Power Basics 01

maychang

Power Basics 01 [Copy link]

Basic knowledge of power supply 01

Introduction

When electronic engineers say "power supply", they usually refer to the part of the circuit that powers the electronic equipment they design and manufacture. In fact, the "power supply" in their minds is also a kind of electronic equipment. From a physical point of view, the so-called "power supply" refers to a device that converts other forms of energy into electrical energy, such as a device that converts mechanical energy into electrical energy (called a generator), a device that converts chemical energy into electrical energy (called a battery), a device that converts thermal energy into electrical energy (thermocouple), and so on. These devices that convert other forms of energy into electrical energy are often called primary power supplies. Compared with primary power supplies, devices that convert one form of electrical energy into another form of electrical energy are often called secondary power supplies. Therefore, the power supply in the minds of electronic engineers often refers to secondary power supplies. Electronic equipment that does not require high power supply quality often uses generators, batteries, etc. to power electronic equipment directly in order to save costs. For example, portable radios are usually powered directly by batteries, and military transceivers during World War II were directly powered by hand-cranked generators. However, as the performance of electronic devices improves, the requirements for electronic device power supply are also increasing. Direct use of primary power supply often cannot meet the requirements - the voltage at both ends of the battery will decrease when it is worn out, and the AC mains voltage is not stable enough. Therefore, secondary power supplies must be used to improve the quality of power supply for increasingly sophisticated electronic devices. These secondary power supplies designed to improve the quality of power supply are the subject of our discussion. In this article, "power supply" refers to secondary power supply for improving the quality of power supply. The so-called "improving the quality of power supply" is nothing more than the power supply voltage or current being not affected by the changes in the primary power supply and the load changes, and being able to remain stable. From this, two categories can be immediately divided: one is the power supply with regulated output, and the other is the power supply with regulated current output. Regulated power supplies are more commonly used in electronic devices, while regulated current power supplies are less used. We mainly discuss regulated power supplies, and only briefly introduce regulated current power supplies. Whether it is a regulated power supply or a regulated current power supply, there are two control modes: linear and switching. In the early days of the development of integrated circuits, linear regulated power supplies were the mainstream. But with the development of integrated circuit technology, switching regulated power supplies have become the mainstream. This "Basic Knowledge of Power Supply" mainly discusses the work of switching regulated power supplies, and only briefly introduces linear regulated power supplies.

Linear voltage regulator

There are two types of linear voltage regulators, one is a series linear voltage regulator and the other is a parallel linear voltage regulator. Figure (01) Figure (01) shows the working principle of a series linear voltage regulator. In the figure, a variable resistor Rx is connected in series with the load. If we can reduce the variable resistor Rx when the voltage Uo across the load Rload is lower than the rated value, the voltage Uo across the load will obviously increase; conversely, if the voltage across the load is higher than the rated value, the variable resistor Rx is increased, and the voltage Uo across the load will obviously decrease. This adjustment process should be carried out until the voltage Uo across the load returns to the rated value. In fact, we certainly cannot manually adjust the variable resistor to stabilize the voltage across the load. Compared with the speed of change of electricity, the speed of human movement is too slow. But we can use electrical methods to adjust it. Figure (02) Figure (02) is a schematic diagram of using electrical methods to adjust the voltage Uo across the load. We can see that the voltage Uo across the load Rload is connected to the inverting input of the amplifier A after being divided by the resistors R1 and R2, while the non-inverting input of the amplifier is connected to a voltage reference Us. The voltage Uo across the load is UoR2/(R1+R2) after voltage division. If UoR2/(R1+R2) is greater than Us, the output of the amplifier decreases, and the transistor (or field effect transistor) T tends to be cut off (which is equivalent to the variable resistor Rx in the previous figure becoming larger), which will reduce the voltage Uo across the load. If UoR2/(R1+R2) is less than Us, the output of the amplifier increases, and the transistor T tends to be turned on (which is equivalent to the variable resistor Rx in the previous figure becoming smaller), which will reduce the voltage Uo across the load. In either case, the voltage across the load will eventually return to a stable value, that is, the state where UoR2/(R1+R2) is equal to Us. The transistor T is connected in series with the load, so this circuit is called a series linear voltage regulator. This type of series linear voltage regulator has been integrated, and the more typical chip models are the 78XX series and the 79XX series. If the voltage divider ratio of resistors R1 and R2 is adjustable, or the reference voltage Us is adjustable, then this is a linear voltage regulator with adjustable output. This type of linear voltage regulator with adjustable output voltage has also been integrated, and the typical models are LM317 and LM337. Figure (03) We redraw Figure (02) into Figure (03), and the two figures only have the amplifier symbols opposite to each other. Netizens who are familiar with negative feedback amplifier circuits can see that the circuit in Figure (03) is actually a negative feedback amplifier, whose input signal is the reference voltage Us, and whose output is Uo. This negative feedback amplifier is formed by cascading amplifier A and transistor T, and it is still an amplifier after cascading. But it is slightly different from ordinary negative feedback amplifiers: this linear voltage regulator only needs to output unidirectional current, while ordinary negative feedback amplifiers often require that the output end can output current in both directions (the output end can flow out current or flow in current). In Figure (02) or Figure (03), since the current must flow out of the emitter of the transistor T and cannot flow in during normal operation, it is a negative feedback amplifier with unidirectional output. Its output voltage can be stable in response to changes in the power supply voltage Ui and the load Rload, which is entirely due to the effect of negative feedback. Figure (04) Figure (04)This is the working principle of parallel linear voltage regulator. In the figure, a variable resistor Rx is connected in parallel with the load, and a resistor R is connected in series in the input circuit. If we can increase the variable resistor Rx when the voltage Uo across the load Rload is lower than the rated value, because the current divided by Rx is reduced, the voltage Uo across the load will obviously increase; conversely, when the voltage across the load is higher than the rated value, the variable resistor Rx is reduced, and the voltage Uo across the load will obviously decrease. This adjustment process should be carried out until the voltage Uo across the load returns to the rated value. Figure (05) Similarly, we cannot manually adjust the variable resistor to stabilize the voltage across the load, and can only do so by electrical means. Figure (05) is a schematic diagram of using electrical methods to adjust the voltage Uo across the load. If the voltage Uo across the load is higher than the reference voltage Us after being divided by R1 and R2, the output of amplifier A becomes higher, and transistor T tends to conduct and flow a larger current, which is equivalent to Rx decreasing in the previous figure, increasing the current divided by the load, and Uo will decrease. If the voltage Uo across the load is less than Us after being divided by R1 and R2, then the output of amplifier A becomes low and transistor T tends to be cut off, which is equivalent to the increase of Rx in the previous figure, the reduction of the current diversion to the load, and Uo will increase. In either case, the voltage across the load will eventually return to a stable value, that is, the state where UoR2/(R1+R2) is equal to Us. This figure is very similar to Figure (03), the only difference is that transistor T is not connected in series with the load but in parallel with the load. Therefore, this voltage regulator circuit is called a parallel linear voltage regulator circuit. Figure (05) is still a negative feedback amplifier, whose input signal is the reference voltage Us, and its output is Uo. Like Figure (03), this amplifier (cascaded A and T) only needs a unidirectional output current. The difference from Figure (03) is that Figure (03) outputs from the emitter of transistor T, and Figure (05) outputs from the collector of transistor T. Therefore, the reference voltage Us in Figure (03) is connected to the non-inverting input terminal of amplifier A, while the reference voltage Us in Figure (05) is connected to the inverting input terminal of amplifier A. This type of parallel linear voltage regulator has also been integrated, and the more typical chip model is TL431 (voltage divider resistors R1 and R2 are external). TL431 has low power and the output voltage is less affected by temperature. It is usually used as an output adjustable voltage reference. Both the series linear voltage regulator circuit and the parallel linear voltage regulator circuit can only generate a stable voltage that is lower than the input voltage. This is the common point of the two voltage regulator circuits. Another common point is that the efficiency is not high, because both voltage regulator circuits rely on the loss of part of the input voltage to achieve voltage regulation. However, the adjustment tube of the series linear voltage regulator circuit generates less heat when the load is light (the load resistance is large), while the adjustment tube of the parallel linear voltage regulator circuit generates less heat when the load is heavy (the load resistance is small). This content is originally created by EEWORLD forum user maychang. If you want to reprint or use it for commercial purposes, you must obtain the author's consent and indicate the source

sh020309

On the way home, I thought about it and it was pretty good.

话说什么呢

The idea of organization is very clear, collect it first

hk6108

Another point to improve the quality of power supply is the sharing issue. The "power supply" must be able to prevent interference from the power supply, and prevent the load disturbance from spreading through the power supply. Multiple loads share one power supply, and the loads may interfere with each other. If filtering alone cannot overcome this problem, it may be necessary to equip some (or all) loads with a "power supply". The voltage and current balancing required for multiple power supplies to supply power to one load can also be "powered".

洌酒游龙

You can learn it.

zxfzb828

Thanks for sharing!!! ...