The primary-side feedback AC / DC control technology is a new AC / DC control technology developed in the past 10 years. Compared with the traditional secondary-side feedback optocoupler plus 431 structure, the biggest advantage is that these two chips are eliminated and A group of components working with it, which saves space on the system board, reduces costs and improves system reliability. It has broad application prospects in markets with high cost pressures, such as mobile phone chargers, and markets with high volume requirements, such as LED drivers.
After omitting this set of components, in order to achieve high-precision constant current / constant voltage (CC / CV) characteristics, new technology must be used to monitor real-time changes in load, power supply and temperature, and the same batch of components Sub-tolerance, which involves primary (primary) adjustment technology, transformer tolerance compensation, cable compensation and EMI optimization technology.
The principle of primary regulation is to detect the load change information by accurately sampling the voltage change of the auxiliary winding (NAUX). When the controller opens the MOS tube, the transformer primary winding current ip rises linearly from 0 to ipeak, the formula is
Indicates that VCS is the voltage on the CS pin, and the meaning of other parameters is shown in Figure 1. This is how the constant voltage (CV) mode works.
Among them C1 is a constant less than 0.5, VCSLMT is CS pin voltage limit value.
After the ratio of the demagnetization time and the switching period is kept constant, the output voltage and current are independent of the inductance of the transformer. Therefore, on the practical level, the requirements of the application solution for the consistency of the inductance of the same batch are reduced. , Thereby reducing the cost of mass production and processing.
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