Just removed the two chargers some time ago, and I can see the circuit inside. In view of the fact that there is a lot of misleading people on the Internet, many websites even have articles saying that there is no transformer isolation in the mobile phone charger. So simply talk about the working principle of the mobile phone charger.
Figure 1: Switching mobile phone charger teardown diagram
The above is the teardown diagram of a cottage charger I have on hand, yes, this is a cottage charger, 5V1A, less than 10 yuan to buy, after the dismantling found that work is still possible, the following other related pictures, not too pit Cottage.
The circuit of the cottage charger is similar to the regular majority. The cottages of some potholes may not have feedback and protection. The quality of the components used is poor or the parts are disassembled. Colleagues still have the problem of parameter virtual standard. The following is the combination of the charger. The machine diagram briefly describes the principle.
Why is the mobile phone charger so small now?
Many people who have a little understanding of electronics remember the traditional big-headed transformer, which converts the AC voltage through the turns ratio of the original secondary winding of the transformer, and then through bridge rectification, voltage regulation filtering, and even through the regulator chip. To achieve a constant voltage output.
Figure 2 Traditional charger and transformer
A big problem with the traditional charger is that the transformer must be made very large. The electric energy is converted into magnetic energy for transmission. In order to transmit enough power, more turns are needed to generate enough magnetic energy to complete the large Voltage conversion of power. Therefore, the larger the current required to be output, the larger the head of the transformer.
So why is the current charger so small? Apple's green dot charger is very mini, what is the principle? This is the advantage of the switch-type power supply, please refer to the internal diagram of the charger in Figure 1, the basic principle of most chargers are the same, so I use this diagram to make a unified description.
First of all, the switch-type power supply also has a transformer, whether it is a mobile phone charger or a computer's switching power supply, this is already indicated in the figure, so why is this transformer so small compared with the traditional transformer? As mentioned earlier, the transformer room transmits energy through electromagnetic conversion. When the current is constant, the current is converted into electromagnetic energy. There are two main factors, the number of turns of the coil, but the frequency of the alternating current. The conventional transformer is fixed by 50 Hz. Frequency, in order to transmit more power, can only change the number of turns, so the greater the power, the larger the transformer.
The switching power supply can keep the transformer in a small volume by satisfying the power requirement by increasing the frequency of the alternating current on the transformer. This feature is very suitable for the needs of modern electronic equipment. Therefore, there are still transformers in the current mobile phone chargers, the difference is the high-frequency transformers, the same is that the output terminals and 220V electricity are still electromagnetically isolated. Those who say that the switch-type mobile phone charger does not have a direct electrical connection to the transformer room are purely misleading.
Through the above analysis, we can know how this type of charger works. Rectification of 200V AC, the cottage transformer in Figure 1 uses four diodes to form a bridge rectifier (a part of the pitted cottage has only one diode, is a half-side rectification, the high-power resistor next to the diode is used for protection), and the high voltage obtained after rectification DC, high-voltage DC through the switch tube (mostly triode, such as 13003, etc.) turn on and off into high-frequency AC, the switch tube conduction and cut-off time can be controlled according to the sampling voltage feedback, thus ensuring a constant voltage output .
Some of the better ones have feedback at the output of the secondary side. As shown in Figure 1, there is a reference voltage source (mostly used TL431). Once the output is abnormal, it can pass the optocoupler (the signal is transmitted through the light, and there is no The electrical connection is fed back to the primary side (high voltage side) for protection.
What are the reasons for the danger of mobile phone chargers?
A simple analysis has been made in the past. It can be seen that there is basically no danger in the case of normal use. What are the dangerous situations that may occur?
1. The trend of miniaturization of mobile phone chargers is very obvious. Compared with the traditional big-headed transformers, the current chargers use switching power supplies instead, and the high-frequency transformers with very small size are replaced by some. The device also has output feedback, and the output feedback is mostly through devices such as optocouplers. The value here shows that whether it is a high-frequency transformer or an optocoupler feedback, one is electromagnetic isolation, one is optically isolated, and there is no electrical connection. Even if the transformer or other components burn out, the 220V power supply will not be stringed to the mobile phone under normal conditions. Come. In the case of an electrocution, the most likely condition is that the charger is flooded and the circuit is shorted.
2, the charger is damaged, the most is the two cases, the first is the explosion capacitor, this movement is relatively large, because it is the switch voltage regulation, once this part of the circuit has problems, the voltage is not normal, the most direct is the explosion capacitance. The second is the explosion transformer, which is mostly caused by the load. Since most chargers have virtual standards, most of them have no feedback and protection circuits, and the transformers are very common. As long as there is no water and no damage, these two are generally not too dangerous for people, but because of the miniaturization of many chargers, the high voltage and low voltage are very close, many chargers have sufficient physical isolation, and a capacitor is used. Energy can easily cause misalignment and adhesion of components inside, posing a hazard.
3, no matter which brand, most of the chargers are made in China, the charger itself has no technical content, the biggest factor is the cost, and the current virtual standard is almost the commonality of all cottage chargers, even if you buy original in Taobao, most It is also very difficult to buy genuine products, so I usually buy chargers for obsolete products such as Palm HP. The price of a dozen pieces is the quality of the first-line products. I will disassemble the other chargers. Look at the bottom.
4, the charger does not necessarily buy the original, but must buy a regular, ordinary mobile phone must be more than 1A current output, iPad and other tablet computers must buy 2A urgent above the current output. The high-current charger can be used on low-current devices, so the iPad's 2A charger is rushed to the 1A iPhone and other phones.
5, the computer's USB output is only 500mA, so the current mobile phone has a detection function, by detecting the resistance between the two wires of USB D+, D- to determine whether it is connected to the computer or USB, all of the chargers do not have this Resistor, in the Android phone battery will display USB instead of AC, affecting the charging speed, this needs attention.
Ok, let's summarize so much and hope to help those in need.
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