What is the tragedy caused by the missing link of “Called by electrocution�
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Figure 1 Electric shock injury in daily life
So why mobile phone charger output with 220V AC? This is back to the isolation voltage problem of our subject power supply. The isolation voltage of the power supply is also known as the electrical strength in the GB4943 national standard. This GB4943 standard is the safety standard for information equipment that we often say. It is a national standard to prevent people from physical and electrical harm, including the protection from electric shock. Physical damage, explosion, and Other injuries. Here we briefly introduce the power isolation voltage and how to select the isolation voltage problem.
How to test the power isolation voltage?
The isolation withstand voltage is mainly to check whether the solid insulation used in the device has sufficient electrical strength. The waveform of the test voltage that the insulation should withstand is basically a sinusoidal waveform and an AC voltage with a frequency of 50 Hz or 60 Hz. The test voltage of a DC power supply is usually a DC voltage.
Most people will confuse the isolation voltage of the power supply with the input impulse voltage of the power supply. The following distinguishes them from the test method. We often say that the power source withstand voltage refers to the electrical strength between the primary circuit and the secondary circuit. To avoid damage to components or insulation that are not related to the test, an integrated circuit or similar circuit may be disconnected or an equipotential bonding may be used. . A simpler equipotential bonding test is generally used for power supply testing. The connection diagram is as follows: 
Figure 2 isolation pressure test diagram
As long as the power supply meets the requirements of the relevant electrical strength (specific requirements can refer to the GB4943 standard), it can ensure that dangerous voltage will not be transmitted to the secondary circuit, and thus will not threaten human life safety.
How to choose the power isolation voltage?
From the above understanding of the power isolation voltage withstand the choice of the key, but it is not the choice of the isolation voltage higher the better, because the higher the isolation voltage will affect the power of other performance, so just choose the right pressure value on the line . The conductors in general circuits that involve dangerous voltages and that are accessible to personnel in the circuit, such as some interface terminals, have strict test requirements for the isolation voltage of the circuit. Let's briefly introduce the isolation and voltage selection of AC-DC and DC-DC power supplies. 
Figure 3 AC-DC power module application
The AC-DC power supply is a primary power supply and converts 220 V mains power into low voltage direct current. It can also be divided into isolated and non-isolated. If you are using a non-isolated power supply, you need to consider sealing all live parts during the product's design to prevent accidental contact with dangerous voltages. If an isolated power supply is used, only the power supply part can meet the relevant isolation withstand voltage and safety requirements. The commonly used industrial power supply voltage can meet 3000VAC. For some special occasions, such as power applications, it is generally required to meet the isolation voltage requirements of 4000VAC and above. Specific pressure resistance requirements can refer to various industry standards. The withstand voltage of the conventional irrigation module power supply is 3000VAC. 
Figure 4 Isolated application of DC-DC power module
The DC-DC power supply is a secondary power supply. Generally, the withstand voltage requirement is based on its application. If communication isolation is generally performed to isolate interference, the isolation withstand voltage can be 1000VDC to 1500VDC. When one end of the circuit may involve a dangerous voltage, it is generally considered to raise the isolation voltage to 3000 VDC. In addition, there are some special application sites, such as medical applications, where many isolated power supplies require isolation voltages of 4000 VAC or 6000 VDC.
According to the above description, the power isolation withstand voltage is varied. How can we achieve this? There are generally two channels. One is to use high-frequency transformers and optocouplers with good isolation characteristics, and the other is to use a power supply with glue injection, which can further improve the isolation performance of the power supply. Generally, in an integrated circuit, in order to save space and reach a corresponding safety requirement, most engineers will consider using an isolated power supply module to design a circuit, which can save space and save time and cost.