The development history of switching power adapter

　　Before the advent of switching power adapters, linear regulated power adapters (referred to as linear power adapters) have been used for a long time. Then, the switching power adapter appeared as an alternative to the linear power adapter, and the name of the switching power adapter was also generated relative to the linear power adapter. Figure 14 is a structural diagram of the linear power adapter. The key component in the figure is the regulator tube V. The output voltage is detected during work, and it is compared with the reference voltage U, and the error is used to perform negative feedback control on the base current of the regulator tube V.

　　In this way, when the input voltage 4 changes, or the load change causes the output voltage a of the power adapter to change, the output voltage a can be stabilized by changing the tube voltage drop a of the regulator tube V. In order for the regulator tube V to exert sufficient regulation, V must work in a linearly amplified state and maintain a certain tube pressure drop. Therefore, this power adapter is called a linear power adapter. The DC input circuit of a linear power adapter is usually composed of a rectifier transformer T working at a power frequency and a diode rectifier plus a capacitor filter. Because the voltage variation range of the AC power adapter is sometimes larger, the variation range is also larger. In addition, the filter capacitor C connected to the diode rectifier circuit cannot be very large, so there will be a certain amount of pulsation. But these can be adjusted by adjusting the tube voltage drop of tube V to make the output voltage a. Both the accuracy and ripple meet higher requirements.

　　There are two functions of the rectifier transformer T in the figure: one is to make n ratio through reasonable calculation of its voltage ratio. A suitable value is higher to ensure that the regulator tube V can work in the amplified state. The second is to electrically isolate the output voltage from the AC input power adapter, which is also very important.

　　Although the linear power adapter in the figure can meet the requirements of the required DC voltage and power supply quality (precision, ripple, etc.), it has two serious shortcomings: one is that the regulator tube V works in a linear amplification state, and the loss is large. Therefore, the efficiency of the entire power adapter is very low; second, a power frequency transformer T is required, which makes the power adapter large in size and heavy in weight.

　　The switching power supply adapter appeared to overcome the shortcomings of the linear power adapter, and its typical structure is shown in Figure 12. The rectifier circuit in Figure 1-2 is to obtain the DC voltage u1 after the AC power adapter is directly filtered by the diode rectifier circuit and the capacitor C, and then the inverter is inverted into a high-frequency AC square wave pulse voltage. Since the audio range that the human ear can hear is generally 20Ha~20kH, the switching frequency of the inverter is mostly selected above 20kH, so as to avoid disturbing noise pollution. The inverter output is isolated by a high-frequency transformer T and transformed into an appropriate AC voltage, which is then rectified and filtered into the required DC output voltage u.

　　When the AC input voltage, load, etc. change, the DC output voltage u. Will also change. At this time, the width of the square wave pulse voltage output by the inverter can be adjusted to make the DC output voltage. keep it steady. It can be seen from Figure 1-2 and the working principle of the switching power supply adapter that the inverter circuit is more complicated and it is the core part of the switching power supply adapter.

　　Although the above circuit structure looks more complicated, it has several outstanding features compared to the linear power adapter shown in Figure 1-1. First of all, the power electronic devices in the inverter circuit that regulates the output voltage in this circuit all work in the switching state, and the loss is small, so that the efficiency of the power adapter can reach 90% or even more than 95%. Secondly, the transformer T, which plays the role of isolation and voltage conversion in the circuit, is a high-frequency transformer, and its working frequency is mostly above 20kHz. Anyone who has handled electronic instruments will have this experience. Electronic instruments are often "sink on one end", and the heavier end is often the end where the power adapter transformer is located. Because the volume of the high-frequency transformer can be made small, the volume of the entire power adapter is greatly reduced, and the weight is greatly reduced. At the same time, due to the high operating frequency, the size of the filter is greatly reduced. Since the power electronic devices in the power adapter shown in FIG. 12 always work in a switching state, it is called a switching power adapter compared to a linear power adapter.

　　The above-mentioned switching power supply adapter belongs to an isolated switching power supply adapter because it is isolated by a high-frequency transformer. There is also a power adapter without a transformer, which is non-isolated and also belongs to the category of switching power adapters. Figure 1-3 is a typical non-isolated switching power adapter circuit. What is drawn in the picture is actually a step-down chopper circuit, which adjusts the output voltage by adjusting the width of the output pulse voltage (that is, adjusting the on-duty ratio D of the switching device V). In addition to the step-down circuit in the figure, there are also a variety of non-isolated switching power supply adapter circuits such as a step-up circuit. The relevant content will be described in detail above.

　　There is also a large type of common DC power adapter, which is the thyristor phase control power adapter (referred to as the phase control power adapter for short) as shown in Figure 14. Shown in the figure is a single-phase fully-controlled bridge rectifier circuit, which is one of the most commonly used phase-controlled power adapter circuits. Regarding this kind of circuit, there is a detailed introduction in the textbook of power electronic technology, so I won't repeat it here. As for the single-phase fully-controlled bridge rectifier circuit shown in Figure 14, the output DC voltage contains 100Hz ripple. If it is changed to a three-phase fully-controlled bridge rectifier circuit, the ripple frequency in the DC output voltage will be It becomes 300Hz. However, no matter which form of phase-controlled rectification circuit is adopted, the switching frequency of the power electronic device (thyristor) is based on the power frequency, which is 50H in my country (100H of a single-phase bridge is twice that of 50Hz, The 300Hz of the three-phase bridge is 6 times that of 50Hz).

　　Like the switching power adapter, the power electronics in the phase-controlled power adapter also work in the on-off state, but its working frequency is power frequency instead of high frequency. In contrast, a significant advantage of the phase-controlled power adapter is simple circuit and convenient control. Its main disadvantage is that it also uses a power frequency transformer T, which makes the entire power adapter large and heavy, which is similar to a linear power adapter. In addition, the DC output voltage ripple frequency of the phase-controlled power adapter is only a few times the power frequency (2 times for a single-phase full-control bridge, 6 times for a three-phase full-control bridge), and a larger filter is required. The filtering effect. The ripple frequency of the DC output voltage of the switching power supply adapter is very high, often above 20kHz, so only a small filter is needed. Due to the low switching frequency of the phase-controlled power adapter, its response speed to control is also slower than that of the switching power adapter.

　　According to current habits, switching power supply adapters specifically refer to DC power adapters in which power electronic devices work under high-frequency switching. Therefore, switching power supply adapters are often referred to as high-frequency switching power supply adapters, while phase-controlled power supply adapters are not included in Inside the switching power adapter. Therefore, it can be said that the switching power adapter is the abbreviation of the high-frequency DC switching power adapter, in which "high frequency" excludes the phase-controlled power adapter, and "DC" excludes the AC power adapter (such as UPs and other power electronic devices are in the on-off state, but It is an AC power adapter).

　　The above briefly introduced three types of DC power adapters: linear power adapter, switching power adapter, and phase-controlled power adapter. Table 12 compares their main features and scope of application.

　　As far as power adapters are concerned, in addition to the above-mentioned DC power adapters, there is a large category of AC power adapters in the standard. For example, UPS provides Constant Voltage Constant Frequency (CvCF) power adapters, and inverters provide Variable Voltage Variable Frequency (wwVF) power adapters. The power electronic devices in them all work on switches. Status and operating frequency are also higher, but they are not switching power adapters.

　　In summary, a power adapter that meets the three conditions at the same time can be called a switching power adapter. These three

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