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Circuit protection scheme for USB power adapter

Article Source：Kinri Energy | Author：Kinri Energy | Issuing Time：2024.05.29
With the continuous upgrading and powerful functions of smart handheld devices such as smartphones and tablets in today's society, entertainment and personalized applications have also made the battery life of devices a blind spot. In real life, we may often see our friends around us carrying a portable power supply. Without a portable power supply, we can only find a place to charge the device at any time. Therefore, power adapters have always been a necessity for people as standard products.


12V ROUTER POWR ADAPTER

Miniaturized adapters, represented by Apple's USB power adapter, are becoming increasingly popular, and the SMD miniaturization packaging of more and more circuit components allows commonly used power chargers to become more compact and portable, combining aesthetics and portability. This article explains from the perspective of the application of fuses, an important safety device in internal circuits, how AEM Technology's innovative SMD250VACFUSE - MF2410 series adapts to the trend and how it is laid out for AC applications on such small-sized AC/DC power adapters. It also explains how to achieve our advocated requirement of "interrupting at the right time, interrupting at the wrong time, ensuring safety at all times!".

As a UMF universal modular fuse, engineers must consider meeting the following requirements at the initial design stage.

1. Structurally maximize the small volume requirements of small-sized power adapters for devices

Taking USB power adapter as an example, at this level, the structure limits the volume of internal components, such as the coin sized PCB area, which makes SMD components the preferred choice for engineers.

2. SMT process suitable for reflow soldering and wave soldering

From the perspective of production technology, AEM's MF fuse material and structure are unique. This SMT production process not only saves a lot of labor and auxiliary material costs, but according to our investigation of customers who use SMDfuse, the bending of the pins of the plug-in caused the fuse body to break, which is also one of the reasons.

Secondly, due to the essential components of power circuit plugins, production factories use peak soldering, which requires fuses to withstand high temperatures. Compared with other SMD ceramic fuses in the industry, AEM's UMF universal modular fuse uses epoxy resin as the matrix, and the electroplated through hole connection method forms a reliable electrical and mechanical connection between the fuse and the end. There is no phenomenon of end welding being heated off, and the ability to withstand high temperatures is outstanding.

3. Application based on power circuit design

The USB power adapter circuit belongs to the AC/DC power supply circuit, which is a typical switch power supply composition

The main circuit of a switching power supply is composed of an input electromagnetic interference filter (EMI), a rectification and filtering circuit, a power conversion circuit, a PWM controller circuit, and an output rectification and filtering circuit. Auxiliary circuits include input overvoltage and undervoltage protection circuit, output overvoltage and undervoltage protection circuit, output overcurrent protection circuit, output short circuit protection circuit, etc.

Common input circuit considerations for lightning strike testing of fuses:

Working principle of AC input lightning protection circuit:

During lightning strike testing, a circuit consisting of MOV1, MOV2, MOV3: F1, F2, F3, FDG1 is used for protection. When the voltage applied to both ends of the varistor exceeds its working voltage, its resistance value decreases, causing high voltage energy to be consumed on the varistor. If the current is too high, F1, F2, F3 will burn out the protection circuit of the later stage. Of course, in practical customer applications, based on cost considerations, most manufacturers only use F2 and F3 in harsh environments.

For more information about the circuit protection scheme for USB power adapters, please contact Kinri Energy. Kinri is a professional enterprise engaged in the development, production, and sales of power adapters.
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