Detailed explanation of the working principle of linear regulated power supply

Working principle: Let us first use the following figure to illustrate the principle of voltage regulation by a linear regulated power supply.

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

Uo=Ui×RL/(RW+RL), so by adjusting the size 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 in our picture, the RW lead-out is not drawn to the left, but to the right. Although there is no difference from the formula, the drawing on the right just reflects the concepts of "sampling" and "feedback" - most of the actual power supplies work in the sampling and feedback mode. Below, 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 triode or field effect transistor to replace the variable resistor in the picture, and control the resistance of this "varistor" by detecting the output voltage to keep the output voltage constant, then we The purpose of voltage stabilization is achieved. This triode or field effect tube is used to adjust the voltage output, so it is called an adjustment tube.

As shown in Figure 1, because the regulating tube is connected in series between the power supply and the load, it is called a series regulated power supply. Correspondingly, there is also a parallel-type regulated power supply, which is to connect the regulating tube in parallel with the load to adjust the output voltage. The typical reference voltage regulator TL431 is a parallel-type voltage regulator. The so-called parallel connection means that like the voltage regulator tube in Figure 2, shunting is used to ensure the "stability" of the emitter voltage of the attenuation amplifier tube. Maybe this picture does not allow you to immediately tell that it is "parallel connection", but Upon closer inspection, it is indeed the case. However, everyone should also pay attention here: the voltage regulator tube here works using its nonlinear region. Therefore, if it is considered a power supply, it is also a nonlinear power supply. In order to make it easier for everyone to understand, let's go back and find a suitable picture to look at 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. Therefore, the adjustment tube working in a linear state will generally generate a large amount of heat, resulting in low efficiency. This is one of the most important shortcomings of linear regulated power supplies. For a more detailed understanding of linear regulated power supplies, please refer to the Analog Electronic Circuits textbook. Here we mainly help you clarify these concepts and the relationship between them.

figure 1

Generally speaking, a linear regulated power supply consists of several basic parts such as a regulating tube, a reference voltage, a sampling circuit, and an error amplifier circuit. In addition, it may also include parts such as protection circuits, startup circuits, etc. The figure below is a relatively simple schematic diagram of a linear regulated power supply (schematic diagram, filter capacitor and other components are omitted). The sampling resistor samples the output voltage and compares it with the reference voltage. After the comparison result is amplified by the error amplification circuit, the adjustment tube is controlled. The degree of conduction keeps the output voltage stable.

figure 2

Commonly used linear series regulated power supply chips are: 78XX series (positive voltage type), 79XX series (negative voltage type) (in actual products, XX is represented by numbers, and the output voltage is what XX is. For example, 7805, the output voltage is 5V); LM317 (adjustable positive voltage type), LM337 (adjustable negative voltage type); 1117 (low voltage dropout type, there are many models, using the mantissa to indicate the voltage value. For example, 1117-3.3 is 3.3V, 1117-ADJ is adjustable).