Adjustable power adapter related technology

　　There are many types of adjustable linear laboratory power adapters. Typically, these power adapters have an adjustable output voltage from zero to a certain maximum value and can be used in constant voltage or constant current control methods. However, no matter which form of control the 5v1a power adapter adopts, the traditional adjustable linear power adapter has three things in common: large, heavy, and low efficiency.

　　When the output voltage is low, most of the rated power of the power adapter is consumed inside the power adapter, and the conversion efficiency will be very low.

　　Using switching converter technology can eliminate additional consumption during operation, and can also reduce size and weight.

　　By using "flyback" or buck-boost-driven switching conversion technology, more advantages can be obtained. The voltage and current provided by a switching power adapter are equal to the voltage and current provided by three independent adjustable linear power adapters.

　　Consumable linear 1A power adapters can provide several different voltage and current ranges, even when the rated output power is the same. For example, a linear power adapter with a rated power of 300W can be in three different situations: 10V, 30A, 30V, 10A or 60V, 5A. When the output voltage of the 30V power adapter is 10V, the maximum output current may be only 10A, then it only outputs 100W of power, and the rest is consumed in the power adapter.

　　Therefore, to include a current range of a certain power level, multiple different linear adjustable power adapters are required.

　　With flyback switch mode technology, a power adapter can provide nearly constant output power for most output voltage and current ranges. A power adapter designed with a capacity of 300W is equivalent to the three linear power adapters mentioned above. The typical output characteristics of this power adapter are shown in the figure.

　　Adjustable switch technology

　　There are many difficulties in the design of adjustable power adapters. Perhaps the most obvious is the relatively large conducted and radiated electronic noise. Laboratory-grade power adapters are often used in research and development applications that are sensitive to electronic noise and ripple. Therefore, there must be good ripple suppression and noise filtering. In this type of application, the filtering cannot be too strong. Faraday shielding of switching devices and filters is a basic requirement, and the application of metal or shielding boxes can reduce EMI (electromagnetic interference) problems.

　　For the power adapter, working at zero output voltage is also a problem, because it requires a very narrow power pulse, and it is difficult to control the required output power at this time. Using fast MOSFET switching and different control methods for low output voltage and current can overcome this limitation.

　　The special nature of digital camera power adapter flyback converter

　　Research on flyback technology has revealed a very useful feature of flyback converters: the energy storage period and the energy conversion period can be regarded as completely independent operations.

　　Consider the simple diagonal half-bridge flyback power part in the figure. When the two MOSFETs are on, the energy is stored in the transformer as magnetic energy. Because the secondary side does not conduct when the MOSFET is turned on, the transformer can be regarded as a single-winding inductor that stores energy. When the conduction period ends, the energy stored in the core is (1/2) LpI2p.

　　At the end of the energy storage period (MOSFET is turned off), the primary side stops conducting, and the transformer works like a single-winding inductor, so that the secondary winding and the output circuit have working conditions.

　　Since these two working processes are independent, there is no direct connection between the work of the primary side and the secondary side (except for necessary dimensions). The output voltage and current of the secondary side are not determined by the input like an ordinary transformer, and the output power can be The sustained power can be obtained from high voltage, small current or low voltage, high current, or any combination of the two, as long as it meets the power conservation requirements.

　　The second obvious feature is that if you continue to maintain the complete energy transfer mode (discontinuous control mode), the number of turns on the secondary side also has no direct effect on the output voltage and current.

　　In short, for certain primary working conditions, the full energy transfer flyback converter transformer can be regarded as a constant energy source.

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