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Safety capacitors in switching power supply

First, safety gauge capacitors
I. Source of the problem
While recently disassembling a low voltage DC power supply circuit board, I came across this capacitor device on the board. From the back of the board, it can be seen that it is connected across the output ground and the input high voltage ground. Here are two questions: What is this device? Why is it necessary to connect this device across the output and input isolation?

Second, what is this device?
The type of this device can be determined by looking at the inscription on it. You can see the above identifier for the JNC, JY102M. Through the network to find the type of device, you can know that it belongs to the safety capacitors. Its capacity is 1000pF, with a withstand voltage of 400V. As for what is a "safety-compliant capacitor", you can find the corresponding description through the network.  I will not repeat it here. In short, here it is a capacitor.

Third, the role of capacitors
Above know that the device is a capacitor. So why add this capacitor between the output ground and input ground? Observe the inverter chip on the circuit board, its model is LP3773. Search the network of this signal design circuit, you can see the output and input ground lines between the safety capacitors. This capacitor connects the output ground to the input ground from the AC. Looking at other switching power supply design circuits, most of them have this capacitance, in this blog post details the role of the switching power supply in the safety capacitor, also known as the Y capacitor, its main role is to prevent the switching power supply external electromagnetic interference.

However, as we know, the input ground of an ordinary switching power supply is actually the common terminal of the rectified AC current, which has a high amplitude AC voltage relative to the zero line. The safety capacitor will actually lead this AC voltage to the output terminal, so in general, the output ground of this kind of low-voltage power supply will be detected with high voltage. Due to the small size of the safety capacitor, this high voltage will not cause people to be electrocuted.

Fourth, cause uneasiness
In one of my previous blog posts: is it leakage or electromagnetic radiation, also recorded a foreigner for the ordinary cell phone charger on the phone to bring the high voltage of the confusion. This is actually the result of the above mentioned safety capacitor leading the high voltage from the AC power supply to the charger's output ground line.

Summary
In this article, we have observed the safety capacitor inside the disassembled switching power supply. It is also known that it not only reduces the external high frequency radiation of the switching power supply, but also brings part of the industrial AC high voltage to the bottom line of the output port. Therefore, it is possible to detect this high voltage when using a charged cell phone.

If you want to know more about switching power supply information, you can consult Xiamen Eahunt Electronics.

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Email: athene-cai@eahunt.com

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