What is the linear regulated power supply _ What are the advantages and disadvantages of linear regulated power supply _ Detailed explanation of the working principle of linear regulated power supply

Linear regulated Power Supplies are a class of DC regulated power supplies that were used earlier. According to the working state of the adjustment tube, we often divide the regulated power supply into two categories: linear regulated power supply and switching regulated power supply. The characteristics of the linear regulated dc power supply are: the output voltage is lower than the input voltage; the reaction speed is fast, the output ripple is small; the noise generated by the work is low; the efficiency is low (the LDO that is often seen now is to solve the efficiency problem) High heat generation (especially high-power power) indirectly adds thermal noise to the system.

According to the working state of the adjustment tube, we often divide the regulated power supply into two categories: linear regulated power supply and switching regulated power supply. In addition, there is a small power supply that uses a Zener. The linear regulated power supply mentioned here refers to the DC stabilized power supply in which the regulating tube operates in a linear state. The adjustment tube works in a linear state, which can be understood as follows: RW (see analysis below) is continuously variable, that is, linear.

In the switching power supply is different, the switch tube (in the switching power supply, we generally call the adjustment tube called the switch tube) is working in the open and closed state: open - the resistance is small; off - the resistance is very Big. The tube operating in the switched state is clearly not linear. Linear regulated power supplies are a class of DC regulated power supplies that were used earlier.

1. Low efficiency;

2. The noise generated by the work is low;

3. The reaction speed is fast and the output ripple is small;

4. The output voltage is lower than the input voltage;

5, large heat (especially high-power power), indirectly add thermal noise to the system.

advantage

High output voltage quality, low ripple, and no need to use inductive components;

Disadvantage

It is a low conversion efficiency, especially in the case of a large input-output voltage difference. If the output current is also large, there will be obvious heat and hot, and even the regulator may be burned out.

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

Let us first illustrate the principle of adjusting the voltage of a 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 size of the RW, the output voltage can be changed. Note that in this equation, 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 diagram does not draw the leading end of the RW to the left and to the right. Although this is no different from the formula, but painted on the right, it just reflects the concept of "sampling" and "feedback"---the actual power supply, most of which are working in the sampling and feedback mode. In the following, there are few feedforward methods, or they are used, and they are just auxiliary methods.

Let's continue: If we use a triode or FET instead of the variable resistor in the figure, and by detecting the magnitude of the output voltage, we can control the resistance of this "varistor" and keep the output voltage constant, so we will The purpose of voltage regulation is achieved. This triode or FET is used to adjust the voltage output, so it is called the 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-type regulated power supply. Correspondingly, there is a shunt regulated power supply, which is to adjust the output voltage by connecting the regulating tube in parallel with the load. The typical reference regulator TL431 is a shunt regulator. The so-called parallel means that, like the Zener tube in Figure 2, the shunting is used to ensure the "stability" of the emitter voltage of the attenuation amplifier. Perhaps this figure does not let you see that it is "parallel" at once, but Take a closer look, it is true. However, everyone should also pay attention here: the Zener here works with its non-linear region, so if it is considered a power supply, it is also a nonlinear power supply. In order to facilitate everyone's understanding, we look back at a suitable map, until we can understand it concisely.

Since the adjustment tube is equivalent to a resistor, the current will flow when the current flows through the resistor, so the adjustment tube operating in a linear state generally generates a large amount of heat, resulting in inefficiency. This is one of the main drawbacks of linear regulated power supplies. For a more detailed understanding of linear regulated power supplies, please refer to Analog Electronic Circuit Textbooks. Here we mainly help you to clarify these concepts and the relationship between them.

Generally speaking, the linear regulated power supply is composed of several basic parts such as an adjustment tube, a reference voltage, a sampling circuit, and an error amplifying circuit. It may also include some parts such as protection circuits, startup circuits, and the like. The following figure is a simple schematic diagram of a linear regulated power supply (schematic diagram, omitting components such as filter capacitors). The sampling resistor samples the output voltage and compares it with the reference voltage. The comparison result is amplified by the error amplifier circuit and the control tube is controlled. The degree of conduction keeps the output voltage stable.