a nonlinear power supply. In order to facilitate everyone's understanding, let's look back at a reasonable figure until we can understand it concisely.

Since the adjustment tube is equivalent to a resistor, it will generate heat when current flows through the resistor, so the adjustment tube working in a linear state will generally generate a lot of heat, resulting in low efficiency. This is one of the main disadvantages of linear regulated power supplies. If you want to know more about linear regulated power supplies, please refer to the analog electronic circuit textbook. Here we mainly help you sort out these concepts and the relationship between them.

Generally speaking, a linear voltage regulator consists of several basic parts, such as an adjustment tube, a reference voltage, a sampling circuit, and an error amplifier circuit. In addition, it may also include some parts such as a protection circuit and a startup circuit. The figure below is a relatively simple linear voltage regulator schematic (schematic diagram, omitting components such as filter capacitors). The sampling resistor samples the output voltage and compares it with the reference voltage. After the comparison result is amplified by the error amplifier circuit, the conduction degree of the adjustment tube is controlled to keep the output voltage stable.

Commonly used linear series voltage-stabilized power supply chips include: 78XX series (positive voltage type), 79XX series (negative voltage type) (in actual products, XX is represented by a number, and the output voltage is the same as XX. For example, 7805 has an output voltage of 5V); LM317 (adjustable positive voltage type), LM337 (adjustable negative voltage type); 1117 (low voltage difference type, there are multiple models, and the voltage value is represented by the decimal point. For example, 1117-3.3 is 3.3V, and 1117-ADJ is an adjustable type).

Linear regulated power supply circuit diagram (I)

The typical discrete component linear voltage regulator circuit is shown in the figure. The core components of this circuit are the error sampling circuit composed of R2, RP, and R3, the reference voltage circuit composed of R4 and VZ, and the error amplifier VT2.

When the input voltage Ui increases or the load becomes lighter, causing the output voltage Uo to increase, the sampling voltage generated by the voltage divider R2, RP, and R3 increases. This voltage is added to the base of VT2. Since the emitter potential of VT2 remains unchanged, VT2 conduction is strengthened, and its collector potential decreases, that is, the base potential of VT1 decreases, causing the output voltage of VT1 to drop to a normal value. When the output voltage Uo increases, the voltage regulation control process is reversed.

Linear regulated power supply circuit diagram (II)

Integrated circuit voltage regulators include non-adjustable and adjustable types. The following introduces the principle of manual adjustment of the output voltage of an adjustable integrated circuit voltage regulator. The typical integrated circuit linear voltage regulator circuit is shown in the figure. The core components of this circuit are potentiometer RP and voltage regulator LM317.

After adjusting the potentiometer RP to increase the voltage at the ADJ terminal of LM317, the output voltage Uo of LM317 will increase, and vice versa.