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The electromagnetic principle of power adapter of flyback topology

source:power adapter     Popular:adapter     release time:2021-04-28 09:07:54     Article author:sznbone    

  It can be seen from the identification of the winding end with the same name in the figure that when the switching tube Q is turned on and current flows through the primary, no current flows in the secondary. This is completely different from the forward topology type converter of the power adapter. In the forward topology type converter of the power adapter, when current flows through the primary, current also flows in the secondary, that is, when the primary current flows into the end with the same name, the secondary current flows out. The end of the same name.

The electromagnetic principle of power adapter of flyback topology(图1)

  Since the primary/secondary load ampere turns cancel each other out, they will not cause the magnetic core to move along the hysteresis loop. Therefore, for the converter of the forward topology type of the power adapter, only the excitation current will cause the magnetic core to move along the hysteresis loop and may saturate it. But the excitation current is only a small part of the total primary current (generally less than 10% of the total current).

  However, in a flyback converter, there is no secondary ampere-turn to offset the primary ampere-turn. All the triangular wave primary currents in the figure will cause the magnetic core to move along the hysteresis loop. In this case, if no precautions are taken, even at very low output power, the core without air gap will almost immediately saturate and damage the switch tube.

  In order to prevent saturation of the core of the flyback transformer, the usual approach is to add an air gap to the core. There are two ways to add an air gap to the core. One is to use a solid ferrite core. The air gap can be formed by grinding away a part of the central column of the EE-type or pot-type magnetic core. It is also possible to insert a plastic sheet at the junction of the two U-shaped or UU-shaped magnetic cores to obtain the required air gap.

  The more common method is to use MPP (permalloy powder) cores. This kind of magnetic core is made by mixing the magnetic powder and baking and hardening, that is, mixing the magnetic powder and the plastic resin binder into a slurry and then casting into an annular shape. In this way, each magnetic particle in the magnetic ring is covered by resin, which acts as an air magnetic gap to prevent saturation of the magnetic core. The basic core material that is ground into magnetic powder is called square  permalloy 80, which is an alloy material manufactured by  Magnetics and  Arnold Magnetics, and contains 79% nickel, 17% iron and 4% molybdenum.

  The magnetic permeability of the magnetic ring can be controlled by controlling the concentration of magnetic particles in the slurry. It can also ensure that the magnetic permeability does not change more than ±5% in a very high temperature range, and the obtainable magnetic permeability ranges from 14 to 550. A core with low permeability can be regarded as a core with a larger internal air gap. To obtain the same required inductance, more turns are required, but the corresponding saturation ampere-turns are also high. A core with high permeability requires relatively few turns, but its saturation ampere turns is also low.

  Such an MPP core is not only used for a flyback converter whose primary current is a unidirectional DC, but it can also be used as an output inductor that needs to withstand a large DC component in a forward converter.


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